Tuesday, March 20, 2012

Activation of the Postsynaptic Cell

Once released from the presynaptic axon terminal, neurotransmitters diffuse across the cleft. A fraction of these molecules bind to receptors on the plasma membrane of the postsynaptic cell. The activated receptors themselves may contain an ion channel, or they may act indirectly, via a G protein, on separate ion channels. In either case, the result of the binding of neurotransmitter to receptor is the opening or closing of specific ion channels in the postsynaptic plasma membrane, which eventually leads to functional changes in that neuron. These channels belong, therefore, to the class of ligand-gated channels whose function is controlled by receptors. Because of the sequence of events involved, there is a very brief synaptic delay—as short as 0.2 sec— between the arrival of an action potential at a presynaptic terminal and the membrane potential changes in the postsynaptic cell.

Neurotransmitter binding to the receptor is a transient event. As with any binding site, the bound ligand—in this case, the neurotransmitter—is in equilibrium with the unbound form. Thus, if the concentration of unbound neurotransmitter in the synaptic cleft is decreased, the number of occupied receptors will decrease. The ion channels in the postsynaptic membrane return to their resting state when the neurotransmitter is no longer bound. Unbound neurotransmitters are removed from the synaptic cleft when they (1) are actively transported back into the presynaptic axon terminal (in a process called reuptake) or, in some cases into nearby glial cells; (2) diffuse away from the receptor site; or (3) are enzymatically transformed into inactive substances, some of which are transported back into the axon terminal for reuse.

The two kinds of chemical synapses (excitatory and inhibitory) are differentiated by the effects of the neurotransmitter on the postsynaptic cell. Whether the effect is excitatory or inhibitory depends on the type of signal transduction mechanism brought into operation when the neurotransmitter binds to a receptor and on the type of channel the receptor influences.