Action potential properties

The key properties of action potentials are shown below:

1) The minimum threshold stimulus is needed to produce an action potential and it works by depolarizing the cell to a critical threshold voltage, usually some 15 mV less than Vm. A threshold stimulus is defined as the current which will cause a neuron to fire on 50% of occasions, and its amplitude depends on the size of the neuron as this affects the electrical resistance of the cell.

2) Physiologically, the action potentials are triggered at the axon hillock, as this area of the neuron has the minimum threshold, and are propagated along the axon towards its terminals.

3) All action potentials are about of the similar size (in a given cell) regardless of the amplitude of the stimulus. The combined effects of this and property 1 are frequently paraphrased as the all-or-none rule; a neuron either fires entirely or not at all.

4) There is a short delay between the onset of the stimulus and the beginning of the action potential. This is termed as the latency. The latency gets shorter since the strength of the stimulus rises.

5) During the spike a neuron becomes entirely inexcitable. This is the absolute refractory period; during that time a nerve cell will not fire again no matter how large is the stimulus. Afterward the spike, while the neuron remains hyperpolarized and then neuron can be excited only by the suprathreshold stimuli. This time period is the relative refractory period and is elucidated by the fact that whenever the cell is hyperpolarized a bigger current is required to drive the membrane potential to the threshold voltage. Those neurons are rendered temporarily refractory has two consequences as shown below:

– It enforces an upper limit on the firing frequency.
– Physiologically, the action potentials are propagated in only one direction, since the segment of axon where the action potential has been is  inexcitable. All the above properties can be elaborated by the behavior of the ion channels dependable for the action potential.