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Nervous System
Spinal Cord & Peripheral Nerves
Spinal Cord
 Continuation of the brain stem
 Tube-like structure w/i spinal cavity
 Diameter similar to width of the thumb
 Extends from foramen magnum of the occipital
bone to the level of the first lumbar vertebrae –
 Lumbar Puncture – this arrangement is the basis
for sampling/withdrawing CSF fluid from the
subarachnoid space between L3/L4 for
evaluation: blood, pathogens, abnormal signs…
 Well protected by vertebrae, meninges, CSF,
blood-brain barreier
Herniated Disc
 What Happens with a herniated disc?
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Spinal disc (cartilaginous disc) becomes
less elastic – can rupture!
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A portion of the spinal disc pushes
outside it’s normal boundary = herniation.
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Spinal nerves/spinal cord can be
pinched/compressed.
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May occur suddenly in an vent such as
fall/accident – or gradually w/ repetitive
straining of the spine. Typically patients
already have spinal stenosis (narrowing
of space around spinal cord/nerves)
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Spinal cord/nerve compression alters the
function – either abnormal signals are
passed or none at all.
Symptoms of Disc Herniation
 Electric Shock Pain
Pressure on the nerve can cause abnormal sensations, commonly
experienced as electric shock pains.
 EX. When the compression occurs in the cervical (neck) region,
the shocks go down your arms, when the compression is in the
lumbar (low back) region, the shocks go down your legs.
 Tingling & Numbness
Patients often have abnormal sensations such as tingling, numbness,
or pins and needles. These symptoms may be experienced in the
same region as painful electric shock sensations.
 Muscle Weakness
Because of the nerve irritation, signals from the brain may be
interrupted causing muscle weakness. Nerve irritation can also be
tested by examining reflexes.
 Bowel or Bladder Problems
These symptoms are important because it may be a sign of cauda
equina syndrome, a possible condition resulting from a herniated disc.
This is a medical emergency,
Gray Matter and White Matter
 Gray Matter
 Center of spinal cord – butterfly shaped
 Consists of cell bodies and interneurons
 2 projections – dorsal horn (posterior horn)
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ventral horn (anterior horn)
 Central canal – opening in the center of gray
matter that extends the length of the spinal
cord
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Open to the ventricular system in the brain and
subarachnoid space – CSF circulates throughout
the subarachnoid space
White Matter
 Outer layer of spinal cord
 Composed primarily of axons
 Neuronal axon are clumped together = nerve tracts
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Sensory Tracts – ascending tracts
 Carry information from the periphery up the spinal cord to the
brain
 Spinothalmic – temperature, pressure pain, light touch
 Dorsal column – proprioception, deep pressure, vibration
 Spinocerebellar - proprioception
Motor Tracts – descending tracts
 Carry information from the brain, down the spinal cord
toward the periphery
 Pyramidal - skeletal muscle tone, voluntary muscle
movement
 Extrapyramidal – skeletal muscle activity, balance/posture
Decussation:
 Most nerve tracts cross over from one side to
another
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EX. Corticospinal tract originates in the Lt –
frontal lobe – descends to the medulla
oblongata – fibers then decussate and
descend down the Rt – side of the spinal cord
– innervate the Lt side of the body.
 Most sensory tracts decussate in the spinal
cord
Injury?
 Neurons of the brain and spinal cord do not
regenerate
 If spinal cord can be severed – w/ neck/back
injuries.
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Quadriplegia – when cord is severed at neck
region (cervical)… trunk and all four
extremities are paralyzed
Paraplegia – when injury occurs to lower
(lumbar) spinal cord and paralysis is from
waist down – still having use of upper
extremities
Spinal Nerves
 Spinal nerves attach to the spinal cord
 Each nerve is attached to the cord by 2 roots
 Dorsal root – contains sensory nerve fibers
that come together to form a dorsal root
ganglion
 Ventral root – contains motor fibers –
distributed to the muscles/glands
 Dorsal/Ventral roots are packaged together
to form a spinal nerve (mixed nerve) –
contains both sensory/motor nerves
Spinal Nerves
Spinal Cord Functions:
 Sensory Pathway – spinal cord provides a
pathway for sensory information traveling from
the periphery to the brain

Information ascends the spinal cord through the dorsal root
ganglion – dorsal root – into spinal cord.
 Motor Pathway – spinal cord provides a pathway for
information descending from the brain to the periphery
out the ventral root of spinal cord to motor neurons
 Reflex Center – spinal cord acts a reflex center
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Ex. When you stick your finger on a tack – you
quickly/automatically withdraw your finger - the spinal cord, not
the brain performs this function
Reflexes:
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Reflex = an involuntary response to a stimulus
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Many activities we engage in everyday occur very rapidly
and w/o conscious control – happens reflexively.
Ex. When you touch a hot surface – your hand is removed
very quickly – hand is safe long before you consciously
say “This is hot!” (withdrawal reflex)
Reflex Arc – nerve pathway involved in a reflex

4 components:
1. Receptor – receives stimulus
2. Afferent (sensory neuron) – nerve impulse is carried by
the sensory neuron to the spinal cord
3. Efferent (motor neuron) – nerve impulse in carried by a
motor neuron the muscle
4. Effector organ – responds to the impulse
Patellar reflex (knee-jerk reflex)
1. Receptor – by tapping the tendon, the mallet
stimulates sensory receptors in the thigh muscles.
2. Afferent – nerve impulse is carried is carried by the
sensory neuron to the spinal cord
3. Efferent – nerve impulse is carried by a motor nerve
to the muscles of the thigh
4. Effector organ – the muscles of the thigh,
specifically the quadriceps femoris, is an effector
organ. In response to the nerv impulse, the
muscles contract and move the leg in an upward
movement.
Patellar
Reflex:
Many, Many Reflexes!
 Organ Reflexes – help regulate organ function
 Physiologic functions:
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Pupillary reflex - regulates the amount of light that enters
the eye
Baroreceptor reflex – when BP changes, this reflex
causes the heart and blood vessels to respond in a way
that restores BP to normal
 Diagnostic – to asses nerve function, abnormal findings
may indicate CNS lesions, tumors, or neurological
diseases
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Abdominal reflex Babinski reflex –
Patellar (knee-jerk reflex)
Peripheral Nervous System
Nerves and ganglia outside the CNS
 Nerve vs Neruon
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Neuron is a single nerve cell
Nerve contains many neuron bundled together
w/ blood vessels – wrapped in connective tissue
Nerves are outside the CNS – within the CNS
they are called tracts
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Sensory Nerve – composed of only sensory neurons
Motor Nerve – composed only of motor neurons
Mixed nerves – both sensory and motor neuron,
most nerves are mixed, all spinal nerves are mixed!
Classification of Peripheral Nerves
Cranial Nerves & Spinal Nerves

(based on origin of the fiber)
Cranial Nerves
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12 Pairs – numbered in order the nerves exit the brain from
front to back
Name – associated w/ the anatomical area served by the
nerve.
Cranial nerves primarily serve the head, face, neck ****except
for the vagus nerve which extends throughout the
thoracic/abdominal cavity.

Functions of Cranial Nerves:
1.
Sensory information special senses: taste, smell, vision, hearing
Sensory information general senses: touch, pressure, pain,
temperature, vibration
Motor information that results in contraction of skeletal muscles
Motor information that results in the secretion of glands and the
contraction of cardiac/smooth muscle
2.
3.
4.
Cranial Nerves
 I
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Olfactory
(S)
sense of smell
II Optic
(S)
sense of sight
III Oculomotor (MX) eyeball/eyelid movement,pupil size
IV Trochlear (MX) movement of eyeball
V Trigeminal (MX) chewing,sensations of face,cornea
VI Abducens (MX) movement of eyeball
VII Facial (MX)
facial expressions, saliva/tears,taste
VIII Vestibulocochlear (S) hearing/balance
IX Glossopharyngeal (MXD) swallowing/saliva/gag/BP
X Vaugus (MXD) Visceral muscle/sensations, BP, digestive system
XI Accessory (MXD) swallowing, head/shoulder movement/speaking
XII Hypogloassal (MXD) speech, swallowing
Cranial Nerve Assessment
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I – Olfactory – smell and identify odor
II – Optic – opthalmoscope, eye chart, peripheral vision
III – Oculomotor – eye movement, pupil constriction, size, shape, equality
IV – Trochlear – ability for eyes to follow moving object
V – Trigeminal – sensations tested (sharp/dull), corneal reflex, motor fxn of jaw
VI – Abducens – ability of eye to follow moving object
VII – Facial – taste, facial motor fxn(smile,close eyes,
whistle)
VIII – Vestibulocochlear – hearing (w/ tuning fork)
IX – Glossopharangeal – gag/swallow reflex, speak, cough
X – Vagus – similar to above – both innervate throat
XI – Accessory – rotate head side/side, shrug shoulders
XII – Hypoglossal – stick tongue out – note deviations
Spinal Nerves
 31 pairs emerge from spinal cord
 Each pair numbered according to level it arises from spinal cord
8 pairs – cervical nerves
 12 pairs of thoracic nerves
 5 pairs of lumbar nerves
 5 pairs of sacral nerves
 1 pair of coccygeal nerves
 Cauda Equina – include the lumbar and sacral nerves – extend from
spinal cord through the spinal cavity before exiting through tiny holes in
the vertebrae (foramina)
 Spinal nerve plexus – when nerve fibers converge after exiting the
vertebral column to form a network (plexus)
 Cervical Plexus (C1-C4) – skin, muscles of the neck shoulder:diaphragm
 Brachial Plexus (C5-C8,T1) – skin, muscles of the upper extremeties
 Lumbosacral plexus (T12, L1-L5, S1-S4) skin, muscles of lower extremeities
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Plexuses
 Cervical Plexus – fibers supply the muscles and skin of the neck.
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Motor fibers pass into the phrenic nerve (stimulates the contraction of
the diaphragm – major breathing muscle)
EX. Injury below C5 level – the person is paralyzed but can still breath
on their own. If the injury is higher (C2) the motor impulses to the
diaphragm are interrupted –can’t breathe w/o assistance
 Brachial Plexus – nerves that emerge from this plexus supply
muscles and skin of the shoulder, arm, forearm, wrist, hand.

EX Axillary nerve in the shoulder suseptible to damage when a person is
using crutches – weight of the body can damage the axillary nerve =
crutch palsey
 Lumbosacral Plexus - gives rise to nerves that supply the
muscles and skin of the lower abdominal wall, external genitalia,
buttocks, and lower extremities

Ex. Sciatic Nerve, the longest nerve in the body arises from this plexus
– it supplies the entire musculature of the leg and foot – it can become
inflamed and cause pain in the buttock and posterior thigh region –
common cause is a ruptured/herniated vertebral disc.
Dermatome…
 Dermatome is where each spinal nerve
innervates a particular area of the skin
 Each dermatome is named for the particular
nerve that serves it ( C4 dermatome is
innervated by the C4 nerve)
 Clinically useful – EX. If the skin of the shoulder region
is stimulated w/ a pin and the person doesn’t feel it – there is
reason to believe that the C4 nerve is impaired
Functional Classification of the
Peripheral Nervous System:
 This type of classification explains where the
nerves go and what they do.
 Somatic Afferent nerves:
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Bring sensory information from the different
parts of the body (skin, muscles) to the CNS
 Somatic Efferent nerves:
 Bring motor information from the CNS to the
skeletal muscles throughout the body
 Autonomic nervous system (ANS):
 Composed of nerves that supply the organs
(viscera) and glands