PHENOMENON OF EXCITABILITY

A. Response to brief Electrical Stimulus (Pulse)

1. Effect of brief depolarization (inside less negative)

Weak stimulus: Brief depolarization followed return to resting potential; due to ion movement, particularly K+ and Cl-

Stronger stimulus: Action potential; due to regenerative Na+ influx

B. Properties of the Action Potential (AP)

1. shape ("spike")

a. brief membrane depolarization
b. overshoots zero (membrane positive), usually
c. duration about 1 msec ( millisecond, 0.001 second)
d. may or may not undershoot resting potential (after potential) when AP is complete, but eventually returns to resting level

2. Threshold: minimum depolarization required to generate an action potential

subtresthold simulus -- below threshold, no action potential generated
superthreshold stimulus -- above threshold, results in normal action potential

3. All-or-None law

An action potential is an "event" that either occurs or does not occur. Once generated, it is independent of the initial stimulus

4. Refractory period

Immediately following an action potential, there is a period of reduced excitability; the reduced excitability dissipates with time and eventually, complete excitability is recovered

a. absolute refractory period: interval following an AP during which no further APs can be generated (order of 1 msec)

b. relative refractory period: interval following the absolute refractory period during which a 2nd AP can be generated only by a higher than normal stimulus (order of several msec); period of increased threshold