All or none phenomenon means -

Ans. is 'b' i.e., Action potential has a fixed value Features of action potential:-o AP has following features1) Self Propagation:# AP, once started, is conducted along the nerve fiber, without any reduction in amplitude or speed, until it comes to the fiber's end, i.e., AP is propagated without decrement. Conduction of nerve impulses although rapid, is much slower than electricity.2) All or none phenomenon :# The action potential is not graded. It is an all or none change. Under a given set of conditions, it has a fixed value. Either all of the action potential is seen or not at all.# If a stimulus of threshold strength is applied, an AP is produced further increase in strength or duration of stimulus does not produce any increase in amplitude or duration of AP.# On the other hand, if the stimulus is of subthreshold intensity, AP fails to occur.3) Membrane excitibility during an AP# An action potential is triggered only if the stimulus raises the membrane potential above firing level of - 55 mV. The minimum strength of stimulus that can trigger an action potential is called a "threshold stimulus".# During action potential, the excitibility of a neuron is reduced. Application of stimulus, whatever its strength, from the onset of deplorization till repolarization is about one-third complete (early part of repolarization) fails to produce any response. This period is called absolute refractory period.# Thereafter, In later part of repolarization till the onset of after depolarization, the membrane is in relative refractory period, i.e., a sufficiently high stimulus can elicit a response.# During after depolarization, the membrane is hyperexicitable; having just come out of the refractory period, the membrane is excitable and being closer to the firing level makes it hyperexciable.# During after-hyperpolarization, the membrane excitibility is low but slowly returns to normal.