Neuronal activity in human brain performs in a complex physiologic environment, and the noise in physiologic environment affects all aspects of nervous-system function. An essential issue of neural information processing is that whether we can estimate and quantify the environmental noise in a neural system in a proper way. This paper calculated the neural energy to estimate the range of critical values of noise intensity, which has remarkable effect on the membrane potential and the energy waveform, so as to define such noisy environment on which neuronal activity relies in a physiologic sense.