Describe the types, functions & properties of nerve fibres 3.2.1 Classify nerve fibres 3.2.2 Classify nerve fibres based on the diameter & conduction velocity 3.2.3 Describe the salient features of Erlanger & Gasser classification of nerve fibres 3.2.4 State the functions of type A, B & C nerve fibres 3.2.5 Compare & contrast the numerical classification with the Erlanger & Gasser classification in the sensory nerve fibres

1. PandianM
Dept.ofPhysiology
DYPMCKOP
PROPERTIES OF
NERVE FIBRES - 1

2. PY 3.2
Describe the types,
functions & 3.2.1 Classify nerve fibres
properties of nerve
fibres
3.2.2 Classify nerve fibres based on the
diameter & conduction velocity
3.2.3 Describe the salient features of Erlanger
& Gasser
classification of nerve fibres
3.2.4 State the functions of type A, B & C
nerve fibres
3.2.5 Compare & contrast the numerical
classification with the
Erlanger & Gasser classification in the
sensory nerve fibres

3. Structure of nerve fibre

4. NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM PERIPHERAL NERVOUS SYSTEM
Brain Spinal Cord Visceral or Autonomic
nervous system
Somatic nervous system
Sympathetic
nervous system
Parasympathetic
nervous system
Classification of nerve
(supply skeletal muscles)
(supply internal organs)

5.  Depending upon functions:
 Sensory nerve fibers (afferent nerve fiber)
 Motor nerve fibers (efferent nerve fibers)
 Depending upon secretion of neurotransmitter
 Adrenergic nerve fibers
 Cholinergic nerve fibers
 Depending upon diameter and conductions of impulse
(Erlanger- gasser classification)
 Classified into three major groups:
 Type A nerve fibers
 Type B nerve fibers
 Type C nerve fibers

6. Erlanger and Gasser Classification
Fiber types Fiber Diameter
(µm)
Conduction
velocity (m/s)
Function
A 12-20 70-120 Somatic motor and
Proprioception
A 5-12 30-70 Touch-Pressure
A 3-6 15-30 Motor to muscle
spindle
A 2-5 12-30 Pain, Cold and Touch
B <3 3-15 Autonomic
preganglionic fibers
C-Dorsal root Fibers 0.4-1.2 0.5-2 Somatic sensations
C-sympathe tic fibers 0.3-1.3 0.7-2.3 Postganglionic
sympathetic fibers

7.  Among these fibers, type A nerve fibers are the thickest
fibers and type C nerve fibers are the
 thinnest fibers.
 Type C fibers are also known as Type IV fibers.
 Except type C fibers, all the nerve fibers are myelinated.
 Type A nerve fibers are divided into four types:
a. Type A alpha or Type I nerve fibers
b. Type A beta or Type II nerve fibers
c. Type A gamma nerve fibers
d. Type A delta or Type III nerve fibers.

8. Numerical Classification
Types Origin Fiber Types
Ia Primary spindle afferent from
from annulospiral endings
A
Ib From Golgi tendon organ A
II Secondary spindle afferent from flower spray
endings also from Touch and pressure receptors
A
III Pain and Cold receptors A
IV Pain and Temperature C fibers in
Dorsal root

9. PROPERTIES OF NERVE FIBRE
1. Excitability
Sub threshold stimulus
Threshold stimulus
Suprathreshold
2. Conductivity
3. Unfatigability
4. Refractory period
5. All or none response
6. Summation
7. Accommodation

10. Excitability
Excitability, i.e. the capability of generating
electrical impulses (action potential).
• Excitability is that property of the nerve fibre by virtue
of which it responds to changes in the external or
internal environments.
• (electrical impulses or the so-called action potentials)
• when it is stimulated by a suitable stimulus which may
be mechanical, thermal, chemical or electrical.

11. Skeletal muscle cannot contract without a
signal from the somatic nervous system
Signal arrives through a motor nerve,
communicated through a synapse called
myoneural or neuromuscular junction

12. ELECTROTONIC POTENTIAL & LOCAL
RESPONSE
When nerve fibre is stimulated by subthreshold stimulus the
A.P. is not produced but there do occur some changes in the
RMP. These local, non-propagated response called E.P.
Two types depending upon the nature-
1. Catelectrotonic potential- localised depolarizing changes in
RMP when subthreshold stimulus applied with cathode.
2.Anelectrotonic potential- localised hyper-polarizing
changes in RMP when subthreshold stimulus applied with
anode.

14. GRADED POTENTIAL
when potential changes with the subthreshold stimulus it is
called generator potential/ Graded potential.
Properties of G.P
.-
 It is local phenomenon.
 Increase in magnitude observed in stepwise with increase
in strength of stimuli.
 Decremental conduction- it decay progressively with time
& distance Depolarizing or Hyperpolarizing Summation
.

15. Factors affecting the excitability
Strength and duration (SD Curve) of the stimulus
Effect of extracellular Ca2+
Decrease in ECF Ca2+ ↑es excitability of neuron by
↓ing the Tp.
↑se in ECF Ca2+ stabilizes the membrane by ↓ ing
excitability.

16. What are threshold, subthreshold and
suprathreshold stimuli?
a) Threshold (minimal) stimulus : Minimum strength of
stimulus required to elicit a response is called as threshold
stimulus.
b) Subthreshold (subminimal) stimulus : A stimulus whose
strength is less than that of threshold stimulus is called
subthreshold stimulus.
c) Suprathreshold stimulus : A stimulus whose strength is
more than that of threshold stimulus is called suprathreshold
stimulus.

17. STRENGTH DURATION CURVE.

18. Define rheobase, utilization time and
chronaxie
Rheobase : Strength of current just sufficient to excite a
tissue when applied for indefinite period of time is called
Rheobase.
Utilization time : The minimum time required to elicit a
response with strength equal to Rheobase is known as
utilization time.
Chronaxie : The minimum time required to excite a tissue
when the current strength is twice a rheobase is called
chronaxie.

19. ALL or NONE RESPONSE.
When a stimulus of sub
threshold intensity is applied
then no AP is produced.
If threshold stimulus applied
response in the form of spike
AP.
If we increase strength of
stimulus more than threshold
no increase in magnitude of AP
is observed.

20. ALL OR NONE LAW

21. MEMBRANE EXCITABILITY
DURING ACTION POTENTIAL
Depending on
response to
stimulus, period of
AP is divided into
REFRACTORY
PERIOD (RP)
Absolute refractory
period (ARP)
Relative refractory
period (RRP)

22. REFRACTORY PERIOD
In this period at which the muscle or nerve does not show
any response to a stimulus.
It is because already one action potential is in progress in
the muscle during this period.
The muscle is unexcitable to further stimulation until it is
repolarized.
Refractory period is of two types.
1. Absolute refractory period
2. Relative refractory period

23. 1. Absolute Refractory Period (ARP)
ARP is the period during which the nerve or
muscle does not show any response at all,
From firing levels to 1/3rd of
Repolarization.
Whatever may be the (2nd stimulus) strength
of stimulus.

25. Refractory period

26. IONIC BASIS
1. During upstroke, m gates of Na+ channels
are opened rapidly &
2. During Repolarization channels are
closed by closure of inactivation gates (h)
gates of Na+ channels.
3. These channels will not reopens until
potential comes back to resting levels

27. 2. Relative Refractory Period
Relative refractory period is the period,
during which the muscle shows some response
if the strength of stimulus is suprathershold.

29. Ionic basis
During this Na+ channels are coming
out of inactivation stage & K +
channels are still opened.
Stronger stimulus open more Na +
channels through m gates & produce
response.

30. Refractory Period in Skeletal Muscle
In skeletal muscle, whole of the latent period is
refractory period.
The absolute refractory period falls during first
half of latent period (0.005 sec). And,
relative refractory period extends during second
half of latent period (0.005 sec). Totally, it is 0.01
sec.

31. Long Refractory Period in Cardiac
Muscle
In cardiac muscle, absolute refractory period
extends throughout contraction period (0.27 sec).
And,
Relative refractory period extends during diastole
period (about 0.26 sec).
 Totally it is about 0.53 sec.
Thus, the refractory period in cardiac muscle is
very long compared to that of skeletal muscle.

32. Significance of long refractory period in
cardiac muscle
Because of the long refractory period, cardiac
muscle does not show:
i. Complete summation of contractions
ii. Fatigue
iii. Tetanus.

33. SUMMATION
 When one subliminal stimulus is applied, it does not produce
any response in the nerve fiber because, the
 subliminal stimulus is very weak.
 However, if two or more subliminal stimuli are applied within a
short interval of about 0.5 millisecond, the response is
produced.
 It is because the subliminal stimuli are summed up together to
become strong enough to produce the response.
 This phenomenon is known as summation.

36. DIFFERENCE BETWEEN GP &
AP
Graded potential Action potential
Amplitude Proportion to strength of Once threshold stimulus is
reached amplitude remain same
Conduction Travel in decremental fashion,
gradually decrese with time &
distance
Conducted in all or none
Summation Can be summated Cannot be
Nature Can be a depolarize &
hyperpolarize
Always a large depolarize
Mechanism Due to opening of ligand gated
leaky ion channels
Due to opening of voltage
ion channels
Properties Does not have threshold or
refractory period
Have both
Example Receptor membrane potential in
sensory nerve endings
Motor End Plate potential
Action potential of nerve fibre ,
cardiac and skeletal muscle

37. ACCOMMODATION
1. While stimulating a nerve fiber
continuously,
2. the excitability of the nerve fiber is
greater in the beginning.
3. Later the response decreases slowly
and finally the nerve fiber does not
show any response at all.
4. This phenomenon of adaptation to the
stimuli is called accommodation

38. IONIC BASIS OF ACCOMMODATION.
Cause forAdaptation
When a nerve fiber is stimulated continuously,
Depolarization occurs continuously.
Continuous depolarization inactivates the sodium pump and
increases the efflux of potassium ions.

39. INFATIGUABILITY.
Nerve fibre cannot fatigued due to its
absolute refractory period.
1. Nerve fiber cannot be fatigued, even if it is stimulated
continuously for a long time.
2. The reason is that nerve fiber can conduct only one action
potential at a time.
3. At that time, it is completely refractory and does not
conduct another action potential.

40. Conductivity
Conductivity, i.e. the ability of propagating the
electrical impulses generated along the entire
length of nerve fibres.
In unmyelinated fibres
In myelinated fibres
Factor affecting velocity of conduction

41. AXOPLASMIC
TRANSPORT

42. Functions of Neuron

43. ZONES OF THE NEURON
From the functional point each neuron is divided
into four Zones
1. Receptor zone (dendritic zone)
2. Site of origin of conducted impulse
3. Zone of all or none transmission
4. Zone of secretion of transmitter (nerve
endings).

44. 1. Receptor zone (dendritic zone) is the region,
where local Potential changes are generated by
integration of the Synaptic connections.
2. Site of origin of conducted impulse is the site,
where Propagated AP are generated.
In case of Spinal motor neuron, initial segment
and in cutaneous Sensory neurons first node of
ranvier is the site of origin of conducted impulses.

45. One-way propagation of an action potential. For simplicity,
potentials are shown only on the upper membrane, local
currents are shown only on the inside of the membrane, &
repolarizing currents are not shown.
(a)Action potential is initiated in region 1, and local currents
depolarize region 2.

46. (a)Action potential in region 2 generates local currents;
region 3 is depolarized toward threshold, but region 1 is
refractory.

47. Action potential in region 3 generates local
currents, but region 2 is refractory.

49. 3. Zone of all or none transmission in the neuron
is the Axon.
4. Zone of secretion of transmitter (nerve
endings).
The Propagated impulses (action potential) to
nerve endings Cause the release of
neurotransmitter.

50. Functional organization of
neurons

51. Factor affecting conductivity
1. Temerature
2. RMP
3. Firing level or level of threshold potential
4. Diameter of the nerve
5. Myelination of nerve
6. Resistance offered by extracellular fluid
7. Concentration of ions

52. Recording Of A.P
.

53. MONOPHASIC- when one electrode is inside &
other is outside of cell

54. BIPHASIC- When both electrode out or
intracellular

55. COMPOUND A.P
.- in mixed nerve like a monophagic

58. With each AP very small difference in conc. of Na+ & K+ in ICF
& ECF.
Types of AP –
1 Spike potential- in nerve and skeletal muscle
2 Plateau potential – in cardiac muscle
3 Slow potential – in smooth muscle

59. Referred :-
 Text book of Medical Physiology
Guyton, 13th edition,
 Text book of Medical Physiology
Indu khurana,
 Text book of Medical Physiology
Vander’s
 Text book of Medical Physiology
Sembulingam &
LPR

60. Thank
You