What principle describes a neuron's reaction of either firing completely or not firing at all?

The all-or-none principle is a fundamental concept in neuroscience that explains how neurons respond to stimuli. According to this principle, a neuron either fires an action potential with full intensity or it does not fire at all. This means that once the threshold for firing is reached, the electrical impulse that travels down the neuron's axon is initiated and will propagate without decreasing in strength.

The action potential itself is the actual electrical signal that occurs after reaching the threshold, but it is the all-or-none principle that defines the binary nature of neuronal firing. If a stimulus does not reach the necessary threshold, the neuron will remain at resting potential and will not fire. This principle is crucial in ensuring that neurons transmit information in a consistent and reliable manner, which is fundamental to the functioning of the nervous system.

Other options, such as resting potential, refer to the state of a neuron when it is not actively sending signals, while the threshold principle serves to describe the point at which a neuron must be depolarized to initiate firing, but does not encapsulate the complete idea of the neuron's firing reaction as effectively as the all-or-none principle does.