what is the significance of node of ranvier???

Node of Ranvier is the periodic gap in the myelin sheath on the axon of certain neurons. At nodes of Ranvier, the axonal membrane is uninsulated and therefore capable of generating electrical activity.

An action potential is a spike of positive and negative ionic discharge that travels along the membrane of a cell. The action potential signal jumps along the axon, from node to node, rather than propagating smoothly (as they do in axons that lack a myelin sheath). The increased speed afforded by this mode of conduction assures faster interaction between neurones.

Thus node of Ranvier helps in rapid and efficient saltatory propagation of action potentials.

 

Synaptic cleft is the tiny space between two nerve cells across which the neurotransmitter acetylcholine diffuses.