Protection Of The Spinal Cord 12 CNS.pdf · •Integration center for spinal reflexes is gray...
Transcript of Protection Of The Spinal Cord 12 CNS.pdf · •Integration center for spinal reflexes is gray...
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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