The function of a synaptic knob is to change the action potential that is carried by axons into a chemical message. The chemical message then interacts with the recipient neuron or effector. This process is called synaptic transmission.
Axons usually have thousands of terminal branches that each end as a bulbous enlargement called a synaptic knob or synaptic terminal. Synaptic knobs contain several membrane-bounded synaptic vesicles that are 40 to 100 nanometers in diameter. The mitochondria, microtubules and other organelles are located in the synaptic knobs. The neurotransmitter is in the synaptic vesicles. The action potentials that arrive at the synaptic knobs trigger the release of the neurotransmitter into the synaptic cleft.