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?

Spinal disc (cartilaginous disc) becomes less elastic – can rupture!

A portion of the spinal disc pushes outside it’s normal boundary = herniation.

Spinal nerves/spinal cord can be pinched/compressed.

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)

Spinal cord/nerve compression alters the function – either abnormal signals are passed or none at all.

Symptoms of Disc Herniation

Electric Shock PainPressure 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 & NumbnessPatients 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 WeaknessBecause 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 ProblemsThese 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) ventral horn (anterior horn) Central canal – opening in the center of gray

matter that extends the length of the spinal cord

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

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 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. 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 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: Reflex = an involuntary response to a stimulus

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:

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 Abdominal reflex - Babinski reflex – Patellar (knee-jerk reflex)

Peripheral Nervous SystemNerves and ganglia outside the CNS

Nerve vs Neruon 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 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 NervesCranial Nerves & Spinal Nerves

(based on origin of the fiber) Cranial Nerves

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

2. Sensory information general senses: touch, pressure, pain, temperature, vibration

3. Motor information that results in contraction of skeletal muscles

4. Motor information that results in the secretion of glands and the contraction of cardiac/smooth muscle

Cranial Nerves I 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

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

Plexuses

Cervical Plexus – fibers supply the muscles and skin of the neck. 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:

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