Ionophore component
There are two types of ionophores- Ion channels (chemically activated) and Second messenger system (enzymatically activated internal metabolic system).
1. Ion channels (chemically activated)
Chemically activated ion channels (ligand-activated channels) are of three types-
Sodium channels
Allow mainly Na+ (Some K+ as well)
Potassium channels
Allow mainly K+
Chloride channels
Allow Cl– and few other anions to pass through. The transmitter that selectively open the Na+ channels excites the post synaptic neuron and called as excitatory transmitters. Those, open K+ and Cl– separately or both of these channels together inhibit the neuron, and referred to as inhibitory transmitters.
2. Second messenger system (enzymatically activated internal metabolic system)
Enzymatically activated internal metabolic system type of receptor causes, in the post synaptic cell – Activation of cellular genes and such genes cause manufacture of additional receptors for the post synaptic membrane and Activation of protein kinases and effects disintegration of receptors and their reduction in number in the post synaptic membrane. These effects on the receptor number of the post synaptic membrane can alter the reactivity of the synapse for minutes, days, months or even years. The transmitters that act on the enzymatically activated system of the cell is called as modulators. The modulators are important in operating memory processes.
The duration of action and fate of the transmitters
- When either an excitatory or inhibitory transmitter is released into the synaptic cleft and excite or inhibit the post synaptic receptors by opening the specific chemically activated ion channels and these channels remained open for only 1 to 2 milliseconds. The reason for these channels be opened through a very short duration is that the transmitter agent is rapidly removed from the cleft in the following ways :
- By diffusion of the transmitter out of the cleft, into the surrounding fluid.
- By enzymatic destruction, within the cleft itself.
- By transmitter re-uptake by the pre-synaptic knobs.
The extent to which each of these methods of removal is operating is different for each type of transmitter.