Hello Shirish,

After the transmission of one impulse, synapse cannot transmit another impulse because of Acetylcholinesterase Enzyme. It is an enzyme that breaks down the neurotransmitter acetylcholine at the synaptic cleft which is the space between two nerve cells, so the next nerve impulse can be transmitted across the synaptic gap. When the impulse is transmitted and other impulse is to be prevented, no acetylcholine is produced.

Hope this helps!