All or None Response
A very weak stimulus does not result in an action potential. The stimulus must have a certain strength to evoke an excitation. This is the threshold value below which no action potential results. Once generated the size of the action potential is not influenced by the magnitude or amplitude of the stimulus. Even if we double or triple the stimulus, the action potential remains the same. The stimulus causes either a full response or none at all. In physiology, this type of response is called all-or-none response.
When a greater stimulus strength is applied to a neuron identical action potentials are produced more frequently. When an entire collection of axons is stimulated as in a nerve, different axons will be stimulated at different frequencies. A low intensity stimulus will activate fibres with low threshold values and as the stimulus intensity is increased more and more fibres will be activated. This process is known as recruitment. Another important property of nerve impulse to be noted is that during the time that a patch of axon membrane is producing an action potential, it is incapable of responding to another stimulus i.e. it is refractory to further stimulation. If the second stimulus is applied when the Na+ gates are open the membrane remains in absolute refractory period but if the stimulus is applied when the K+ gates are open the membrane is in relative refractory period and can be depolarised only if the stimulus is very strong.