Synaptic Transmission
The process by which neurons communicate across synapses using neurotransmitters, and how it is modulated and terminated.
Synapses are the junctions through which neurons communicate. At a chemical synapse, an action potential arriving at the presynaptic terminal opens voltage-gated calcium channels; the calcium influx triggers the fusion of synaptic vesicles with the membrane, releasing neurotransmitter molecules into the synaptic cleft, a gap of about 20 to 40 nanometers. The transmitters diffuse across and bind to receptors on the postsynaptic membrane, opening ion channels that produce a small excitatory (EPSP) or inhibitory (IPSP) postsynaptic potential.
The signal is then terminated so that the synapse can respond again: enzymes degrade neurotransmitters (acetylcholinesterase breaks down acetylcholine), and transporter proteins reuptake them into the presynaptic terminal or surrounding cells. This termination is why drugs that block reuptake — such as SSRIs for serotonin — amplify signaling.
A single neuron receives thousands of synaptic inputs, and its output depends on the sum: when the integrated potentials at the axon hillock reach threshold, the neuron fires. Synapses are not fixed relays: long-term potentiation (LTP) and long-term depression (LTD) strengthen or weaken specific connections, and these activity-dependent changes are the leading cellular model of learning and memory.
The major neurotransmitters — glutamate (the main excitatory transmitter), GABA (the main inhibitory one), acetylcholine, dopamine, serotonin, and norepinephrine — define pathways that are the targets of most psychiatric and neurological drugs: antipsychotics block dopamine receptors, benzodiazepines enhance GABA signaling, and nicotine and many toxins act on acetylcholine receptors. The synapse is thus both the elementary unit of neural computation and a primary site of pharmacological action, with a synaptic delay of about a millisecond setting the speed limit of neural circuits.
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neurons neuroscience neurotransmitters synapses
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