Final answer:
Sub-threshold depolarization does not trigger an action potential. The neuron remains inactive if the threshold of -55 mV isn't reached, returning to its resting state without firing. Action potentials operate on an 'all or none' basis, conducting a uniform response only when the threshold is surpassed.
Step-by-step explanation:
After a sub-threshold depolarization, which is a small change in membrane potential that does not reach the threshold needed to trigger an action potential, nothing further happens in terms of neuron firing. In biology, the concept of an action potential is akin to a digital event; it is described as being 'all or none'. If the depolarization does not reach the critical threshold level of -55 mV, the voltage-gated Na+ channels do not open and an action potential will not be initiated. Consequently, the neuron will not transmit a signal, and the cell returns to its resting potential without conducting an impulse.
During the process of depolarization, stimuli open voltage-gated Na+ channels, helping to depolarize the membrane from -70 mV towards the threshold. Unless this threshold is attained, the significant changes required to propagate a neuronal signal do not occur. If the membrane potential does reach -55 mV, then the action potential unfolds in its entirety, peaking at +30 mV before repolarization occurs, initiated by K+ leaving the cell. This action potential response is always the same, regardless of stimulus strength, highlighting the 'all or none' principle.