Chapter 3.2

Action potentials are brief but large changes in membrane potential that originate in the axon hillock and are propagated along the axon.

Patterns of action potentials carry information to presynaptic membranes.

If the membrane reaches the threshold, about -40mV, it triggers an action potential at the axon hillock.

Once the threshold is reached, an action potential occurs and the membrane potential reverses and the inside of the cell becomes positive.

As you may recall, ions have been entering the dendrites, and causing graded potentials that depolarize and hyperpolarize the dendrites and the cell body (soma).

If the overall sum, of excitatory postsynaptic potentials (ESPS) and inhibitory postsynaptic potentials (ISPS), can depolarize the cell at the axon hillock, an action potential will occur.

Temporal summation is the summoning of potentials that arrive at the axon hillock at different times.

The closer together in time that they arrive, the greater the summation and possibility of an action potential.

Spatial summation is the summing of potentials that come from different parts of the cell.

If the overall sum, of EPSPs and IPSPs, can depolarize the cell at the axon hillock and action potential will occur.

Neurons calculate the overall postsynaptic input and indicate an action potential if depolarization exceeds the threshold and reaches its axon hillock.

Starting at the axon hillock, the cell membrane is also studded with voltage-gated ion channels.

Gated ion channels open and close in response to voltage changes, chemicals, or mechanical action.

Voltage-gated Na+ channels are the unique channels which open in response to the initial depolarization, which is when the membrane potential is -40 mV.

Sodium is drawn into the intracellular space due to the low concentration of it, as well as the overall negative charge inside.

The influx of Na+ continues until the membrane potential reaches the Na+ equilibrium potential of +40 mV.

The voltage-gated sodium channel closes when the intracellular space reaches +40mV.

At this time we begin the absolute refractory period, because we cannot initiate an action potential since there is already one in progress.

Extracellular space — the sodium has left and there is a low concentration of sodium and potassium, leaving the extracellular space 40 mV more negatively charged in the intracellular space.

Intracellular space — lots of sodium raises the electro static pressure of the intracellular space, saturated with sodium and potassium.

Voltage-gated K+ channels are the unique channels which open when the membrane potential is +40 mV.

When voltage-gated K+ channels open, K+ quickly exits the neuron due to electrostatic pressure and diffusion.

Voltage-gated potassium channel closes when the intracellular space reaches -40mV.

Once the potassium is mostly concentrated in the extracellular space, and the sodium in the intercellular space, the sodium potassium pump goes back to work.

The sodium potassium pump push sodium out of the neuron and draw potassium into the neuron.

The charge largely remains the same in the intracellular space; however, the distribution of ions is much different.

All-or-none property of action potentials: the neuron fires at full amplitude or not at all – does not reflect increase stimulus strength.

This is different from the graded local potentials which can be a various strengths.

Refractory period: only some stimuli can produce an action potential.

Absolute refractory phase: no action potentials are produced.

Relative refractory phase: only strong stimulation can produce an action potential.

Action potentials are regenerated along the axon—each adjacent section is depolarized any new action potential occurs.

Action potential travel in one direction because of the refractory state of the membrane after depolarization.

Myelin increases the magnitude of the energy in the action potential and it increases the speed as well.

Saltatory conduction: the action potential travels inside the axon and jumps from node to node.

This is quicker than if there was no myelin.