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Postsynaptic elements and event by chemical synapses
Chemical synapses are biological junctions through which neurons signal can be exchanged to
each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses
allow neurons to form circuits within the central nervous system. They are crucial to the
biological computations that underlie perception and thought. They allow the nervous system
to connect to and control other systems of the body.
After the opening of the vesicles, their neurotransmitter contents are dumped into the synaptic
cleft, the narrow space between the membranes of the pre- and postsynaptic cells.
The neurotransmitter diffuses within the cleft. Some of it escapes, but some of it binds to
chemical receptor molecules located on the membrane of the postsynaptic cell, the opposite
side of the synaptic gap.
Receptors can respond in either of two general ways, and this response is the key step by which
the synaptic process affects the behavior of the postsynaptic cell.
First, the receptors may directly open ligand-gated ion channels in the postsynaptic cell
membrane, causing ions to enter or exit the cell and changing the local transmembrane potential.
The resulting change in voltage is called a postsynaptic potential. In general, the result is
excitatory in the case of depolarizing currents, and inhibitory in the case of hyperpolarizing
currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel
conduct the postsynaptic current(s), which in turn is a function of the type of receptors and
neurotransmitter employed at the synapse.
The second way a receptor can affect membrane potential is by modulating the production of
chemical messengers inside the postsynaptic neuron. These second messengers can then
amplify the inhibitory or excitatory response to neurotransmitters.
After a neurotransmitter molecule binds to a receptor molecule, it must be removed to allow for
the postsynaptic membrane to continue to relay subsequent EPSPs and/or IPSPs (excitatory
and/or inhibitory postsynaptic potencial). This removal can happen through one or more
processes:
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The neurotransmitter may diffuse away due to thermally-induced oscillations/vibrations
of both it and the receptor, making it available to be broken down metabolically outside
the neuron or to be reabsorbed.
Enzymes within the subsynaptic membrane may inactivate/metabolize(break down) the
neurotransmitter.
Reuptake pumps may actively pump the neurotransmitter back into the presynaptic axon
terminal for reprocessing and repackaging for future release.