First, the neurotransmitter must be synthesized and stored in vesicles so that when an action potential arrives at the nerve ending, the cell is ready to pass it along to the next neuron. Next, when an action potential does arrive at the terminal, the neurotransmitter must be quickly and efficiently released from the terminal and into the synaptic cleft. The neurotransmitter must then be recognized by selective receptors on the postsynaptic cell so that it can pass along the signal and initiate another action potential. Or, in some cases, the receptors act to block the signals of other neurons also connecting to that postsynaptic neuron. After its recognition by the receptor, the neurotransmitter must be inactivated so that it does not continually occupy the receptor sites of the postsynaptic cell.
Most neurotransmitters are specific for the kind of information that they are used to convey.
As a result, a certain neurotransmitter may be more highly concentrated in one area of the brain than it is in another. In addition, the same neurotransmitter may elicit a variety of different responses based on the type of tissue being targeted and which other neurotransmitters, if any, are co-released. The integral role of neurotransmitters on the normal functioning of the brain makes it clear to see how an imbalance in any one of these chemicals could very possibly have serious clinical implications for an individual. Whether due to genetics, drug use, the aging process, or other various causes, biological disfunction at any of the four steps of synaptic transmission often leads to such imbalances and is the ultimately source of conditions such as schizophrenia, Parkinson's disease, and Alzheimer's disease. The causes and characteristics of these conditions and others will be studied more closely are as we focus specifically on the four steps of synaptic transmission, and trace the actions of several important neurotransmitters.