What is a nerve impulse?

All cells maintain a difference in concentration of metal ions between their inside surfaces and the liquid outside. The difference in concentrations of the metal ions leads to an electrochemical potential, this is the ability to generate a small electric current along a fine wire if the inner and outer liquids are connected by it. Nerve cells have enhanced this general feature so that the electrochemical potential can be rapidly changed and then reestablished and so that the change in potential can be propagated along the cell membrane. The cells have a sodium/potassium pump in the cell membrane which pushes Na⁺ ions out of the cell and brings K⁺ ions into the cell. This pumping action requires the use of ATP as an energy source.

 

Transmembrane proteins in the axon membrane form channels through which Na⁺ and K⁺ ions can diffuse along their concentration gradients. K⁺ can diffuse about 20 times faster than the Na⁺. As the pumps push Na⁺ out and K⁺ in the K⁺ leaks back out faster than the Na⁺ leaks back in. This leads to a build up of negative ions in the cytoplasm of the axon. The inside of the axon has a negative electrochemical potential of about -65 mv compared to the outside of the membrane. This is known as the resting potential. When the cell membrane is stimulated, the channels through which the Na⁺ is diffusing become easier to pass through and the Na⁺ move more rapidly back into the cell. This reduces the negative charge inside the cell as more positive ions flood in until the charge inside becomes positive, about +45 mv. The membrane is said to be depolarised at this time. During this time the sodium/potassium pump remains fully active. Once the charge inside the cell membrane has become positive the Na⁺channels change their permeability slowing down the entry of more Na⁺. Now the pumps are moving Na⁺ out of the cell faster than they can diffuse back in so the resting potential is re-established.

 

The period during which the recovery to a resting potential is taking place is referred to as the refractory period. There are two phases to this refractory period: absolute refractory period is a time of approximately 1 millisecond when a new action potential cannot be produced, relative refractory period is a time of approximately 10 ms during which an action potential can only be produced with abnormally strong stimulation of the axon membrane.

Propagation of the action potential:

The change in the charge inside the cell membrane is very localised and the adjacent area of membrane is still in the resting stage. This creates a local circuit between the action potential site and the adjacent resting potential site. The local circuit has the effect of increasing the permeability of the adjacent membrane section to Na⁺ by altering the Na⁺channel shape so that increased Na⁺diffusion into this section will depolarise the membrane causing a new action potential to occur at this point. In this way the action potential moves along the axon from one end to the other. It cannot move back on itself because the refractory period is occurring at the site it occupied before it changed position.

 

To increase the speed of nerve impulse transmission the nerve axon may be myelinated. Schwann cells grow around the axon and wrap themselves tightly around it to form an insulating layer.

 

Each Schwann cell is about 1 mm in length and the gaps between the cells, called Nodes of Ranvier are the only points at which the sodium/potassium pumps and diffusion channels are found. The action potentials can only form at these sites and the insulation provided by the Schwann cells extends the local circuit to its maximum working distance. Instead of the action potential moving slowly from point to point along the axon it will now ‘jump’ from node to node and consequently will travel faster. Such impulses travel at about 120 ms^-1 and it is called saltatory conduction.

 Other factors which increase nerve impulse transmission rates are temperature and increased axon diameter. Many simple organisms which have unmyelinated axons possess giant nerve cells which have much larger axon diameters than normal to increase speed of impulse transmission. The earthworm has these running from one end of the body to the other and they carry impulses which stimulate the escape response which needs to be as fast a response as possible.