positive charge occurs on the outside of the membrane of a neuron while
a negative charge occurs on the inside of the membrane. This difference
is called the resting potential. The cell maintains this difference via
active transport. Propagation of the nerve message along the membrane
occurs when there is an action potential. This is the temporary change
in electrical charge of the membrane. The spreading of the action
potential after it occurs at a particular site on the membrane passes
the message along. After the message is passed along, the brief
refractory period occurs, when the membrane cannot generate another
action potential. Hence messages are transmitted only in one direction
along the membrane.
Before the action potential occurs, the outside of the cell membrane contains positive sodium ions and the inside contains positive potassium ions as well as other negative ions. Hence the outside has a higher positive charge than the inside. An action potential occurs when sodium ions move to the inside, causing the inside to have a greater positive charge than the outside. Potassium ions then move outside of the cell to restore higher positive charges on the outside, but not the original distribution. To restore this, the cell uses active transport to move the sodium ions back to the outside and potassium ions back to the inside.
Nerve impulses are transmitted from the synaptic terminals of a neuron (remember the bottom of the “pyramid”?) to the dendrites of another neuron (the apex of another “pyramid”) across a minute space, the synaptic cleft. Where the synaptic terminals of a neuron and a muscle fibre meet, a motor end plate is formed on the muscle fibre.
Small synaptic vesicles at the tip of the axon store neurotransmitters, which are used to pass a message across a synaptic cleft. Some vesicles move to the end of axons and release neurotransmitters into the cleft upon meeting an action potential. The neurotransmitters then bind to the dendrites of another neuron, either generating another action potential or stopping action potentials. Thus, various types of message are transmitted from one neuron to another and finally to a motor end plate of a muscle fibre, which then causes movement of the muscle.
[©Martin and Ambrose 2001]