Question

Describe how presynaptic inhibition and presynaptic facilitation occur, and how they affect the post-synaptic neuron.

Describe how presynaptic inhibition and presynaptic facilitation occur, and how they affect the post-synaptic neuron.

Homework Answers

Answer #1

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:-

  1. 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.
  2. 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:-

  1. 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.
  2. 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.
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