In the nervous system, information flows from nerve cells, called neurons, to other nerve cells or non-neuronal cells by means of a specialized junction called a synapse. The components of a synapse include the presynaptic ending, the postsynaptic ending, and the synaptic cleft, a 20-nanometer (7.874015748 X 10-7 inch) gap between the cells. When a nerve impulse travels to the presynaptic ending, the presynaptic neuron releases small packets or vesicles of neurotransmitter chemicals into the synaptic cleft. The chemicals diffuse across the synaptic cleft and bind to the postsynaptic ending, upon which receptors for the signaling chemical reside. The nerve signal cannot proceed directly across the synaptic cleft; rather, the binding of the neurotransmitter to the postsynaptic cell creates an influx of sodium into the cell, which initiates another nerve signal.
There are several reasons for the synaptic cleft. First, the gap ensures that the information signal moves in only one direction. Second, the synaptic cleft allows one neuron to communicate with several postsynaptic cells, or conversely, several neurons to converge a signal to one postsynaptic cell. This allows the body to distribute a signal quickly or add them together to create a graded response. Finally, repetitive background stimuli, such as the contact between clothes and skin, may spend the supply of neurotransmitter in the presynaptic neuron, preventing passage of the unnecessary signal through the cleft.
Synapses contain one of two key neurotransmitters. In both the central and the peripheral nervous systems, acetylcholine serves as the main signal molecule in the synaptic cleft. Acetylcholine receptors activate skeletal muscle and various nerve cells in the parasympathetic nervous system. Noradrenalin, the other key neurotransmitter, primarily functions within the sympathetic nervous system, involved in mediating “fight or flight” reactions.
The human body contains five types of synapse, each with a different neuroreceptor and neurotransmitter in the synaptic cleft. Neuromuscular junctions between nerve cells and muscle cells stimulate the muscle and induce contraction of the muscle by the release of acetylcholine in the synaptic cleft. Excitatory and inhibitory ion channel synapses incite or impede a nerve impulse, respectively. Non-channel synapses result in the production of messenger chemicals within the postsynaptic cell. A rare synapse, called an electrical synapse, shares some proteins with adjacent cells by direct contact.
Drugs, both medicinal and recreational, produce their effects by switching synapses on or off within the human body. For example, amphetamines, ecstasy, cocaine, and caffeine all work by promoting the release of noradrenalin into synapses throughout the body, with a stimulant effect. On the other hand, alcohol, marijuana, and benzodiazepines, such as Valium, promote the release of gamma-aminobutyric acid (GABA), which has a tranquilizing effect. Drugs work by mimicking the action of neurotransmitters, blocking the binding of neurotransmitters, or the shutting down of ion channels normally affected by neurotransmitters.