What does the axon bulb do?

It starts the transmission of a neural signal from one neuron to another.

An axon ends in an axon terminal, which ends in a small rounded tip called the axon bulb. Each terminal lies very close to either a dendrite or a cell body of another neuron, and the combination of the end of the first neuron, the beginning of the next, and the space between them is known as a synapse; when a neural impulse reaches the axon bulb, it is stimulated to release chemical messengers called neurotransmitters into the synaptic cleft (gap), which diffuse over to the second neuron, conveying the signal to the second neuron.