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Define synapse and describe its features.
The junction between two neurons across which the nerve impulse passes from one neuron to another is called synapse. There is no actual contact between the neurons, they are separated by a narrow gap called synaptic cleft. The characteristic features of synapse are:
- There is no physical contact between two neurons.
- A small gap, the synaptic cleft exists between the presynaptic & the postsynaptic neuron It is 20-30nm wide.
- The plasma membrane of the neurons opposed to each other are called presynaptic and postsynaptic membranes.
- Presynaptic axon ends in a bulb-like structure (boutons terminaux).
- The end bulb of presynaptic neuron contains synaptic vesicles that release neurotransmitter in the synaptic cleft.
- Post synaptic membrane has receptors with which the neurotransmitter binds.
Describe the anatomy of transmission of impulse across the synapse.
- The arrival of the nerve impulse at the presynaptic terminal, causes the synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.
- The neurotransmitter diffusing across the synaptic cleft and binds to receptors on the post synaptic membrane.
- It leads to the generation of a nerve impulse, called action potential in the postsynaptic neuron.
- Later the neurotransmitter molecules are deactivated by enzymes in the synaptic cleft.
Classify Synapses.
Anatomically, based on the parts of the neurons that participate in formation of synapse and also the direction in which the transmission of nerve impulse occurs, the synapses are classified into following types:
- Axo-dendritic (commonest)
- Axo-somatic
- Axo-axonic
- Dendro-dendritic
- Dendro-somatic
- Dendro-axonic
