nerve fibres

2.  Excitability
 Conductivity
 Unfatigability
 Refractive period
 All or none response
 Summation
 Accommodation

3.  Excitability:
› nerve fibres are highly excitable tissue
› respond to various stimuli
› Capable of generating electrical impulse
Conductivity:
› action potential is generated in the nerve fibre,
which is propagated along its entire length to the
axon terminal.

4.  Refractive period:
 during action potential the excitability of a
nerve become reduced
 i.e a new impulse cannot be generated
during a AP
 Types:
› a. Absolute refractory period (ARP)
› b. Relative refractory period( RRP)
 Note :- once initiated moving impulse
wont depolarize the area behind it

7.  Unfatiguability :
› Nerve fibres can not be fatigued even when they
are stimulated continuously.
All or none response:
› Either all of the action potential is seen or none
at all
› If a stimulus of threshold strength is applied AP
will be generated
› Further increase in strength of stimulus or
duration has no effect on amplitude of AP
› But can affect frequency

8.  Summation:
 › Application of a sub threshold
stimulus does not evoke an action
potential. However if sub threshold
stimuli are applied in rapid
succession they are added and they
produce an action potential.
Accommodation:
›
Application of continuous stimuli may
decrease the excitability of nerve fibre.

9. EXCITABILITY
 Covered in properties of AP
 Property of showing response when an adequate
stimulus is applied to a living cell. The response is in
form of AP.
 Cause: due to alteration in ionic movement disturbing
their equilibrium.

10. Electrical stimuli are commonly used for stimulation in
experimental work because they are:
- Easily applied.
- Accurately controlled as regard: strength & duration.
- Similar to the physiological process of excitation. So,
they cause no (or minimal) damage to the tissues &
can be repeated.

11. 2.conductivity
 Active process
 Discussed already in AP
 1.orthodromic: forward. From Initial ssegment to axon
terminal.
 2.antidromic:in reverse direction.from terminal to
Initial segment.
 seen in sensory nerve supplying blood vessel
 as synapse permits condn in one direction only, the
ADC dies when it is crossing 1st synapse.
 Both types occur when axon is stimulated in middle.

13. Factors affecting conduction
 1.temperature:
 2.diameter
 3.myelination
 4.drugs: blocking channels
 5.hypoxia: less oxygen

14. The chronaxia (time factor):
- It is the time needed to stimulate the tissue by a stimulus which
is double the rheobase.
- It is used:
a- to compare the excitability of different tissues.
b- to compare the excitability of the same tissue under different
conditions.
The shorter the chronaxia, the greater the excitability and vice
versa.

16. All or none response
 AP is all or none response to stimulus
 Sub threshold stimulus – no response( AP)
( none response)
 Threshold stimulus – spike potential
 Supra threshold stimulus : no increase in
magnitude of AP( all response)
Single nerve fiber
Single skeletal muscle f obey all or none law
Entire atrial
ventricular musculature obey all or none law

18. Refractory period
Period following AP during which 2nd
stimulus will not produce any response
AP can be divided into 3 periods
- refractory period
- super normal period
- sub normal period

22. Refractory period
 Types : absolute & relative
ARP : Absolute refractory period
- Short period following AP
- 2nd stimulus cannot evolve response
- Membrane completely loses excitability
- Extends from firing level to completion of
1/3rd of repolarization
- Neither fresh AP can be generated nor
propagated

23.  Ionic basis of ARP :
- DURING DEPOLARIZATION
- ‘m’ gates of sodium channels opened rapidly
- DURING EARLY REPOLARIZATION
- Na + channels closed due to inactivation of ‘h’
gates
- slow K+ channels are not yet opened
- Na+ channel gates donot open unless
potential comes back to resting level

24. Ionic basis of RRP
- Na+ channels are coming out of inactive stage
- Voltage gated K+ channels – still opened
- Stronger stimulus- open more Na+ channels
through ‘m’ gates
- AP elicited during RRP has lower upstroke velocity,
lower overshoot potential
- EFFECTIVE REFRACTORY PERIOD: ARP+ Early
part of RRP
- At the end of effective refractory period the cell
membrane is able to produce & propagate AP

25. ACCOMMODATION
DEF: adoptation of cell membrane to slowly increasing
strength of stimulus which produces no AP
- Quick threshold stimulus  produce AP
( square pulse stimulus )
- Slow threshold stimulus  produce no AP
( Saw tooth pulse stimulus)

27.  IONIC BASIS OF ACCOMMODATION:
-more & more Na+ channels open up
- Get inactivated after 1msec
- K+ channels remain open & restore membrane
potential
- Repolarizing forces overwhelm the depolarizing forces
so no AP is produced

28.  FACTORS INHIBITING EXCITABILITY OF CELL
MEMBRANE:
- High extracellular CA++ conc - ↓ Na+ permability to
inside
- Local anaesthetics –
-bind to activated Na+ gates
- inhibit Na+ channels
- myelinated fibers less sensitive

29. CONDUCTIVITY ( PROPAGATION OF
AP)
 In the form of wave of depolarization
 In the body – in one direction
 In experimental conditions – in either direction
 Conductivity in unmyelinated fibers
 Conductivity in myelinated fibers
 Orthodromic conduction
 Antidromic conduction
 Factors affecting conduction velocity

33. CONDUCTION VELOCITY
 FACTORS AFFECTING CONDUCTION VELOCITY :
 A) FACTORS ATTRIBUTABLE TO AP GENERATION
 1 TEMP
 2 LEVEL OF RMP
 3 LEVEL OF THRESHOLD POTENTIAL
( FIRING LEVEL )
 B) FACTORS ATTRIBUTABLE TO ELECTROTONIC
CONDUCTION
 1 AXON DIAMETER
 2 MYELINATION
 3 RESISTANCE OFFERED BY ECF