nerve fibre.ppt 0
DESCRIPTION
pptTRANSCRIPT
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