Setting up a nerve impulse

Nerves convey information rapidly from one part of the body to another, enabling animals to respond quickly to changes in their external and internal environments. The information is carried in the form of electrical signals called nerve impulses.Most of our understanding of the nature of nerve impulses comes from work done on giant axons of squids. These are the nerve fibres responsible for the rapid escape movements of squids. Their large diameter (up to 1 mm) makes it possible to measure the electrical activity in a giant axon when it is at rest and when it is conveying a nerve impulse.

A fine glass microelectrode is inserted inside an axon, and the voltage (potential difference; p.d.) between it and a reference electrode on the surface of the axon can be displayed on a cathode ray oscilloscope. By convention, the potential difference of the inside of the cell is always measured relative to that on the outside, so that the outside potential is taken as zero.

Resting potential

A resting neurone is so called because it does not convey a nerve impulse, not because it is inactive. On the contrary, a resting neurone expends much energy in maintaining a potential difference across its membrane. This is called the resting potentialand measures about -70 millivolts.

During the resting potential, the inside of the neurone is negative relative to the outside because of an unequal distribution of charged ions. On the outside, sodium ions (Na⁺), chloride ions (Cl^-), and calcium ions Ca²⁺) are present in higher concentrations than inside the cell. By contrast, the inside of the cell has a higher concentration of potassium ions (K⁺) and organic anions (negative ions).

This unequal distribution of ions results from a combination of active transport and diffusion of sodium and potassium ions across the cell membrane, and the inability of large organic anions to pass out of the cell. A sodium-potassium pumpactively transports sodium ions out of the neurone and potassium ions in. For every three sodium ions pumped out, only two potassium ions are pumped inwards. On its own, this would result in only a slight potential difference across the membrane. However, this difference is amplified by the membrane being about 50 times more permeable to potassium ions than to sodium ions. Potassium ions are able to diffuse freely back out of the cell down their concentration gradient, but the sodium ions diffuse back into the cell only very slowly. This creates a negative electrical charge inside compared with outside. Without active transport, an equilibrium would eventually be reached and there would be no potential difference across the membrane.

Action potential

A nerve impulse occurs when the resting potential across the membrane of a neurone has a sufficiently high stimulus. A stimulusis any disturbance in the external or internal environment which changes the potential difference across a membrane. The stimulus may be chemical, mechanical, thermal, or electrical, or it may be a change in light intensity.

The recording on the cathode ray oscilloscope shows the effects of a stimulus on a giant axon. When the stimulus is applied, the axon becomes depolarised;that is, the inside becomes temporarily less negative. If the stimulus is strong enough (if it exceeds the threshold level), an actionpotential occurs. There is a complete reversal of the charge across the nerve cell: the interior becomes positively charged relative to the outside. Typically, the action potential reaches a peak of about +35 millivolts. The potential difference then drops back down, undershoots the resting potential and finally returns to it. The return of the potential difference towards the resting potential is called repolarisation.The entire action potential takes about 7 milliseconds. Although this example refers specifically to a giant axon, its general features apply to all animal neurones.