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

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