action-potential by-dr-roomi

UNITS OF EXCITABILITY

1. RHEOBASE

2. UTILIZATION TIME

3. CHRONAXIE

RHEOBASE, UTILIZATION TIME, CHRONAXIE

• RHEOBASE: Voltage / strength of stimulus required just to excite the tissue, e.g., 1 mV.

• UTILIZATION TIME: The time for which Rheobase must be applied to excite the tissue.

• CHRONAXIE: A time for which a stimulus, double the rheobase when applied, just excites the tissue, e.g., 2 mV. (Chronological is from time).

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 chronaxie more excitability)

2. In nerve injury repair procedure We assess

the recovery by finding chronaxie of nerve affected & muscle affected. – Damage to nerve fiber is determined by measuring

chronaxie. – 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 in membrane potential, lasting for fraction of a second.

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

• Nerve impulse is being conducted along a nerve fiber = 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 or suprathreshold action potential is produced with its maximum amplitude, if subthreshold stimulus not produced at all).

6. Self propagating. (automatically propagated in both directions).

7. Has a refractory period. (when there won’t be response to 2nd 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 closed.

Ionic basis of action potential

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

closed – For fraction of m sec in presence of threshold stimulus, rapid sodium

influx 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 to stimulate.

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

• Cause: It is because when potential has reached resting value, some K channels are still open & K efflux continues membrane becomes more negative.

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

• Part of action potential between threshold value & beginning of after depolarization 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