Final answer:
An axon 'fires' when an action potential travels down the axon, triggering the release of neurotransmitters at the synapse. This all-or-none event is essential for neuronal communication and is propagated either continuously in unmyelinated axons or through saltatory conduction in myelinated axons.
Step-by-step explanation:
When an axon 'fires', it is referring to the process of an action potential being triggered within a neuron. This action potential is an all-or-none phenomenon; it either occurs at full strength or does not occur at all. As the action potential travels down the axon, it maintains its strength much like a lit fuse, and does not fade. It is propagated along the axon via the opening and closing of voltage-gated ion channels.
In unmyelinated axons, the propagation occurs continuously because voltage-gated channels are evenly distributed along the axon. However, in myelinated axons, the propagation occurs through saltatory conduction where the action potential 'jumps' from one node of Ranvier to another, which speeds up the signal. Once the action potential reaches the axon terminal, it causes the release of neurotransmitters, which then communicate with other neurons or muscle fibers at the synapse.
This electrochemical event is crucial for neuronal communication and allows the organism to respond to stimuli, regulate functions, and process information.