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Cervical Plexuses C1-C4, some of C5. Deep to sternocleidomastoid muscle. Serves muscles and skin of neck and

shoulder and some head. Phrenic nerve serves diaphragm,

chief muscle for breathing. Irritation can give hiccups Damage to the spinal cord above origin of

phrenic nerves leads to respiratory arrest (“C3, C4, C5 keep the diaphragm alive”).

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Brachial Plexuses C5-T1 mainly Serves shoulder, some thorax

muscles, and upper limb Roots (really rami) -> trunks ->

divisions -> cords-> major nerves (Real tired, drink coffee black)

Injuries are common - often from stretching

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Brachial Plexus: Major Nerves Axillary nerve

Deltoid and teres minor Sensory to superolateral arm Damage: difficulty with abduction and anesthesia

along the superolateral skin of the arm. Musculocutaneous nerve

Elbow flexors Sensory to lateral forearm

Radial nerve Extensors Posterior skin Damage: wrist-drop

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Brachial Plexus: Major Nerves Median nerve

Wrist and hand flexors Sensory to hand Damage: wrist slashing, carpal tunnel

Characteristic “ape hand” deformity Ulnar nerve

Wrist and hand flexors Sensory to hand Damage: “funny bone”, clawhand

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Brachial Plexus Injuries Fairly common esp. for persons 18-22 Axillary nerve injury Radial nerve injury Posterior cord injury: crutch palsy and

drunkard’s paralysis Median nerve injury Ulnar nerve injury Superior trunk injury affects C5 and C6

anterior rami Inferior trunk injury from excessive abduction.

Involves the C8 and T1 anterior rami.

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Lumbar Plexuses L1-L4, within the psoas major muscle Motor supply to anterior and medial

thigh muscles and cutaneous supply to anterior thigh and part of leg and muscles and skin of anteriolateral abdominal wall, genitals

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Lumbar Plexuses Chief nerves

Femoral Thigh flexors and leg extensors (anterior

compartment of thigh, quadriceps) Stab or gunshot would -> inability to

extend leg and loss of sensation over anteriomedial thigh

Obturator Adductor muscles (medial compartment) Childbirth -> paralysis of adductor muscles

and loss of sensation of medial thigh

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Sacral Plexuses L4-S4 Posterior muscles, buttocks, pelvis, and

skin of lower limb Principal nerve: sciatic nerve

Thickest and longest Tibial and common peroneal usually split

near the knee Damage: footdrop (fibular nerve damage),

sciatica (usually from herniated disc, also from dislocated hip, osteoarthritis of spine, PG, gluteal injection)

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Reflexes Rapid, automatic, involuntary reactions of

muscles or glands to a stimulus. All reflexes have similar properties.

a stimulus is required to initiate a response to sensory input

a rapid response requires that few neurons be involved and synaptic delay be minimal

an automatic response occurs the same way every time Awareness of the stimulus occurs after the reflex action

has been completed, in time to correct or avoid a potentially dangerous situation.

Spinal vs. cranial reflexes Somatic vs. autonomic reflexes

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Components of a Reflex Arc The neural “wiring” of a single reflex. Always begins at a receptor in the PNS. Communicates with the CNS. Ends at a peripheral effector (muscle or

gland) cell.

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Ipsilateral and Contralateral Reflex Arcs Ipsilateral is when both the receptor and effector

organs of the reflex are on the same side of the spinal cord.

for example, an ipsilateral effect occurs when the muscles in your left arm contract to pull your left hand away from a hot object

Contralateral is when the sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs in the opposite limb.

for example, contralateral effect occurs when you step on a sharp object with your left foot and then contract the muscles in your right leg to maintain balance as you withdraw your left leg from the damaging object

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Monosynaptic Reflexes The simplest of all reflexes. Interneurons are not involved in this reflex. The patellar (knee-jerk) reflex is a

monosynaptic reflex that physicians use to assess the functioning of the spinal cord.

By tapping the patellar ligament with a reflex hammer, the muscle spindles in the quadriceps muscles are stretched.

Produces a noticeable kick of the leg.

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Polysynaptic Reflexes Have more complex neural pathways

that exhibit a number of synapses involving interneurons within the reflex arc.

Because this reflex arc has more components, there is a more prolonged delay between stimulus and response.

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Examples of Spinal Reflexes

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Withdrawal (Flexor) Reflex Polysynaptic reflex initiated by a

painful stimulus

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Stretch Reflexes Monosynaptic reflex that monitors

and regulates skeletal muscle length. When a stimulus results in the stretching of a

muscle, that muscle reflexively contracts. The patellar (knee-jerk) reflex is an example of

a stretch reflex. The stimulus (the tap on the patellar tendon)

initiates contraction of the quadriceps femoris muscle and extension of the knee joint.

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Golgi Tendon Reflex Prevents skeletal muscles from

tensing excessively. Golgi tendon organs are nerve endings located

within tendons near a muscle–tendon junction. activation of the Golgi tendon organ signal

interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons

The associated muscle is allowed to relax, thus protecting the muscle and tendon from excessive tension damage.

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Reflex Testing in a Clinical Setting Reflexes can be used to test specific

muscle groups and specific spinal nerves or segments of the spinal cord.

Consistently abnormal reflex response may indicate damage to the nervous system or muscles.

A reflex response may be normal, hypoactive, or hyperactive.

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Spinal Cord Injury Many causes: tumors, herniated discs,

clots, trauma Possible consequences

Paralysis: total loss of voluntary motor functions from nerve or muscle damage

Monoplegia: one limb Diplegia: upper or lower limbs Paraplegia: lower limbs Hemiplegia: one side Quadriplegia: upper and lower limbs

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Spinal Cord Injury Following transection, spinal

shock, that lasts a few days to a few weeks Areflexia temporarily below the lesion

Usually, lasts a few hours If 48 hours or longer, then permanent

paralysis usually Anti-inflammatory drug,

methylprednisolone, may help if given within 8 hours

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Spinal Cord Injury Complete transection

All tracts cut -> lose all sensation and voluntary movement below cut (paraplegia or quadriplegia, spastic paralysis)

Hemisection Partial transection -> partial loss below

transection Posterior column-medial lemniscus pathway: loss

on same side Lateral corticospinal tracts: loss on same side Anterior corticospinal tracts: loss on opposite side Spinothalamic tracts: loss on opposite side

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Poliomyelitis (Polio) Most serious form is Bulbar ->

paralysis from destruction of cell bodies of motor neurons in anterior horn and nuclei of cranial nerves in medulla

Death may occur from respiratory arrest or heart failure if virus invades vital medullary centers

Salk and Sabin vaccine eradicated it

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Spinal Cord Development The central nervous system forms from

the embryonic neural tube. Cranial and spinal nerves form from

neural crest cells that have split off from the developing neural tube.

The cranial (superior) part of the neural tube expands and develops into the brain.

The caudal (inferior) part of the neural tube forms the spinal cord.

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