Action Potential

Summary of an action potential

• Graded potentials (IPSPs and EPSPs) enter trigger zone (axon hillock)

• If the sum of these graded potentials depolarizes the membrane potential greater than threshold voltage (-55mV) then…

• Voltage-gated Na+ channels open and Na+ enters neuron

• A wave of positive charge (an action potential) spreads along adjacent sections of axon by local current flow

• An action potential at one section of the axon causes voltage-gated channels at adjacent sections to depolarize to threshold, and so on

→ this is an example of positive feedback

→ Propagation is conduction of action potential along a non-myelinated axons

→ Saltatory conduction is conduction of action potential along myelinated axons (is faster than propagation)

• Efflux of K+ repolarized the membrane at each section

• The refractory period prevented backward conduction

Action Potentials: The all-or-none signals generated by neurons

that

Voltage gated Na+ channels have 3 states and 2 gates

1. At resting membrane potential, the channel is voltage sensitive because the activation gate is CLOSED and inactivation gate is open

Voltage gated Na+ channels have 3 states and 2 gates

2. When the neuron is stimulated past threshold, the activation gate OPENS

Voltage gated Na+ channels have 3 states and 2 gates

3. Small amount of Na+ influxes through channel and the neuron starts to depolarize

Voltage gated Na+ channels have 3 states and 2 gates

4. Intermediate step: inactivation gate closes (channel INACTIVATED) after ~0.5 msec

Voltage gated Na+ channels have 3 states and 2 gates

5. Then both gates return to their original positions (as seen in step 1) after a few milliseconds

So, these channels cycle through CLOSED, OPEN, and INACTIVATED states during the action potential

Voltage gated K+ channels have only ONE gate

• At resting membrane potential, their gate is closed

• When threshold is met, their gate starts to SLOWLY OPEN

• These channels are NOT FULLY OPEN until peak depolarization is reached (and Na+ channels are inactive)

• K+ channels then SLOWLY CLOSE during the repolarization and hyperpolarization phases

The second action potential will not fire during the absolute refractory period

• Na+ channels still inactivated, but not completely closed

• They must be completely re-set (closed) to be opened again

• K+ channels are slow to close, resulting in brief hyperpolarization

However, a second action potential CAN fire during the relative refractory period

• But only if the EPSP is higher than normal

The action potential!