Question

Describe the physiology of a neuronal impulse, including all relevant definitions and steps, to explain what happens for depolarization to occur and transmission of stimuli across a synapse.

Answer 1

Transmission of a stimuli across a synapse:

Synaptic transmission is the process by which information from presynaptic neuron passes to the post synaptic neuron through the synapse.

It occurs due to release of neurotransmitters from presynaptic nerve terminal that initiates action potential in the postsynaptic neuron.

Pre synaptic mechanisms:

1) Vesicles containing neurotransmitter molecules that are concentrated at active zones of presynaptic axon terminal undergo docking and priming.

Docking: Process by which vesicles attach with the membrane.

Priming: Process by which vesicles become ready to discharge their contents in response to a stimuli.

2) The action potential that arrives at presynaptic axon terminal depolarizes the presynaptic membrane.

3) Depolarization of membrane causes opening of voltage gated calcium channels that allows calcium to enter the axon terminal through active zone.

4) Increase in calcium concentration in presynaptic terminal increases calcium mediated exocytosis of vesicles. Calcium causes fusion of vesicles to presynaptic membrane by causing contraction of microfilaments that faciliates their movement, and then help to discharge their content into the cleft.

5) Transmitter is released into synaptic cleft in a quantized amount that diffuses passively across the cleft to post synaptic membrane.

Post synpatic mechanisms:

1) Neurotransmitter binds with receptors in the post synaptic membrane and brings about conformational change in the receptor that either opens an ion channel or triggers a cascade of biochemical reactions that generate a second messenger, which in turn generates change in ionic permeability of cell.

2) Some of the transmitter molecules diffuse away from the postsynaptic receptor that are cleaved either by enzymatic degradation or taken back into the presynaptic cell by endocytosis.

3) After the binding of the neurotransmitter with the receptors, the ion channels in the post synaptic membrane open up and movement of the ions takes place.

4) The membrane potential of post synaptic membrane changes either towards depolarization or hyperpolarization.

5) This change in membrane potential, also called synaptic potential, creates signal in post synaptic neuron.