Nerve Anatomy and Function

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Nerve Anatomy and Function

• Basic Science Lecture
• Chapter 13
• Victor Ursua, M.D. 5/18/2007

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Neurons

• neuron
• axon hillock
• dendrites
• axonmyelin, nodes of ranvier
• axoplasmic transportdepend on ATP, calcium and
  microtubules, carrier proteins antegrade and
  retrograde

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types of nerve cells

• Neurons
• astrocytesmost common glial for supportive
  structure, scavenging, nutritive
• oligodendrocytesin cns
• schwann cellin pns
• microglia

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How signals are sent

• resting membrane potential(-) 50 to (-) 80 mV
• action potentials
• Large diameter vs small
• Myelin vs unmyelin
• graded potential
• Higher length constant, lower time constant

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Myelin

• Formed by schwann cells in pns and
  oligodendrocytes in cns
• Myelin in cns contain proteolipid and myelin
  basic protein
• Myelin in pns contain P0 glycoprotein,
  myelin-associated glycoprotein, plasma membrane
  protein-22

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Time constant

• RCm
• Internal resistance
• capacitance
• Goal is to lessen the time constant

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Length constant

• Rm/Ri
• Rm membrane resistance
• Ri internal resistance
• Goal is to increase the length constant

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Structure of the nerve fiber

• endoneurium
• fascicles covered with perineurium
• groups of fascicles covered with epineurium

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Nerve fiber classification

• A delta, A gamma, A alpha, A beta
• A alpha spindle afferents, motor efferentsnvc
  100ms
• A betaskin touch and pressure afferentsnvc 50
  ms
• A gammamotor efferent to spindlencv 20 ms
• A deltaskin temperature and painncv 15 ms
• B ---sympathetic preganglionicncv 7 ms
• C skin pain, sympathetic postganglionicncv 1
  ms

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Classification of Nerve injury

• I
• Endoneurium in tact neuropraxia, quick
  recovery
• II
• Axon lesion within intact endoneurium
  axonotmesis
• Wallerian degeneration
• Complete regeneration
• III
• Axon injury with endoneurial injury
• Complete loss of motor and sensory
• Recovery is delayed, variable
• More proximal injury, worse prognosis
• IV
• Injury to axon, endoneurium, and perineurium
• Complete loss of motor and sensory
• Recovery requires surgical repair, otherwise poor
  prognosis
• V
• Injury of axon to epineurium
• Must have surgical repair

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Types of nerve injury

• compression
• nerve fibers in small bundles and little
  epineurium at higher risk
• nerves against bony surfaces, unyielding
  compartments
• double crush
• Stretch
• nerves in small bundles with little epineurium at
  higher risk of injury
• Ischemic
• tourniquet in upper extremity
• lt 50 to 100 mm Hg above SBP lt 2 hrs
• tourniquet in lower extremity
• lt 2x SBP, lt 2 hrs

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Nerve response to injury

• conduction block at injury site occurs
  immediately
• reduced amplitude on distal stimulation occurs
  within 7 days after injury
• denervation changes seen within 2 to 5 wks after
  injury
• reinnervation on EMG if partial lesion seen after
  6-8 wks

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wallerian degeneration

• changes distal to the lesion
• axonal degeneration myelin removed by
  macrophages, schwann cells
• schwann cell response proliferation with 3 days
  produce laminin, fibronectin promoting neurite
  growth
• macrophage response day 1 to 3

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Proximal response to injury

• 1) chromatolytic phase,
• 2) recovery phase,
• 3) degenerative phase
• if cell body survives, proximal nerve fiber
  reduces diameter and myelin thickness, associated
  with slower ncv

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axon regeneration

• average growth in humans 1 to 2 mm/ day

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Surgical repair for neurotemesis

• Objectives to maximize no of axons that
  regenerate across the lesion, improve accuracy of
  re-innervation
• Resection of ends preparation of stump
• Suture re approximation with alignment ie,
  coaptation and suturing
• Maintaining coaptation
• Direct nerve to nerve ends
• Indirect via nerve graft

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Organization of nerves

• Afferent fibers
• Efferent fibers
• Autonomic fibers
• Somatic fibers

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Central nervous system

• Brain and spinal cord
• To corticospinal tract

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Peripheral nervous system

• Spinal cord to end organs
• Begins at motor neurons

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Brachial plexus
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Somatic nerves

• Innervate muscles
• Receive signals from sensory end organs

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Autonomic nervous system

• Parasympathetic nerves
• Vegetative
• Craniosacral
• Pacemaker to heart
• Maintain normal tone to blood vessels
• gut
• Sympathetic
• Fight or flight
• Thoracolumbar
• Constricts blood vessels to gut
• Increase blood flow to heart, lungs, muscles

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Horners syndrome

• Ptosis, anhydrosis, miosis

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spinal nerve
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spinal cord embryology

• derived from the neural plate which is a portion
  of the ectoderm
• neural plate folds into neural tube
• if neural fold do not meet, rachisis or
  anencephaly develops
• if neural tube folds but overlying vertebrae fail
  to develop, spina bifida occurs

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Spinal cord
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Lumbar disc herniation
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Spinal cord

• White matter
• Gray matter
• Coverings
• Blood supply

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Spinal cord

• Dorsal gray lamina
• Ventral gray lamina
• ventral horn cells, 3 types
• alpha and gamma motor neurons axons leave cns
• interneuronsaxons remain in spinal cord
  function as a link between the pns, descending
  pathways, and spinal motor neurons
• tract cellsaxons ascend to supra spinal centers

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sensory system

• receptor types
• nociceptors
• cutaneous mechanoreceptors
• thermal receptors
• muscle and skeletal mechanoreceptors
• pain free nerve ending, A delta, gamma, C
  fibers
• temperature--same
• pressureMerkels receptor (slow adapting)A beta
• pressuremeissners (rapidly adapting)A beta
• vibrationpacinian corpuscleA beta
• muscle stretchspindleA alpha, beta
• tendon tension golgi tendon organA alpha
• joint capsule mechanoreceptorA beta

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Ascending tracts
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motor system

• 4 divisions
• spinal cord
• brain stems and reticular formation
• motor cortex
• premotor cortex including basal ganglia and
  cerebellum
• descending pathways
• pyramidal (corticospinal) and extrapyramidal
  systems (rubrospinal, reticulospinal,
  vestibulospinal

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Spinal cord injury syndromes

• Brown Sequard hemisection
• Loss of ipsilateral motor and contralateral pain/
  temp
• Best prognosis
• Central cord
• Loss of motor and sensory, uppers gt lowers
• Fair prognosis
• Anterior cord
• Incomplete motor and sensory
• Poor prognosis
• Complete
• Poor prognosis

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Neuromuscular junction
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motor units

• basic functional unit of a muscle
• consists of alpha motor neuron, its motor axon,
  all muscle fibers it innervates
• final common pathway

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Muscle fiber types

• slow fatigue resistant (types S, type I)low
  tension, slow contraction times, most fatigue
  resistant slow oxidative fibers smaller fibers
• fast fatigue resistant (type FR type II A)fast
  oxidative glycolytic fibers
• fast fatigue intermediate (type FI)
• fast fatigable (type FF type II B)highest
  tension, fast contractions, easily fatigable
  fast glycolytic fibers larger fibers

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reflexes

• stereotyped response to a specific sensory
  stimulus

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Muscle spindle

• 2 types of intrafusal fibers
• nuclear chain fibers and nuclear bag fibers
• Nuclear bag sensitive to sudden change in length
  phasic
• Nuclear chain sensitive to steady changes tonic

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flexion reflex
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autogenic inhibition
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Upper motor neuron

• Characteristics spasticity, hyper reflexia,
  clonus, no atrophy
• Negative symptoms loss of dexterity, weakness,
  fatigability
• Positive symptoms abnormal posture,
  exaggeration of reflexes, babinski (upward
  going)
• Spasticity

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Lower motor neuron

• Characteristics flaccid tone, atrophy,
  areflexia, fasciculation present, negative
  babinski
• Begins at alpha, gamma motor neurons in ventral
  gray
• Innervate extrafusal fibers in striated muscle

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summary