CHAPTER THIRTEEN
Anatomy of the Nervous System
13.1 The Embryological Perspective
During fertilization, a sperm cell and an egg cell fuse to
become a zygote (a fertilized egg cell). Then the zygote
divides to generate cells that make up an entire organism.
16 days after fertilization, the developing embryo’s cells
belong to one of three embryonic germ layers that give rise
to different tissues in the body. The endoderm (inner germ
layer) is responsible for generating the lining tissues of
various spaces within the body, such as the mucosae of the
digestive and respiratory systems. The mesoderm (middle
germ layer) gives rise to most of the muscle and
connective tissues. Finally, the ectoderm (outer germ
layer) develops into the integumentary system (the skin)
and the nervous system.
A. The Neural Tube
1. The brain and all other nervous tissue forms from
ectoderm.
2. During embryonic development, a portion
of the ectoderm differentiates into a
specialized region of neuroectoderm to
form the neural plate which folds inward
to form the neural groove.
3. Eventually the two sides of the neural
groove will converge to form the neural
tube which lies beneath the ectoderm.
The neural tube will go on to form the
CNS while the neural
Neural Tube
Anterior End
Primary Vesicle
Secondary Vesicle
Telencephalon
Prosencephalon
(forebrain)
Diencephalon
Mesencephalon
(midbrain)
Mesencephalon
Adult Structures
Cerebrum (cerebral cortex, white matter,
and basal nuclei)
Thalamus, pineal gland, and
hypothalamus
Brain stem (midbrain)
Metencephalon
Brain stem (pons) and cerebellum
Fourth Ventricle
Myelencephalon
Brain stem (medulla oblongata)
Fourth Ventricle
Spinal Cord
Central Canal
Rhombencephalon
(hindbrain)
Posterior
End
crest (cluster of cells located near tube)
will develop into the PNS.
B. Primary Vesicles
1. The anterior end of the neural tube
develops into the brain. It begins to
enlarge forming three prominent
divisions called the primary brain
vesicles: Prosencephalon (forebrain),
Mesencephalon (midbrain), and
Rhombencephalon (hindbrain).
2. The posterior end of the neural tube will become
the spinal cord.
C. Secondary Vesicles
1. By week 5 of development, the
primary brain vesicles have changed
position and the Prosencephalon and
Rhombencephalon have further
subdivided to form the secondary
brain vesicles. The mesencephalon
does not change significantly during
development.
2. As development continues, the cerebrum
enlarges to the point where it covers the
other portions of the brain.
D. Spinal Bifida = a birth defect of the spinal cord
caused when the neural tube fails to close
completely, and the result is the emergence of
meninges and neural tissue through the vertebral
column in the form a cyst.
Ventricles
Lateral Ventricles
Third Ventricle
Cerebral Aqueduct
13.2 The Central Nervous System
A. The brain and spinal cord are the central nervous system. The spinal cord is a single structure while the adult brain is
described in terms of four major regions: cerebrum, diencephalon, brain stem, and cerebellum. The brain contains almost
97% of the body’s neural tissue and a typical adult brain weighs about 3 lbs. and has a volume of 1200 ml. Brain size varies
considerably among individuals. On average the brain of males are about 10% larger than those of females, owing to
differences in body size. No correlation exists between brain size and intelligence. Individuals with the smallest brains and
the largest brains are functionally normal.
B. Cerebrum = accounts for 80% of the brain’s mass.
1. Anatomy of Cerebrum
a. The cerebrum functions in conscious thought, memory storage and processing, sensory processing, and
the regulation of skeletal muscle contractions.
b. The surface of the cerebrum is highly folded (wrinkled) and covered with a superficial layer of gray
matter called the cerebral cortex. It consists of:
a. Fissures = deep grooves
b. Sulcus (plural: sulci) = shallow groove that separates the folds or wrinkles
c. Gyrus (plural: gyri) = fold or wrinkle that increases surface area
c. The cerebrum is divided into two distinct halves, a right and left cerebral hemisphere by the longitudinal
fissure.
d. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for
communication between the two hemispheres of the cerebral cortex.
e. Each cerebral hemisphere is subdivided into four major lobes: frontal, parietal, occipital, and
temporal. Deep within the lateral sulcus is a fifth lobe called the insula.
f. The central sulcus separates the frontal lobe from the parietal lobe. The wrinkle immediately in
front of the central sulcus is called the precentral gyrus while the wrinkle immediately behind the
central sulcus is called the postcentral gyrus.
g. The lateral sulcus separates the frontal and parietal lobes from the temporal lobe.
h. The parieto-occipital sulcus separates the parietal and occipital lobes.
2.
Cerebrum in more detail
a. As mentioned earlier the cerebrum functions in conscious thought, memory storage and processing,
sensory processing, and the regulation of skeletal muscle contractions.
b. The cerebral cortex is the GRAY matter of the cerebrum and is responsible for all qualities
associated with “consciousness.”
c. Each hemisphere is concerned with the sensory and motor functions of the opposite (contralateral)
side of the body.
d. Although symmetrical in structure, the two hemispheres are not equal in function. Instead, there is
lateralization or specialization of the cortical functions:
i. Right brain = analyzes sensory information and relates the body to the sensory environment;
interpretive centers in this hemisphere enable you to identify familiar objects by touch, sight,
smell, taste, or feel. Right brained individuals are often more artistic, musically inclined, or
attuned to their emotions.
ii. Left brain = possesses the general interpretive and speech centers and is important in languagebased skills, important in reading, writing, speaking, math, and logic.
e. The German neuroscientist, Korbinian Brodmann, extensively studied the microscopic anatomy of the
cerebral cortex and divided it into 52 separate regions classified as
Brodmann’s Areas.
f. The cerebral cortex contains three functional areas (COMPOSED OF
INTERNEURONS!!!)
i. Motor areas = control voluntary motor functions.
ii. Sensory areas = provide conscious awareness of sensations.
iii. Association areas = integrate all sensory and motor information.
iv. No functional area acts alone, and conscious behavior involves the entire cerebral cortex in
one way or another.
g.
MOTOR AREAS OF THE CEREBRAL CORTEX
i. Primary Motor Cortex (Brodmann Area 4) = located within the precentral gyrus of the
frontal lobe
1. Initiates and coordinates motor movements.
2. Possesses large neurons called pyramidal cells. These cells are the upper motor neurons
that instruct cells in the spinal cord (lower motor neurons) to move skeletal muscles.
3. Allows conscious control of skilled voluntary movements of skeletal muscles.
4. The motor areas of the precentral gyrus have been spatially mapped (somatotopy).
Specific parts of the primary motor cortex correspond to specific regions of the body.
ii. Premotor Cortex (Brodmann Area 6) = located anterior to the precentral gyrus of the frontal lobe
1. Responsible for planning movements.
2. Controls learned motor skills that are repeated or patterned (such as playing an
instrument or typing). Some people say this is called muscle memory.
3. Also coordinates the movements of muscles simultaneously and/or sequentially by
sending activating impulses to the primary motor cortex.
iii. Broca’s Area (Brodmann Areas 44, 45) = also known as the speech center, is located
anterior to the lower part of the premotor cortex
1. Responsible for the production of language (motor speech including thinking about or
planning to speak) or controlling movements responsible for speech.
2. It is located only on the left hemisphere in most people.
iv. Frontal Eye Fields (Brodmann Area 8) = located anterior to the premotor cortex and superior to
the Broca’s
1. Controls the voluntary movements of the eye
2. Attend to visual stimuli but has no role in the interpretation of visual stimuli.
h.
SENSORY AREAS OF THE CEREBRAL CORTEX
i. Somatosensory Cortex = located within the parietal lobe of the cerebrum
1. Primary Somatosensory Cortex (Brodmann Areas 1, 2, & 3) = located in the postcentral
gyrus
a. Region where sensory information is initially received from the thalamus for
conscious perception.
b. All tactile senses (touch, pressure, tickle, pain, itch, and vibration) are processed
in this area, as well as proprioception (sense of body position) and kinesthesia
(sense of body movement).
c. Neurons receive information from the somatic sensory receptors of the skin and
from proprioceptors in skeletal muscles.
d. Endows special discrimination, or rather allow you to interpret what body
region is being stimulated.
2. Somatosensory Association Area = lies posterior to the primary
ii.
iii.
iv.
somatosensory cortex
a. Integrates and analyzes the somatic sensory inputs form the primary
somatosensory cortex and memories of previous experience to produce an
understanding about what is being felt.
b. Allows you to recognize the cold, flat, round thing in your pocket is a quarter.
Visual Cortex = located within the occipital lobe of the cerebrum
1. Primary Visual Cortex (Brodmann Area 17) processes and interprets light
information from the retina of the eye such as color, form, texture, and movement.
2. Visual Association Area (Brodmann Area 18, 19) integrates and analyzes the visual
information coming from the primary visual cortex and past experiences to interpret what
the image is or means (a face, a flower, a car, a stop sign).
Auditory Cortex = located within the temporal lobes
1. Primary Auditory Cortex (Brodmann Area 22) receives and processes sound
information from the ear such as pitch, rhythm, and volume.
2. Auditory Association Area (Brodmann Area 23) integrates and analyzes the
information from the primary auditory cortex and past experiences to interpret what the
sound stimulus is or means (a scream, music, a fire alarm, thunder, etc.)
Other sensory areas:
1. Wernicke’s Area (Brodmann Areas 39, 40) = located only on the left temporal lobe and
is responsible for understanding and interpreting written and spoken language.
2. Olfactory Cortex = located on the medial aspect of the temporal lobe and is responsible
for the conscious perception of odors or smells.
3. Gustatory Cortex = located in the insula and portions of the frontal lobe; is involved in
the conscious perception of taste.
4. Visceral Sensory Cortex = located in the insula and is involved in the conscious
perception of visceral sensations (upset stomach, full bladder, urge to defecate, etc.)
i. INTERPRETIVE AREAS OF THE CEREBRUM
i. Prefrontal Cortex (Brodmann Areas 9, 10)
1. Involved with intellect, complex learning abilities, short-term memory, and
personality
2. Necessary for abstract ideas, judgment, reasoning, planning, concern for others and a
ii.
j.
conscience. Tumors in this area frequently lead to personality disorders.
General Interpretive Areas = also called the posterior association area
1. Encompasses parts of the temporal, occipital, and parietal lobes of one hemisphere
(usually the left)
2. Seamlessly integrates sensory and motor information with emotions.
3. Plays a role in recognizing patterns and faces, localizing us and our surroundings,
and in binding different sensory inputs into a coherent whole.
White Matter of the Cerebrum
i. The white matter of the cerebrum is located deep to the gray matter and is composed
primarily of myelinated fibers bundled into tracts.
ii. Provides communication between the different cortical areas of the cerebral cortex and
between the cerebral cortex and the lower CNS centers.
k.
Basal Nuclei (also known as the Basal Ganglia) = a set of interconnected nuclei located deep in the
cerebrum. The basal nuclei communicate with the motor cortex to coordinate movement. It acts to
facilitate and inhibit movement. The basal nuclei consist of the caudate nucleus, putamen, globus
pallidus (internus & externus), subthalamic nucleus, and the substantia nigra. Collectively, the caudate
nucleus and putamen are called the striatum. The basal nuclei utilize excitatory neurons (release the
neurotransmitter glutamate) and inhibitory neurons (release the neurotransmitter GABA) in forming
direct and indirect motor pathways between the motor cortex and thalamus.
i. Direct pathway = initiates motor activity. The motor cortex projects excitatory neurons (release
glutamate) into the striatum which then projects inhibitory neurons (release GABA) into the globus
pallidus internus (GPi) and the substantia nigra pars reticulata (SNr). The GPi/SNr then sends
inhibitory neurons to the thalamus which then projects excitatory neurons back to the motor cortex.
The direct pathway causes the disinhibition of the thalamus which allows it to excite the motor
cortex to initiate motor movements.
ii. Indirect pathway = inhibits motor activity. The motor cortex sends excitatory neurons (release
glutamate) into the striatum which then projects inhibitory neurons (release GABA) into the globus
pallidus externus (GPe). The GPe sends inhibitory neurons into the subthalamic nucleus (STN)
which then projects excitatory neurons into the globus pallidus internus (GPi) and the substantia
nigra pars reticulata (SNr). The GPi/SNr then sends inhibitory neurons to the thalamus which then
projects excitatory neurons back to the motor cortex. The indirect pathway causes, or reinforces, the
normal inhibition of the thalamus which fails to excite the motor cortex; thereby, inhibiting motor
movements.
iii. Nigrostriatal Pathway = a dopamine pathway from the substantia nigra pars compacta (SNc)
to the striatum to modulate the activities of the direct and indirect motor pathways. The SNc releases
the neurotransmitter dopamine. The activity of the direct motor pathway is amplified by dopamine
via the excitatory D1-type receptors. In contrast, the indirect motor pathway is inhibited by
dopamine via the inhibitory D2-type receptors which results in increased motor activity. When the
substantia nigra pars compacta is firing, it signals to the basal nuclei that the body is in an active
state, and movement will be more likely. When the substantia nigra pars compacta is silent, the body
is in a passive state, and movement is inhibited. For example, while a student is sitting listening to
a lecture, the substantia nigra pars compacta would be silent and the student less likely to get up
and walk around. Likewise, while the professor is lecturing, and walking around at the front of the
classroom, the professor’s substantia nigra pars compacta would be active, in keeping with their
activity level.
iv. Parkinson’s Disease = caused by the degeneration of dopaminergic neurons of the substantia nigra
pars compacta. It impairs the ability of the basal nuclei to cause the release of inhibition necessary
to make a movement. Individuals with Parkinson's disease have difficulty initiating and
maintaining movements, resulting in muscle rigidity, decreased bodily movement, and postural
imbalance.
v. Huntington’s Disease = an inherited neurodegeneration of the basal nuclei (especially the
striatum) which causes the inhibitory capabilities of the basal nuclei to be diminished thereby
causing exaggerated abnormal involuntary movements such as jerky or writhing movement.
l.
Limbic System = is a group of brain structures located around the medial aspects of the cerebral
hemispheres.
i. The structures of the limbic system are involved in processing and regulating emotions,
memory, and behavior. Some of the structures are the:
1. Amygdala = involved in emotional responses to fearful and threatening stimuli.
Assesses danger and elicits fear responses associated with fight or flight. It’s involved in
forming memories associated with fear-inducing events and positive moments.
2. Cingulate Gyrus = plays a role in expressing emotions through gestures, posture, and
movement. It regulates aggressive behavior and emotional responses to pain.
3. Hippocampus = involved in long-term memory formation.
C. Diencephalon = serves as the structural and functional link between the cerebral hemispheres and the rest of the central
nervous system.
1. The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle.
2. The diencephalon is subdivided into the:
a. Thalamus = forms the superiolateral walls of the third ventricle
i. A collection of nuclei that process and relay information between the cerebral cortex and the
periphery, spinal cord, or brain stem.
ii. All sensory information (except the sense of smell - olfaction) passes through the thalamus before
processing by the cerebral cortex.
iii. Composed of masses of gray matter held together by a midline of commissure fibers known
as the intermediate mass.
iv. It works with the basal nuclei and cerebral cortex to coordinate motor activity.
b. Hypothalamus = forms the inferiolateral walls of the third ventricle
i. Inferior and slightly anterior to the thalamus
ii. Primarily responsible for regulating homeostasis.
iii. Walls of the hypothalamus meet and extend to form the infundibulum, from which the
pituitary gland is suspended.
iv. Serves as the main visceral control center
v. Its homeostatic roles include:
1. Autonomic control center: influences BP, HR, GI motility, respiration rate and depth,
pupil size
2. Center for emotional response and behavior
3. Body temperature regulation and initiates sweating and shivering
4. Regulates food intake
5. Regulates water balance through the thirst mechanism
6. Sleep/Wake cycle regulation
7. Releases hormones that control the secretions of the anterior pituitary gland.
c. Epithalamus = forms the roof of the third ventricle; contains an extensive area of choroid plexus, a
posterior mass of commissure fibers, and the pineal gland, which secretes melatonin and is
responsible for regulating the sleep-wake cycle.
D. Cerebellum = accounts for ~10% of the brain’s mass.
1. The cerebellum functions in the coordination of skeletal muscle movements, modulation of motor commands
from the cerebral cortex, and maintaining balance and equilibrium.
2. The cerebellum is partially hidden by the cerebral hemispheres and is the second largest structure in the brain.
3. The cerebellum is separated from the cerebrum by the transverse fissure.
4.
5.
6.
The cerebellum also possesses fold-like wrinkles called folia, is divided into two hemispheres, and further
subdivided into lobes: the anterior lobe and posterior lobe.
The two cerebellar hemispheres are separated by the vermis while the anterior and posterior lobes are
separated by the primary fissure.
The white matter of the cerebellum is called the arbor vitae and is surrounded by gray matter called the
cerebellar cortex.
E. Brain Stem = only portions of the brain stem are visible underneath the cerebrum.
1. Though small, this is an extremely important part of the brain as the nerve connections of the motor and sensory
systems from the main part of the brain to the rest of the body pass through the brain stem.
2. The brain stem is subdivided into the:
a. Midbrain = contains nuclei that process visual and auditory information and control reflexes
triggered by these stimuli. It also contains centers that help maintain consciousness.
i. Corpora quadrigemina – made of two nuclei on each side
a. Superior colliculi – visual reflexes; head-eye coordination
b. Inferior colliculi – auditory and startle reflexes
ii. Substantia nigra – has a dark appearance due to large amounts of neuromelanin made from LDOPA, precursor to dopamine. It regulates the activity of the basal nuclei of the brain, thus,
regulating movement. It has been linked to Parkinson’s disease because it contains neurons that
produce dopamine.
iii. Red nucleus – contains large amounts of hemoglobin and iron and issues subconscious muscle
commands that affect upper limb position and background muscle tone
iv. Cerebral aqueduct – passes through the center of the midbrain and allows for CSF to flow
from the third ventricle down to the fourth ventricle
b. Pons = located below the midbrain and above the medulla oblongata
i. Possesses projection fibers between the higher and lower brain centers and between the
pons and the cerebellum
ii. Possesses nuclei that function in somatic and visceral motor control
iii. Some pons nuclei are respiratory centers that help to maintain the normal rhythm of
breathing
c. Medulla oblongata = controls autonomic functions and connects the higher levels of the brain to the spinal
cord. The medulla contains major centers for regulating autonomic functions like those of the
hypothalamus:
i. Controls the force and rate of heart contraction
ii. Regulates BP by regulating the smooth muscle of blood vessels
iii. Regulates the rate and depth of breathing
iv. Regulates visceral reflexes such as vomiting, hiccupping, swallowing, coughing, and sneezing
3. Reticular Formation = a collection of neuronal cell bodies whose projection fibers extends through the
central core of the medulla oblongata, pons and midbrain.
a. Maintains cerebral cortical awareness (wakefulness) via the Reticular Activating System
b. Filters out repetitive or weak stimuli entering the brain
c. Helps regulate skeletal and visceral muscle activity
F. The Spinal Cord
1. The adult spinal cord measures approximately 45 cm (18 inches) in length and has a maximum width of roughly
14 mm (0.55 inch). The spinal cord is located within the vertebral foramen (also known as the vertebral canal).
2. Anatomy of the Spinal Cord
A. The cervical enlargement supplies nerves to the shoulder and upper limbs.
B. The lumbar enlargement provides innervation to the structures of the pelvis and lower limb.
C. The conus medullaris is the tapered, conical portion of the spinal cord inferior to the lumbar
enlargement.
D. Because the adult spinal cord ends at the level of the first or second lumbar vertebra, the dorsal and ventral
roots of the spinal segments L2 and S5 extend inferiorly. When seen in gross dissection, the filum terminale
and the long bundles of ventral and dorsal roots resemble a horse’s tail and is named the cauda equina.
E. The filum terminale is a slender strand of fibrous tissue that extends from the tip of the conus medullaris
to the second sacral vertebra. It provides longitudinal support to the spinal cord as a component of the
coccygeal ligament.
F. The posterior median sulcus is a shallow longitudinal groove on the posterior (or dorsal) surface
of the spinal cord.
G. The anterior median fissure is a deep groove along the anterior (ventral) surface.
H. The central canal is a longitudinal passageway that extends the length of the spinal cord that contains
cerebrospinal fluid.
I. The spinal cord contains gray matter and white matter:
1. The gray matter, dominated by the cell bodies of neurons, neuroglia, and unmyelinated
axons, surrounds the narrow central canal and forms a butterfly shape. The gray matter can
be organized into structural and functional areas:
a. Structural Organization: The projections of gray matter toward the outer surface
of the spinal cord are called horns.
i. The posterior gray horn contains somatic and visceral sensory nuclei for
sensory processing.
ii. The lateral gray horn (located only in the thoracic and lumbar segments)
contains visceral motor nuclei (cell bodies of motor neurons of the
autonomic nervous system).
iii. The anterior gray horn contains somatic motor nuclei to send out motor
signals to skeletal muscles.
b. Functional Organization: The cell bodies of the neurons in the gray matter of the
spinal cord are organized into functional groups called nuclei.
i. Sensory nuclei: receive and relay sensory information from the receptors of
the body to the CNS.
ii. Motor nuclei: issue motor commands to the peripheral effectors.
2. The superficial white matter contains large numbers of myelinated axons. Like gray matter,
white matter is also divided into functional and structural areas:
a. Structural Organization: The structural components of white mater are divided into
columns.
i. The posterior white column lies between the posterior gray horns and posterior
median sulcus.
ii. The lateral white column includes the white matter on either side of the spinal
cord between the anterior and posterior columns.
iii. The anterior white column lies between the anterior gray horns and the
anterior median fissure.
b. Functional Organization: the functional component of the white matter is divided into
tracts.
i. Ascending tracts carry sensory information up to the brain.
ii. Descending tracts carry motor commands from the brain to the spinal cord.
J. The entire spinal cord is be divided into 31 segments, each of which gives rise to a pair of
spinal nerves.
1. The dorsal root contains the axons of the sensory neurons entering the spinal cord. Their cell
bodies are in the dorsal root ganglion.
2. A dorsal root ganglion contains the cell bodies of sensory neurons whose axons carry
information to the spinal cord. Each segment of the spinal cord has a pair of dorsal root ganglia,
one on each side.
3. The ventral root contains the axons of motor neurons that exit the spinal cord and extend into the
periphery to control somatic and visceral effectors in the body.
4. A single spinal nerve contains the axons of BOTH sensory and motor neurons. The sensory
fibers enter the CNS through the dorsal root. The motor fibers emerge for the CNS within the
ventral root.
13.3 Circulation and the Central Nervous System
A. Arterial Blood Supply = the internal carotid arteries and vertebral arteries are responsible for oxygenated blood supply
to the brain. The right and left vertebral arteries merge to form the basilar artery which gives rise to branches that supply
the brain stem and cerebellum. The internal carotid arteries and the branches of the basilar artery merge to form the circle of
Willis (a circular blood vessel to ensure constant perfusion of the cerebrum in the event of a blockage of one of the
arteries).
B. Venous Return = After passing through the CNS, deoxygenated blood returns to the circulation via a series of dural
sinuses and veins. Through the jugular veins, blood returns to the heart.
C. Stroke = a disruption of blood supply to the brain caused by a blockage to an artery in the brain. It results in cell death
within the brain which can cause the loss of specific functions in the body.
D. Protective Coverings of the Brain and Spinal Cord
1. The outer surface of the CNS is covered by a series of membranes composed of connective tissue called the
meninges, which protect the brain and spinal cord. The meninges provide the necessary physical stability and shock
absorption for the brain and spinal cord especially during damaging contact with the surrounding bony walls of the
skull and vertebral canal.
2. The meninges consist of three layers:
a. The dura mater is a thick fibrous, outermost meningeal layer, and a protective sheath over the entire
brain and spinal cord. The dura mater is anchored to the inner surface of the cranium and vertebral cavity.
In the CNS, a narrow subdural space separates the dura mater from the arachnoid mater. In the spinal
cord, an epidural space lies between the dura mater and the wall of the ventral canal. The epidural space
contains aerolar connective, blood vessels, and a protective layer of adipose tissue.
b. The arachnoid mater is a thin fibrous, middle meningeal layer that forms a loose sac around the CNS.
It consists of a simple squamous epithelium named the arachnoid membrane and the subarachnoid
space that extends between the arachnoid mater and the pia mater.
c. The pia mater is a thin fibrous, inner meningeal layer that is firmly bound to the underlying
neural tissue.
3. The subarachnoid space contains the arachnoid trabeculae (spider web-like), a network of collagen and elastic
fibers that attaches the arachnoid mater to the pia mater. The subarachnoid space is filled with cerebrospinal
fluid (CSF), which acts as a shock absorber and a diffusion medium for dissolved gases, nutrients, chemical
messengers, and waste products.
4.
5.
6.
The spinal meninges accompany the dorsal and ventral roots as they pass through the intervertebral foramina.
The meningeal membranes are continuous with the connective tissues that surround the spinal nerves and their
peripheral branches.
In adults, CSF can be safely withdrawn in a procedure known as a lumbar puncture or spinal tap. A needle is
inserted into the subarachnoid space in the lumbar region inferior to the tip of the conus medullaris.
Meningitis = an inflammation of the meninges caused by bacteria or viruses. The primary test for meningitis is a
lumbar puncture.
E. The Ventricular System
1. Within the brain are four, fluid-filled cavities (open spaces) called ventricles.
2. The ventricles contain and circulate cerebrospinal fluid. In all the ventricles, CSF is produced by the choroid
plexus, an intricate network of capillaries and ependymal cells. After circulating in the CNS, dirty CSF is
reabsorbed into the venous dural sinus via arachnoid granulations.
3. The ventricles are lined with ependymal cells which whip their cilia to circulate the CSF.
4. There are four ventricles:
a. Two lateral ventricles (right and left), one within each cerebral hemisphere.
b. Third ventricle in the diencephalon.
c. Fourth ventricle which begins in the metencephalon and extends into the superior portion of the medulla
oblongata. Then it narrows and becomes continuous with the central canal of the spinal cord. CSF from
the 4th ventricle also flows into the subarachnoid space through the median and lateral apertures to circulate
around the brain and spinal cord.
5. The lateral ventricles are connected to the third ventricle via an interventricular foramen, also called the
foramen of Monro.
6. The two lateral ventricles are separated from each other by a partition known as the septum pellucidum.
7. The third ventricle is connected to the fourth ventricle via the cerebral aqueduct. This slender canal passes
through the midbrain.
13.4 Peripheral Nervous System
A.
Cranial Nerves
1. There are 12 pairs of cranial nerves.
2. Each pair of nerves is classified as either sensory (containing sensory neurons only), motor (containing motor
neurons only), or mixed (containing both sensory and motor neurons).
3. For each of the 12 cranial nerves, be sure you can: name, number, describe the function, identify if it is
sensory, motor, or mixed, and name a disorder (if applicable).
4. The following table summarizes this information:
Cranial Nerve
Number
Disease or
Disorder
Name
Function
Foramen
CN I
(sensory)
Olfactory
Smell
Olfactory foramina of
the cribriform plate
Anosmia
CN II
(sensory)
Optic
Vision
Optic canal
Anopsias
Superior orbital
fissure
External
strabismus
CN III
(motor)
Oculomotor
Movement of eyelid and eyeball (via
superior rectus, inferior rectus, medial
rectus, and inferior oblique), shape of lens,
contracts pupil size
CN IV
(motor)
Trochlear
Trigeminal
CN V
(both)
(the largest cranial
nerve with three
Movement of eye by the superior oblique
Superior orbital
fissure
Double vision
General sensations of touch, pain, &
temperature of the face, and sensory fibers
from teeth and tongue, also innervates the
muscles of chewing
Superior orbital
fissure
Tic
douloureux
Movement of the eyeball by the lateral
Superior orbital
Internal
branches)
CN VI
(motor)
Abducens
rectus
fissure
strabismus
Muscles controlling facial expressions,
CN VII
(both)
Facial
secretion of saliva by the submandibular and
sublingual glands and tears by the lacrimal
Internal acoustic
meatus
Bell's Palsy
and ageusia
gland, and sensory function for taste from the
anterior 2/3 of the tongue
CN VIII
(sensory)
Vestibulocochlear
CN IX
(both)
Glossopharyngeal
CN X
(both)
Vagus
CN XI
(motor)
Accessory
CN XII
(motor)
Hypoglossal
Hearing and equilibrium
Secretion of saliva by the parotid glands,
elevation of pharynx during swallowing,
and taste
Taste (gag reflex), smooth muscle
contraction and relaxation of visceral organs
and for the secretion of
digestive fluids
Rotation of head and shrugging the shoulders
Motor function of tongue for speech,
swallowing, and chewing
Internal acoustic
meatus
Jugular Foramen
Nerve
deafness or
dizziness
Impaired
swallowing
and ageusia
Jugular Foramen
Jugular Foramen
Hypoglossal canal
Loss of voice and
death
Inability to turn
the head and
shrug shoulders
Difficulty
with speech or
loss of speech
B. The Spinal Nerves
1. Anatomy of a Spinal Nerve:
a. Spinal nerves are designed similarly to the design of a muscle.
i.
A spinal nerve is composed of bundles called fascicles.
ii. Each fascicle is composed of numerous nerve cells known as neurons.
iii. The entire nerve is surrounded in a layer of connective tissue called the
epineurium (outer layer).
iv.
Each fascicle is wrapped in a perineurium (middle layer).
v.
Each neuron is surrounded by a connective tissue called the endoneurium (inner layer).
b. Each spinal nerve branches to form rami. Some of these rami carry visceral motor fibers of the autonomic
nervous system (ANS). Spinal nerves in the thoracic and upper lumbar segments of the spinal cord carry
the motor output of the sympathetic division that is responsible for “fight or flight” responses.
i.
Dorsal ramus – innervates the muscles, joints, and skin of the back.
ii. Ventral ramus – innervates the structures in the lateral and anterior trunk as well as the limbs.
Communicating rami – are present in the thoracic and superior lumbar segments of the spinal
cord. These rami contain the axons of the sympathetic neurons.
c. The specific bilateral region of the skin surface monitored by a single pair of spinal nerves is known
as a dermatome. Each pair of spinal nerves services its own dermatome, but the boundaries of
adjacent dermatomes overlap to some
degree. Dermatomes are clinically important because damage or infection of a spinal nerve or dorsal root
ganglion produces a characteristic loss of sensation in the corresponding regions of the skin.
There are 31 pair of spinal nerves. All are mixed nerves.
a. A complex interwoven network of the ventral rami of spinal nerves is called a plexus.
b. The ventral rami form four major plexuses: a) the cervical plexus, b) the brachial plexus, c) the
lumbar plexus, and 4) the sacral plexus.
Cervical plexus – consists of the ventral rami of spinal nerves C1 – C5.
a. Mostly cutaneous nerves that supply the skin.
b. A few innervate muscles of the anterior neck.
c. The phrenic nerve is the single most important cervical nerve that innervates the diaphragm for
breathing.
Brachial plexus – innervates the pectoral girdle and upper limbs, with contributions from the ventral rami of the
spinal nerves C4 – T1.
a. Radial nerve is the largest branch of the brachial plexus, and it produces elbow extension,
forearm supination, wrist and finger extension, and thumb abduction.
b. Ulnar nerve produces wrist and finger flexion and adduction as well as abduction of the medial fingers.
c. Median nerve pronates the forearm, flexes the wrist and fingers, and opposes the thumb.
d. Suprascapular nerve innervates the supraspinatus and infraspinatus muscles for movement of
the shoulder.
e. The distribution of the cutaneous nerves of the wrist and hand is very important in clinical medicine.
Nerve damage or injury in this region can be precisely localized by carefully testing the sensory
functions of the hand.
Thoracic Region (12) – does not form a plexus; none of the intercostal nerves intertwine.
a. 12 pairs of intercostal nerves that give rise to many cutaneous branches to supply the chest and torso
area and innervate the external intercostal muscles for breathing.
Lumbar Plexus (5) – innervates the pelvic girdle and portions of the lower limb.
a. The lumbar plexus arises from spinal nerves T12 - L4.
b. The femoral nerve is the largest terminal nerve of this plexus and innervates the anterior muscles of the
thigh (thigh flexors and knee extensors). Branches to form the saphenous nerve on the medial thigh and
knee.
c. The obturator nerve innervates the adductor muscles of the leg.
Sacral Plexus (5)
a. The sacral plexus arises from spinal nerves L4 - S4.
b. The largest branch of the sacral plexus is the sciatic nerve which supplies the entire lower limb (leg) except
the anteromedial thigh.
c. The Sciatic nerve is the thickest and longest nerve in the body and branches to form the:
i.
Tibial nerve – flexors of the knee and extensors of the ankle, flexors of the toes, and plantar
surface of the foot.
ii. Fibular nerve. – biceps femoris muscle and tibialis anterior muscle; extensors of the toes, surface
of the leg and dorsal surface of the foot.
iii. Pudendal nerve innervates muscles of the perineum, urogenital diaphragm, and external anal
and urethral sphincter muscles; skin of the genitalia.
iii.
2.
3.
4.
5.
6.
7.