Action Potential

Are propagated changes in the transmembrane potential that, once initiated, affect an entire excitable membrane

Are also Action potentials which are electrical events

1. voltage gated sodium ions channels open usually at the initial segment of the axon
2.Na+ move into the axon and depolarizes adjacent sites
3. That triggers the opening of additional voltage gated channels
4. Causes chain reation down the surface of the membrane ultimately reaching the synapic terminals

Is the transmembrane potential at shich an action potential beigins
-on an axon typically -60mV to -55mV

Properties of the action potential are independent of the relative strength of the depolarizing stimulus, as long as the stimulus exceeds the threshold
-applies to all excitable membranes

Starting at the resting state-(activation gates of the V-gated Na+ channels are closed)
1.depolarization to threshold
2.activation of Na+ channels and rapid depolarization
3.inactivation of Na+ channels and activation of K+ channels
4.Closing of K+ channels

During an action potential the membrane does not respond normally to additional depolarizing stimuli from the time an action potential begins until the normal restiong potential has stabilized.

Absolute refractory period- lasts .4-1.0 msec.
Relative refractory period- begins when the Na+ channels regain their normal resiting condition and continues until resting levels

J

-actioin potential is relayed from one location to another in a series of steps

Continuos Propogation- unmyilinated axons
Saltatory Propogation-myilinated axons

1.action potential begins at the axon's initial segment and the transmembrane potential becomes more positive than negative momentarily
2. A local current then developes as Na+ begins moving in the cytosol and the extracellular fluid
3.The process then continues in a chain reaction

In the CNS and PNS carries potentials along the axon much mor rapidly than contiuous popatation due in internodes and nodes

May be electrical with direct physical contat between the cells or may be chemical involving NT