Dr. Kaan Yücel
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INTRODUCTION TO NERVOUS SYSTEM
The nervous system comprises the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS is surrounded and protected by the skull (neurocranium) and vertebral column and consists
of the brain and the spinal cord. The PNS exists primarily outside these bony structures.
The entire nervous system is composed of neurons, which are characterized by their ability to
conduct information in the form of impulses (action potentials), and their supporting cells plus some
connective tissue. A neuron has a cell body (perikaryon) with its nucleus and organelles that support the
functions of the cell and its processes. Dendrites are the numerous short processes that carry an action
potential toward the neuron’s cell body, and an axon is the long process that carries the action potential
away from the cell body. Many axons are ensheathed with a substance called myelin, which acts as an
insulator. Myelinated axons transmit impulses much faster than nonmyelinated axons.
One neuron communicates with other neurons or glands or muscle cells across a junction between
cells called a synapse. Typically, communication is transmitted across a synapse by means of specific
neurotransmitters, such as acetylcholine, epinephrine, and norepinephrine, but in some cases in the CNS by
means of electric current passing from cell to cell.
The central nervous system consists of the brain and spinal cord, and the peripheral nervous system
consists of the sensory and motor nerves that are distributed throughout the body and that convey
information to and from the brain (via 12 pairs of cranial nerves) and the spinal cord (via 31 pairs of spinal
nerves). The peripheral nervous system is divided into the somatic nervous system and the autonomic
nervous system.
The somatic nervous system is the part of the PNS that innervates the skin, joints, and skeletal
muscles.
The autonomic nervous system (ANS) is the part of the PNS that innervates internal organs, blood
vessels,and glands.
2. NEURONS & GLIAL CELLS
Information coming from peripheral receptors that sense the environment is analyzed by the brain
into components that give rise to perceptions, some of which are stored in memory. On the basis of this
information, the brain gives commands for the proper action (motor, emotional, autonomic, cognitive, etc.
responses). The brain does all this with nerve cells and the connections between them. Despite the simplicity
of the basic units, the complexity of behavior –evident in our capability for perception, information storage,
and action-, is achieved by the concerted signaling of an enormous number of neurons. The best estimate is
that the human brain contains about 100 billion neurons. Although nerve cells can be classified into perhaps
as many as 10,000 different types, they share many common features.
A key discovery in the organization of the brain is that nerve cells with basically similar properties
are able to produce very different actions because of precise connections with each other and with sensory
receptors and muscle.
Nerve cells differ from other cells in the body because of their ability to communicate rapidly with
one another, sometimes over great distances and with great precision. The rapid and precise communication
is made possible by two mechanisms- axonal conduction and synaptic transmission. Synaptic transmission
can be electrical or chemical. At chemical synapses the pre-and post-synaptic elements are separated by a
synaptic cleft.
A typical neuron has four morphologically defined regions: the cell body [also called the soma,
consisting of the nucleus and perikaryon]; dendrites, axon and presynaptic terminals. Each of these regions
has a distinct function in the generation of signals. Nerve cells differ most at the molecular level. The cell
body is the metabolic center of the neuron. The cell body usually gives rise to two types of processes called
the dendrites and the axon. A neuron usually has several dendrites; the branch out in tree-like fashion and
serve as the main apparatus for receiving the input to the neuron from other nerve cells. Often the cell body
is triangular or pyramidal in shape.
The cell body also gives rise to one axon, a tubular process with a diameter ranging that can ramify
and extend up to 1 meter. The axon is the main conducting unit of the neuron; it is capable of conveying
information great distances by propagating the signal and producing the transient electrical signal, called
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action potential. Large axons are surrounded by a fatty insulating sheath called myelin, which is essential for
high-speed conduction of action potentials.
Near its end the axon divides into fine branches that have specialized swellings called presynaptic
terminals; these are the transmitting elements of a neuron. By means of its terminals, one neuron transmits
information about its activity to the receptor surfaces (for example dendrites) of other neurons. The point of
contact is known as synapse. The neuron sending out the information, therefore is called the presynaptic
neuron, the neuron receiving the information is called the postsynaptic neuron. The space separating the
presynaptic from the postsynaptic cell is called the synaptic cleft. Most presynaptic neurons terminate near
the postsynaptic neuron’s dendrites, but communication may occur with the cell body or, less often, with the
initial segment or terminal portion of the axons.
Neuron types:
On the basis of the number of processes that arise from the cell body, neurons are classified into
three large groups:
1) Unipolar neurons: have one primary process that may give rise to many branches. One branch is
the axon and other branches serve as dendritic receiving structures.
2) Bipolar neurons: have an ovoid soma and two processes; a peripheral process or dendrite which
conveys information from the periphery, and a central process or axon, which carries information toward the
CNS. Many bipolar cells are sensory.
3) Multipolar neurons: predominate in the vertebrate nervous system. These cells have a single axon
and one or more dendritic branches that typically emerge from all parts of the cell body. The size and shape
of
cells vary. The larger dendritic tree of the Purkinje cell of the cerebellum receives approximately 150.000
contacts.
The neurons of the brain can be classified functionally into three major groups: afferent, motor, and
interneurons. Afferent or sensory neurons carry information into the nervous system both for conscious
perception and for motor coordination. Motor neurons carry commands to muscles and glands. Interneurons
constitute by far the largest class and consist of all the remaining cells in the nervous system that are not
specifically sensory or motor. Interneurons process information locally or convey information from one site
within the nervous system to another.
Glial cells
Nerve cell bodies and axons are surrounded by glial cells [Greek glia, “glue”]. There are between 10
and 50 times more glial cells than neurons in the CNS. Glial cells have other roles than processing
information. Some of the functions of the glial cells are as follows:
1- They serve as supporting elements, providing firmness and structures to the brain. They also
separate and occasionally insulate groups of neurons from each other.
2- Oligodendrocyte in the CNS forms myelin, the insulating sheath that covers most large axons.
3- Some glial cells remove debris after injury or neuronal death.
4- Some glial cells take up and remove chemical transmitters released by neurons during synaptic
transmission.
5- Some glial cells have nutritive functions for nerve cells.
Glial cells are divided into two major classes: microglia and macroglia. Ependymal cells are also
considered as glial cells.
TYPES OF NERVES
The PNS is anatomically and operationally continuous with the CNS. Its afferent (sensory) fibers
convey neural impulses to the CNS from the sense organs (e.g., the eyes) and from sensory receptors in
various parts of the body (e.g., in the skin). Its efferent (motor) fibers convey neural impulses from the CNS
to effector organs (muscles and glands). Nerves are either cranial nerves or spinal nerves, or derivatives of
them.
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SPINAL CORD
The spinal cord is a vital communication link between the brain and the peripheral nervous
system. Within the spinal cord, sensory nerves carry messages from the body to the brain for interpretation,
and motor nerves relay messages from the brain to the effectors. The spinal cord is also the primary reflex
centre, coordinating rapidly incoming and outgoing neural information.
The spinal cord is the major reflex center and conduction pathway between the body and brain. This
cylindrical structure, slightly flattened anteriorly and posteriorly, is protected by the vertebrae, their
associated ligaments and muscles, the spinal meninges, and the cerebrospinal fluid (CSF).
The spinal cord begins as a continuation of the medulla oblongata (commonly called the medulla),
the caudal part of the brainstem In adults, the spinal cord is 42-45 cm long and extends from the foramen
magnum in the occipital bone to the level of the L1 or L2 vertebra. However, its tapering inferior end, the
conus medullaris, may terminate as high as T12 vertebra or as low as L3 vertebra. Thus the spinal cord
occupies only the superior two thirds of the vertebral canal. In neonates, the spinal cord extends
approximately to vertebra LIII, but can reach as low as vertebra LIV. In the young child, it is relatively
longer and ends at the upper border of the third lumbar vertebra. The distal end of the cord (the conus
medullaris) is cone shaped. A fine filament of connective tissue (the pial part of the filum terminale)
continues inferiorly from the apex of the conus medullaris.
Although the spinal cord terminates at the level of first or second lumbar vertebra, the filum
terminale and the spinal nerve roots from the lumbosacral part of the spinal cord that form the cauda equina
continue inferiorly within the lumbar cistern containing CSF. This bundle of spinal nerve roots arising
inferior to the L1 vertebra, known as the cauda equina (L. horse tail), descends past the termination of the
spinal cord.
The spinal cord is not uniform in diameter along its length. It has two major swellings or
enlargements in regions associated with the origin of spinal nerves that innervate the upper and lower limbs.
A cervical enlargement occurs in the region associated with the origins of spinal nerves C5 to T1, which
innervate the upper limbs (brachial plexus). A lumbosacral enlargement occurs in the region associated with
the origins of spinal nerves L1 to S4, which innervate the lower limbs (lumbosacral plexus).
Inferiorly, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation
of the pia mater, the filum terminale, descends to be attached to the back of the coccyx . The cord possesses
in the midline anteriorly a deep longitudinal fissure, the anterior median fissure, and on the posterior surface
a shallow furrow, the posterior median sulcus. Internally, the cord has a small central canal (containing CSF)
surrounded by gray and white matter:
The gray matter is rich in nerve cell bodies, which form longitudinal columns along the cord, and in
cross-section these columns form a characteristic H-shaped appearance in the central regions of the cord;
The white matter surrounds the gray matter and is rich in nerve cell processes, which form large bundles or
tracts that ascend and descend in the cord to other spinal cord levels or carry information to and from the
brain.
The spinal cord is a long tubular structure that is divided into a peripheral white matter (composed of
myelinated axons) and a central gray matter (cell bodies and their connecting fibers). When viewed in cross
section, the gray matter has pairs of horn-like projections into the surrounding white matter. These horns are
called ventral horns, dorsal horns, and lateral horns, but in three dimensions they represent columns that run
the length of the spinal cord.
The ventral horns contain the cell bodies of motor neurons and their axons. A collection of neuronal
cell bodies in the CNS is a nucleus. Axons of the ventral horn nuclei leave the spinal cord in bundles called
ventral roots. These motor fibers innervate skeletal muscles.
The lateral (intermediolateral) horns contain the cell bodies for the sympathetic nervous system at
spinal cord levels T1–L2 and for the parasympathetic nervous system at spinal cord levels S2–S4. The axons
from these neurons also leave the spinal cord through the ventral root and will synapse in various peripheral
ganglia. A collection of neuronal cell bodies in the PNS is a ganglion.
The dorsal horns receive the sensory fibers originating in the peripheral nervous system. Sensory
fibers reach the dorsal horn by means of a bundle called the dorsal root. The central axons of the sensory
neuron enter the dorsal horn of the gray matter. Some of these fibers will run in tracts (a bundle of fibers in
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the CNS) of the white matter to reach other parts of the CNS. Other axons will synapse with intercalated
neurons (interneurons), which in turn synapse with motor neurons in the ventral horn to form a reflex arc.
The arterial supply to the spinal cord comes from two sources. It consists of: longitudinally oriented
vessels, arising superior to the cervical portion of the cord, which descend on the surface of the cord; and
feeder arteries that enter the vertebral canal through the intervertebral foramina at every level; these feeder
vessels, or segmental spinal arteries, arise predominantly from the vertebral and deep cervical arteries in
the neck, the posterior intercostal arteries in the thorax, and the lumbar arteries in the abdomen. After
entering an intervertebral foramen, the segmental spinal arteries give rise to anterior and posterior radicular
arteries. This occurs at every vertebral level.
The longitudinal vessels consist of: a single anterior spinal artery, which originates from the
vertebral arteries- passes inferiorly, approximately parallel to the anterior median fissure, along the surface
of the spinal cord; and two posterior spinal arteries, which also originate in the cranial cavity, usually
arising directly from a terminal branch of each vertebral artery. The right and left posterior spinal arteries
descend along the spinal cord, bracket the posterolateral sulcus.
Veins that drain the spinal cord form a number of longitudinal channels. These longitudinal channels
drain into an extensive internal vertebral plexus in the extradural (epidural) space of the vertebral canal.
.Spinal Nerves
Spinal nerves initially arise from the spinal cord as rootlets; the rootlets converge to form two nerve
roots. An anterior (ventral) nerve root, consisting of motor (efferent) fibers passing from nerve cell bodies in
the anterior horn of spinal cord gray matter to effector organs located peripherally.
A posterior (dorsal) nerve root, consisting of sensory (afferent) fibers from cell bodies in the spinal
sensory or posterior (dorsal) root ganglion that extend peripherally to sensory endings and centrally to the
posterior horn of spinal cord gray matter.
The posterior and anterior nerve roots unite, within or just proximal to the intervertebral foramen, to
form a mixed (both motor and sensory) spinal nerve, which immediately divides into two rami (L.,
branches): a posterior (dorsal) ramus and an anterior (ventral) ramus. As branches of the mixed spinal nerve,
the posterior and anterior rami carry both motor and sensory fibers, as do all their subsequent branches. The
terms motor nerve and sensory nerve are almost always relative terms, referring to the majority of fiber
types conveyed by that nerve. Nerves supplying muscles of the trunk or limbs (motor nerves) also contain
about 40% sensory fibers, which convey pain and proprioceptive information. Conversely, cutaneous
(sensory) nerves contain motor fibers, which serve sweat glands and the smooth muscle of blood vessels and
hair follicles.
Posterior rami are distributed to the synovial joints of the vertebral column, deep muscles of the
back, and the overlying skin. The remaining anterolateral body wall, including the limbs, is supplied by
anterior rami. Posterior rami and the anterior rami of spinal nerves T2-T12 generally do not merge with the
rami of adjacent spinal nerves to form plexuses.
Spinal (segmental) nerves exit the vertebral column (spine) through intervertebral foramina. Spinal
nerves arise in bilateral pairs from a specific segment of the spinal cord. The 31 spinal cord segments and
the 31 pairs of nerves arising from them are identified by a letter and number (e.g., “T4”) designating the
region of the spinal cord and their superior-to-inferior order (C, cervical; T, thoracic; L, lumbar; S, sacral;
Co, coccygeal). A spinal cord segment is the portion of the spinal cord that gives rise to a pair of spinal
nerves. Thus, the spinal cord gives rise to 8 pairs of cervical nerves (C1–C8), 12 pairs of thoracic nerves
(T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal
nerves (Co1). The spinal cord segments are numbered in the same manner as these nerves.
The first cervical nerve (C1) emerges from the vertebral canal between the skull and vertebra CI.
Therefore cervical nerves C2 to C7 also emerge from the vertebral canal above their respective vertebrae.
Because there are only seven cervical vertebrae, C8 emerges between vertebrae CVII and TI. As a
consequence, all remaining spinal nerves, beginning with T1, emerge from the vertebral canal below their
respective vertebrae. Cervical spinal nerves (except C8) bear the same alphanumeric designation as the
vertebrae forming the inferior margin of the IV foramina through which the nerve exits the vertebral canal.
The more inferior spinal (T1 through Co1) nerves bear the same alphanumeric designation as the vertebrae
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forming the superior margin of their exit. The first cervical nerves lack posterior roots in 50% of people, and
the coccygeal nerve may be absent.
Although the dorsal root is essentially sensory and the ventral root is motor, the two roots come
together within the bony intervertebral foramen to form a mixed spinal nerve (i.e., it contains both sensory
and motor fibers). The spinal cord is defined as part of the CNS, but the ventral and dorsal roots are
considered parts of the PNS. Outside the intervertebral foramen, the mixed nerve divides into a ventral
ramus (from the Latin for “branch”) and a dorsal ramus.
The larger ventral ramus supplies the ventrolateral body wall and the limbs; the smaller dorsal ramus
supplies the back. Since the ventral and dorsal rami are branches of the mixed nerve, they both carry sensory
and motor fibers.
The term “peripheral nerve” such as sciatic nerve, ulnar nerve etc. should not be confused by the
spinal nerve. Peripheral nerve is the last product of these somatic networks; somatic plexuses.
The anterior rami form plexuses (network). All major somatic plexuses (cervical, brachial, lumbar,
and sacral) are formed by anterior rami (ramus=branch, rami=branches).
The peripheral nervous system contains two systems; one working voluntarily; somatic nervous
system (soma, in ancient Greek, body), and one involuntarily, as it name implies, autonomic nervous
system.
The unilateral area of skin innervated by the sensory fibers of a single spinal nerve is called a
dermatome; the unilateral muscle mass receiving innervation from the fibers conveyed by a single spinal
nerve is a myotome. Generally, at least two adjacent spinal nerves (or posterior roots) must be interrupted to
produce a discernible area of numbness.
As they emerge from the intervertebral foramina, spinal nerves are divided into two rami
 Posterior (primary) rami of spinal nerves supply nerve fibers to the synovial joints of the vertebral
column, deep muscles of the back, and the overlying skin in a segmental pattern. As a general rule, the
posterior rami remain separate from each other (do not merge to form major somatic nerve plexuses).
 Anterior (primary) rami of spinal nerves supply nerve fibers to the much larger remaining area, consisting
of the anterior and lateral regions of the trunk and the upper and lower limbs. The anterior rami that are
distributed exclusively to the trunk generally remain separate from each other, also innervating muscles and
skin in a segmental pattern. However, primarily in relationship to the innervation of the limbs, the majority
of anterior rami merge with one or more adjacent anterior rami, forming the major somatic nerve plexuses
(networks) in which their fibers intermingle and from which a new set of multisegmental peripheral nerves
emerges. The anterior rami of spinal nerves participating in plexus formation contribute fibers to multiple
peripheral nerves arising from the plexus; conversely, most peripheral nerves arising from the plexus contain
fibers from multiple spinal nerves.
Spinal meninges
The spinal dura mater is the outermost meningeal membrane and is separated from the bones forming
the vertebral canal by an extradural space. Superiorly, it is continuous with the inner meningeal layer of
cranial dura mater at the foramen magnum of the skull. Inferiorly, the dural sac dramatically narrows at the
level of the lower border of vertebra SII and forms an investing sheath for the pial part of the filum terminale
of the spinal cord. This terminal cord-like extension of dura mater (the dural part of the filum terminale)
attaches to the posterior surface of the vertebral bodies of the coccyx.
The arachnoid mater is a thin delicate membrane against, but not adherent to, the deep surface of the
dura mater. It is separated from the pia mater by the subarachnoid space. The arachnoid mater ends at the
level of vertebra SII.
The subarachnoid space between the arachnoid and pia mater contains CSF (Cerebrospinal fluidBeyin-omurilik sıvısı-BOS). The subarachnoid space around the spinal cord is continuous at the foramen
magnum with the subarachnoid space surrounding the brain.
The spinal pia mater is a vascular membrane that firmly adheres to the surface of the spinal cord.
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INTRODUCTION TO THE BRAIN
The brain (encephalon; Greek, within the head) is divided into three major divisions. These are, in
ascending order from the spinal cord;
1) Hindbrain (Rhombencephalon)
I. Medulla oblongata
II. Pons
III. Cerebellum
Pons and cerebellum are called as metencephalon.
2) Midbrain (Mesencephalon)
3) Forebrain (Prosencephalon)
I. Telencephalon (Cerebrum)
II. Diencephalon (between brain)
The brainstem (a collective term for the medulla oblongata, pons, and midbrain) is that part of the
brain that remains after the cerebral hemispheres and cerebellum are removed.
1. Telencephalon (Cerebrum): Telencephalon by far forms the largest region of the brain. They
consist of the cerebral hemispheres, basal ganglia and ventricles. The basal ganglia participate in regulating
motor performance.
2. Diencephalon: Diencephalon or between-brain is so called because it lies between the cerebral
hemispheres and the midbrain. The main structures of the diencephalon are the thalamus and hypothalamus.
The thalamus processes most of the information reaching the cerebral cortex from the rest of the central
nervous system. Hypothalamus regulates autonomic, endocrine, and visceral function.
3. Mesencephalon (Midbrain): is the smallest brain stem component and lies anterior to pons.
Several regions of the midbrain play a dominant role in the direct control of eye movements, whereas others
are involved in motor control of skeletal muscles. The midbrain is also involved with the coordination of
visual and auditory reflexes.
4. Metencephalon [Pons+Cerebellum]
Pons (Latin, bridge), which lies above the medulla, conveys information about movement from the
cerebral hemisphere to the cerebellum.
Cerebellum, lies behind the pons and is connected to the brain stem by several major fiber tracts
called peduncles. The cerebellum modulates the force and range of movement and is involved in learning of
motor skills.
5. Medulla oblongata (medulla): which lies directly above the spinal cord, includes several centers
responsible for such vital autonomic functions as digestion, breathing, and the control of heart rate.
BRAINSTEM
The brainstem is the oldest part of the CNS. The brainstem is made up of the medulla oblongata, the pons,
and the midbrain and occupies the posterior cranial fossa of the skull. It is stalklike in shape and connects
the narrow spinal cord with the expanded forebrain.
The brain stem contains 10 cranial nerves, and most of the motor and sensory systems pass through this
important region. It is a relatively small region (approximately 7 cm long) that links the forebrain (i.e.,
cerebral cortex) and the spinal cord and all messages going between the two areas must go through the brain
stem.
The brainstem has three broad functions: (1) it serves as a conduit for the ascending tracts and descending
tracts connecting the spinal cord to the different parts of the higher centers in the forebrain; (2) it contains
important reflex centers associated with the control of respiration and the cardiovascular system and with the
control of consciousness; and (3) it contains the important nuclei of cranial nerves III through XII.
Midbrain [Mesencephalon]
The midbrain measures about 0.8 inch (2 cm) in length and connects the pons and cerebellum with the
forebrain. The cerebral hemispheres are connected to the brainstem by two large fiber tracts , the cerebral
peduncles. The narrow cavity of the midbrain is the cerebral aqueduct, which connects the third and fourth
ventricles.
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The dorsal aspect of the midbrain is known as the tectum (L., roof] and incorporates the paired superior
and inferior colluculi (singular, colliculus). The superior and inferior colliculi are also known as corpora
quadrigemina. These are rounded eminences that are divided into superior and inferior pairs by a vertical
and a transverse groove. The superior colliculi are centers for visual reflexes, and the inferior colliculi are
lower auditory centers.
The region of the mesencephalon below the cerebral aqueduct is known as the midbrain (mesencephalic)
tegmentum (L., cover).
The midbrain comprises two lateral halves, called the cerebral peduncles; each of these is divided into an
anterior part, the crus cerebri, and a posterior part, the tegmentum, by a pigmented band of gray matter, the
substantia nigra.
The substantia nigra is a large motor nucleus situated between the tegmentum, and the crus cerebri and is
found throughout the midbrain. The substantia nigra is concerned with muscle tone and is connected to the
cerebral cortex, spinal cord, hypothalamus, and basal nuclei.
The crus cerebri contains important descending tracts and is separated from the tegmentum by the
substantia nigra. These descending tracts connect the cerebral cortex to the anterior gray column cells of the
spinal cord, the cranial nerve nuclei, the pons, and the cerebellum.
Pons
The pons is anterior to the cerebellum and connects the medulla oblongata to the midbrain. It is about 1 inch
(2.5 cm) long and owes its name to the appearance presented on the anterior surface, which is that of a
bridge connecting the right and left cerebellar hemispheres.
The anterior surface is convex from side to side and shows many transverse fibers that converge on each
side to form the middle cerebellar peduncle.
Pons has a convex anterior surface marked by transversely running fibers which laterally forms a bundle
called middle cerebellar peduncle.
Main Features
- The trigeminal nerve emerges from the anterior surface at its junction with middle cerebellar peduncle.
- Presents a basilar sulcus in the midline which lodges basilar artery
- In the groove between Pons and the medulla oblongata, there emerge, from medial to lateral, abducent,
facial and vestibulocochlear nerves.
Posterior surface of the pons is limited laterally by superior cerebellar peduncle and forms the upper part of
the floor of the 4th ventricle.
Main Features:
The floor is divided into symmetrical halves by a median sulcus.
Lateral to this sulcus is an elongated elevation, the medial eminence, which is bounded laterally by a
sulcus limitans.
- Inferior end of medial eminence is slightly expanded to form facial colliculus, which is produced by
facial nerve
- The upper end of sulcus limitans presents a bluish-gray coloration and the area is called substantia
ferruginosa.
Area vestibule lies lateral to sulcus limitans
Parts of the Pons
·
a posterior part, the tegmentum, and
·
an anterior basilar part
Medulla Oblongata
The medulla oblongata is situated in the posterior cranial fossa, lying beneath the tentorium cerebelli
and above the foramen magnum. It is related anteriorly to the basal portion of the occipital bone and the
upper part of the odontoid process of the axis and posteriorly to the cerebellum.
The medulla oblongata not only contains many cranial nerve nuclei that are concerned with vital functions
(e.g., regulation of heart rate and respiration), but it also serves as a conduit for the passage of ascending and
descending tracts connecting the spinal cord to the higher centers of the nervous system.
The medulla oblongata is conical in shape. Its broad part joins the pons above and narrow part
becomes continuous with the spinal cord. The junction between medulla and spinal cord coincides with the
level of the upper border of atlas (first cervical vertebra).
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Its length is about 3 cm and its width is about 2cm at its upper end.
It is divided into
1. A lower closed part with central canal and
2. An upper open part posteriorly which is related to the lower part of the 4th ventricle
Features on the anterior surface of Medulla Oblongata
Anterior median fissure, is an upward continuation of similar fissure present on the spinal cord
Anterolateral sulcus, on each side, is in line with the ventral roots of spinal cord
- Gives attachment to the rootlets of the hypoglossal nerve
Pyramid is an elevation on each side of the midline between anterior median fissure and anterolateral sulcus.
- Composed of bundles of nerve fibers of corticospinal tract that descends from the cerebral cortex to the
spinal cord
- Tapers inferiorly where the majority of fibers cross over to the opposite side, obliterating the medulla.
These crossing fibers constitute the decussation of the pyramid.
Olive is a prominent, elongated oval swelling that lies in the upper part of medulla posterolateral to the
pyramid separated by anterolateral sulcus.The elevation is produced by the underlying inferior olivary
nucleus.
Features on posterior surface of the medulla oblongata
Posterior median sulcus is upward continuation of the similar fissure on the spinal cord.
Posterolateral sulcus lies in line with the dorsal roots of spinal nerves.
- Gives attachment to the rootlets of 9th, 10th and 11th cranial nerves.
Between the posterior median sulcus and posterolateral sulcus, the medulla contains tracts (asccending) that
enter it from the posterior funiculus of the spinal cord.
- Fasciculus gracilis lies medially and fasciculus cuneatus lies laterally
- Both fasciculi end in rounded elevations called gracile tubercle (nucleus gracilis) and cuneate tubercle
(nucleus cuneatus) respectively.
Just above these tubercles, medulla is occupied by a triangular fossa which forms the lower part of the 4th
ventricle. This fossa is bounded on each side by inferior cerebellar peduncle which connect the medulla to
cerebellum.
Features on the posterior part of the medulla that forms the floor of the 4th ventricle:
Presents median sulcus, on each side of which there is a longitudinal elevation called the median eminence
(continuous above in the pontine part of the floor of 4th ventricle). The eminence is bounded laterally by
sulcus limitans.
The sulcus limitans is marked by a depression called inferior fovea. The part of the medulla below fovea
presents hypoglossal triangle medially and vagal triangle laterally.
The inferior angle where the lateral margins of the floor meet is called obex.
CRANIAL NERVES
The 12 pairs of cranial nerves are part of the peripheral nervous system (PNS) and pass through
foramina or fissures in the cranial cavity. All nerves except one, the accessory nerve [XI], originate from the
brain. In addition to having similar somatic and visceral components as spinal nerves, some cranial nerves
also contain special sensory (such as hearing, seeing, smelling, balancing, and tasting). The special sensory
components are associated with.
Nuclei of 12 cranial nerves
10 of them in the brainstem
Midbrain Of the IV & III
Pons
Of the other 4 - VIII,VII,VI,V
Medulla Of the last 4 - XII,XI,X, IX
I
Olfactory
Purely sensory
Telencephalon
Smelling
II
Optic Sensory
Retinal ganglion cells
Seeing
III
Oculomotor Mainly motor
Midbrain
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Eye movements & pupillary reflex
IV
Trochlear
Motor
Midbrain
Intorts the eyeball.
V
Trigeminal Both sensory and motor
Pons
Receives sensation from the face and innervates the muscles of mastication
VI
Abducens
Mainly motor Pons
Abducts the eye.
VII
Facial Both sensory and motor
Pons
Provides motor innervation to the muscles of facial expression. Also receives the special sense of
taste from the anterior 2/3 of the tongue and provides secretomotor innervation to the salivary glands
(except parotid) and the lacrimal gland.
VIII
Vestibulocochlear Mostly sensory Pons
Hearing and balance
IX
Glossopharyngeal Both sensory and motor
Medulla
Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the
parotid gland, and provides motor innervation to the stylopharyngeus. Some sensation is also relayed to
the brain from the palatine tonsils.
X
Vagus Both sensory and motor
Medulla
Supplies branchiomotor innervation to most laryngeal and pharyngeal muscles (except the
stylopharyngeus, which is innervated by the glossopharyngeal). Also provides parasympathetic fibers to
nearly all thoracic and abdominal viscera till the proximal two-thirds of the transverse colon. Receives the
special sense of taste from the epiglottis. A major function: controls muscles for voice and resonance and
the soft palate.
XI
Accessory (often separated into the cranial accessory and spinal accessory nerves)
Medulla
Mainly motor Cranial and Spinal Roots
Controls the sternocleidomastoid and trapezius muscles, and overlaps with functions of the vagus
nerve (CN X). Symptoms of damage: inability to shrug, weak head movement.
XII
Hypoglossal mainly motor Medulla
Provides motor innervation to the muscles of the tongue (except for the palatoglossus, which is
innervated by the vagus nerve) and other glossal muscles. Important for swallowing (bolus formation) and
speech articulation.
Reticular formation
The reticular formation (L. reticulum, “little net”) consists of various distinct populations of cells embed
in a network of cell processes occupying the central core of the brainstem. From an evolutionary
perspective, the reticular formation is phylogenetically an ancient neural complex that is closely associated
with two other ancient neural systems, the olfactory system which mediates the visceral sense of smell, and
the limbic system which functions in the visceral and behavioral responses to emotions.
The reticular formation consists of an intricate mixture of neuronal cell bodies and fascicles of axons
running in small bundles, which are oriented in many different directions. Thus, as its name suggest, it is
like a network extending from the spinal cord through the medulla, the pons, the midbrain, the subthalamus,
hypothalamus and the thalamus.
The reticular formation and the olfactory and limbic systems are interrelated as a result of their participation
in visceral functions and behavioral responses. The reticular formation is continually informed of activity
occurring in almost all areas of the nervous system and responds by influencing the following: skeletal
muscle motor activity; somatic and visceral sensation; autonomic nervous system; endocrine functions;
biological rhythms, via reciprocal connections to the hypothalamus, and the level of consciousness.
More than 100 nuclei scattered throughout the tegmentum of the midbrain, pons and medulla have been
identified as being part of the brainstem reticular formation. Most of the nuclei of the reticular formation are
not clearly as defined as are other nuclei of the CNS. Although the nuclei of the reticular formation have a
number of diverse functions, they are classified according to the following four general functions:
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1- The regulation of the level of consciousness, and ultimately cortical alertness
2- The control of somatic motor movements
3- The regulation of visceral motor or autonomic functions
4- The control of sensory information
Autonomic Nervous System
The autonomic nervous system is distributed throughout the central and peripheral nervous systems. It is
divided into two parts, the sympathetic and the parasympathetic and, consists of both afferent and efferent
fibers. This division between sympathetic and parasympathetic is made on the basis of anatomical
differences, differences in the neurotransmitters, and differences in the physiologic effects.
The autonomic nervous system and the endocrine system control the internal environment of the body. It is
the autonomic nervous system that provides a fine discrete control over the functions of many organs and
tissues, including heart muscle, smooth muscle, and the exocrine glands.
The autonomic nervous system functions for the most part at the subconscious level. We are not aware, for
example, that our pupils are dilating or that our arteries are constricting. The system should not be regarded
as an isolated
portion of the nervous system, for it is known that it can play a role with somatic activity in expressing
emotion and that certain autonomic activities, such as micturition, can be brought under voluntary control.
The various activities of the autonomic and endocrine systems are integrated within the hypothalamus.
The sympathetic part of the autonomic system has the efferent fibers originating from the spinal cord.
The function of the sympathetic system is to prepare the body for an emergency. The heart rate is increased,
arterioles of the skin and intestine are constricted, arterioles of skeletal muscle are dilated, and the blood
pressure is raised. There is a redistribution of blood; thus, it leaves the skin and gastrointestinal tract and
passes to the brain, heart, and skeletal muscle. In addition, the sympathetic nerves dilate the pupils; inhibit
smooth muscle of the bronchi, intestine, and bladder wall; and close the sphincters. The hair is made to stand
on end, and sweating occurs.
The activities of the parasympathetic part of the autonomic system are directed toward conserving and
restoring energy. The heart rate is slowed, pupils are constricted, peristalsis and glandular activity is
increased, sphincters are opened, and the bladder wall is contracted.
The connector nerve cells of the parasympathetic part of the autonomic nervous system are located in the
brainstem and the sacral segments of the spinal cord.
Parasympathetic System in the Brainstem
 Edinger-Westfall nucleus in the midbrain (mediates the diameter of the pupil in response to light)
 Superior and inferior salivatory nuclei in the pons and medulla (mediate salivary secretion and the
production of tears)
 Dorsal motor nucleus of the vagus nerve in the medulla. The parasympathetic system controls the motor
responses of the heart, lungs, and gut elicited by the vagus nerve (e.g., slowing of the heart rate and
constriction of the bronchioles).
The sympathetic and parasympathetic components of the autonomic system cooperate in maintaining the
stability of the internal environment. The sympathetic part prepares and mobilizes the body in an
emergency, when there is sudden severe exercise, fear, or rage.
The parasympathetic part aims at conserving and storing energy, for example, in the promotion of digestion
and the absorption of food by increasing the secretions of the glands of the gastrointestinal tract and
stimulating peristalsis.
The sympathetic and parasympathetic parts of the autonomic system usually have antagonistic control over a
viscus. For example, the sympathetic activity will increase the heart rate, whereas the parasympathetic
activity will slow the heart rate. The sympathetic activity will make the bronchial smooth muscle relax, but
the muscle is contracted by parasympathetic activity.
CEREBELLUM
The cerebellum (L. “little brain”) is the largest part of the hindbrain and lies posterior to the fourth
ventricle, the pons, and the medulla oblongata. The cerebellum is situated in the posterior cranial fossa and
is covered superiorly by the tentorium cerebelli. It is made up of two lateral cerebellar hemispheres and a
median vermis (L. “worm”). The surface of the cerebellum displays slender and parallel elevations (ridges)
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known as folia and depressions (grooves) known as sulci that facilitate a great increase in the surface area of
the cerebellar cortex.
Though apparently smaller than the cerebral cortex, the cerebellum contains ~2X as many neurons
as the cerebral cortex. Its function is to make our movements as fast, accurate, smooth, and consistent as
possible. Cerebellar damage does not cause paralysis but renders movements slow, inaccurate, inconsistent,
and jerky. Though the cerebellum sends very few fibers to the spinal cord it exerts a powerful influence on
movements via projections, either direct or via one relay neuron, to the structures from which all four major
descending motor tracks originate.
The cerebellum is connected to the dorsal aspect of the brainstem by three pairs of prominent fiber
bundles, the superior, middle, and inferior cerebellar peduncles. On its ventral surface, near the middle
cerebellar peduncle, a small, bulb-like region of each cerebellar hemisphere, known as the flocculus, is
connected to a region of the vermis known as the nodulus.
The effect of this fissuring is to give the cerebellum in section the appearance of a many branched tree
which is called as arbor vitae; the tree of life.
The cerebellum is somewhat ovoid in shape and constricted in its median part.
The cerebellum has traditionally been recognized as having three anterior-posterior divisions: the anterior
lobe (lobules I – V) is separated from the posterior lobe by the primary fissure, and the posterior
lobe (lobules VI – IX) is separated from the flocculonodular lobe (lobule X) by the posterolateral fissure
(posterior fissure or uvulonodular fissure). A deep horizontal fissure that is found along the margin of the
cerebellum separates the superior from the inferior surfaces.
The anterior lobe receives its input primarily from the spinal cord and is referred to as the paleocerebellum
The posterior lobe receives its input primarily from the cerebral cortex via relay neurons in the pontine
nuclei and is called the neocerebellum. The flocculonodular lobe receives most of its input from the
vestibular system and is called the vestibulocerebellum.
The cerebellum plays a very important role in the control of posture and voluntary movements. It
unconsciously influences the smooth contraction of voluntary muscles and carefully coordinates their
actions, together with the relaxation of their antagonists. Each cerebellar hemisphere controls muscular
movements on the same side of the body and that the cerebellum has no direct pathway to the lower motor
neurons but exerts its control via the cerebral cortex and the brainstem.
The cerebellum is composed of an outer covering of gray matter called the cortex and inner white
matter. Embedded in the white matter of each hemisphere are three masses of gray matter forming the
intracerebellar nuclei (deep cerebellar nuclei).
The human cerebellum contains more than 100 billion neurons, a number that represents about 80%
of the total number of neurons in the brain. Unlike the cerebral cortex, the cortex of the cerebellum has
uniform anatomical structure, suggesting that there may be a similar mode of operation for all its possible
functions. The cortex of the cerebellum is deeply folded. If one looks at the human cerebral cortex,one can
see about one third of it; two thirds is buried on the banks and depths of the fissures. For the cerebellar
cortex, one would see only about one tenth; 90% is buried within the fissures.
The output from the cortex is mainly to the intracerebellar nuclei, which are buried within the white
matter of the cerebellum. The white matter consists of fibers coming into the cerebellum and the axons of
the output neurons of the cerebellar cortex, Purkinje cells, coursing to terminate in the cerebellar nuclei.
There are four nuclei on each side. Most laterally is the dentate or lateral nucleus. Most medially is
the fastigial or medial nucleus. Between the lateral and medial nuclei are the intermediate, or interpositus
nuclei, the globose (more lateral) and emboliform (more medial).
Thirty million Purkinje cells are the only neurons whose axons carry information from the cortex to
the nuclei. Purkinje cell axons run through the white matter to terminate in the cerebellar nuclei. All output
from the cerebellar cortex leaves the cortex via Purkinje cell axons. Climbing fibers terminate directly on the
dendrites of ~10 Purkinje cells. An action potential in a climbing fiber always causes an action potential in
the Purkinje cells that it contacts. Thus a mossy fiber has a small effect on many Purkinje cells, whereas
climbing fibers have a large effect on a small number of Purkinje cells.
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Cerebellar Peduncles
The cerebellum is linked to other parts of the central nervous system by numerous efferent and afferent
fibers that are grouped together on each side into three large bundles, or peduncles. The superior cerebellar
peduncles connect the cerebellum to the midbrain, the middle cerebellar peduncles connect the cerebellum
to the pons, and the inferior cerebellar peduncles connect the cerebellum to the medulla oblongata.
Functions of the cerebellum
Cerebellum makes an important contribution to the control of voluntary movement and movement
coordination, as well as control of balance, gait, and posture.
Functions- 3 major functional roles
1. Coordination of Movement-the cerebellum controls the timing and pattern of muscle activation during
movement.
2. Maintenance of Equilibrium (in conjunction with the vestibular system)
3. Regulation of Muscle Tone-modulates spinal cord and brain stem mechanisms involved in postural
control.
There is also strong evidence for a cerebellar role in cognition (memory, attention, language and executive
functions), emotions, and anxiety.
DIENCEPHALON
The diencephalon is located at the dorsal end of the brain stem surrounded by the internal capsule
laterally and the lateral ventricles and corpus callosum superiorly. It is divided into symmetrical halves
separated by the narrow third ventricle but connected by the massa intermedia.
As you will see the structures of the diencephalon are named according to their position to the thalamus. See
yourself below:
The diencephalon can be divided into four parts:
(1) thalamus
(2) subthalamus [-sub: 'inferior to”]
(3) epithalamus [-epi: “superior to”]
(4) hypothalamus [-hypo: “under” ]
The diencephalon extends posteriorly to the point where the third ventricle becomes continuous with the
cerebral aqueduct and anteriorly as far as the interventricular foramina. Thus, the diencephalon is a midline
structure with symmetrical right and left halves. Obviously, these subdivisions of the brain are made for
convenience, and from a functional point of view, nerve fibers freely cross the boundaries.
Gross Features
The inferior surface of the diencephalon is the only area exposed to the surface in the intact brain. It
is formed by hypothalamic and other structures, which include, from anterior to posterior:
1. optic chiasma, with the optic tract on either side
2. infundibulum, with the tuber cinereum
3. mammillary bodies.
The superior surface of the diencephalon is concealed by the fornix. The actual superior wall of the
diencephalon is formed by the roof of the third ventricle. The roof contains a thin epithelial membrane called
ependyma. It is continuous with the rest of the ependymal lining of the third ventricle. The ependymal is
involved in CSF production. It is covered superiorly by a vascular fold of pia mater, called the tela choroidea
of the third ventricle. From the roof of the third ventricle, a pair of vascular processes, the choroid plexuses
of the third ventricle, project downward from the midline into the cavity of the third ventricle. The choroid
plexus is the place where the CSF is produced.
The lateral surface of the diencephalon is bounded by the internal capsule of white matter and
consists of nerve fibers that connect the cerebral cortex with parts of the brainstem and spinal cord.
Since the diencephalon is divided into symmetrical halves by the slitlike third ventricle, it also has a
medial surface. The medial surface of the diencephalon (i.e., the lateral wall of the third ventricle) is formed
in its superior part by the medial surface of the thalamus and in its inferior part by the hypothalamus. These
two areas are separated from one another by a shallow sulcus, the hypothalamic sulcus. A bundle of nerve
fibers, which are afferent fibers to the habenular nucleus, forms a ridge along the superior margin of the
medial surface of the diencephalon and is called the stria medullaris thalami.
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1. Thalamus
L. thalamus "inner chamber," from Gk. thalamos "inner chamber, bedroom"
The thalamus is a large ovoid mass of gray matter that forms the major part of the
diencephalon. The thalamus is situated on each side of the third ventricle. The superior surface of the
thalamus is covered medially by the tela choroidea and the fornix, and laterally, it is covered by ependyma
and forms part of the floor of the lateral ventricle; the lateral part is partially hidden by the choroid plexus of
the lateral ventricle. The inferior surface is continuous with the tegmentum of the midbrain.
The medial surface of the thalamus forms the superior part of the lateral wall of the third ventricle and is
usually connected to the opposite thalamus by a band of gray matter, the interthalamic connection
(interthalamic adhesion; adhesio interthalamica; massa interrmedia). The interthalamic adhesion is found in
70-80% of humans.
The lateral surface of the thalamus is separated from the lentiform nucleus by the very important band of
white matter called the internal capsule.
The thalamus is a very important cell station that receives the main sensory tracts (except the
olfactory pathway). It should be regarded as a station where much of the information is integrated and
relayed to the cerebral cortex and many other subcortical regions. It also plays a key role in the integration
of visceral and somatic functions. The activities of the thalamus are closely related to that of the cerebral
cortex and damage to the thalamus causes great loss of cerebral function.
The thalamus is actually a relay centre subserving both sensory and motor mechanisms.
Thalamic nuclei (50–60 nuclei) project to one or a few well-defined cortical areas. Multiple cortical areas
receive afferents from a single thalamic nucleus and send back information to different thalamic nuclei.
The anterior part of the thalamus contains the anterior thalamic nuclei, which receive the
mammilothalamic tract from the mammillary nuclei. The function of the anterior thalamic nuclei is closely
associated with of that of the limbic system and is concerned with emotional tone and the mechanisms of
recent memory.
The medial part of the thalamus contains the large dorsomedial nucleus and several smaller nuclei.
The dorsomedial nucleus has 2 connections with the whole prefrontal cortex of the frontal lobe of the
cerebral hemisphere. It also has similar connections with the hypothalamic nuclei. The medial part of the
thalamus is responsible for the integration of a large variety of sensory information, including somatic
visceral
and olfactory information and the relation of this information to one’s emotions. The lateral part is
subdivided in dorsal and ventral components.
2. Subthalamus
The subthalamus lies inferior to the thalamus and, therefore, is situated between the thalamus and the
tegmentum of the midbrain; craniomedially, it is related to the hypothalamus. The nucleus has important
connections with the corpus striatum; as a result, it is involved in the control of muscle activity.
3. Epithalamus (dorsal thalamus)
The epithalamus consists of the habenular nuclei and their connections (stria medullaris thalami &
habenulointerpeduncular tract ; fasciculus retroflexus).) and the pineal gland.
Habenular Nucleus
The habenular nucleus is a small group of neurons situated just medial to the posterior surface of the
thalamus. The habenular nucleus is believed to be a center for integration of olfactory, visceral, and somatic
afferent pathways.
Pineal Gland (Body)
The pineal gland is a small, conical structure that is attached by the pineal stalk to the diencephalon. The
superior part of the base of the stalk contains the habenular commissure; the inferior part of the base of the
stalk contains the posterior commissure. The pineal gland possesses no nerve cells, but adrenergic
sympathetic fibers derived from the superior cervical sympathetic ganglia enter the gland and run in
association with the blood vessels and the pinealocytes.
The pineal gland, once thought to be of little significance, is now recognized as an important endocrine
gland capable of influencing the activities of the pituitary gland, the islets of Langerhans of the pancreas, the
parathyroids, the adrenal cortex and the adrenal medulla, and the gonads. The pineal secretions, produced by
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the pinealocytes, reach their target organs via the bloodstream or through the cerebrospinal fluid. Their
actions are mainly inhibitory and either directly inhibit the production of hormones or indirectly inhibit the
secretion of releasing factors by the hypothalamus. Animal experiments have shown that pineal activity
exhibits a circadian rhythm that is influenced by light. The gland has been found to be most active during
darkness. Melatonin and the enzymes needed for its production are present in high concentrations within the
pineal gland.
4. Hypothalamus
The hypothalamus is that part of the diencephalon that extends from the region of the optic chiasma to the
caudal border of the mammillary bodies. It lies below the hypothalamic sulcus on the lateral wall of the
third ventricle. It is thus seen that anatomically the hypothalamus is a relatively small area of the brain that is
strategically well placed close to the limbic system, the thalamus, the ascending and descending tracts, and
the hypophysis. Microscopically, the hypothalamus is composed of small nerve cells that are arranged in
groups or nuclei. Physiologically, there is hardly any activity in the body that is not influenced by the
hypothalamus. The hypothalamus controls and integrates the functions of the autonomic nervous system and
the endocrine systems and plays a vital role in maintaining body homeostasis. It is involved in such activities
as regulation of body temperature, body fluids, drives to eat and drink, sexual behavior, and emotion.
Relations of the Hypothalamus
Anterior to the hypothalamus is an area that extends forward from the optic chiasma to the lamina terminalis
and the anterior commissure; it is referred to as the preoptic area. Caudally, the hypothalamus merges into
the tegmentum of the midbrain. The thalamus lies superior to the hypothalamus, and the subthalamic region
lies inferolaterally to the hypothalamus.
The hypothalamus can be loosely divided into four distinct groups in the rostral-caudal plane of the third
ventricle: preoptic (above and in front of the optic chiasm - actually telencephalic extension of the basal
forebrain, but functionally considered with the diencephalon), chiasmatic (above and around the optic
chiasm), tuberal (above and around the "tuber cinereum", i.e. pituitary stalk) and the posterior region which
includes the mammillary bodies.
When observed from below, the hypothalamus is seen to be related to the following structures, from anterior
to posterior: (1) the optic chiasma, (2) the tuber cinereum and the infundibulum, and (3) the mammillary
bodies.
Optic Chiasma
The optic chiasma is a flattened bundle of nerve fibers situated at the junction of the anterior wall and floor
of the third ventricle. The superior surface is attached to the lamina terminalis, and inferiorly, it is related to
the hypophysis cerebri, from which it is separated by the diaphragma sellae. A small recess, the optic recess
of the third ventricle, lies on its superior surface.
Pituitary gland
Let me do an anology here. Pituitary gland is the “switch” of the body. Look at the functions of the gland for
God’s sake. Metabolism in the body, reproduction, water balance, growing…. Pituitary gland releases
hormones under the influence of the hormones released by the hypothalamus.
The hypothalamus is considered as a part of the diencephalon but they do not count the pituitary gland in the
diencephalon but still we talk about it when we talk diencephalon. It lies under the hypothalamus and sits on
the sella turcicae part called “fossa hypophysis”. It has a stalk called infundibulum and has two parts; the
anterior pituitary and posterior pituitary. The posterior pituitary is specific as it is formed by the axons
coming from the distinct nuclei in the hypothalamus. The anterior pituitary is regulated by the hypothalamus
by the help of a vascular network.
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Hormone
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Stimulated by the hypothalamic
Does
hormone
Anterior pituitary gland (Adenohypoysis)
Growth Hormone (GH)
Growth Hormone-Releasing Hormone Growing
(GHRH)
Thyroid-stimulating
Thyrotropin-Releasing Hormone (TRH) Metabolism of the body
hormone (TSH)
Adrenocorticotropic
Corticotropin-Releasing Hormone
Production and release of corticosteroids from
hormone (ACTH)
(CRH)
the adrenal glands
Prolactin (PRL)
Long list of chemical substances,
Stimulation of milk production in breasts
inhibited by dopamine
Luteinizing hormone (LH) Gonadotropin-Releasing Hormone
(GnRH)
Triggers ovulation
ICHS production of testosterone
Follicle-stimulating
hormone (FSH)
Gonadotropin-Releasing Hormone
(GnRH)
Regulates the development, growth, pubertal
maturation, and reproductive processes of the
body
Posterior pituitary gland (Neurohypoysis)
Oxytocin
Secreted from the hypothalamus and Distension of the cervix and uterus during labor,
carried to the pituitary gland
facilitating birth, and after stimulation of the
nipples, facilitating breastfeeding.
Antidiuretic hormone
Secreted from the hypothalamus and Increases water absorption in the the kidney
(ADH)
carried to the pituitary gland
Tuber Cinereum
The tuber cinereum is a convex mass of gray matter, as seen from the inferior surface. It is continuous
inferiorly with the infundibulum. The infundibulum is hollow and becomes continuous with the posterior
lobe of the pituitary gland. The median eminence is a raised part of the tuber cinereum to which is attached
the infundibulum.
Mammillary Bodies
The mammillary bodies are two small hemispherical bodies situated side by side posterior to the tuber
cinereum. They possess a central core of gray matter invested by a capsule of myelinated nerve fibers.
They are parts of the limbic system.
5. Third Ventricle
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Anterior commissure (AC)
The anterior commissure (AC) of the primate brain is a tract of axons that primarily connects the right and
left neocortex of the middle and inferior temporal lobes.
Posterior commissure (PC)
The posterior commissure bridges the upper part of the midbrain and lies adjacent to the posterior end of the
third ventricle. The posterior commissure interconnects the pretectal nuclei, mediating the consensual
pupillary light reflex. It is also related to superior colluculi related to light reflex.
TELENCEPHALON
The telencephalon (sometimes refered to as “cerebrum”) is the largest part of the brain. The majority
of the telencephalon is formed by the right and left hemispheres divided by the interhemispheric fissure.
Each hemisphere contains the frontal, parietal, temporal, occipital lobes as well as the insula.
The lateral ventricles and some part of the basal ganglia are other components of the telencephalon.
LOBES OF THE BRAIN
FRONTAL LOBE
The frontal lobe occupies the area anterior to the central sulcus and superior to the lateral sulcus. The
superolateral surface of the frontal lobe is divided by three sulci into four gyri. The precentral sulcus runs
parallel to the central sulcus, and the precentral gyrus lies between them. The superior frontal gyrus lies
superior to the superior frontal sulcus, the middle frontal gyrus lies between the superior and inferior
frontal sulci, and the inferior frontal gyrus lies inferior to the inferior frontal sulcus.
PARIETAL LOBE
The parietal lobe occupies the area posterior to the central sulcus and superior to the lateral sulcus; it
extends posteriorly as far as the parieto-occipital sulcus. The lateral surface of the parietal lobe is divided by
two sulci into three gyri. The postcentral sulcus runs parallel to the central sulcus, and the postcentral gyrus
lies between them. Running posteriorly from the middle of the postcentral sulcus is the intraparietal sulcus.
Superior to the intraparietal sulcus is the superior parietal lobule (gyrus), and inferior to the intraparietal
sulcus is the inferior parietal lobule (gyrus). The superior parietal lobule is an association area involved in
somatosensory function. The inferior parietal lobule is divided into the supramarginal gyrus gyrus
[surrounding the posterior end of the posterior ramus of the Sylvian fissure], which integrates auditory,
visual and somatosensory information, and the angular gyrus [surrounding the posterior end of the superior
temporal sulcus], which receives visual input.
TEMPORAL LOBE
The temporal lobe occupies the area inferior to the lateral sulcus. The lateral surface of the temporal lobe is
divided into three gyri by two sulci. The superior and middle temporal sulci run parallel to the posterior
ramus of the lateral sulcus and divide the temporal lobe into the superior, middle, and inferior temporal
gyri; the inferior temporal gyrus is continued onto the inferior surface of the hemisphere.
OCCIPITAL LOBE
The occipital lobe occupies the small area behind the parieto-occipital sulcus. The sulcus parieto-occipitalis
lies between the parietal and occipital lobes in the medial surface and also lies on the lateral surface.
Another sulcus is the transverse occipital sulcus lies among the unnamed gyri on the lateral surface of the
lobe. The most caudal end of the occipital lobe is called occipital pole.
INSULA
The insula is an area of the cortex that is buried within the lateral sulcus and forms its floor. The “little lids”
surrounding the Sylvian (lateral) fissure are called “operculum. The insula has been the area of interest in
studies with patients with psychiatric disorders, and is a mysterious brain structure. It has been suggested has
it has functions related to autonomic system, cognition, speech, etc.
MEDIAL AND INFERIOR SURFACES OF THE HEMISPHERE
The lobes of the cerebral hemisphere are not clearly defined on the medial and inferior surfaces. However,
there are many important areas that should be recognized. The corpus callosum, which is the largest
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commissure of the brain, forms a striking feature on this surface. The cingulate gyrus begins beneath the
anterior end of the corpus callosum and continues above the corpus callosum until it reaches its posterior
end. The gyrus is separated from the corpus callosum by the callosal sulcus. The cingulate gyrus is
separated from the superior frontal gyrus by the cingulate sulcus.
The paracentral lobule is the area of the cerebral cortex that surrounds the indentation produced by the
central sulcus on the superior border. The anterior part of this lobule is a continuation of the precentral gyrus
on the superior lateral surface, and the posterior part of the lobule is a continuation of the postcentral gyrus.
The precuneus is an area of cortex bounded anteriorly by the upturned posterior end of the cingulate sulcus
and posteriorly by the parieto-occipital sulcus.
The cuneus is a triangular area of cortex bounded above by the parieto-occipital sulcus, inferiorly by the
calcarine sulcus, and posteriorly by the superior medial margin. On the inferior surface of the frontal lobe,
the olfactory bulb and tract overlie a sulcus called the olfactory sulcus. Medial to the olfactory sulcus is the
gyrus rectus, and lateral to the sulcus are a number of orbital gyri.
BRODMANN AREAS
The best accepted system of functional regionalization of the cerebral cortex was developed by the German
neuroanatomist, Korbinian Brodmann (1868-1918). In 1909, Brodmann mapped the cortex into 47 unique
areas, each associated with specific morphological charecteristics. Although later, investigators refined and
expanded his map into more than 200 areas and assigned functional characteristics to them, Brodmann’s
original classification is still widely used.
The cerebral cortex is well designed with neurons, neuroglia, nerve fibers and a rich vascular supply. The
organization of the six layers of the neocortex is known as cytoarchitecture, whereas each layer has a name
and associated Roman numeral. It is important, however, to realize that not all areas of the cerebral cortex
possess six layers. Over the past century, clinicopathologic studies in humans and electrophysiologic and
ablation studies in animals have produced evidence that different areas of the cerebral cortex are
functionally specialized. However, the precise division of the cortex into different areas of specialization, as
described by Brodmann, oversimplifies and misleads the reader. The simple division of cortical areas into
motor and sensory is erroneous, for many of the sensory areas are far more extensive than originally
described, and it is known that motor responses can be obtained by stimulation of sensory areas.
Some of the Main Anatomical Connections of the Cerebral Cortex
Function
Origin
Cortical Area
Destination
Somatosensory
Ventral posterior lateral and ventral
Primary somesthetic area (BA 3, 1,
Secondary somesthetic area; primary
(most to contralateral side of body; oral to
posterior medial nuclei of thalamus
and 2), posterior central gyrus
motor area
Sensory
BA = Brodmann area
same side; pharynx, larynx, and perineum
bilateral)
Vision
Auditory
Taste
Lateral geniculate body
Medial geniculate body
Nucleus solitarius
Primary visual area
Secondary visual area
(BA 17)
(BA 18 and 19)
Primary auditory area
Secondary auditory area
(BA 41 and 42)
(BA 22)
Posterior central gyrus
(BA 43)
Smell
Olfactory bulb
Primary olfactory area;
Secondary olfactory area (B28)
periamygdaloid and prepiriform
areas
Motor
Function
Origin
Cortical Area
Destination
Fine movements
Thalamus from cerebellum, basal
Primary motor area
Motor nuclei of brainstem and
(most to contralateral side of body;
ganglia; somatosensory area;
(BA 4)
anterior horn cells of spinal cord;
extraocular muscles, upper face, tongue,
premotor area
corpus striatum
mandible, larynx, bilateral)
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WhIte matter of the cerebral hemispheres
The white matter is composed of myelinated nerve fibers of different diameters supported by neuroglia. The
nerve fibers may be classified into three groups according to their connections: (1) commissural fibers, (2)
association fibers, and (3) projection fibers.
Commissure Fibers
Commissure fibers essentially connect corresponding regions of the two hemispheres. They are as follows:
the corpus callosum, the anterior commissure, the posterior commissure, the fornix, and the habenular
commissure.
Corpus callosum
The corpus callosum, the largest commissure of the brain, connects the two cerebral hemispheres. It lies at
the bottom of the longitudinal fissure. For purposes of description, it is divided into the rostrum, the genu,
the body, and the splenium.
The rostrum is the thin part of the anterior end of the corpus callosum, which is prolonged posteriorly to be
continuous with the upper end of the lamina terminalis. The genu is the curved anterior end of the corpus
callosum that bends inferiorly in front of the septum pellucidum. The body of the corpus callosum arches
posteriorly and ends as the thickened posterior portion called the splenium.
The anterior commissure is a small bundle of nerve fibers that crosses the midline in the lamina terminalis.
The posterior commissure is a bundle of nerve fibers that crosses the midline immediately above the
opening of the cerebral aqueduct into the third ventricle; it is related to the inferior part of the stalk of the
pineal gland.
Association Fibers
Association fibers are nerve fibers that essentially connect various cortical regions within the same
hemisphere and may be divided into short and long groups. The short association fibers lie immediately
beneath the cortex and connect adjacent gyri; these fibers run transversely to the long axis of the sulci. The
long association fibers are collected into named bundles.
The uncinate fasciculus connects the first motor speech area and the gyri on the inferior surface of the
frontal lobe with the cortex of the pole of the temporal lobe.
The cingulum is a long, curved fasciculus lying within the white matter of the cingulate gyrus. It connects
the frontal and parietal lobes with parahippocampal and adjacent temporal cortical regions.
The superior longitudinal fasciculus is the largest bundle of nerve fibers. It connects the anterior part of
the frontal lobe to the occipital and temporal lobes.
The inferior longitudinal fasciculus runs anteriorly from the occipital lobe, passing lateral to the optic
radiation, and is distributed to the temporal lobe.
Projection Fibers
Afferent and efferent nerve fibers passing to and from the brainstem to the entire cerebral cortex must travel
between large nuclear masses of gray matter within the cerebral hemisphere. At the upper part of the
brainstem, these fibers form a compact band known as the internal capsule. Once the nerve fibers have
emerged superiorly from between the nuclear masses, they radiate in all directions to the cerebral cortex.
These radiating projection fibers are known as the corona radiata.
INTERNAL STRUCTURE OF THE CEREBRAL HEMISPHERES
Located in the interior of the cerebral hemispheres are the lateral ventricles, masses of gray matter, the
basal nuclei, and nerve fibers. The nerve fibers are embedded in neuroglia and constitute the white matter.
VENTRICLES
LATERAL VENTRICLES
Lateral ventricles are the largest ventricles. They are paired and horse-shaped cavities separated from each
other by the septum pellucidum. They are located both in the right and left hemispheres. Actually, in
coherence with the shape of the hemispheres, they lie in in the shape the letter “C”.
Each lateral ventricle is divided into a body, which occupies the parietal lobe, and from which anterior,
posterior, and inferior horns extend into the frontal, occipital, and temporal lobes, respectively. The lateral
ventricle communicates with the cavity of the third ventricle through the interventricular foramen (of
Monro). 70% of the entire CSF (cerebrospinal fluid) in the brain is produced by the relatively extensive
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choroid plexus of the lateral ventricles. The total volume of the entire CSF is 150 ml with a daily production
of 500-750 ml. 1/5 of the CSF stays in the ventricles, and the remaining 4/5 stays in the subarachnoid space.
THIRD VENTRICLE
The third ventricle is quadrilateral and slit-like. It is vertically placed between the walls of the right and left
thalami. It is interrupted by a mass of grey matter massa intermedia – interthalamic adhesion; adhesio
interthalamica- ; that forms a bridge between two thalami. The interthalamic adhesion is found in 70-80% of
humans.
FOURTH VENTRICLE
The fourth ventricle has a classical diamond shape in sagittal sections and lies between the brainstem and the
cerebellum in the hindbrain. The fourth ventricle extends from the cerebral aqueduct (of Sylvius) posteriorly
to the obex anteriorly. It is continuous with the central canal of the spinal cord. The lateral aspect of the
fourth ventricle has three foramina; the right and left foramina of Luschka and the single,median foramen of
Magendie, which drain the CSF from the fourth ventricle into the subarachnoid space. The CSF can enter
the spinal cord or the subarachnoid space through these three foramina. The fluid then flows around the
superior sagittal sinus to be reabsorbed via the arachnoid villi into the venous system.
Cisternae
In certain areas of the brain the arachnoid mater completely diverges from the pia mater, forming expanded
subarachnoid spaces which are called subarachnoid cisterns. As the CSF is circulating through the
subarachnoid spaces it also enters the subarachnoid citerns, filling them. The major subarachnoid cisterns
are:Cisterna magna (Cisterna cerebromedullaris) the largest subarachnoid cistern, Pontine cistern,
Interpeduncular cistern, Chiasmatic cistern (cisterna basalis), Superior cistern (Cistern of the great cerebral
vein).
BASAL GANGLIA
The basal ganglia comprise a distributed set of brain structures in the telencephalon, diencephalon,
and mesencephalon. The forebrain structures include the caudate nucleus, the putamen, the nucleus
accumbens (or ventral striatum) and the globus pallidus. Together, these structures are named the corpus
striatum.
The caudate nucleus is a C-shaped structure that is closely associated with the lateral wall of the lateral
ventricle. It is largest at its anterior pole (the head), and its size diminishes posteriorly as it follows the
course of the lateral ventricle (the body) all the way to the temporal lobe (the tail), where it terminates at the
amygdaloid nuclei.
The putamen is also a large structure that is separated from the caudate nucleus by the anterior limb of the
internal capsule. The putamen is connected to the caudate head by bridges of cells that cut across the internal
capsule. Because of the striated appearance of these cell bridges, the caudate and putamen are collectively
referred to as the striatum or neostriatum, and the nucleus accumbens is often called the ventral striatum.
Functionally, the caudate nucleus and the putamen are considered equivalent to each other; indeed, most
mammals have only a single nucleus called the striatum. The putamen and the globus pallidus are
collectively called the lenticular nucleus, or lentiform nucleus. The globus pallidus is divided into two
segments: the internal (or medial) segment and the external (or lateral) segment.
The subthalamic nucleus is part of the diencephalon; as its name implies, it is located just below the
thalamus. The substantia nigra is a midbrain structure, composed of two distinct parts: the pars compacta
and the pars reticulata. The substantia nigra is located between the red nucleus and the crus cerebri (cerebral
peduncle) on the ventral part of the midbrain. The pars compacta is the source of a clinically important
dopaminergic pathway to the striatum; loss of neurons in this area is the cause of Parkinson’s disease.
Basal Ganglia Afferents
The striatum is the main recipient of afferents to the basal ganglia. These excitatory afferents arise from the
entire cerebral cortex and from the intralaminar nuclei of the thalamus (primarily the centromedian nucleus
and parafascicularis nucleus).
Basal Ganglia Efferents
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The major output structures of the basal ganglia are the globus pallidus internal segment (GPint) and the
substantia nigra pars reticulata (SNr). Both of these structures make GABAergic, inhibitory connections on
their targets.
Functions of the Basal Ganglia
MOTOR FUNCTIONS
The function of the basal ganglia in motor control is not understood in detail. It appears that the basal
ganglia is involved in the enabling of practiced motor acts and in gating the initiation of voluntary
movements by modulating motor programs stored in the motor cortex and elsewhere in the motor hierarchy.
Thus, voluntary movements are not initiated in the basal ganglia (they are initiated in the cortex); however,
proper functioning of the basal ganglia appears to be necessary in order for the motor cortex to relay the
appropriate motor commands to the lower levels of the hierarchy.
COGNITIVE FUNCTIONS
There are a number of cortical loops through the basal ganglia that involve prefrontal association cortex and
limbic cortex. Through these loops, the basal ganglia are thought to play a role in cognitive function that is
similar to their role in motor control. That is, the basal ganglia are involved in selecting and enabling various
cognitive, executive, or emotional programs that are stored in these other cortical areas.
BLOOD SUPPLY OF THE BRAIN
The brain receives its arterial supply from two pairs of vessels, the vertebral and internal carotid
arteries, which are interconnected in the cranial cavity to produce a cerebral arterial circle (of Willis).
The two vertebral arteries enter the cranial cavity through the foramen magnum and just inferior to
the pons fuse to form the basilar artery.
The two internal carotid arteries enter the cranial cavity through the carotid canals on either side.
Each vertebral artery arises from the first part of each subclavian artery in the lower part of the neck,
and passes superiorly through the transverse foramina of the upper six cervical vertebrae.
The two internal carotid arteries arise as one of the two terminal branches of the common carotid arteries.
They proceed superiorly to the base of the skull where they enter the carotid canal. Entering the cranial
cavity each internal carotid artery gives off the ophthalmic artery, the posterior communicating artery, the
middle cerebral artery, and the anterior cerebral artery
The cerebral arterial circle (of Willis) is formed at the base of the brain by the interconnecting
vertebrobasilar and internal carotid systems of vessels. This anastomotic interconnection is accomplished
by:
 an anterior communicating artery connecting the left and right anterior cerebral arteries to each other;
 two posterior communicating arteries, one on each side, connecting the internal carotid artery with the
posterior cerebral artery.
Venous drainage of the brain begins internally as networks of small venous channels lead to larger
cerebral veins, cerebellar veins, and veins draining the brainstem, which eventually empty into dural venous
sinuses. The dural venous sinuses are endothelial-lined spaces between the outer periosteal and the inner
meningeal layers of the dura mater, and eventually lead to the internal jugular veins. Also emptying into the
dural venous sinuses are diploic veins, which run between the internal and external tables of compact bone
in the roof of the cranial cavity, and emissary veins, which pass from outside the cranial cavity to the dural
venous sinuses. The emissary veins are important clinically because they can be a conduit through which
infections can enter the cranial cavity because they have no valves.
The dural venous sinuses include the superior sagittal, inferior sagittal, straight, transverse, sigmoid,
and occipital sinuses, the confluence of sinuses, and the cavernous, sphenoparietal, superior petrosal, inferior
petrosal, and basilar sinuses. As the transverse sinuses leave the surface of the occipital bone, they become
the sigmoid sinuses, which turn inferiorly and end at the beginning of the internal jugular veins.
The paired cavernous sinuses are against the lateral aspect of the body of the sphenoid bone on either
side of the sella turcica. They are of great clinical importance because of their connections and the structures
that pass through them.
Structures passing through each cavernous sinus are: internal carotid artery; and abducent nerve [VI].
Structures in the lateral wall of each cavernous sinus are, from superior to inferior:
oculomotor nerve [III];trochlear nerve [IV];ophthalmic nerve [V1]; and maxillary nerve [V2].
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http://imueos.wordpress.com/2010/10/08/ascending-descending-tracts-of-spinal-cord
Ascending tracts
Sensory
Descending tracts
Motor
General arrangement of both tracts
1st order neuron
2nd order neuron
3rd order neuron
The only difference is the different locations where each order of neuron ends.
Decussation is the cross-over of the tract from one side to the other. Therefore, there are instances where the
left side of the body is controlled by the right brain hemisphere. Decussation occurs at different locations for
each tracts.
DESCENDING TRACTS
General arrangement of descending tracts
1st order neuron
starts at the cerebral cortex in the primary motor cortex
2nd order neuron
axon of the 1st order neuron will synapse with the 2nd order neuron at the level of the brain stem, which
commonly decussate (crosses over) to the opposite side.
3rd order neuron
The 3rd order neuron is located in the ventral horn of the spinal cord, which will exit with the spinal nerve to
supply the muscle.
Types of descending tracts:
Lateral corticospinal tract
Anterior corticospinal tract
Therefore, the descending tract is also known as corticospinal tract.
Corticospinal tract arise from long axons of the pyramidal cells of the precentral gyrus (primary motor
centre of the cerebral cortex) which lies in front of the central sulcus
Homunculus arrangement: arranged upside down; the finer the movement, the more the cortical
representation
fingers, face, tongue – more
trunk, lower limbs – less
medial surface: lower limbs
superolateral surface: everything else
1st order neuron
Fibres of the 1st order neuron arise from the precentral gyrus
These fibres converge and enter a small area
internal capsule
ALL the fibers (from ascending & descending tracts) converge here
Function: separates the caudate nucleus and the thalamus from the lenticular nucleus (putamen+ globus
pallidus)
internal capsule: bounded medially by the thalamus and caudate nucleus and bounded laterally by the
lenticular nucleus
Parts of internal capsule (not homunculus arrangement, normal head to toe)
anterior limb: head & neck fibres most anterior
posterior limb: lower limb fibres most posterior
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The descending fibres passes through the LATERAL half of the posterior limb of internal capsule
After the internal capsule, the fibres enter the brain stem; midbrain, pons and medulla.
2nd order neuron
Fibres of the 1st order neuron ends when it enters the brain stem and synapse with the 2nd order neuron
The fibres pass through the brainstem
1st – through the (mid 5th) crus cerebri of midbrain
2nd – through the anterior part of the pons
3rd – in the medulla oblongata
80-85% of the fibres cross to the opposite side: Motor decussation
Enters the spinal cord
3rd order neuron
2nd order neuron fibres in the medulla oblongata enters the spinal cord and synapse with the 3rd order
neuron
Motor decussation in the spinal tract, the crossed tract descend as the lateral corticospinal tract
Therefore, the motor cortex of the cerebral hemisphere controls the opposite side of the body (L – R, R – L)
contra-lateral side.
In upper motor neuron lesions: above the motor decussation (above medulla), opposite side of body affected
below the motor decussation same side of body affected ipsilateral side
Uncrossed fibres: in the spinal tract, the uncrossed tract descent as the anterior corticospinal tract
its fibres cross at spinal level?
ASCENDING TRACTS
Types of ascending tracts:
Spinothalamic tracts
Lateral spinothalamic tract
pain & temperature
Anterior spinothalamic tract
light touch & pressure
Dorsal column tract
deep touch & pressure
proprioception
vibration sensation
Spinocerrebellar tract
posture & coordination
SPINOTHALAMIC TRACTS
1st order neuron: Arise from sensory receptors of the body
The fibres enter the white mater from the tip of posterior gray horn
2nd order neuron:The fibres of 1st order neuron synapse with the 2nd order neuron at the substantia
gelatinosa. These fibres then cross to the opposite side
Pain & temperature fibres enters the lateral spinothalamic tract
Light touch & pressure fibres enters the anterior spinothalamic tract
These tracts ascends to brainstem to medulla oblongata, pons and midbrain
tracts flattened in the brainstem: spinal lemniscus
Reaches the ventral posterolateral nucleus of the thalamus and ends here.
3rd order neuron: The 3rd order neurons arise from the thalamus and pass through the internal capsule
thalamocortical fibres pass through the medial part of the posterior limb of the internal capsule
Enters the postcentral gyrus - sensory cortex of the cerebrum, behind the central sulcus.
Same homunculus arrangement; more sensitive areas in the body have a greater representation.
DORSAL COLUMN TRACT
1st order neuron:
 Arise from the sensory receptors of the body
 Fibres enter the dorsal column of the SAME side (post column of spinal cord)
 ascends to the medulla oblongata
 (does not synapse and end here like spinothalamic tract)
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 Enters medulla oblongata
 ends in the gracile and cuneate nucleus
2nd order neuron: Starts at the gracile & cuneate nucleus of the medulla oblongata
These fibres crosses to the opposite side of the medulla oblongata.
Ascends through the brain stem as flattened bundle medial lemniscus
Ends in the ventral posterolateral nucleus of the thalamus.
3rd order of nucleus: Arise from the thalamus Pass through the internal capsule; medial aspect of the
posterior limb of internal capsule.
Reaches the postcentral gyrus and ends here.
SPINOCEREBELLAR TRACT
1st order neurons:
Arise from the sensory receptors of the body
Enters the spinal cord
Ends in the Clarke’s Column of the posterior grey horn
synapse
2nd order neurons:
Arise from the Clarke’s Column
synapse with 1st order neurons
Ascends in the spinocerebellar tracts, enters the cerebellum through the interior and superior cerebellar
peduncles
the only tract that enters the cerebellum
These tracts decussate 2 times; therefore cerebellum controls same side of body
ipsilateral
eg. right spinocerebellar tract controls the right side vice versa
The limbic system has two main functions:
Emotional processing
Motivation
Another function of the system; short-term memory (also emotional memory) is also important for
“survival”.
The limbic system works to process our emotions and is related to motivation and with its connections with
the cognitive parts of the brain helps us to “use our mind” a.k.a. accomplish mental processes.
The limbic system structures are telencephalic & subcortical structures.
The complex network for the process of emotions and is also related to memory and learning in addition to
hippocampus, amygdala and parahippocampus includes:
 Cingulate gyrus
 Hypothalamus
 Major areas in the prefrontal cortex
 Striatum
 Some thalamic nuclei
 Orbitofrontal cortex
 Septal area
 Some medial components of the midbrain (e.g. VTA)
 Habenula …
 + white matter tracts
In 1937 James Papez proposed the Papez circuit: A list of structures in the brain and a closed circuit related
to emotions
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Hippocampal formation (Subiculum) → fornix → mammillary bodies
Mammillary bodies → mammillothalamic tract → anterior thalamic nucleus
Anterior thalamic nucleus → genu of the internal capsule → cingulate gyrus
Cingulate gyrus → cingulum → parahippocampal gyrus
Parahippocampal gyrus → entorhinal cortex → perforant pathway → hippocampus.
In 1952 Paul D. McLean added
 Amygdala
 Septum
 Pre-frontal cortex
to the Papez circuit and came up with the idea of a system: Limbic System.
In 2014, we now know that the system is more complex than it was first proposed and discussed in
th
mid-20 century.
1939
Klüver-Bucy Syndrome
bilateral removal of amygdala and hippocampal formation
What happens if we remove the medial temporal lobe of an animal, a monkey?
 Became docile;”good monkeys”.
 A tendency towards oral behaviour such as attempting to ingest inedible objects.
 Hypersexualized behaviour by mounting females of the same and different species.
 A compulsion to attend and react to every visual stimulus
 No fear.
 Change in dietary habits
The most famous two members of the limbic system are hippocampus & amygdala.
Hippocampus (sea horse; hippocampal formation) is located in the medial temporal lobe under the inferior
(temporal) horn of the lateral ventricle. Amygdala (almond) resides at the tip of the temporal lobe anteriorly,
and is posterior to anterior part of hippocampus.
Hippocampus is the site of short-term memory. It is also an important structure in mood regulation with its
connections with the hypothalamus.
Amygdala is important in emotion processing with ventromedial prefrontal cortex and acts as an emotional
memory box.
Anterior cingulate cortex (ACC), medial part of prefrontal cortex, the basal ganglia (particularly caudate,
putamen, and nucleus accumbens; the site of pleasure), anterior and dorsomedial thalamic nuclei are some of
the important limbic system structures.
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TWO MAIN CIRCUITS IN THE BRAIN
COGNITIVE CIRCUIT EMOTION CIRCUIT
DORSAL CIRCUIT
VENTRAL CIRCUIT
The cognitive networks inhibit the ventral circuit.
Dorsal (cognitive) circuit
Hippocampus
Dorsolateral prefrontal cortex (DLPFC)
Dorsal regions of the anterior cingulate cortex (ACC)
Parietal cortex
Posterior insular region
Modulates selective attention, planning and effortful regulation of affective state.
Ventral (limbic) circuit structures:
Amygdala
Insula (Particularly, anterior insula)
Ventral striatum
Ventral regions of the anterior cingulate cortex (ACC)
Orbitofrontal cortex (OFC) and medial PFC
It is possible that the altered emotional regulation or cognition found in all of these syndromes involves
aberrant function of these circuits, but perhaps with different patterns on a molecular level. (Phillips et al.
2003).
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