external anatomy of spinal cord - 1.cdn.edl.io · pdf fileaxons that form ascending &...
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Chapter 13The Spinal Cord & Spinal Nerves
Together with brain forms the CNS Functions spinal cord reflexes integration (summation of inhibitory and excitatory)
of nerve impulses highway for upward and downward travel of sensory
and motor information
We are only going to cover Pages 420-434 and 447
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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 thin layer with many BVs denticulate ligs hold in place
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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
External Anatomy of Spinal Cord
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Conus medullaris cone-shaped end of spinal cord
Filum terminale thread-like extension of pia mater stabilizes spinal cord in canal
Caudae equinae (horses tail) dorsal & ventral roots of lowest
spinal nerves Spinal segment area of cord from which each pair
of spinal nerves arises
Inferior End of Spinal Cord
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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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Gray Matter of the Spinal Cord
Gray matter is shaped like the letter H or a butterfly contains cell bodies, unmyelinated axons & dendrites paired dorsal and ventral gray horns lateral horns only present in thoracic spinal cord gray commissure crosses the midline
Central canal continuous with 4th ventricle of brain
Note: colors in reverse due to staining of tissue
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White Matter of the Spinal Cord
White matter covers gray matter Anterior median fissure deeper than Posterior median
sulcus Anterior, Lateral and Posterior White Columns contain
axons that form ascending & descending tracts 9
Tracts of the Spinal Cord
Function of tracts highway for sensory & motor information sensory tracts ascend motor tracts descend
Naming of tracts indicates position & direction of signal example = anterior spinothalamic tract impulses travel from spinal cord towards brain
(thalamus) found in anterior part of spinal cord
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Motor tracts!! ! ! Sensory tracts Direct (pyramidal) tract ---spinothalamic tracts Indirect(extrapyramidal) tract! ---posterior columns
see page 515 ---spinocerebellar
Location of Tracts inside Cord
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Function of Spinal Tracts Spinothalamic tract pain, temperature, deep pressure & crude touch
Posterior columns proprioception, discriminative touch, two-point
discrimination, pressure and vibration Direct pathways (corticospinal & corticobulbar) precise, voluntary movements
Indirect pathways (rubrospinal, vestibulospinal) programming automatic movements, posture &
muscle tone, equilibrium & coordination of visual reflexes
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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 nerves13
Reflex Arc Specific nerve impulse pathway 5 components of reflex arc receptor sensory neuron integrating center motor neuron effector
4 important somatic spinal reflexes stretch, tendon, flexor(withdrawal) & crossed
extensor reflexes
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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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Tendon Reflex
Controls muscle tension by causing muscle relaxation that prevents tendon damage
Golgi tendon organs in tendon activated by stretching of tendon inhibitory neuron is stimulated (polysynaptic) motor neuron is hyperpolarized and muscle relaxes
Both tendon & muscle are protected Reciprocal innervation (polysynaptic) causes contraction of ipsilateral muscle group
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Illustration of Tendon 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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Crossed Extensor Reflex Lifting left foot requires
extension of right leg to maintain ones balance
Pain signals cross to opposite spinal cord
Contralateral extensor muscles are stimulated by interneurons to hold up the body weight
Reciprocal innervation - when extensors contract flexors relax, etc
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Endoneurium - around individual nerve fibers- Fascicles - a bundle of axons/nerve fibers Perineurium - around fascicles Epineurium - the superficial covering around the whole nerve
Connective Tissue Coverings of the Spinal Nerves
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Endoneurium - around individual nerve fibers- Fascicles - a bundle of axons/nerve fibers Perineurium - around fascicles Epineurium - the superficial covering around the whole nerve
Connective Tissue Coverings of the Spinal Nerves
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Clinical Considerations
Checking a patients 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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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
Infusing local anesthetics or cutting roots must be done over 3 adjacent spinal nerves.
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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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 nerves27
The End
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