PRESYNAPTIC INHIBITION
This is also known as indirect inhibition as inhibitory
postsynaptic potential is not produced. The excitability of
postsynaptic cell is not diminished, whereas in postsynaptic
inhibition the IPSP reduces the effectiveness of all excitatory
input to a cell. Presynaptic inhibition occurs due to failure of
the release of excitatory neurotransmitter from the presynaptic
axon terminal. This occurs in axo-axonic synapses. there are two
mechanisms by which presynaptic release of neurotransmitter is
decreased:-
- By opening chloride channels of presynaptic terminal. the
inhibitory neurone releases and inhibitory neurotransmitter GABA
which binds to chloride channels on the presynaptic neurone
terminal. Increase in chloride permeability results in
hyperpolarization of the axon terminal. When an action potential
arrives the presynaptic terminal, the size of action potential is
reduced because of the increased chloride conductance. Because of
the smaller size of action potential, less calcium enters the nerve
terminal and thus the amount of excitatory neurotransmitter
released is markedly decreased.
- By activation of G protein. When the inhibitory transmitter
GABArelease from the inhibitory neuron binds to receptor called in
GABA receptor, it activates a G protein. The G protein aids in
reducing the amount of excitatory neurotransmitter released from
the presynaptic terminal by acting in one of the two ways:- either
by opening potassium channels or by directly blocking the calcium
channels.
POST SYNAPTIC INHIBITION
It means the inhibition of the postsynaptic membrane. It occurs
by following two mechanisms:-
- Direct postsynaptic inhibition by development of inhibitory
postsynaptic potential (IPSP). IPSP means hyperpolarization of
postsynaptic membrane is produced by the inhibitory
neurotransmitters released in the synaptic cleft. The most common
inhibitory neurotransmitter inside the CNS is GABA. The inhibitory
transmitter released at the synaptic cleft causes opening of either
potassium channels or chloride channels in the postsynaptic
membrane, leading to diffusion of a large number of K ions from the
neuron to the extracellular fluid or large number of Cl ions to
diffuse to the interior of the neuron. This causes post synaptic
membrane potential to become more negative ie
hyperpolarization.
- Postsynaptic inhibition due to refractory period. sometimes the
postsynaptic membrane can be refractory to the excitation because
it has just fired and is in its refractory period.