According to UCC.edu, a "synaptic knob" is formed by each axonal end branch adjacent to where the synapse and a tiny cleft are located. A neurotransmitter is released into the synapse from the vesicles as a nerve reaches the synaptic knob.
The neurotransmitter that is released binds to receptors of the membrane that is adjacent to the muscle cell or neuron and diffuses across the gap. This process introduces an electrical change in the adjacent neuron's membrane, causing a nerve impulse within that cell. This entire process is called "synaptic transmission." According to BiologyReference.com, the "synaptic knob" is also called the "synaptic terminal."