Action potential step by step (E1)

Threshold

voltage-gated Na+ channels open

Rising phase

Na+ influx

Overshoot

membrane potential approaches ENa

Falling phase

voltage-gated Na+ channels inactivate (close), voltage-gated K+ channels open, K+ efflux

Undershoot

membrane potential approaches EK, voltage-gated K+ channels close

Absolute refractory period

Na+ channels remain inactivated for some time

Relative refractory period

Hyperpolarization moves the membrane away from threshold for some time

Triggering the Action Potential

Action potentials can be triggered by injecting electrical current through a microelectrode

Resting potential

restored by passive diffusion of K+, Na-K pump

Electrodes

can be used to stimulate individual neurons

Modest depolarizations

local potentials

Strong depolarizations

Action potential

Excitation

Activate Na+ channels -depolarization.

“Excitatory Post - Synaptic Potentials” = EPSP

Inhibition

Activate Cl– channels, Cl– enters cell and results in hyperpolarization, which is inhibitory (more

negative, like -90)

Open gated K+ channels, K+ leaves the cell, also results in hyperpolarization

IPSCs

spike initiation

axon hillock

How to speed up conduction

increase axon diameter

The axonal action potential:

Chain reaction moves along unmyelinated axon

 Na+ ions spread passively to nearby regions

 Changes the membrane potential to local threshold

 Opens more Na+ channels

In myelinated axons, regeneration of the action potential occurs …

only at nodes of Ranvier

 Na+ channels

Saltatory Conduction

Requires less energy and is faster

Saxitoxin,

blocks Na+ channels after it is ingested

 Conduction failure results in death by suffocation

Tetrodotoxin

Made by bacteria such as Alteromonas, Shewanella, and Vibrio species