Structure of a Synapse

The synapse is a gap between two neurons, which is filled with a chemical substance called as neurotransmitters. The mechanism by which neurons stimulate other neurons is called synaptic transmission. The entire junction is the synapse, the transmitting cell is the presynaptic neuron, the receiving cell is called the postsynaptic neuron and the space in between them is the synaptic cleft. The terminal button of the presynaptic neuron contains many synaptic vesicles full of the neuron’s chemical transmitter substance, called a neurotransmitter. The membrane of the postsynaptic neuron contains the synaptic receptor sites.

When an impulse in the axon of the presynaptic neuron reaches the terminal button, one or more of the synaptic vesicles release their contents, the neurotransmitter, into the synaptic cleft. After diffusing to the other side of the synaptic cleft, the neurotransmitter molecules attach to the receptor sites and stimulate the post synoptic neuron.

After the neurotransmitter has stimulated the postsynaptic neuron by attaching to its receptor sites, the molecules are ejected or fragmented by another chemical, the deactivating enzyme. The presynaptic neuron may reabsorb unused whole molecules of transmitter. The duration of the process from arrival of the impulse at the terminal button to stimulation of the post synaptic neuron is about 0.3 to 1.0 msec.

The chemical reactions that take place at the synapses are of a very precise kind. Only one kind of neurotransmitter can stimulate a given post synaptic receptor site and only one kind of receptor site can accept a given neurotransmitter.