Question

Explain the difference between pre-synaptic inhibition and facilitation and physiologically how this happens.

Answer 1

1. PRESYNAPTIC INHIBITION :

– It is also called indirect inhibition.

– it occurs due to failure of presynaptic axon terminal to release sufficient quantity of excitatory neurotransmitteer substance

– this type of inhibition is mediated by axo axonal synapses. In spinal cord, a modulatory neuron called presynaptic inhibitiory neuron forms axo axonic synapse with presynaptic neuron.

–the inhibitory neuron inhibits the presynaptic neuron and decreases the magnitude of action potential in presynaptic neuron.

–the smaller action potential reduces the calcium influx. This in turn decreases the quantity of neurotransmitter released.

–so, the magnitude of EPSP in postsynaptic neuron is decreased resulting in synaptic inhibition.

2. POSTSYNAPTIC INHIBITION :

–It is also called direct inhibition

–it occurs due to release of inhibitory neurotransmitter.

– when inhibitory neurotransmitter acts on the post synaptic neuron

–as a result, opening of potassium channels occurs instead of sodium channels and there is efflux of potassium ions.

–along with it there is influx of chloride ions due to opening of chloride channels

–the infkux of chloride ions and efflux of potassium ions leads to more negativity inside leading to hyperpolarization.

– synaptic transmission does not occur in hyperpolarized state.