Neorotransmitters are synthesized in the cell body and transported in vesicles to the axon endings by microtubules. Electrical impulses arriving at the axon ending cause the release of the neurotransmitters into the synapse, as shown in the diagram on the left. The neurotransmitter travels quickly across the gap and binds with molecules (receptors) found on the surface of the neighbouring cell. The neurotransmitters have a specific shape that fit perfectly onto the receptors. Once the receptors are occupied they cause a change in the membrane of the cell which generates an electrical signal that travels along the neighbouring cell. The neurotransmitter is then quickly removed. Nerve activity therefore can be considered as a repeated pattern of electrical-chemical signalling.
There are many neurotransmitters active in the brain. Each neuron uses only one or two of the main neurotransmitters.
Once the neurotransmitter is released into the synapse and activates the signal, it is either reabsorbed into the nerve cell or destroyed by special chemicals. If the neurotransmitter is allowed to persist in the synapse, constant excitation of the nerve takes place. During muscle contraction a single nerve impulse results in one muscle contraction. If prolonged muscle contraction is needed, repeated electrical impulses arrive at the nerve endings and neurotransmitter is constantly released into the synapse.
As we shall see on the next page, some drugs can interfere with the action of neurotransmitter. Curare was a poison used by South American Indians to kill animals. The poison was placed on the tips of arrows and would cause paralysis. Curare actually blocked the receptors that lined the membrane of the muscle cells. When neutransmitter arrived at the synapse it could not bind to the receptors and cause the muscle to contract. As no communication could reach the muscles the animal was totally paralysed.