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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

• eripheral nerves have

different conduction

velocity

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Conduction velocity in nerves

and fiber tracts

• roportional to fiber diameter 

• eripheral nerves! "#$ meter%sec

cm%msec'

• Spinal descending tracts! "($)*$$ m%sec

• Cranial nerves! varies

&Auditory nerve! +$ m%sec'

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 ,ig -./ From: Møller: Sensory Systems, 2003

Nerve fibers with different

diameter have different

conduction velocity

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Epineurium

Perineurium

Endoneurium

Fascicles

Nerve fber

Node o Ranvier

Schwann cell

Myelin

Axon

From: Møller: Sensory Systems, 2003

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0any nerves are mixed nerves

• Contains nerve fibers 1ith 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 dorsalhorn of the

spinal cord

,230 423DA5 *667

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0any nerves are bipolar nerves

• Examples are dorsal roots

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 ,rom 4rodal *667

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athologies of peripheral nerves

Nerves!• Neurapraxia

•  Axonotmesis• Neurotmesis Nuclei!

•  Altered discharge pattern &burst activity'

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Causes of in8ury to peripheral

nerves

• 9rauma

• Compression &entrapment'

• Irritation

• 0etabolic disorders

• Inflammatory &neuritis'

• :irus

•  Age related changes

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9rauma to peripheral nerves

• Interruption of nerve trun; &neurotmesis'

• Interruption of axons &axonotmesis'

• 9otal conduction failure &neurapraxia' • Impaired conduction &no morphologic

change'

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Normal

Cenral porion Peripheral porion

Sreched

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Axon

Epineurium

PerineuriumEndoneuriu

Neurapraxia

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9otal conduction failure

&neurapraxia'

• No function

• 2ecovers spontaneously over days or

1ee;s &1hen the cause is resolved'

• 2esults of spontaneous recovery are

almost al1ays good

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Interruption of axons

&axonotmesis'

• No function

• Ne1 axon gro1s from cell body

&spontaneously'

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 Axonotmesis

• Nerve may regenerate from in8ured

location a1ay from the cell body

• 2egeneration! * mm per day &approx. *inch per month'

• 2esults of spontaneous recovery are good

to moderate depending on distance

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Interruption of nerve trun;

&neurotmesis'

• No function

• Irreversible< grafting is re=uired

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Neurotmesis

• Does not regenerate spontaneously

• >rafting is necessary to restore function

• 2esults of grating are good to moderate tofailures

I 8 d

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In8ured nerves

 Axon interrupted&?allerian

degeneration'

Interruption of axon and

endoneurial sheet

Interruption of 

perineurial sheet

Interruption of

nerve trun;

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Axon

Epineurium

PerineuriumEndoneuriu

9ype +

9ype /

9ype -

9ype #

 Axonotmesis

Neurotmesis

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Interrupted axons

• Degenerate distally &a1ay from cell body'

• Wallerian degeneration• Interrupted axons regenerate from in8ury<

provided that endoneural tube is intact

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?allerian degeneration means!

9he degenerative changes thedistal segment of a peripheral nerve

fiber &axon and myelin' undergoes1hen its continuity 1ith its cell bodyis interrupted by a focal lesion.Syn! orthograde degeneration<

secondary degeneration.

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S9A29 6%(%$#

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2emaining symptoms after nerve

healing of in8ury

• Syn;inesis

• @yperactivity

&0ostly 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

• 0echanical &compression< stretching'

• @eat

• 0etabolic• Inflammation

• Iatrogenic &from medical treatment'

• Idiopathic &un;no1n'•  Age

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9rauma

• >unshot to limbs

•  Accidents

• Surgery &iatrogenic'

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Sprouting

• Caused by in8ury

• Caused by regeneration

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,ormation of neuroma

• Sprouting of axons at cut of a nerve

• In8ured perineurium

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Neuroma are mechanically

sensitive

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Compression

• No ;no1n cause

• Scar tissue

• Changes in bone formation

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4loc; of axoplasmatic flo1

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Irritation

• Scar tissue

• 4lood vessels

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0etabolic and chemical induced

peripheral neuropathy

• Diabetes

• remic< hepatic and vitamin &4*

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Inflammatory &neuritis'

• >uillain)4arre syndrome

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:irus

 

• @erpes simplex &causes severe pain'

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Demyelination

• Ephaptic transmission

• 2eflection of neural activity

• 0echanosensitivity

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In8ury to a peripheral nerve can cause

transneural degeneration of the target cell

, ti l i li ti f l

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,unctional implications of neural

in8uries

• Change the function of the target

central neurons!

 – 0ay cause expression of neural plasticity

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 Abnormal activity in a peripheral

nerve can cause changes in thefunction of the target cells

C f ;i i d

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Cause of syn;inesis and

hyperactivity

• lastic changes in target neurons induced

by the in8ury

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Expression of neural plasticityfrom in8ury to sensory nerves!

•  Deprivation of input

•  3verstimulation

I i d d ti

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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

 – ain

 – @ypersensitivity

 – Spasm

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Irritation of nerve roots

• Cause change in central processing

• 9ransition bet1een peripheral and central

myelin is especially sensitive

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Irritation of nerve roots

• ain

• @yperalgesia

•Spasm

&Expression of neural plasticity'

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Start 6%*+%$#

Signs of peripheral nerve

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Signs of peripheral nerve

disorders

• 0otor nerves!

 – aralysis

 – Spasm

 – Electrophysiologic changes

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Explanation

 A sign is any abnormality

indicative of disease< discoverableon examination of the patient.

Symptoms of peripheral nerve

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Symptoms of peripheral nerve

disorders

• Sensory nerves

 – Numbness – 9ingling

 – ain

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 A symptom is any morbid

phenomenon or departure fromthe normal in structure< function<or sensation< experienced by thepatient and indicative of disease.

Explanation

Slightly in8ured nerves

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Slightly in8ured nerves

&demyelination'

• Decreased conduction velocity

•  Abnormal firing pattern• eripheral nerves as impulse generators

• ossible ephapticB' transmission

B' Direct transfer of impulse activity from onenerve fiber to another 

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Decreased conduction velocity

• 9emporal dispersion of neural activity

• Change in neural coding of sensory stimuli

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Decreased conduction velocity

• 9he decrease is not the same for all fibers

• 9emporal dispersion of neural activity

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Increased spatial dispersion

• Can decrease excitation

• Can increase excitation• Can degrade temporal information

Acion poenial

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 !hreshold

 !hreshold

EPSP

EPSP

,rom 0ller +$$#

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Temporal dispersion

1. Can decrease the activation of the target neuron

2. Can increase the duration of firing of the target neuron,

which can increase or decrease activation of the following

neuron.

3. 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

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 Abnormal firing pattern

• 4urst firing

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eripheral nerves as impulse

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eripheral nerves as impulse

generators

• Ectopic activity

 – After demyelination

 – Diabetic neuropathy

 – Adrenergic substances facilitate impulse

generation

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Ectopic firing

• 4urst

• 2hythmic

•  After)discharges to stimulation