Upon activation, some postsynaptic receptors cause excitation of the postsynaptic neuron, and others cause inhibition. The importance of having inhibitory, as well as excitatory, types of receptors is that this feature gives an additional dimension to nervous function, allowing restraint of nervous action and excitation.
The different molecular and membrane mechanisms used by the different receptors to cause excitation or inhibition include the following.
Excitation
1. Opening of sodium channels to allow large numbers of positive electrical charges to flow to the interior of the postsynaptic cell. This action raises the intra cellular membrane potential in the positive direction up toward the threshold level for excitation. It is by far the most widely used means for causing excitation.
2. Depressed conduction through chloride or potassium channels, or both. This action decreases the diffusion of negatively charged chloride ions to the inside of the postsynaptic neuron or decreases the diffusion of positively charged potassium ions to the outside. In either instance, the effect is to make the internal membrane potential more positive than normal, which is excitatory.
3. Various changes in the internal metabolism of the postsynaptic neuron to excite cell activity or, in some instances, to increase the number of excitatory membrane receptors or decrease the number of inhibitory membrane receptors.
Inhibition
1. Opening of chloride ion channels through the post synaptic neuronal membrane. This action allows rapid diffusion of negatively charged chloride ions from outside the postsynaptic neuron to the inside, thereby carrying negative charges inward and increasing the negativity inside, which is inhibitory.
2. Increase in conductance of potassium ions out of the neuron. This action allows positive ions to diffuse to the exterior, which causes increased negativity inside the neuron; this is inhibitory.
3. Activation of receptor enzymes that inhibit cellular metabolic functions that increase the number of inhibitory synaptic receptors or decrease the number of excitatory receptors.
