A stochastic nonlinear model of neuronal activity in a neuronal population is proposed in this paper, where the combined dynamics of phase and amplitude is taken into account. An average number density is introduced to describe collective behavior of neuronal population, and a firing density of neurons in the neuronal population is referred to be neural coding. The numerical simulations show that with a weaker stimulation, the response of the neuronal population to stimulation grows up gradually, the coupling configuration among neurons dominates the evolution of the average number density, and new neural coding emerges. Whereas, with a stronger stimulation, the neuronal population responds to the stimulation rapidly, the stimulation dominates the evolution of the average number density, and changes the coupling configuration in the neuronal population.