ANATOMY OF THE NERVOUS
SYSTEM
Lecture 9 &10
Functions of the nervous system
• 1. Initiate and/or regulate movement of body parts
• 2. Regulate secretions from glands
• 3. Gather information about the external environment and the
internal environment of the body
– using senses (sight, hearing, touch, balance, taste) and
mechanisms to detect pain, temperature, pressure, and
certain chemicals, such as carbon dioxide, hydrogen, and
oxygen
• 4. Maintain an appropriate state of consciousness
• 5. Stimulate thirst, hunger, fear, rage, and sexual behaviors
appropriate for survival
• The entire nervous system can be
divided into two parts:
• 1- Central nervous system (CNS)
– includes the brain and spinal cord
• 2- Peripheral nervous system (PNS),
which consists of:
– Cranial nerves
– spinal nerves
• A further distinction is often made:
• the autonomic nervous system (ANS)
– which integrates activity of visceral
structures
• smooth muscle, cardiac muscle, and glands
• The ANS has elements in both:
– Central nervous system.
– Peripheral nervous system.
• In the PNS:
1- sensory (afferent) nerves
- gather information about the external and
internal environments and relay this information to
the CNS.
- The specialized organs or cells that detect
specific stimuli are sensory receptors.
• CNS interprets information arriving via the
PNS, integrates that information, and
initiates
• appropriate movement of body parts
• glandular secretion
• behavior response.
2- Motor efferent
- Communication between the CNS and
muscles and accomplished via nerves of
the PNS
Microscopic Neuroanatomy
• The individual nerve cell is called a neuron.
• Each neuronal cell body gives rise to one or more
nerve processes and cytoplasmic extensions of the
cell.
• The nerve processes are
called dendrites if they
transmit electrical signals
toward the cell bodies
• They are called axons if they
conduct electrical signals
away from the cell bodies.
• Neurons may be classified morphologically
according to their number of nerve
processes:
– Unipolar neurons have one process
– Bipolar neurons have one dendrite and one axon
– Multipolar neurons have a number of dendrites in
addition to their single axon.
• We do not have true unipolar neurons, but
many sensory neurons have their single
dendrite and axon fused so as to give the
appearance of a single process
– This configuration is pseudounipolar.
• Nervous tissue consists not only of neurons but also
of supportive cells.
– In the CNS, these supportive cells are the neuroglia,
comprising a variety of glial cells
• Groups of nerve cell bodies within the CNS are
generally called nuclei, while groups of nerve cell
bodies in the PNS are called ganglia.
• In general terms:
– Aggregates of neuronal cell bodies form the gray matter of
the CNS
– Regions characterized primarily by tracts are white matter.
Development of CNS
• Shortly after gastrulation,
ectodermal cells on the
dorsum just cranial to the
primitive streak begin to
proliferate and
differentiate into a neural
plate.
• The neural plate
proliferates faster along
its lateral margins than on
the midline, creating the
neural groove
Development of CNS
• The edges of which (the
neural folds) ultimately
meet dorsally to form the
neural tube.
• The entire CNS is formed
from the cells of the neural
tube.
• The lumen of the neural
tube persists in the adult
as the central canal of the
spinal cord and as the
ventricles of the brain.
• Development of the spinal cord continues
by an increase in the thickness of the wall of
the neural tube.
• As cells divide and differentiate, three
concentric layers of the neural tube emerge:
– an inner (ventricular zone)
– a middle (intermediate zone)
– a superficial (marginal zone)
• The thin ventricular zone of cells (also
called ependymal zone) surrounds the
lumen of the neural tube
• The site of mitosis of neuronal and glial
precursors in the developing nervous
system.
– It will ultimately form the ependyma of the
central canal of the spinal cord and of the
ventricles of the brain.
• As cells are born in the germinal layer, they
migrate outward to form the intermediate
zone (also called mantle zone).
• The intermediate zone comprises neurons
and neuroglia and becomes the gray matter
near the center of the cord.
– The dorsal parts of the intermediate zone
develop into the dorsal horns.
- The ventral intermediate zone becomes
the ventral horns
• The marginal zone, which is most
superficial, consists of nerve processes that
make up the white matter of the spinal cord.
– The spinal cord white matter is divided into
dorsal, lateral, and ventral funiculi, which are
delimited by the dorsal and ventral horns of
gray matter.
• Development of the brain:
• The first gross subdivisions of the brain create the three-vesicle
stage.
• These subdivisions, which consist of three dilations of the
presumptive brain, are
– prosencephalon, or forebrain
– mesencephalon, or midbrain
– rhombencephalon, or hindbrain.
– In the five-vesicle stage of development
– the prosencephalon further subdivides to form the telencephalon (future
cerebrum) and the diencephalons ,
– the rhombencephalons divides into the Metencephalon (future pons and
cerebellum) and the myelencephalon (future medulla oblongata).
– The mesencephalon does not subdivide.
Central Nervous System
• Brain
• The gross subdivisions of the adult brain include:
– cerebrum
– cerebellum
– brainstem
•
The cerebrum develops from the embryonic
Telencephalon.
• The components of the brainstem are defined in a
number of ways
– include the diencephalon, midbrain, pons, medulla oblongata
• Cerebrum comprises:
– the two cerebral hemispheres, including the cerebral
cortex, the basal nuclei
• The surface area of the cerebrum in domestic mammals
is increased by numerous foldings to form convex ridges,
called gyri (singular gyrus), which are separated by
furrows called fissures or sulci.
• A particularly prominent fissure, the longitudinal fissure,
lies on the median plane and separates the cerebrum
into its right and left hemispheres.
•
Deep to the cerebral cortex are aggregates of
subcortical gray matter called the basal nuclei
Diencephalon
• Is a derivative of the prosencephalon.
– thalamus
– epithalamus
– hypothalamus
– the third ventricle are included in the diencephalon.
• The thalamus is an important relay center for nerve fibers
connecting the cerebral hemispheres to the brainstem
and spinal cord.
• The epithalamus, dorsal to the thalamus, includes a
number of structures, the pineal gland, which is an
endocrine organ in mammals.
• The hypothalamus, ventral to the thalamus, surrounds
the ventral part of the third ventricle and comprises
many nuclei that function in autonomic activities and
behavior.
– Attached to the ventral part of the hypothalamus is
the hypophysis, or pituitary gland
Mesencephalon
• The mesencephalon, or midbrain
• lies between the diencephalon rostrally and the pons caudally.
– The two cerebral peduncles
– four colliculi are the most prominent features of the midbrain.
• The cerebral peduncles, also called crura cerebrii, are large
bundles of nerve fibers connecting the spinal cord and
brainstem to the cerebral hemispheres.
– These peduncles consist of both sensory and motor fiber tracts.
• The colliculi are four small bumps (colliculus is Latin for little
hill) on the dorsal side of the midbrain.
• They consist of right and left rostral colliculi and right and
left caudal colliculi.
– The rostral colliculi coordinate certain visual reflexes,
– The caudal colliculi are relay nuclei for audition (hearing).
Metencephalon.
• The metencephalon includes
– the cerebellum dorsally and the pons ventrally.
• The cerebellum features two lateral hemispheres and a median
ridge called the vermis.
•
The surface of the cerebellum consists of many laminae called
folia. In the cerebellum, like the cerebrum, the white matter is
central, and the gray matter is peripheral in the cerebellar
cortex.
•
The cerebellum is critical to the accurate timing and execution
of movements; it acts to smooth and coordinate muscle
activity.
• The pons is ventral to the cerebellum, and its surface
possesses visible transverse fibers that form a bridge from one
hemisphere of the cerebellum to the other.
Myelencephalon
• The myelencephalon becomes the medulla
oblongata in the adult.
• It is the cranial continuation of the spinal
cord
• The medulla oblongata (often simply called
the medulla)
• contains a number of important autonomic
centers and nuclei for cranial nerves.
Ventricular System
• The ventricles of the brain are the remnants of the lumen of the
embryonic neural tube.
•
Right and left lateral ventricles lie within the respective cerebral
hemispheres.
• They communicate with the midline third ventricle by way of the
interventricular foramina.
• Most of the third ventricle is surrounded by the diencephalon.
•
The third ventricle connects with the fourth ventricle by way of the
mesencephalic aqueduct (cerebral aqueduct) passing through the
midbrain.
• The fourth ventricle, between the cerebellum above and pons and
medulla below, communicates with the subarachnoid space
surrounding the CNS through two lateral apertures.
• Each ventricle features a choroid plexus
– a tuft of blood capillaries that protrudes into the
lumen of the ventricle.
– The plexus of capillaries is covered by a layer of
ependymal cells that are continuous with the
lining membrane of the ventricles.
• Cerebrospinal fluid (CSF), filling the
ventricular system and surrounding the CNS,
is formed primarily by the choroid plexuses,
with a smaller contribution made by the
ependyma lining the ventricles.
• CSF is a modified transudate, formed
primarily through active secretion by the
ependymal cells, especially those of the
choroid plexuses.
Meninges
• The coverings of the brain and spinal cord are
the meninges (singular meninx).
• They include, from deep to superficial
– the pia mater
– the arachnoid
– the dura mater.
• The pia mater, the deepest of the meninges, is
a delicate membrane that invests the brain and
spinal cord, following the grooves and
depressions closely.
• The pia mater forms a sheath around the blood
vessels and follows them into the substance of
the CNS.
Meninges
• The arachnoid
– arises embryologically from the same layer as the pia
mater but separates from it during development so
that a space forms between them.
– Because of the weblike appearance of these
filaments, this middle layer is called the arachnoid
(arachnoidea, arachnoid mater).
• Together, the pia mater and arachnoid constitute
the Ieptomeninges (from the Latin word lepto,
delicate), reflecting their fine, delicate nature.
• The space between the two layers is the
subarachnoid space.
– It is filled with CSF.
Meninges
• The Dura mater is the tough fibrous outer covering
of the CNS.
• Within the cranial cavity the dura mater is intimately
attached to the inside of the cranial bones and so
fulfills the role of periosteum.
– It also forms the falx cerebri, a median sickle-shaped fold
that lies in the longitudinal fissure and partially separates
the cerebral hemispheres.
– Another fold of dura mater, the tentorium cerebelli, runs
transversely between the cerebellum and the cerebrum.
• In some locations within the skull, the dura mater splits into
two layers divided by channels filled with blood. These dural
sinuses receive blood from the veins of the brain and empty
into the jugular veins.
– They are also the site of reabsorption of CSF back into the
circulation.
Spinal Cord
• The spinal cord is the caudal continuation of the
medulla oblongata.
•
Unlike that of the cerebrum, the spinal cord’s gray
matter is found at the center of the cord, forming a
butterfly shape on cross-section.
• Myelinated fiber tracts, the white matter, surround
this core of gray matter.
• A spinal cord segment is defined by the presence of
a pair of spinal nerves. Spinal nerves are formed by
the conjoining of dorsal and ventral roots.
Spinal Cord
• Sensory neurons of neural crest origin are present in
aggregates, called dorsal root ganglia, lateral to the spinal
cord.
– The neurons within these ganglia are pseudounipolar
– they give rise to axons that enter the dorsal horn of the
spinal cord and other fibers that join with motor fibers from
the ventral horn neurons to become a spinal nerve
extending into the periphery.
• The ventral root of the spinal nerve consists largely of motor
fibers that arise from the nerve cells in the ventral horn of the
spinal cord.
• The dorsal and ventral roots unite to form the spinal nerve
close to the intervertebral foramen between adjacent vertebrae.