Question

How does the presynaptic neuron transmit an impulse (action potential) across the synaptic cleft to the postsynaptic neuron?

Answer 1

Part 1: Introduction
Synapse is a gap present between two consecutive neurons. It consists of three elements :
1) The presynaptic membrane - formed by the terminal button of an axon
2) The postsynaptic membrane - composed of a segment of dendrite or cell body
3) Synaptic cleft - the space between presynaptic and postsynaptic membrane.

There are two types of synapses- chemical synapses and electrical synapses. The transmission of impulse varies in both of these synapses.

Part 2:Transmission through chemical synapse
The calcium ions present at the synaptic cleft enter the synaptic knobs located at the axon terminal of the presynaptic neuron when an impulse reaches the axon terminal. The synaptic knobs have the synaptic vesicles that consist of neurotransmitters. In the synaptic cleft, the vesicles release the neurotransmitter acetylcholine. The acetylcholine molecules tend to bind to the protein receptors located on the plasma membrane of the postsynaptic neurons The binding opens up channels for sodium ions to enter the postsynaptic neuron. Simultaneously, the potassium ions exit the postsynaptic neuron. This causes an action potential in the postsynaptic neuron membrane. Thus, the impulse is transmitted to the postsynaptic neuron.

Part 3:Transmission through electrical synapse
In chemical synapse, the cells were separated and the transmission required neurotransmitters and receptors. But electrical synapses are gap junctions through which the impulse is directly transmitted from one neuron to another. Electrical synapses do not involve neurotransmitters as the impulse is directly transmitted.