THE ACTION POTENTIAL

Stimulating electrode: Introduces current that can depolarize or hyper-polarize Recording electrode: Records change in Potential of the membrane At a distance away

Resting Membrane Potential Membrane potential at which neuron membrane is at rest, ie does not fire action potential Membrane Potential (potential difference across the plasma membrane) at which the net flow of an ion type = zero The number of ions moving into the cell = the number of ions moving out of the cell for a particular species of ion Ionic Equilibrium Potential

Time (msec) Voltage (mVolts) along Y axis At Threshold Na influx equals K efflux

The Action Potential Oscilloscope to visualize an AP -Rising phase : rapid depolarization to reach the peak of 40mV -Overshoot : part where inside neurons are more positive than outside (> 0mV) -Falling phase : rapid repolarization -Undershoot : after-hyperpolarization

The Generation of Multiple Action Potentials Continuous depolarizing current injection can cause multiple action potential generation

The Generation of Multiple Action Potentials Firing frequency reflects the magnitude of the depolarizing current - One way that stimulation intensity is encoded There is a limit! -Maximum firing frequency ~ 1000 Hz -Absolute refractory period : time required to initiate the next AP once an AP is initiated ~ 1 msec -Relative refractory period : for a few miliseconds after the end of absolute refractory period, current needed to reach threshold is above normal

The Action Potential, In Reality Key Properties of the Action Potential Threshold Rising phase Overshoot Falling phase Undershoot Absolute refractory period -sodium channel deinactivation Relative refractory period - potassium channel closure (hyperpolarization)

Action Potential Conduction Propagation Depolarized to threshold Sodium channels open Influx of Na + Positive charges coming in depolarize the membrane just ahead to threshold Next population of sodium channels open

Action Potential Conduction Propagation of the action potential Orthodromic -Action potential travels in one direction - down axon to the axon terminal Antidromic (experimental) - Backward propagation is possible if the initiation of AP occurs in the middle of axon Cannot turn back on itself -Refractory (inactivated sodium channels) Typical conduction velocity: 10 m/sec

Six Characteristics of an Action Potential #1 Triggered by depolarization a less negative membrane potential that occurs transiently Understand depolarization, repolarization and hyperpolarization

#2 Threshold Threshold depolarization needed to trigger the action potential mV depolarization must occur to trigger action potential

#3 All or None Are all-or- none event Amplitude of AP is the same regardless of whether the depolarizing event was weak (+20mV) or strong (+40mV).

#4 No Change in Size Propagates without decrement along axon The shape (amplitude & time) of the action potential does not change as it travels along the axon

#5 Reverses Polarity At peak of action potential the membrane potential reverses polarity Becomes positive inside as predicted by the E na Called OVERSHOOT Return to membrane potential to a more negative potential than at rest Called UNDERSHOOT

#6 Refractory Period Absolute refractory period follows an action potential. Lasts 1 msec During this time another action potential CANNOT be fired even if there is a transient depolarization. Limits firing rate to 1000AP/sec