2nd lecture on action potential by Dr. Roomi

7/29/2019 2nd lecture on action potential by Dr. Roomi.

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UNITS OF EXCITABILITY

1. RHEOBASE

2. UTILIZATION TIME

3. CHRONAXIE



RHEOBASE, UTILIZATION TIME, CHRONAXIE

• RHEOBASE: Voltage /strength of stimulusrequired just to excite thetissue, e.g., 1 mV.

• UTILIZATION TIME: The timefor which Rheobase mustbe applied to excite thetissue.

• CHRONAXIE: A time forwhich a stimulus, double

the rheobase when applied, just excites the tissue, e.g.,2 mV. (Chronological is fromtime).



CLINICAL APPLICATION / SIGNIFICANCE OF

CHRONAXIE

1. A particular value of it for a particular tissue. – Type A-alpha nerve fiber has minimum value of chronaxie,

i.e. they are more excitable as compared to cardiac muscle.(less chronaxiemore excitability)

2. In nerve injury repair procedureWe assessthe recovery by finding chronaxie of nerve affected& muscle affected. –

Damage to nerve fiber is determined by measuringchronaxie.

– It improves with recovery.



Action potential

By

Dr. Mudassar Ali Roomi (MBBS, M. Phil)



ACTION POTENTIAL OF NERVE FIBER /

SKELETAL MUSCLE

• Defintion: it is an abrupt pulse like change inmembrane potential, lasting for fraction of asecond.

•During action potential there is reversal of potential. (inside +, outside -).

• Nerve impulse is being conducted along a nervefiber = action potential is being conducted.

• Depolarization = loss of negativity inside.

• Repolarization = return of negativity inside.



RMP -90 mV

Threshold -65 mV

0 mV

+35 to 40 mV

After Hyper-polarization

(Undershoot) / Sub-normal period ( -95 mV )

[K+ efflux continues, K+ channels remain

open for some time after RMP is reached].

Here tissue is difficult to be excited.

Depolarization Repolarization

[K+ efflux]

Time (msec)

(Overshoot)

Peak

Membrane Voltage(mV)

RMP -90 mV

[Rapid

Na+influx]

Complete

opening of

fast Na+ channels

Slow Repolarization/ K+accumulate

Excitable/ Super-normal period

After Depolarization

(70% of repolarization / start of

After-Depolarization)Spike potential

(First 1/3 of repolarization)

Relative Refractory period

Absolute Refractory period

[Na+ inactivation gates are still closed]

-65mV



Properties of action potential

1. Sudden / abrupt in onset.

2. Of limited magnitude / amplitude.

3. It goes to +35 to 40 mV & comes back. (biphasic)

4. Short duration (may be few millisec).

5. It obeys all or none law. (if a stimulus is threshold orsuprathreshold action potential is produced with itsmaximum amplitude, if subthreshold stimulus notproduced at all).

6. Self propagating. (automatically propagated in bothdirections).

7. Has a refractory period. (when there won’t be response to2nd stimulus if the fiber is already stimulated).



Ionic basis of action

potential



Remains closed at rest

Rapid sodium influx

for fraction of millisec

due to threshold

stimulus

Delayed closure of

inactivation gate

at the end of depolarization

Potassium gates closed at rest.

Repolarization due to activation of K channels,

+ ions move out repolarization.Only 1 gate for K on inside

K efflux. Threshold stimulus simultaneously

to Na channels, a slow change in K channels.

K channels will open when Na gates are



Ionic basis of action potential

•

Voltage gated Na+ channels: – At rest (-90mV) sodium activation gates on outside of membrane remainclosed

– For fraction of m sec in presence of threshold stimulus, rapid sodiuminflux takes place depolarization (-90mV to +35mV)

– There is delayed closure of inactivation gate on the inside.

• Voltage gated K+ channels: – Repolarization due to activation of potassium channels, +ions move out regain of negativity inside (repolarization)

– At rest potassium gates situated on inside are closed (-90mV)

– There is slow activation of potassium gates between +35mV to -90mV.

Rapid potassium efflux occurs. – Threshold stimulus causes a slow change in potassium channels. It will

open when sodium gates are closed.



threshold / firing / critical value: - 65 mV for sodium channels. It

causes change in activation gate of sodium channels at -65 mV,

complete opening of fast sodium channels.



• K+ becomes accumulated on outer side of membrane during later part of

repolarization, which slows down further K+ efflux after 70% of repolarization

slow repolarization is called after depolarization.

• Super normal period: During After depolarization, there is super normal period.

Tissue is most excitable. Here potential is – 65 mV, so small change is required tostimulate.



• When potential has reached the resting value, it does not stay there & becomesmore negative & called After hyperpolarization.

• Cause: It is because when potential has reached resting value, some K channels arestill open & K efflux continuesmembrane becomes more negative.

• Sub-normal period: During After hyper-polarization it occurs, because tissue isdifficult to be excited because potential becomes – 95 mV.

• Part of action potential between threshold value & beginning of afterdepolarization is called SPIKE POTENTIAL.



Electrotonic potential or

graded potential or localized

potential

Action potential

1. Proportional to stimulus strength(graded) Independent of stimulus strength(all or none)

2. Not propagated but decremental

with distance

Propagated, unchanged in magnitude

3. Exhibits summation Summation not possible

4. magnitude: low Magnitude: high

5. Refractory period: absent Refractory period: present

6. duration: upto 20 msec duration: upto 2 msec

7. examples: subthreshold potential, EPSP,

IPSP

Action potential in nerve fibers

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2nd lecture on action potential by Dr. Roomi