Question

What is the mechanism of nerve impulse transmission at the synapse?

Answer 1

In general, the site of contact between two adjacent nerves or neurons is known as a synapse. There are two types of synaptic connections based on the transmission of the signal between the neurons, and they are electrical synapses and chemical synapses. In electrical synapsis, the membranes of presynaptic and postsynaptic neurons are connected by a gap junction, and electrical current passes through them.

Complete answer:
In a chemical synapse, either excitatory neurotransmitters or inhibitory neurotransmitters are released from presynaptic neurons into the synaptic cleft and from the synaptic cleft; these chemicals bind to the receptors of the postsynaptic neuron. Chemical synapses can be further categorized based on the neurotransmitter that is released, such as adrenergic, glutamatergic, cholinergic, and GABAergic chemical synapses.
Neurotransmitters are chemicals that are stored inside the synaptic vesicles present in the axon terminal of the presynaptic neuron. The action potential travels along the axon and reaches the axon terminal, but that cannot itself cause synaptic vesicles to release neurotransmitters. Due to the arrival of an action potential, the depolarization of the plasma membrane occurs, which leads to the opening of calcium ion channels. This permits the influx of calcium ions into the cytosol from the extracellular medium and a localized rise in cytosolic calcium ions concentration near the synaptic vesicles. The bivalent calcium ions bind to the proteins that connect the synaptic vesicle with the plasma membrane of the axon terminal, which induces membrane fusion and exocytosis of neurotransmitters into the synaptic cleft.
The neurotransmitters bind to the ligand-gated receptors of the postsynaptic neuron and cause an immediate conformational change which opens other ion channels. The opening of cation gated ion channels such as sodium and potassium channels cause rapid depolarization of the postsynaptic membrane. This action potential travels through the axon of the postsynaptic neuron, and thus, a nerve impulse is transferred from one neuron to another. In contrast, inhibitory neurotransmitters induce the opening of chloride ion channels which lead to hyperpolarization of the plasma membrane.

Note:
Chemical synapse is not only found between neurons but also between muscle fibres and neurons. The synaptic connection between a postsynaptic motor endplate (muscle fibre) region and a presynaptic axon terminal end of a motor neuron is known as the neuromuscular junction or myoneural junction. It helps in action potential transmission from the nerve to the muscle. Neuromuscular junction consists of cholinergic chemical synapsis.