Nerves and Synapses
Nerves are made of special cells called neurones. They are capable of carrying electrical impulses from one part of the body to the other. Neurones can be very long, over a metre in length in some cases. They are wrapped in fatty cells called a myelin sheath which helps the neurones conduct their electrical impulses faster. There are different types of neurone.
The sensory neurone gathers information from the senses and pass it on to the central nervous system (CNS). It is attached to special receptor cells or in some cases the nerve's end is a receptor itself. When stimulated it carries an electrical impulse along its length, passed the cell body and down the axon to the nerve endings. It is here that the cell meets with another neurone (or neurone) at a junction called a synapse. The cell bodies of sensory neurones can all be found together in a nerve. This causes a swelling called a ganglion.
These neurones carry impulses away from the CNS towards effector organs like muscles or glands. These cells have very long axons at the end of which are motor end plates where the nerve cell can stimulate the effector organ.
The synapse is a junction where two or more nerve cells meet. The synapse allows the nerve cells to pass on their electrical impulse to another cell. The synapse is also a way of controlling the direction in which impulses travel. They can only travel one way through a synapse. When an impulse reaches the synaptic knob, it releases vesicles of a chemical called a neurotransmitter to be released into the synaptic cleft. They quickly diffuse across the gap and bind with receptors on the surface of the connecting neurone. When enough of the receptors have been filled an electrical impulse is triggered in this neurone and off it travels. Enzymes in the synaptic cleft quickly destroy the neurotransmitter so it is only short lived. There can be a number of nerves which meet at a particular synapse and so any neurone can be stimulated by a number of other neurones. Also, some neurotransmitters can have an inhibitory effect making neurones less likely to be stimulated. Whether a neurone is stimulated or not is decided by the balance of stimulating and inhibiting neurotransmitters.