6. 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.
7. 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
8.
9.
10. 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
11. 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.
12. 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.
13. 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.
14. 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.
17. 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.
18.
19. 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
20.
21. 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.
23.
24.
25. 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
26. 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
27. 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
28. 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)
29.
30. 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
31. 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
32. 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.
34.
35.
36. 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