Synapse: the junction between the axon terminal of one neuron and dendrites, cell body or axon of another neuron is called synapse. The neuromuscular junction is also known as synapse.

Synaptic knob: the swelling terminal of axon or dendrites is known as synaptic knob.

Pre-synaptic neuron: the neuron carrying impulse toward synapse

Post synaptic neuron: the neuron which receive the impulse and carry the impulse away from the synapse.

Two theories have been put forward to explain the conduction of nerve impulse across the synapse. They are;

1. Electrical transmission theory
2. Chemical transmission theory

Electrical transmission theory:

Impulse transmission through synapse is accomplished by electric current. When the impulse reaches the pre synaptic knob, the impulse itself act as stimulus for the post synaptic neuron causing depolarization. Now the action potential generate in second neuron.

Chemical transmission theory:

Nerve impulse are conducted across the synapse with the help of chemical substances called neurotransmitter. The process of chemical transmission was discovered by Henry (1936).

Mechanism

Figure: Nerve impulse transmission through Synapse

• When nerve impulse reaches the pre-synaptic knob, it depolarized the presynaptic membrane and causes the opening of voltage gated calcium channel.
• Diffusion of Ca++ ion in the presynaptic knob causes movement of synaptic vesicle to the surface of the knob. Synaptic vesicle carries the neurotransmitter.
• Synaptic vesicles then fused with the presynaptic membrane and get rupture to discharge its content ie. Neurotransmitter (Acetlcholine) into synaptic cleft.
• Synaptic vesicles then return to the cytoplasm of pre-synaptic knob for refilling.
• Some of the released neurotransmitter binds with the protein receptor present on the post synaptic membrane of another neuron and change the membrane potential.
• Other unbound neurotransmitter immediately get lost from the synaptic cleft.
• The depolarization of the post synaptic membrane opens the Sodium channel causing influx of Na+ ion. Thus causing depolarization and generate action potential. In this way, the impulse get transmitted to next neuron along the synapse.

Nerve Impulse Transmission across Synapse

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