EXCITABLE TISSUES 

In an earlier module inthis lesson we saw that action potentials are large, fast, complex changes in the membrane potential that are the fundamental elements of communication in excitable tissues. In this section we will investigate the ionic basis of the action potential.

The action potential consists of three distinct phases and the easiest way to dissect the ion movements that are responsible for each phase is to analyse the changes that occur at three distinct points along its course (see diagram opposite).

1. Threshold

The diagram opposite highlights some of the key elements of excitable tissue cells that are important to the initiation of the action potential at threshold:

• With high concentration of Na⁺ outside cells and a low concentration inside the cell there is a concentration gradient that favours the movement of Na⁺ into the cell.
• At rest the inside of the cell is also negatively charged so there is an electrical gradient that attracts Na⁺ into the cell.
• Located in the membrane of excitable tissue cells we have a population of voltage-gated Na⁺ channels. These clever little channels are closed at the resting membrane potential but are programmed to open when the membrane potential reaches threshold.

When the membrane potential reaches threshold the voltage-gated Na⁺ channels open and the membrane suddenly becomes highly permeable to Na⁺. As a consequence, Na⁺ rushes into the cell along both its concentration and electrical gradients. This means that there is positive charge pouring into the cell and so not surprisingly the inside of the cell becomes less negative (i.e. the membrane depolarises).

This depolarisation in turn opens more voltage-gated Na⁺ channels and more and more Na⁺ enters the cell.

This positive feedback loop is responsible for the explosive nature of the depolarising phase of the action potential. Because the equilibrium potential for Na⁺ is around +56 mV, Na⁺ continues to flow into the cell even after the membrane potential reaches 0 mV and in fact continues on until it peaks at around +30 mV.

So the depolarising phase of the action potential is a direct consequence of Na⁺ influx.

2. Peak

As we approach the peak of the action potential the voltage-gated Na⁺ channels that are responsible for the depolarising phase of the action potential close.

In addition, a population of voltage-gated K⁺ channels begin to open. (These K⁺ channels are different from the passive K⁺ channels responsible for the resting membrane potential). Both the concentration and electrical gradients favours the movement of K⁺ out of the cell.

The net effect of these two events is that the influx of Na⁺ rapidly declines and there is a marked increase in K⁺ leaving the cell along its concentration and electrical gradients.

As a result of the declining influx of positive charge (carried by Na⁺) and an increase in the efflux of positive charge (carried by K⁺) the action potential peaks and then rapidly repolarises.

So the repolarising phase is a direct consequence of K⁺ efflux.

3. Hyperpolarising Phase

Towards the end of the action potential many cells exhibit a marked undershoot where the membrane potential is more negative than the resting membrane potential. This is known as the hyperpolarising phase.

The hyperpolarising phase is a consequence of the relatively slow closing of the voltage-gated K⁺ channels that are responsible for the repolarising phase of the action potential. Because these channels remain open in addition to the resting K⁺ channels that are responsible for the resting membrane potential, the permeability of the membrane to K⁺ is actually higher than it is at rest.

So it is the slow closing of voltage-gated K⁺ channels that is responsible for the hyperpolarising phase of the action potential.