EXCITABLE TISSUES: Graded Potentials

The second type of membrane potentials that are important in excitable tissues are what are collectively referred to as graded potentials. These potentials are observed wherever an excitable tissue cell is subjected to an excitatory or inhibitory stimulus. The nature of these stimuli need not concern us right now but they usually come from other excitable tissue cells or from the environment.

Graded potentials are relatively small changes in the membrane potential (in the order of 1-30 mV) and are fairly transient (only lasting 10’s of ms).

The term graded means that the size of the potential is not consistent but is directly proportional to the size of the stimulus. Thus the larger the stimulus the bigger the graded potential is.

As is shown in the animation opposite, graded potentials don’t affect the whole cell but only the part of the cell surrounding the site of the stimulus (i.e. they produce local, not global effects).

Animation showing localised effects of excitatory and inhibitoy stimuli on a cell.

There are actually two major types of graded potential and these are named according to the direction that the membrane moves in response to the stimulus:

Depolarising Graded Potentials

Depolarising graded potentials are produced by an excitatory stimulus applied to the cell. As you can see from the figure opposite shortly after an excitatory (+) stimulus is applied to the cell, we observe a transient depolarisation of the membrane.

Hyperpolarising Graded Potentials

Hyperpolarising Graded Potentials are produced by an inhibitory stimulus applied to the cell. In the figure opposite you can see that this inhibitory (-) stimulus causes a transient increase in the membrane potential (i.e. it becomes more negative than the RMP) or hyperpolarisation.

Animation showing effcets of excitatory and inhibitoy stimuli on the membrane potential.

We will consider how some forms of graded potentials are generated in the lesson on synaptic transmission.