CHAPTER THIRTEEN
Anatomy of the Nervous System
13.1 The Embryological Perspective
A. The Neural Tube
1. The brain and all other nervous tissue forms from ectoderm.
2. During embryonic development, a thin flat layer of ectoderm begins to thicken to form the
neural plate which then invaginates to form the neural groove (also called the neural fold).
3. Eventually the two sides of the neural groove touch forming and internal passageway called the
neurocoel. The structure is now called the neural tube. The neural tube will go on to form the
CNS while the neural crest (located near tube) will develop into the PNS.
B. Primary Vesicles
1. The anterior end of the neural tube 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.
Primary Vesicle
Secondary Vesicle
Adult Structure
Prosencephalon
(forebrain)
Mesencephalon
(midbrain)
Rhombencephalon
(hindbrain)
Telencephalon
Cerebrum (cerebral cortex, white
matter, and basal nuclei)
Diencephalon
Thalamus, pineal gland, and
hypothalamus
Mesencephalon
Brain stem (midbrain)
Metencephalon
Brain stem (pons) and cerebellum
Myelencephalon
Brain stem (medulla oblongata)
13.2 The Central Nervous System
A. 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 and covered with a superficial layer of gray
matter called the cerebral cortex.
c.
d.
e.
f.
g.
a. Fissures = deep grooves
b. Sulci (sulcus) = shallow depressions that separate the folds or wrinkles
c. Gyri (gyrus) = folds or wrinkles that increase surface area
The cerebrum is divided into a pair of large, cerebral hemispheres by the longitudinal
fissure.
Each cerebral hemisphere is further subdivided into lobes. There are five lobes: the
frontal, parietal, occipital, temporal, and insula.
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.
The lateral sulcus separates the frontal and parietal lobes from the temporal lobe.
white matter of the corpus callosum provides the major pathway for communication
between the two hemispheres of the cerebral cortex
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 side
of the body (contralateral)
d. Even though 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 language-based skills; important in reading, writing, speaking, math, and logic.
e. The cerebral cortex was analyzed microscopically and divided into 52 separate regions
called Brodmann’s regions.
f. The cerebral cortex contains three functional areas (COMPOSED OF
INTERNEURONS!!!)
i. Motor areas = which control voluntary motor functions
ii. Sensory areas – which 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 (Area 4) = located within the precentral gyrus of the
frontal lobe
1. Possesses large neurons called pyramidal cells
2. Allows conscious control of skilled voluntary movements of skeletal
muscles.
3. The motor areas of the precentral gyrus have been spatially mapped =
somatotopy.
ii. Premotor Cortex = located anterior to the precentral gyrus of the frontal lobe
1. Regions that control learned motor skills that are repeated or patterned
(such as playing an instrument or typing). Some people say this is called
muscle memory.
2. Also coordinates the movements of muscles simultaneously and/or
sequentially by sending activating impulses to the primary motor cortex.
iii. Broca’s Area (area 44/45) = also known as the speech center, is located anterior
to the lower part of the premotor cortex
1. Involved in directing motor speech including thinking about or planning to
speak
2. Present in only one hemisphere (typically the left)
iv. Frontal Eye Field = located anterior to the premotor cortex and superior to the
Broca’s
1. Controls the voluntary movements of the eye
2. 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 = located in the postcentral gyrus
a. Neurons receive touch information (i.e. temperature, pressure,
vibration, pain, etc.) from the somatic sensory receptors of the skin
and from proprioceptors in skeletal muscle.
b. Endows special discrimination, or rather allow you to interpret
what body region is being stimulated.
2. Somatosensory association area = lies posterior to the primary
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.
ii. Visual Cortex = located within the occipital lobe of the cerebrum
1. Primary visual cortex receives light information from the retina of the
eye such as color, form, texture, and movement.
2. Visual association area 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).
iii. Auditory Cortex = located within the temporal lobes
1. Primary auditory cortex (areas 22) receives sound information from the
ear such as pitch, rhythm, and volume.
2. Auditory association area 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.)
iv. Other sensory areas:
1. Olfactory cortex = located on the medial aspect of the temporal lobe and
is responsible for the conscious perception of odors or smells.
2. Gustatory cortex = located in the insula and portions of the frontal lobe;
is involved in the conscious perception of taste.
3. 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 = also called the anterior association area
1. Involved with intellect, complex learning abilities, recall, and personality
2. Necessary for abstract ideas, judgment, reasoning, planning, concern for
others and a conscience. Tumors in this area frequently lead to personality
disorders.
ii. 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.
4. Also contains the Wernicke’s area involved in understanding written and
spoken language and sounding out unfamiliar words.
j. 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 = There are islands of gray matter located deep within the white matter of
the cerebrum.
i. The function of these islands of gray matter is to subconsciously control large
automatic skeletal muscle contractions (arm swinging when walking), play in a
role in maintaining attention, and to produce dopamine.
ii. Disorder of the basal nuclei result in too much or too little movements as
exemplified by Huntington’s (too much) and Parkinson’s (too little).
l. Limbic System = not an isolated part of the brain but actually spans large areas and
several parts particularly around the medial aspects of the cerebral hemispheres.
i. The limbic system is our emotional brain.
1. The amygdala recognizes angry and fearful facial expressions, assesses
danger, and elicits fear responses associated with “fight or flight”.
2. The cingulated gyrus plays a role in expressing our emotions through
gestures and in resolving mental conflicts when frustrated.
ii. The limbic system also holds the hippocampus region which (along with the
amygdala) plays a role in formation of memories.
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 completely covered by the cerebrum and is not visible by external
examination.
2. The diencephalon is further subdivided into the:
a. Thalamus = forms the superiolateral walls of the third ventricle
i. Composed of masses of gray matter held together by a midline of commissure
fibers known as the intermediate mass.
ii. Contains important relay and processing centers for sensory information.
Sometimes called the gatekeeper or relay station.
iii. Contains many nuclei, each projecting fibers to and receiving fibers from a
specific region of the cerebral cortex
b. Hypothalamus = forms the inferiolateral walls of the third ventricle
i. Walls of the hypothalamus meet and extend to form the infundibulum, from
which the pituitary gland is suspended.
ii. Serves as the main visceral control center
iii. 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 cycles.
D. Cerebellum = accounts for 11% of the brain’s mass.
1. The cerebellum functions in the coordination and modulation of motor command 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. 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.
5. The two cerebellar hemispheres are separated by the vermis while the anterior and posterior
lobes are separated by the primary fissure.
6. 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 further subdivided into:
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
1. Superior colliculi – visual reflexes; head-eye coordination
2. Inferior colliculi – auditory and startle reflexes
ii. Substantia nigra – contains copious amounts of melanin, the precursor to
dopamine, and regulates the activity of the basal nuclei of the brain, has also been
linked to Parkinson’s disease
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 – 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 also
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 = relays sensory information to other portions of the brain stem and
to the thalamus. The medulla also 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 = extends through the central core of the medulla oblongata, pons and
midbrain.
a. Maintains cerebral cortical awareness via the Reticular Activating System
b. Filters out repetitive or weak stimuli
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 ventral and dorsal roots resemble a
horse’s tail. Hence, this complex is called 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.
ii. The lateral gray horn (located only in the thoracic and lumbar
segments) contains visceral and motor nuclei.
iii. The anterior gray horn contains somatic motor nuclei.
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 and
unmyelinated 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 toward the brain.
ii. Descending tracts carry motor information from the brain to the
spinal cord.
J. The entire spinal cord can 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 neurons whose 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 extend into the
periphery to control somatic and visceral effectors.
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. Meninges of the spinal cord
1. The delicate neural tissues must be protected from shocks, including damaging contact with the
surrounding bony walls of the vertebral canal. The spinal meninges are a series of specialized
membranes surrounding the spinal cord and providing the necessary physical stability and shock
absorption.
2. The spinal meninges consist of three layers:
a. The tough fibrous dura mater is the outermost covering of the spinal cord. It contains
dense collagen fibers that are oriented along the longitudinal axis of the cord. Between
the dura mater and the walls of the ventral canal lies the epidural space, a region that
contains areolar connective tissue, blood vessels, and a protective padding of adipose. A
narrow subdural space separates the dura mater from the arachnoid mater.
b. The arachnoid mater is the middle meningeal layer. It includes as simple squamous
epithelium, called the arachnoid membrane, and the subarachnoid space that extends
between the arachnoid mater and the pia mater.
c. The pia mater consists of a meshwork of elastic and collagen fibers that is firmly bound
to the underlying neural tissue.
3. The subarachnoid space contains the arachnoid trabeculae, a network of collagen and elastic
fibers that attaches the arachnoid mater to the pia mater. It is filled with cerebral spinal fluid
(CSF), which acts as a shock absorber and a diffusion medium for dissolved gases, nutrients,
chemical messengers, and waste products.
4. 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.
5. 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.
B. The brain possesses four, fluid-filled chambers called ventricles.
1. The ventricles contain CSF, or cerebrospinal fluid. CSF is formed by choroid plexus, an
intricate network of capillaries, and is absorbed by the arachnoid trabeculae.
2. The ventricles are lined with ependymal cells which whip their cilia to circulate the CSF.
3. There are four ventricles:
a. There are two lateral ventricles (right and left) each within one of the cerebral
hemispheres.
b. The third ventricle is located in the diencephalon.
c. The fourth ventricle begins in the metencephalon and extends into the superior portion
of the medulla oblongata. It then narrows and is continuous with the central canal of the
spinal cord.
4. Each lateral ventricle communicates with the third ventricle via an interventricular foramen,
also called the foramen of Monro.
5. The two lateral ventricles are separated from each other by a partition known as the septum
pellucidum.
6. The third ventricle communicates with the fourth ventricle via the cerebral aqueduct. This
slender canal passes through the midbrain.
13.4Peripheral 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
Name
Function
Foramen
Disease or
Disorder
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
Superior orbital
fissure
Double vision
CN III
(motor)
Oculomotor
CN IV
(motor)
Trochlear
Trigeminal
CN V
(both)
(the largest cranial
nerve with three
branches)
CN VI
(motor)
Abducens
CN VII
(both)
Facial
Movement of eyelid and eyeball (via
superior rectus, inferior rectus,
medial rectus, and inferior oblique),
shape of lens, contracts pupil size
Movement of eye by the superior
oblique
General sensations of touch, pain, &
temperature of the face, and sensory
fibers from teeth and tongue, also
innervates the muscles of chewing
Movement of the eyeball by the
lateral rectus
Muscles controlling facial
expressions, secretion of saliva by the
submandibular and sublingual glands
and tears by the lacrimal gland, and
sensory function for taste from the
anterior 2/3 of the tongue
Superior orbital
fissure
Tic
douloureux
Superior orbital
fissure
Internal
strabismus
Internal acoustic
meatus
Bell's Palsy
and aguesia
CN VIII
(sensory)
Vestibulocochlear
CN IX
(both)
Glossopharyngeal
CN X
(both)
Vagus
Hearing and equilibrium
Secretion of saliva by the parotid
glands, elevation of pharynx during
swallowing, and taste
Taste, smooth muscle contraction and
relaxation of visceral organs and for
the secretion of digestive fluids
CN XI
(motor)
Accessory
Rotation of head and shrugging the
shoulders
CN XII
(motor)
Hypoglossal
Motor function of tongue for speech,
swallowing, and chewing
Internal acoustic
meatus
Jugular Foramen
Jugular Foramen
Nerve
deafness or
dizziness
Impaired
swallowing
and aguesia
Loss of voice
and death
Inability to
turn the head
Jugular Foramen
and shrug
shoulders
Difficulty
Hypoglossal canal 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.
iii. 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.
2. 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.
3. 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.
4. 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.
5. 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 the chest and
torso area and innervate the external intercostal muscles for breathing.
6. 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.
7. 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.