Functions of Synapse

FUNCTIONS OF SYNAPSE

Main function of the synapse is to transmit the impulses, i.e. action potential from one neuron to another. However, some of the synapses inhibit these impulses. So the impulses are not transmitted to the postsynaptic neuron.

On the basis of functions, synapses are divided into two types:

1. Excitatory synapses, which transmit the impulses (excitatory function)

2. Inhibitory synapses, which inhibit the transmission of impulses (inhibitory function).

Synapse classification and function


EXCITATORY FUNCTION

Excitatory Postsynaptic Potential

Excitatory postsynaptic potential (EPSP) is the nonpropagated electrical potential that develops during

the process of synaptic transmission. When the action potential reaches the presynaptic axon terminal, the voltagegated calcium channels at the presynaptic membrane are opened. Now, the calcium ions enter the axon terminal from ECF.

Calcium ions cause the release of neurotransmitter substance from the vesicles by means of exocytosis.

Neurotransmitter, which is excitatory in function (excitatory neurotransmitter) passes through presy from ECF enter the cell body of postsynaptic neuron. As the sodium ions are positively charged, resting

membrane potential inside the cell body is altered and mild depolarization develops. This type of mild

depolarization is called EPSP. It is a local potential (response) in the synapse.

Properties of EPSP

EPSP is confined only to the synapse. It is a graded potential. It is similar to receptor potential

and endplate potential.

EPSP has two properties:

1. It is nonpropagated

2. It does not obey allornone law.

Significance of EPSP

EPSP is not transmitted into the axon of postsynaptic neuron. However, it causes development of action

potential in the axon. When EPSP is strong enough, it causes the opening

of voltagegated sodium channels in the initial segment of axon. Now, due to the entrance of sodium ions, the depolarization occurs in the initial segment of axon and thus, the action potential develops. From here, the action potential spreads to other segment of the axon.

INHIBITORY FUNCTION

Inhibition of synaptic transmission is classified into five types:

1. Postsynaptic or direct inhibition

2. Presynaptic or indirect inhibition

3. Negative feedback or Renshaw cell inhibition

4. Feedforward inhibition

5. Reciprocal inhibition.

1. Postsynaptic or Direct Inhibition

Postsynaptic inhibition is the type of synaptic inhibition that occurs due to the release of an inhibitory neurotransmitter from presynaptic terminal instead of an excitatory neurotransmitter substance. It is also called direct inhibition. Inhibitory neurotransmitters are gammaaminobutyric acid (GABA), dopamine and glycine.

Action of GABA – development of inhibitory postsynaptic potential

Inhibitory postsynaptic potential (IPSP) is the electrical potential in the form of hyperpolarization that develops during postsynaptic inhibition. Inhibitory neurotransmitter substance acts on postsynaptic membrane by binding with receptor. Transmitterreceptor complex opens the ligandgated

potassium channels instead of sodium channels. Now, the potassium ions, which are available

in plenty in the cell body of postsynaptic neuron move to ECF. Simultaneously, chloride channels also open and chloride ions (which are more in ECF) move inside the cell body of postsynaptic neuron. The exit of potassium ions and influx of chloride ions cause more negativity inside, leading to hyperpolarization. Hyperpolarized state of the synapse inhibits synaptic transmission.

2. Presynaptic or Indirect Inhibition

Presynaptic inhibition occurs due to the failure of presynaptic axon terminal to release sufficient quantity

of excitatory neurotransmitter substance. It is also called indirect inhibition. channels. Now, the potassium ions, which are available in plenty in the cell body of postsynaptic neuron move to ECF. Simultaneously, chloride channels also open and chloride ions (which are more in ECF) move inside the cell body of postsynaptic neuron. The exit of potassium ions and influx of chloride ions cause more negativity inside,

leading to hyperpolarization. Hyperpolarized state of the synapse inhibits synaptic transmission.

2. Presynaptic or Indirect Inhibition

Presynaptic inhibition occurs due to the failure of presynaptic axon terminal to release sufficient quantity

of excitatory neurotransmitter substance. It is also called indirect inhibition.

from ECF enter the cell body of postsynaptic neuron. As the sodium ions are positively charged, resting

membrane potential inside the cell body is altered and mild depolarization develops. This type of mild

depolarization is called EPSP. It is a local potential (response) in the synapse.

Properties of EPSP

EPSP is confined only to the synapse. It is a graded potential. It is similar to receptor potential

and endplate potential.

EPSP has two properties:

1. It is nonpropagated

2. It does not obey allornone law.

Significance of EPSP

EPSP is not transmitted into the axon of postsynaptic neuron. However, it causes development of action

potential in the axon. When EPSP is strong enough, it causes the opening of voltagegated

sodium channels in the initial segment of axon. Now, due to the entrance of sodium ions, the depolarization occurs in the initial segment of axon and thus, the action potential develops. From here, the action potential spreads to other segment of the axon.

INHIBITORY FUNCTION

Inhibition of synaptic transmission is classified into five types:

1. Postsynaptic or direct inhibition

2. Presynaptic or indirect inhibition

3. Negative feedback or Renshaw cell inhibition

4. Feedforward inhibition

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