Nerve fibre

Nerve fiber

Generalized Neuron

Classification of nerve fiber

Structure:

Myelinated nerve fibers

Non myelinated nerve fibers

Distribution:

Somatic-supply the muscles of the body

Visceral or autonomic supply varies internal organ.

Classification continue.,

Origin:

Cranial nerves arise from the brain

Spinal nerves arise from the spinal cord

Functional:

Motor carry motor impulses from the CNS

Sensory Carry sensory impulses towards higher centre.

I -a

Muscle spindle,annulospiral ending

I -b

Golgitendon organ

II

Muscle spindleflower spray endingTouch, pressure

III

Pain,temperature

IV

Pain and other receptors

Numerical classification

Number

Origin

Erlanger gasser classification

Fibertype

Functions

Diameter-mm

ConductionVelocity/M/se

A-

Proprioception somaticmotor

12 - 20

70 - 120

A-

Touch, Pressure

5 - 12

30-70

A-

Motor to spindle

3 - 6

15 - 30

A-

Pain,tem,touch

2 - 5

12 - 30

B

Preganglionic autonomic

3

3 - 15

C

Dorsal root pain-reflex

0.4-1.2

0.5-2

Sympathetic post gang

0.3-1.3

0.7-2.3

Nerve injury

Nerve injuries were classified by seddon in 1944

Types:

Neuropraxia

Axonotmesis

neurotmesis

Neuropraxia

Paralysis is incomplete

Temporary functional loss

Recovery is rapid and complete

No degenerative changes

Caused by pressure on the nerve

Eg.Saturday night paralysis

Susceptibility

Most

Intermediate

Least

Hypoxia

B

A

C

Pressure

A

B

C

Localanaesthetics

C

B

A

Neuropraxia

Axonotmesis

Axons are damaged

Surrounding connective tissue are intact

Distal segments undergoes wallerian degeneration

Functional recovery slow and complete

E.g. .crushinjury,traction and compression

Axonotmesis

Neurotmesis

Complete section of the nerve trunk

Loss of motor and sensory function

Possibility of recovery is remote due to loss of continuity of the nerve trunk

Neurotomesis

Wallerian degeneration

When a pheripheral nerve is cut, the part of the nerve separated from the cell body shows a series of chemical and physical degenerative changes wallerian degeneration.

Now wallerian degeneration includes changes in the distal part,changes in proximal part and changes in the cell body of the neuron.

Functional changes:

Decrease in the conduction velocity

Failure in the conduction of nerve impulse

Changes in cell body:

Starts after 48 hrs of nerve injury

First nissle granules disintegrates into fragments - chromotolysis

Golgi apparatus disintegrates

Cell body swells due to accumulation of fluid and becomes round.

Neurofibrils disappear

Nucleus is pushed to the periphery.

Wallerian degeneration cont.,

Changes in the nerve fiber distal part:

Axis cylinder swells, breaks into fragments, debris collects in the axis cylinder place

Myelin sheath slowly disintegrates into fat droplets

Neurilemma remains intact

Schwann cells proliferate rapidly

Macrophages remove debris of axis cylinder

Neurilemmal tube becomes empty (ghost tube)

Schwann cell cytoplasm fills the neurilemmal tube

Changes in the proximal part:

Same degenerative changes as in the distal part (anterograde degeneration)

Wallerian degeneration

Regeneration

Criteria for a nerve to regenerate:

Gap between the cut ends is less than 3 mm

The neurilemma is intact

The nucleus is intact

There should not be any block

The two cut ends remains in the same line

Regeneration continue

Schwann cells grow in all direction forming pseudopodia like fibrils

Some of the fibers enter into the peripheral stump

Schwann cells proliferate and form a continuous tube

Axis cylinder gets fully established

The fibers grow towards the degenerated muscle fibers due to some chemical attraction called neurotrophism

Schwann cells produce myelin sheath

Gradual increase in diameter of fiber occur

Nerve cell body nissle granules appear followed by Golgi apparatus

Cell loses excess fluid, nucleus comes to the centre

Anatomical regeneration occurs

Functional recovery takes long time

The rate of regeneration is 1-4mm/24hrs.

Neuroma