Spinal Cord, Spinal Nerves, and Spinal Reflexes

Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives 1. Describe the structures and functions of the

spinal cord, including its extent, enlargements, and spinal meninges.

2. Differentiate the gray and white matter of the spinal cord and discuss its importance.

3. Describe the major components of the spinal nerves and its pattern of distribution.

4. Describe the reflex arc.

5. Discuss types of spinal reflexes and explain its functions.

6. Identify common spinal cord injuries and their importance.

Spinal Cord• Extends through vertebral canal from __ to __. • Cervical and lumbar enlargements• 31 pairs of spinal nerves• ___ = tapered tip of cord• ___ = resemble horse’s tail (sacral roots and filum terminale)

• ___ = site of spinal anesthesia

a. Cauda equina b. L3-L4 vertebraec. conus medullaris d. L1 vertebraee. Foramen magnum

Meninges of the Spinal Cord• Dura mater

– epidural space = site of epidural block

• Arachnoid mater– subarachnoid space is filled with _______.

• Pia mater– adherent to spinal cord– filum terminale (inferior end)– denticulate ligaments (sides)

Cross-Section of the Spinal Cord• Gray matter - surrounds the central canal; has projections (gray horns);

contains neurons, neuroglial (glial) cells, and nerve fibers a) Posterior horns

(somatic and visceral sensory nuclei= association neurons) b) Lateral horns (visceral motor nuclei= sympathetic motor)c) Anterior horns(somatic motor nuclei= motor neuron)

- connected by gray commissure

• White matter - contains nerve fibers and neuroglial (glial) cells

White Matter of the Spinal Cord

• 3 pairs of white columns (funiculi): 1. _____ funiculus- between the posterior median sulcus

and posterior gray horn2. _____ funiculus- between the posterior gray horn and

anterior gray horn3. _____ funiculus- between the anterior gray horn and

anterior median fissure a. lateral b. anterior c. posterior

• Each column (funiculus) is filled with named fiber Tracts (fasciculi), 2 types:

1. Ascending tract- are ____ 2. Descending tract- are ____

a. Motor b. Sensory

Poliomyelitis

• Diseases causing destruction of ___ and results to skeletal muscle ___.

• Poliomyelitis caused by poliovirus spread by fecally contaminated water– weakness progresses to paralysis and

respiratory arresta. posterior horn cells b. anterior horn cells

c. atrophy d. hypertrophy

Anatomy of a Nerve and Ganglia

• Spinal Nerve – surrounded by connective tissue ______ outer layer, covers whole nerve.

______ middle layer, surrounds a fascicle (bundle).______ inner layer, separates individual axons (nerve fibers)

• Spinal Ganglion - a cluster of cell bodies in PNS (DRG/ Sensory Neuron)

a. endoneurium b. perineurium c. epineurium

Peripheral Distribution of Spinal Nerves

• 31 pairs of spinal nerves – mixed nerves

• Proximal branches– dorsal root is sensory input to spinal cord– ventral root is motor output of spinal cord

• Distal branches– dorsal ramus supplies dorsal body muscle

and skin– ventral ramus to ventral skin and muscles

of body wall and limbs

Cutaneous Innervation • Dermatome = bilateral region of skin surface monitored

by a specific pair of spinal nerves. • _____________ = are regional losses of sensory and

motor function resulting from nerve trauma or compression.

• Shingles, a viral infection causing skin eruptions along path of nerve

Nerve Plexuses

• Ventral rami branch and anastomose, control muscles of neck and limbs

• 4 major plexuses – cervical in the neck, C1 to C5 – brachial in the armpit, C5 to T1– lumbar in the low back, L1 to L4– sacral in the pelvis, L4, L5 and S1 to S4

Cervical Plexus• Cervical plexus - innervates neck, thoracic cavity, and

diaphragm; phrenic nerve is the major nerve that innervates the diaphragm.

• Brachial plexus- innervates the shoulder, arm, and hand (pectoral girdle and upper limbs); it forms:

- Trunks – superior, middle, and inferior - Cords – lateral, medial, and posterior - Peripheral nerveso Musculocutaneouso Mediano Ulnaro Axillaryo Radial

Brachial Plexus Injury

• Carpal tunnel syndrome- injury to median nerve; inability to flex the wrist and fingers

• Wrist drop- injury to radial nerve; inability to extend the wrist

• Claw hand- injury to ulnar nerve, difficulty of flexion of the 5th finger

Lumbar Plexus

• Lumbar plexus- branches:Genitofemoral n., Lateral femoral cutaneous n., Femoral n.; innervates abdominal wall, anterior thigh and genitalia

• Sacral plexus- innervates the lower limbs; branches: a. Sciatic nerve

- fibular (peroneal) nerve, sural nerve (sensory innervating the lateral part of foot; used in nerve grafts - tibial nerve

b. Pudendal nerve- remainder of lower trunk and lower limb

___ What nerve is compressed that produces the sensation that your leg has fallen asleep?

a. Femoral nerve b. sciatic nerve

Reflexes• Quick, automatic response to specific stimuli• Events/steps in a neural reflex arc

1. Stimulation of somatic receptor (sensory nerve endings)2. Activation of a sensory neuron (DRG) 3. Information processing (CNS) 4. Activation of motor neuron (CNS)5. Response of effector

Classification of Reflexes• Development of reflexes

a. innate

b. acquired• Nature of response

a. somatic

b. visceral • Complexity of circuit

a. monosynaptic

b. polysynaptic

State the types of reflexes of the examples below.

• Driver steps on the brake suddenly when a deer appears.

• Salivation upon smell of fried chicken

• Cremasteric reflex• Infant suckling on

mother’s breast.• Knee jerk reflex• Flexor- extensor reflex

Muscle Spindle

• Receptor in stretch reflexes• Composed of intrafusal muscle fibers, innervated

by both sensory and motor neurons• Surrounded by extrafusal muscle fibers, maintain

muscle tone and contract muscle

Patellar Tendon (Knee Jerk) Reflex

• Type of reflex- ___• Receptor stimulated- ___• Sensory neuron- ___• Motor neuron- ___• Effector response- ___

a.anterior horn cells b.monosynapticc.contraction of quadriceps femoris musclesd.DRGe.muscle spindles

Flexor Withdrawal Reflexes

• _________ type of reflex• Occurs during withdrawal of foot from pain• Maintains balance by extending other leg• Intersegmental reflex extends up and down the

spinal corda. polysynaptic reflex b. monosynaptic reflex

Babinski Reflexes• Normal in infants • Plantar reflex or negative Babinski reflex

- curling of toes in adult produced by stroking the sole of the foot.

• Babinski sign (positive Babinski reflex) - fanning of toes in adult indicates CNS injury (higher centers or descending tracts are damaged)= Upper motor neuron injury (ex: CVA or stroke)

• Spinal cord trauma- 10,000 -12,000 people/ year are paralyzed; 55% occur in traffic accidents

Homework (self Review) 1. Define the following: conus medullaris, filum terminale,

cauda equina,dorsal root, ventral root, denticulate ligaments, funiculi, fasciculi, nerve plexus, ganglion, reflex, muscle spindle, peripheral neuropathy, and positive babinski reflex.

2. Describe the anterior gray horn, lateral gray horn, posterior gray horn, and central gray commisure as to its function.

3. Differentiate ascending and descending tracts in white matter of the spinal cord.

4. Identify the nerve and plexus that would be affected by: a) difficulty of breathing, b) wrist drop, c) sensation that leg has fallen asleep, d) paresthesia of shoulder and upper limb, e) muscle pain at anterior thigh.

5. Give the areas of the body supplied by dorsal ramus and ventral ramus of the spinal nerves.

6. Compare and contrast monosynaptic reflex and polysynaptic reflex and cite an example for each.