THE ACTION POTENTIALTHE ACTION POTENTIALTHE ACTION POTENTIALTHE ACTION POTENTIAL

Stimulating electrode:Introduces current that candepolarize or hyper-polarize

Recording electrode:Records change in Potential of the membraneAt a distance away

Resting Membrane PotentialResting 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 = zeroThe 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 RealityThe 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 ConductionAction 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 ConductionAction 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 PotentialSix Characteristics of an Action Potential

#1 Triggered by depolarization

• a less negative membrane potential that occurs transiently

• Understand depolarization, repolarization and hyperpolarization

#2 Threshold #2 Threshold

• Threshold depolarization needed to trigger the action potential

• 10-20 mV depolarization must occur to trigger action potential

#3 All or None#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#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 #5 Reverses Polarity

• At peak of action potential the membrane potential reverses polarity

• Becomes positive inside as predicted by the Ena Called OVERSHOOT

• Return to membrane potential to a more negative potential than at rest

• Called UNDERSHOOT

#6 Refractory Period#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