Two connecting neurones are not in physical contact with each other but there is a gap between the presynaptic knob and the postsynaptic neurone.The junction between two neurones or a neurone and a muscle or gland cell is called a synapse. The gap separating the axon of one neurone from the dendrite of the next is called the synaptic cleft. It is approx 20 - 30 nm across.Impulses are transmitted across the synapse by neurotransmitters, such as acetyl choline in cholinergic synapses, diffusing across the synaptic cleft.