Answer
Neurons have resting membrane potentials. This means that there is a voltage difference between the inside and outside of their cell membranes. This is called the resting membrane potential, and is usually taken to be about -70 mV. For a stimulus to generate a neural impulse, it must be strong enough to depolarize the nerve cell --it must cause enough cations to flow into the cytosol to raise the cell membrane potential to threshold ( or above 0.0 mV to +30 mV)) for that cell. The threshold of nerve cells is usually cited as -55mV. When a stimulus is strong enough to raise the membrane potential above threshold, an action potential will be triggered at the axon hillock and will tend to propagate distally from the cyton.
Work Step by Step
"All or None" refers to the fact that each axon has a threshold value. An action potential will not be triggered in an axon unless the stimulus is strong enough to raise the membranne potential to threshold value for that cell. A subthreshold stimulus will not trigger an AP at all, and a super strong stimulus will not trigger an AP with an abnormally high amplitude for a given cell. The super strong stimulus may increase the frequency and number of APs generated but the amplitude of the AP remains the same.
An adequate stimulus will generate an AP of a certain amplitude, ; an inadequate stimulus, no matter how close to threshold value, will not trigger an AP. Also a super powerful stimulus will not increase the amplitude of the triggered APs. The triggering of the AP takes place or not and the amplitude is the same once the threshold is achieved.
Graded or generator potentials are produced when ligand gated channels open and permit changes in resting membrane potentials of postsynaptic neurons. These potentials vary in size ( 1-50 mV) and are not usually strong enough or able to travel far enough to trigger an AP at the axon hillock. However, these graded potentials change the sensitivities of axons in the direction of depolarization or of hyperpolarization (EPSP or IPSP). Several EPSPs may thus facilitate the generation of an AP
