Final answer:
At the end of each branch is a synaptic end bulb, a structure essential for neurotransmitter release and neuronal communication at the synapse when an action potential triggers Ca²+ entry and vesicle fusion.
Step-by-step explanation:
At the end of each branch is a synaptic end bulb, which is a knob-like structure at axon terminals. The synaptic end bulb plays a key role in neuronal communication by releasing neurotransmitter molecules onto a target cell across a synapse. When an action potential reaches the axon terminal, voltage-gated Ca²+ channels in the membrane of the synaptic end bulb open, leading to an increased concentration of Ca²+ inside the end bulb. This triggers vesicle fusion and neurotransmitter release into the synaptic cleft, where they can bind to receptors on the membrane of the target cell, such as another neuron or a muscle fiber at the neuromuscular junction.