Final answer:
The point when the membrane potential reaches -55mV and an axon 'fires' is called the threshold potential, which is the precursor to an action potential, a critical event in neuronal signaling.
Step-by-step explanation:
The point when the membrane potential reaches -55mV and the axon "fires" is referred to as the threshold potential. This event is part of what is known as an action potential, a fundamental process in neuronal communication. Once the threshold potential is reached, voltage-gated Na+ channels open, allowing an influx of sodium ions, which causes the depolarization of the membrane, reversing its charge to approximately +40 mV from the resting potential of -70 mV. Following depolarization, K+ channels open, allowing potassium ions to exit the neuron and initiate repolarization. Subsequently, the neuron enters a refractory period before returning to resting potential, preventing the generation of a subsequent action potential for a brief period.