PROPERTIES OF NERVE FIBRES.pptx

properties of nerve fibres

1. PROPERTIES OF NERVE FIBRES

2. PROPERTIES OF NERVE FIBRES 1. EXCITABILITY 2. ALL OR NONE RESPONSE 3. REFRACTORY PERIOD ABSOLUTE REFRACTORY PERIOD RELATIVE REFRACTORY PERIOD SUPERNORMAL PERIOD’ SUBNORMAL PERIOD 4. SUMMATION 5.ACCOMODATION 6.INFATIGUABILITY 7.CONDUCTIVITY

3. properties Excitability Conductivity All or none phen . Refractory period Accommodation Infatiguability summation

4. Excitability C o n d uctivity Unfa t ig a bility Ref r active period All o r no n e response Sum m ation A c commo d ation

5. Exci t ab i li t y: › 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.

6. Refracti ve per i od: 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. U n fa t i g ua b il i t y : › 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. Summat i on: › 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 1 st synapse. Both types occur when axon is stimulated in middle.

12. Factors affecting conduction 1.temperature: 2.diameter 3.myelination 4.drugs: blocking channels 5.hypoxia: less oxygsen

13. 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.

14. 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

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

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

17. 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

18. 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

19. 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)

20. 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

21. 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

22. 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

23. 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