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The Spinal Cord & Spinal Nerves

• Together with brain forms the CNS

• Functions

– spinal cord reflexes

– integration (summation of inhibitory and

excitatory) nerve impulses

– highway for upward and downward travel of

sensory and motor information

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Spinal Cord Protection

By the vertebral column, meninges, cerebrospinal fluid, and vertebral ligaments.

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Structures Covering the Spinal Cord

• Vertebrae

• Epidural space filled with fat

• Dura mater

– dense irregular CT tube

• Subdural space filled with

interstitial fluid

• Arachnoid = spider web of

collagen fibers

• Subarachnoid space = CSF

• Pia mater

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External Anatomy of Spinal Cord

• Flattened cylinder

• 16-18 Inches long &

3/4 inch diameter

• In adult ends at L2

• In newborn ends at L4

• Growth of cord stops at

age 5

• Cervical enlargement

– upper limbs

• Lumbar enlargement

– lower limbs

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Inferior End of

Spinal Cord

• Conus medullaris

– cone-shaped end of spinal cord

• Caudae equinae (horse’s tail)

– dorsal & ventral roots of lowest

spinal nerves

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Spinal Cord & Spinal Nerves

• Spinal nerves begin as roots

• Dorsal or posterior root is incoming sensory fibers

– dorsal root ganglion (swelling) = cell bodies of sensory nerves

• Ventral or anterior root is outgoing motor fibers

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Spinal tap or Lumbar Puncture

• Technique

– long needle into subarachnoid space

– safe from L3 to L5

• Purpose

– sampling CSF for diagnosis

– injection of antibiotics, anesthetics or

chemotherapy

– measurement of CSF pressure

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Spinal Reflexes

• Automatic response to change in environment

• Integration center for spinal reflexes is gray matter

of spinal cord

• Examples

– somatic reflexes result in skeletal muscle contraction

– autonomic (visceral) reflexes involve smooth & cardiac

muscle and glands.

• heart rate, respiration, digestion, urination, etc

• Note: cranial reflexes involve cranial nerves

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Reflex Arc

• Specific nerve impulse pathway

• 5 components of reflex arc

– receptor

– sensory neuron

– integrating center

– motor neuron

– effector

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Stretch Reflex (patellar reflex)

• Monosynaptic,ipsilateral reflex arc

• Prevents injury from over stretching because

muscle contracts when it is stretched

• Events of stretch reflex

– muscle spindle signals stretch of muscle

– motor neuron activated & muscle contracts

• Brain sets muscle spindle sensitivity as it sets

muscle tone (degree of muscle contraction at rest)

• Reciprocal innervation (polysynaptic- interneuron)

– antagonistic muscles relax as part of reflex

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Illustration of the Stretch Reflex

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Flexor (withdrawal) Reflex

• Step on tack (pain fibers

send signal to spinal

cord

• Interneurons branch to

different spinal cord

segments

• Motor fibers in several

segments are activated

• More than one muscle

group activated to lift

foot off of tack

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Clinical Considerations

• Checking a patient’s reflexes may help to detect

disorders/injury

• Plantar flexion reflex -- stroke the lateral margin

of the sole

– normal response is curling under the toes

– abnormal response or response of children under 18

months is called Babinski sign (upward fanning of

toes due to incomplete myelination in child)

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Spinal Nerves• 31 Pairs of spinal nerves

• Named & numbered by the cord level of their origin

– 8 pairs of cervical nerves(C1 to C8)

– 12 pairs of thoracic nerves (T1 to T12)

– 5 pairs of lumbar nerves(L1 to L5)

– 5 pairs of sacral nerves (S1 to S5)

– 1 pair of coccygeal nerves

• Mixed sensory & motor nerves

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Dermatomes & Myotomes

• Each spinal nerve contains both sensory & motor

nerve fibers

• Dermatome

– area of skin supplied by one spinal nerve

– overlap prevents loss of sensation if one damaged

– sensory anesthesia requires 3 spinal nerves to be

blocked

• Skin on face supplied by Cranial Nerve V

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Dermatomes

• Damaged regions of the

spinal cord can be

distinguished by patterns

of numbness over a

dermatome region

• Spinal cord transection

– injury that severs the cord

loss of sensation & motor

control below the injury

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Disorders• Neuritis

– inflammation of nerves

– caused by injury, vitamin deficiency or poison

• Shingles

– infection of peripheral nerve by chicken pox virus

– causes pain, skin discoloration, line of skin blisters

• Poliomyelitis

– viral infection causing motor neuron death and

possible death from cardiac failure or respiratory

arrest