Action potential (nerve fiber) Definition If adequate / threshold stimulus is applied to the excitable tissues, their membrane potential show series of changes mainly reversal of membrane potential (called depolarization phase) followed by its recovery (called repolarization phase). This is action potential.

-If stimulus is applied to the excitable tissues, -Membrane potential becomes less –ve or mem. Depolarizes (-90 mv towards zero) -If sub-threshold stimulus is applied, change is less than 15 mv (ex. -90 to -85 mv), it is graded or local potential. - But if threshold stimulus is applied, mem. Potential reaches to firing level (-75 mv) & AP is produced.

Phases & ionic basis of AP i) Depolarization -reversal of mem. Potential or change of mem. Potential from -90 mv to +35 mv - ionic basis or cause is opening of fast V.G. Na channels that result Na influx ii) Repolarization -recovery of mem. Potential or change of mem. Potential from + 35 mv back to – 90 mv - ionic basis or cause is opening of V.G. K channels that result K efflux & closure of V.G. Na channels

Depolarization -ionic basis or cause is - Na influx thro. Voltage gated Na channels Repolarization -ionic basis or cause is - K efflux thro. Voltage gated K channels

Resting / polarized state (-90mv) During Depolarization (-90 to + 35 mv) During Repolarization (+ 35 mv to -90 mv) Open slowly & Close slowly Open fast & Close fast

iii) After-Depolarization -It is slow phase of Repolarization -Cause is K efflux thro. V. G. K channels But rate of Repolarization is slow as gradient of K+ becomes less iv) After-Hypeolarization -It is Hyperpolarizing phase of AP -Cause is continuation of K efflux thro. V. G. K channels even after resting potential (-90 mv) -due to slow closure of K channels

1)RMP (Polarized S.) 2)Stimulus artifact 3)Latent period 4)Firing level 5)Depolarization 6)Repolarization 7)After-Depolarization 8)After-Hyperpolarization 9)Overshoot 10)Spike Potential 11)Refractory Period (absolute & relative)

v) Stimulus artifact It is deflection of baseline & mark point of stimulus Cause is- current leak from stimulating electrode to recording electrode. vi) Latent period Is time interval between stimulus & response Cause is time taken by AP to reach recording E from stimulating electrode. vii) Threshold / Firing level Is -75 mv at which VG fast Na channels start opening

viii) overshoot It is part of AP above 0 mv ix) Spike potential steep part of AP (fast D & R) x) Absolute Refectory period Is the period after stimulus during which excitable tissues do not response to next stimulus Cause-Inactivation of VG fast Na by inactivation gates Absolute R. P.- excitability is totally lost. Relative R.P.- excitability is reduced.

Absolute Refectory period Is the period from point of stimulation to 1/3 of repolarization excitability is totally lost. Relative R.P.- Is the period from 1/3 of repolarization to start of after-depolarization. excitability is reduced but very strong stimulus can produce AP.

Local PotentialAction Potential -Local Potential is non- propagated and graded response to stimulus. -It does not obey all or none law -It don ’ t show RP & LP -Example- 1) receptor P. 2) EPSP 3) End plate P. -AP is propagated without decrement in both direction till end of membrane. -AP obeys all or none law -AP show RP & LP -Example- 1) ECG 2) EMG