What are the Chemicals that cross the synaptic gap and bind to receptors on another neuron called?

Synapse is a narrow gap containing communicating junction between two neurons where an axon terminal comes near contact with dendrite terminal of next neuron. A narrow fluid filled space, called synaptic cleft, occurs between the two.As the impulse reaches the presynaptic knob, it stimulates release of neurotransmitter into the cleft.