Question

What is a synapse? How does the nerve impulse across the synapse?

Answer 1

Synapse is the junction between two neurons. The major function of the synapse is to transmit the impulses, i.e. action potential from one neuron to another neuron.

Complete answer:
Synapse is formed by the axon of one neuron ending on the cell body, dendrite, or axon of the next neuron. The functional classification of a synapse is on the basis of the mode of impulse transmission.

According to this, the synapse is classified into two categories:

Electrical synapse: Electrical Synapse is the synapse in which the physiological continuity between the presynaptic and the postsynaptic neurons is provided by a gap junction between the two neurons.
Chemical synapse: Chemical synapse is the junction between two nerve fibers through which the impulses are transmitted by the release of a chemical transmitter.

Generally, synapse refers to the chemical synapse.
The neuron from which the axon arises is called the presynaptic neuron. The neuron on which the axon ends (dendrites of another neuron) is called the postsynaptic neurons. A postsynaptic neuron contains some receptor proteins. Axon of the presynaptic neuron divides into small branches before forming the synapse. These branches are known as axon terminals. Some of the terminals are enlarged slightly like knobs called terminal knobs which contain neurotransmitters.

In the chemical synapse, there is the presence of a space called synaptic cleft between the two neurons. Action potential reaching the presynaptic terminal causes the release of neurotransmitter substances from the vesicles of this terminal. Neurotransmitter reaches the postsynaptic neuron through synaptic cleft and causes the production of the action potential.

Note: Transmission of impulse across electrical synapse: There is a direct exchange of ions between the two neurons through the gap junction. The action potential reaching the terminal portion of the presynaptic neuron directly enters the postsynaptic neuron. In an electrical synapse, the synaptic delay is very less because of the direct flow of current. This type of impulse transmission occurs in some tissues like the cardiac muscle fibers, smooth muscle fibers of the intestine, and the epithelial cells of the lens in the eye.