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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