Disorders of peripheral nerves

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Disorders of peripheral nerves
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Symptoms and signs of disorders of nerves

• Caused by changes in axons
• Increased conduction time
• Increased temporal dispersion
• Expression of neural plasticity causing changes
  in the function of CNS structures

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Anatomy of peripheral nerves
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Anatomy of peripheral nerves

• Peripheral nerves have different conduction
  velocity

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Conduction velocity in nerves and fiber tracts

• Proportional to fiber diameter
• Peripheral nerves 50 meter/sec
• (5 cm/msec)
• Spinal descending tracts 70-100 m/sec
• Cranial nerves varies
• (Auditory nerve 20 m/sec)

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Nerve fibers with different diameter have
different conduction velocity
Fig 4.3
From Møller Sensory Systems, 2003
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From Møller Sensory Systems, 2003
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Many nerves are mixed nerves

• Contains nerve fibers with different conduction
  velocity

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Recording from a long nerve composed of fibers
with different diameter thus different conduction
velocity
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Organization of the dorsal horn of the spinal cord
FROM BRODAL 1998
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Many nerves are bipolar nerves

• Examples are dorsal roots

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From Brodal 1998
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Pathologies of peripheral nerves

• Nerves
• Neurapraxia
• Axonotmesis
• Neurotmesis
• Nuclei
• Altered discharge pattern (burst activity)

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Causes of injury to peripheral nerves

• Trauma
• Compression (entrapment)
• Irritation
• Metabolic disorders
• Inflammatory (neuritis)
• Virus
• Age related changes

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Trauma to peripheral nerves

• Interruption of nerve trunk (neurotmesis)
• Interruption of axons (axonotmesis)
• Total conduction failure (neurapraxia)
• Impaired conduction (no morphologic change)

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Neurapraxia
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Total conduction failure (neurapraxia)

• No function
• Recovers spontaneously over days or weeks (when
  the cause is resolved)
• Results of spontaneous recovery are almost always
  good

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Interruption of axons (axonotmesis)

• No function
• New axon grows from cell body (spontaneously)

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Axonotmesis

• Nerve may regenerate from injured location away
  from the cell body
• Regeneration 1 mm per day (approx. 1 inch per
  month)
• Results of spontaneous recovery are good to
  moderate depending on distance

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Interruption of nerve trunk (neurotmesis)

• No function
• Irreversible, grafting is required

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Neurotmesis

• Does not regenerate spontaneously
• Grafting is necessary to restore function
• Results of grating are good to moderate to
  failures

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Injured nerves
Axon interrupted (Wallerian degeneration) Interr
uption of axon and endoneurial sheet Interruptio
n of perineurial sheet Interruption of nerve
trunk
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Axonotmesis
Type 2
Neurotmesis
Type 3
Type 4
Type 5
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Interrupted axons

• Degenerate distally (away from cell body)
• Wallerian degeneration
• Interrupted axons regenerate from injury,
  provided that endoneural tube is intact

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Wallerian degeneration means

• The degenerative changes the distal segment of a
  peripheral nerve fiber (axon and myelin)
  undergoes when its continuity with its cell body
  is interrupted by a focal lesion.
• Syn orthograde degeneration, secondary
  degeneration.

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START 9/7/05
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Remaining symptoms after nerve healing of injury

• Synkinesis
• Hyperactivity
• (Mostly caused by effect on central nervous
  system structures)

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Electrophysiological manifestations of
pathologies of peripheral nerves

• Nerves
• Increased conduction times
• Increased or decreased discharge activity
• Dispersion of neural activity
• Altered discharge pattern (burst activity)

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Cause of neural pathologies

• Mechanical (compression, stretching)
• Heat
• Metabolic
• Inflammation
• Iatrogenic (from medical treatment)
• Idiopathic (unknown)
• Age

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Trauma

• Gunshot to limbs
• Accidents
• Surgery (iatrogenic)

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Sprouting

• Caused by injury
• Caused by regeneration

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Formation of neuroma

• Sprouting of axons at cut of a nerve
• Injured perineurium

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Neuroma are mechanically sensitive
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Compression

• No known cause
• Scar tissue
• Changes in bone formation

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Block of axoplasmatic flow
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Irritation

• Scar tissue
• Blood vessels

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Metabolic and chemical induced peripheral
neuropathy

• Diabetes
• Uremic, hepatic and vitamin (B1,B2,B12) deficits
• Alcohol
• Chemical

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Inflammatory (neuritis)

• Guillain-Barre syndrome

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Virus

• Herpes simplex (causes severe pain)

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Demyelination

• Ephaptic transmission
• Reflection of neural activity
• Mechanosensitivity

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Injury to a peripheral nerve can cause
transneural degeneration of the target cell
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Functional implications of neural injuries

• Change the function of the target central
  neurons
• May cause expression of neural plasticity

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Abnormal activity in a peripheral nerve can cause
changes in the function of the target cells
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Cause of synkinesis and hyperactivity

• Plastic changes in target neurons induced by the
  injury

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Expression of neural plasticity from injury to
sensory nerves

• Deprivation of input
• Overstimulation

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Impaired conduction

• Decreased conduction velocity
• Increased refractory period
• Ectopic (out of place) activity

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Irritation of peripheral nerves

• Cause change in central processing
• Pain
• Hypersensitivity
• Spasm

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Irritation of nerve roots

• Cause change in central processing
• Transition between peripheral and central myelin
  is especially sensitive

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Irritation of nerve roots

• Pain
• Hyperalgesia
• Spasm
• (Expression of neural plasticity)

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Start 9/12/05
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Signs of peripheral nerve disorders

• Motor nerves
• Paralysis
• Spasm
• Electrophysiologic changes

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Explanation

• A sign is any abnormality indicative of disease,
  discoverable on examination of the patient.

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Symptoms of peripheral nerve disorders

• Sensory nerves
• Numbness
• Tingling
• Pain

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Explanation

• A symptom is any morbid phenomenon or departure
  from the normal in structure, function, or
  sensation, experienced by the patient and
  indicative of disease.

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Slightly injured nerves (demyelination)

• Decreased conduction velocity
• Abnormal firing pattern
• Peripheral nerves as impulse generators
• Possible ephaptic) transmission
• ) Direct transfer of impulse activity from one
  nerve fiber to another

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Decreased conduction velocity

• Temporal dispersion of neural activity
• Change in neural coding of sensory stimuli

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Decreased conduction velocity

• The decrease is not the same for all fibers
• Temporal dispersion of neural activity

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Increased spatial dispersion

• Can decrease excitation
• Can increase excitation
• Can degrade temporal information

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From Møller 2005
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Temporal dispersion

• 1. Can decrease the activation of the target
  neuron
• Can increase the duration of firing of the target
  neuron, which can increase or decrease activation
  of the following neuron.
• Increased temporal dispersion may prevent
  activation of the next neuron in a chain, or it
  may make it possible to activate neurons that are
  not normally activated.
• 4. Increased temporal dispersion may degrade
  information when temporal coding is important
  such as in sensory systems, most pronounced in
  hearing

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Abnormal firing pattern

• Burst firing

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From Møller 2005
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Peripheral nerves as impulse generators

• Ectopic activity
• After demyelination
• Diabetic neuropathy
• Adrenergic substances facilitate impulse
  generation

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Ectopic firing

• Burst
• Rhythmic
• After-discharges to stimulation