EXCITABLE TISSUES 

Action potentials are large, fast complex changes in the membrane potential elicited by a large excitatory stimulus. Once initiated they travel over the surface of the cell and consequently affect the whole cell.

From the previous module we know that an excitatory stimulus produces a depolarising graded potential. To get an action potential this depolarising graded potential must be large enough to reach a value referred to as threshold (see opposite). Like the membrane potential, the value of the threshold varies from cell to cell but for simplicity we will imagine that it is -65 mV.

If the stimulus is too small (and the membrane potential does not reach threshold) then we simply get a depolarising graded potential.

If the stimulus is big enough so that the depolarising graded potential reaches threshold then we get an action potential.

The action potential has three characteristic phases that are named according to the direction that the membrane potential is moving during that time:

The depolarising phase is the period between threshold and the peak of the action potential. Note that the peak of the action potential is actually around +30 mV. This means that at this point the inside of the cell is positive compared to the outside.

The repolarising phase is the period between the peak of the action potential and the resting membrane potential.

Notice however that the action potential has a third phase where it becomes more negative before returning to the resting membrane potential. Because this type of movement is referred to as hyperpolarisation, we refer to thus undershoot in the action potential as the hyperpolarising phase.

At this stage it is probably worth highlighting a few of key differences between action potentials and graded potentials:

(i) Unlike graded potentials, action potentials are always the same size. We don’t get small action potentials and big action potentials (as is the case for graded potentials). We either get the whole thing or no action potential at all. If threshold is reached we get the whole sequence (depolarising, repolarising and hyperpolarising phases) of the action potential. If threshold is not reached we get no action potential. This property of action potentials is known as the all or none principle.

(ii) Whilst graded potentials may last tens of milliseconds, in most cells action potentials are over in a few milliseconds. So although action potentials are bigger than graded potentials they are very much shorter in duration.

(iii) One of the key elements about graded potentials is that they are localised changes in the membrane potential (i.e. they don’t affect the whole cell). Action potentials on the other hand travel along the length of cells and consequently affect the whole cell. The movement of action potentials is referred to as action potential propagation. We will come back to the mechanism of action potential propagation a little later in this lesson.

The speed of action potential propagation is fairly rapid and usually referred to as conduction velocity. Different cells have different conduction velocities but these are usually in the order of 0.5 - 130 m.sec^-1. The speed at which these electrical signals travel along cells is of course the secret to the very rapid form of communication that is enabled by excitable tissues. In fact it is action potentials travelling along excitable tissues that are responsible for every sensation, thought and movement we make because these large, fast changes in the membrane potential encode all the information that is communicated in nervous and muscle tissue.

But how do action potentials encode information? I have stressed that action potentials are always the same size, so how does the nervous system use action potentials to provide the information that indicates the magnitude of a sensation or force of contraction of a muscle? The key element here is that it is not the size of the action potential that encodes information but the frequency of action potentials. Frequency is usually expressed in Hertz (Hz) which means number of cycles per second. So when we talk about frequency in excitable tissues we are talking about the number of action potentials per second.

For example, humans are very good at distinguishing between very small changes in skin deformation. This is because we have neurones in our skin that are exquisitely sensitive to pressure applied to its surface. But how do these neurones encode the difference in skin deformation?

The answer is of course action potential frequency and we can illustrate this by recording from one of the neurones that in the skin of the finger that is activated by skin deformation. In the experiment on the right we are recording the membrane potential of one such neurone (red) in response to a skin indentation produced by a blunt probe. The magnitude and duration of the skin indentation is shown (black) and the neurone response in blue. (Note that because of the duration of the experiment, each action potential appears as a straight line). As the probe is advanced we see a series of action potentials in the neurone that last as long as the skin deformation and have a fairly consistent frequency of action potentials. If we repeat the skin deformation but increase the amount of skin deformation (by increasing the force applied to the probe) then we see a higher frequency of action potentials. So in this example the higher stimulus intensity is encoded by a higher frequency of action potentials.

So it is not the size of the action potentials that the nervous system uses to encode information, but the frequency (i.e. number of action potentials per second). This is an important concept in excitable tissues and is referred to as frequency coding.

Action potentials are integral to the way in which excitable tissues work. Throughout this semester (and next) we will return again and again to these important elements to see how they enable communication in the nervous system, musculature and other excitable tissues. We will consider what actually causes action potentials towards the end of this lesson.