Minimum stimulus required to activate a neuron?

For a neuron to fire, an action potential is sent down the axon. An action potential is an "all or nothing" phenomenon, which means that it is either sent, or it isn't. There is no small firing, or big firing, etc. Leave that for the graded potential, which occurs when the neurotransmitters stimulate the dendrites on the cell body, which changes the polarity of the cell and creates the action potential. Graded potential = graded.... which is not an all or nothing phenomenon, but instead is measured (hence "graded").