Nerve impulses are transmitted through a functional gap or intercellular space between neural cells (neurons) termed the synapse (also termed the synaptic gap). Although nerve impulses are conducted electrically within the neuron, in the synapse they are continued (propogated) via a special group of chemicals termed neurotransmitters.
The synapse is more properly described in structural terms as a synaptic cleft. The cleft is filled with extra cellular fluid and free neurotransmitters.
The neural synapse is bound by the presynaptic terminal end of one neuron, and the dendrite of the postsynaptic neuron. Neuromuscular synapses are created when neurons terminate on a muscle. Neuroglandular synapses occur when neurons terminate on a gland. The major types of neural synapses include axodendritic synapses, axosomatic synapses, and axoaxonic synapses—each corresponding to the termination point of the presynaptic neuron.
The arrival of an action potential (a moving wave of electrical changes resulting from rapid exchanges of ions across the neural cell membrane) at the presynaptic terminus of a neuron, expels synaptic vesicles into the synaptic gap.
The four major neurotransmitters found in synaptic vesicles are noradrenaline, actylcholine, dopamine, and serotoin. Acetylchomine is derived from acetic acid and is found in both the central nervous system and the peripheral nervous system. Dopamine, epinephrine, and norepinephrine are catecholamines derived from tyrosine. Dopamine, epinephrine, and norepinephrine are also found in both the central nervous system and the peripheral nervous systems. Serotonin and histamine neurotransmitters are indolamines that primarily function in the central nervous system. Other amino acids, including gama-aminobutyric acid (GABA), aspartate, glutamate, and glycine along with neuropeptides containing bound amino acids also serve as neurotransmitters. Specialized neuropeptides include tachykinins and endorphins (including enkephalins) that function as natural painkillers.
Neurotransmitters diffuse across the synaptic gap and bind to neurotransmitter specific receptor sites on the dendrites of the postsynaptic neurons. When neurotransmitters bind to the dendrites of neurons across the synaptic gap they can, depending on the specific neurotransmitter, type of neuron, and timing of binding, excite or inhibit postsynaptic neurons.
After binding, the neurotransmitter may be degraded by enzymes or be released back into the synaptic cleft where in some cases it is subject to reuptake by a presynaptic neuron.
A number of neurons may contribute neurotransmitter molecules to a synaptic space. Neural transmission across the synapse is rarely a one-to-one direct diffusion across a synapse that separates individual presynapticpostsynaptic neurons. Many neurons can converge on a postsynaptic neuron and, accordingly, presynaptic neurons are often able to affect the many other postsynaptic neurons. In some cases, one neuron may be able to communicate with hundreds of thousands of postsynaptic neurons through the synaptic gap.
Excitatory neurotransmitters work by causing ion shifts across the postsynaptic neural cell membrane. If sufficient excitatory neurotransmitter binds to dendrite receptors and the postsynaptic neuron is not in a refractory period, the postsynaptic neuron reaches threshold potential and fires off an electrical action potential that sweeps down the post synaptic neuron.
A summation of chemical neurotransmitters released from several presynaptic neurons can also excite or inhibit a particular postsynaptic neuron. Because neurotransmitters remain bound to their receptors for a time, excitation or inhibition can also result from an increased rate of release of neurotransmitter from the presynaptic neuron or delayed reuptake of neurotransmitter by the presynaptic neuron.
Bridge junctions composed of tubular proteins capable of carrying the action potential are found in the early embryo. During development, the bridges degrade and the synapses become the traditional chemical synapse.
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K. Lee Lerner