Download MBBS Physiology Presentations 70 Type Of Nerve Fibers Lecture Notes

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Objectives

Should be able to describe,

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I. Types of injuries
I . Responses of nerve injury in

CNS and PNS
End Organs (e.g. Muscles)

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I I. Factors that effect nerve regeneration

Nerve injury may occur due to :

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? Obstruction of the blood flow
? Toxic substances
? Pressure over the fibre- crushing of the

fibre

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? Transection of the fibre
Classification of nerve injuries

(Seddon Classification)

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1. Neuropraxia

2. Axonotmesis

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3. Neurotmesis

Neuropraxia

? Injuries due to pressure

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? Local conduction block only
? This is the mildest for of nerve injury. Mild, blunt

blows, including some low-velocity missile injuries

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close to the nerve

? Recovery takes place without wallerian

degeneration.

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? Pressure over the fibre- short period -

Obstruction of the blood flow, hypoxia

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? The axon is not destroyed mild

demyelination looses the function for a

short period ,conduction block.returns

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within few hrs ? few wks
Axonotmesis

? Usually traction injury

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? Endoneurial tubes are intact

Axonotmesis

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? Severe pressure over the fibre- long period

? endoneurium is intact

? Repair of function 18 months

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Endoneurium

? Each nerve fibre is

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covered by

endoneurium

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? Endoneurium is interrupted
? Epineurium and perineurium are intact
? Recovery is slow
? Neurotmesis

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Each fasciculus is covered by

Perineurium

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Epineurium

Whole nerve is covered by a sheath
Degenerative changes

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- axonotmesis- Wallerian degeneration

? Changes in cell body
? Changes in axon

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Changes in cell body

1. Cellular edema
2. Chromatolysis starts near axon hillock.

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(Dispersion of Nissl fine granules -Cytoplasmic

RNA)

3. Moving of nucleus to periphery

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Changes in axon

1. Degeneration process

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a. Distal segment

? Swelling and fragmentation of axon &

branches called wallerian degeneration

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? Debris digested by Schwann cells and

tissue macrophages

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b. Proximal segment

? Degenerate til first node of Ranvier

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2.Regeneration process

? Schwann cells rapidly proliferate and forms

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parallel cords within basement membrane

? Endoneurial sheath and contained cords of

Schwann cells called band fiber

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? The band fiber extends from first node of Ranvier

in proximal segment up to end organ.

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? When there is gap in the injury site

Schwann cell wil form the codes and bridge

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the gap if only endoneurial tube is intact.



(In CNS microglial cells phagocytosed the

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debris and astrocytes form a scar and no

band fiber

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formation

? So regeneration of PNS depends on endoneurial

tubes and Schwann cells

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? Multiple sprouts arise from proximal axon and cross

the gap through the codes of Schwann cell and

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enter in to distal segment.


But only one filament wil persist

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and grows and reach the end organ.

? When axon reaches end organ Schwann

cells begin to lay down the myelin sheath.

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? It starts from injury site and spread distally.
? The time may be months to complete the

process depending on the severity of injury

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Recovery- reappearance of Nissl gran: due

to Protein synthesis

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Reduction of edema

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Repositioning of nucleus

Recovered peripheral nerve may not

be that efficient compare with the

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original nerve.

Why?

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1.Reduced conduction velocity



(Axon that reaches end organ wil have 80%

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original diameter)

2.Muscle control wil be less precise
(Innervation of more muscle fibers)

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No nerve regeneration in Central nervous

system as in PNS.
Why?

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1. Absence of endoneurial tubes
2. Failure of oligodendrocytes to serve as in

the same manner as schwann cells in PNS

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3. Laying down of scar tissue by active

astrocytes cells

4. Absence of nerve growth factors, or

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5. Production of nerve inhibitory factors in CNS

Changes of end organs supplies by the

nerve

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A. Loss of function

B. Denervation hypersensitivity

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(supersensitivity)

Loss of function

1. Skeletal muscles ? atrophy

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2. Sensory loss -cutaneous

3. Vasomotor-loss of sympathetic control

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impaired blood supply.

4. Sudomotor ?loss of sweating & skin become

dry

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5. Trophic changes

Local tissue changes due to:

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Nutrition/blood supply

Disuse & Loss of sensation

(E.g. in nail, bone)

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Denervation hypersensitivity

(Supersensitivity)

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Hypersensitivity of end organs supplied by

denervated nerve due to increase response to

neurotransmitter

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Denervation hypersensitivity can occur in;

? Skeletal muscles

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Fine irregular contraction of individual fibers

called fibril ation.

? smooth muscles

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? Exocrine glands except sweat glands



This is due to :

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1. Receptor up regulation in end Organs
2. Increase neurotransmitter levels at the site due to

reduce uptake by the nerve

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The factors that effect regeneration

? Type of injury
Gap between nerve ends

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Distance to cell body

(Worse with proximal injuries)

Damage to adjacent tissue

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Presence of foreign bodies
? Ischaemia &Infections
? Delay between injury and repair
? After care