Final answer:
An action potential is what enters the center of the axon, causing it to fire the message to the synaptic knob. This event is an all-or-none signal that triggers the release of neurotransmitters at the axon terminal.
Step-by-step explanation:
The statement refers to an action potential entering the center of the axon, which causes the axon to fire the message to the synaptic knob, also known as the axon terminal. An action potential is a rapid rise and subsequent fall in voltage or membrane potential across a cellular membrane with a characteristic pattern. When a neuron receives a sufficient stimulus to reach the threshold of excitation, an action potential is generated at the axon hillock and travels down the axon. This is an all-or-none event—the signal is either propagated with full strength or not at all. As the action potential arrives at the axon terminal, it triggers the release of neurotransmitters, which then communicate with the next neuron or target tissue such as muscle fibers.