Nerve fibers are classified based on their structure, distribution, origin and function. They can be myelinated or unmyelinated. When injured, the distal portion undergoes Wallerian degeneration over 3 months as the axon and myelin sheath break down. The cell body shows chromatolysis. Regeneration is possible if the nerve ends are close together. New axonal growth occurs rapidly, entering the distal stump at 3-4mm/day. Myelination resumes over a year. Though anatomy recovers, full function returns slowly.

1. CLASSIFICATION &
PROPERTIES OF NERVE
FIBERS
DR. NABEEL BEERAN
MBBS, MD
DEPARTMENT OF PHYSIOLOGY, YMC
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2. CLASSIFICATION OF NERVE FIBERS
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3. Depending upon structure :
 Myelinated Nerve Fibres
 Un Myelinated Nerve Fibres
Depending upon distribution:
 Somatic- supply skeletal muscles
 Visceral /autonomic nerve fiber
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4. Depending upon origin:
 Cranial nerves
 Spinal nerves
Depending upon function:
 Motor nerve fibers
 Sensory nerve fibers
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5. Erlanger and Gasser classification
According to diameter & conduction velocity of impulse
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6. Numerical classification of sensory fibers
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7. Physio - clinical classification
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8. 8

9. PROPERTIES OF NERVE FIBERS
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10. 10

11. PROPERTIES OF NERVE FIBER
1. EXCITABILITY
2. CONDUCTIVITY
3. ALL OR NONE LAW
4. REFRACTORY PERIOD
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12. EXCITABILITY
It’s the ability of a cell to produce
action potential in response to a
stimulus.
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13. Stimulus –
A change in environment which
brings about a change in
potential across a membrane in
an excitable tissue
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14. Types of stimuli
 Electrical
 Chemical
 Thermal
 Mechanical
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15. Depending on the strength of stimulus
 Minimal (threshold)
 Subliminal
 Sub-maximal
 Maximal
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16. STRENGTH-DURATION CURVE
TIME
UTILISATION TIME
STRENGTH
RHEOBASE
2 X RHEOBASE
CHRONAXIE
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17. RHEOBASE- minimum strength (voltage)
of stimulus which can excite the tissue,
whatever may be the duration of the
stimulus
UTILIZATION TIME- Minimum time
required for a rheobasic current (threshold
strength) to excite the tissue.
CHRONAXIE- Minimum time at which the
stimulus with double the rheobasic
strength (Voltage) can excite the tissue.
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18. Chronaxie
 Is an index of excitability of a tissue
 Can be used to compare the excitability
of the various tissues
Longer the chronaxie lesser is the
excitability
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19. Factors affecting Excitability
1) Temperature
2) Mechanical pressure
3) Blood supply
4) Chemicals- CO2 & narcotics
5) pH- increased excitability in alkaline and
reduced excitability in acidic media.
6) Ions- Na+, Mg++ and K+ are neuro-
excitatory and Ca++ is neurosedative
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20. Nerve and muscle are excitable
Capable of self-generation of
electrochemical impulses at
their membrane
Transmit signals along the
membrane
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21. CONDUCTIVITY
 Ability to conduct an impulse is called
Conductivity.
TWO TYPES:
 Orthodromic conduction
 Antidromic conduction
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22. Factors affecting Conductivity
 Temperature
 Mechanical pressure
 Blood supply
 Chemicals
 pH
 Ions
 Size of the nerve
 Myelination
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23. 23

24. ALL OR NONE RESPONSE
A single nerve fiber obeys “all or none law”
When stimulus of sub-threshold intensity is
applied to the axon , then no action potential is
produced (NONE RESPONSE)
A response in the spike of action potential is
observed when the stimulus if of threshold intensity
There occurs no increase in the magnitude of
action potential when the strength of stimulus is
more than the threshold level (ALL RESPONSE) 24

25. REFRACTORY PERIOD
The period of time during
which an excitable cell
cannot generate another
action potential is called the
refractory period.
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26. 1) Absolute refractory period-
 it is the period during an action potential,
during which a second stimulus can’t
produce a second response.
 period of action potential from firing level
until repolarization is almost 1/3rd
complete (spike potential)
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27. 2) Relative refractory period-
 it is the period during an action
potential, during which a stimulus of
higher intensity can produce a second
response
 It extends FROM the end of absolute
refractory period to the start of after
depolarization phase of action potential
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28. 28

29. CONDUCTION OF IMPULSES IN THE
NERVE FIBRE
• Unmyelinated neurons undergo the
Continuous conduction of an AP.
• Myelinated neurons undergo Saltatory
conduction of an AP.
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30. Continuous Conduction
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31. 31

32. • There are two advantages of saltatory
conduction.
1.Energy is saved as sodium-potassium
pumps are only required at specific points
along the axon.
2. Conduction of an action potential is much
faster within a myelinated axon (around
120 m/sec as opposed to around 35 m/sec)
in unmyelinated ones.
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33. 33
Saltatory Conduction

34. Myelinated axons conduct
nerve impulse faster than non-
myelinated.
Conduction velocity is
proportional to diameter of
axon (larger  faster).
Myelination allows small
diameter axons to conduct
signals quickly.
.
Saltatory Conduction
Action potential jumps from
one node to the next
Speeds conduction velocity
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35. – one node of Ranvier depolarizes the next,
so that action potentials can skip between
nodes
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36. 36

37. A “Nerve impulse” consists
of two components:
a local potential
and
an action potential
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38. LOCAL RESPONSE
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39. GRADED POTENTIAL ACTION POTENTIAL
Produced due to application of
subthreshold stimulus
Produced due application of
threshold stimulus
It is a local response Propagative type of response
It is a graded response All or nothing response
It has no latent period It has a latent period
It has no refractory period It has a refractory period
Not affected by hypoxia,
anaesthesia
Not produced during hypoxia,
anaesthesia
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40. DEGENERATION
AND
REGENERATION
OF
NERVE FIBERS
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41. • When the axon of the neuron is injured,
a series of degenerative changes are
seen at three levels
In the axon distal to the site of injury
In the axon proximal to injury
In the cell body
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42. Common causes of injury
• Transections
• Crushing of nerve fibers
• Local injection of toxic substance
• Ischemia due to obstruction of blod flow
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43. SUNDERLAND had graded the injury of
• First degree injury- transient loss of
function (mild pressure)
• Second degree injury – severe nerve
damage with intact endometrial tube
• Third degree injury – endoneural tube is
degenerated
• Fourth degree injury- Fasciculi
disorganized
• Fifth degree injury- Nerve trunk is cut 43

44. • The degenerative changes which occur in the
part of axon distal to the site of injury are
referred as anterogade degeneration or
wallerian degeneration
• The degenerative changes occurring in the
neuron proximal to the injury are referred as
retrograde degeneration . These changes
takes place in the cell body and the axon
proximal to the injury
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45. Cell Body
Axon cut
ADRD
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46. • The degenerative changes
start within few hours of
injury and continue for about
3 months
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47. CHANGES IN THE PART OF THE AXON DISTAL
TO INJURY
• Axis cylinder becomes swollen and breaks
up into small pieces
• After few days, the broken pieces appear as
debris in the space occupied by the axis
cylinder
• The myelin sheath slowly disintegrates into
fat droplets (8th to 35 day)
• Neurilemmal sheath is unaffected 47

48. • Schwann cell multiply rapidly
• The macrophages remove the debris of
axis cylinder and the fat droplets of
disintegrated myelin sheath
• Neurilemmal tube becomes empty
• Changes takes place for about 2 months
from day of injury
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49. Changes in the action
potential and the ability of
the nerve to conduct an
impulse decreases
markedly.
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50. Changes in the cell bodyCHANGES IN THE CELL BODY OF THE
NEURON (start within 48 hours and
continue for 15-20 days)
 Nissl substances undergo disintegration
and dissolution (chromatolysis)
 Golgi apparatus, mitochondria and
neurofibrils are fragmented and
eventually disappear
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51. Changes in the cell body
Cell body draws in more fluid, enlarges
and become spherical
 Nucleus is displaced to the periphery ,
sometimes extruded out of the cell
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52. 52

53.  FACTORS AFFECTING REGENERATION
 1. Chances of regeneration are considerably
increased if the two cut ends are near each
other (gap does not exceed 3mm)
2. Presence of neurilemma is a must
3. Presence of nucleus I the neuron cell
body is also must
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54. REGENERATION IN
PERIPHERAL NERVES
 Recovery in the nerve cell body begins in 20days
and is completed in 80days.
 Changes in the distal stump and at site of injury.
 (i) Axis cylinder from the proximal stump elongates
and then grows out in all directions.
 (ii) Initial rate of growth 0.25mm/day but once it
enters the distal stump, it becomes 3-4mm/day.
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55. Regeneration of nerve
 First, pseudopodia like
extensions grow from
the proximal cut end of
the nerve
(regenerative sprouts)
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56. Regeneration of nerve
 Fibrils move towards
the distal cut end of
the nerve fiber
 Some of the fibers
enter the neurilemmal
tube of the distal end
 And form axis cylinder
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57.  In about 15 days, schwann cell starts laying down
myelin sheath round the successful fibril, which is
completed within one year.
 In the nerve cell body , first the nissl granules
appear followed by golgi apparatus
 cell looses excess fluid .
 The nucleus occupies the central portion
 Though anatomical regeneration occur the
functional recovery occurs after long period
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59. Thank you………
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