We present a mathematical model to quantitatively describe the neuronal dynamics in
. Since calcium imaging is a popular technique to visualize the neuronal activity in
, the model includes the variable of the fluorescence intensity in addition to the membrane potential and the intracellular calcium concentration. The fluorescence intensity is a quantity which is comparable with the experimental data. The parameters in the model are determined to reproduce the neurophysiological experimental data. Our model exhibits good agreement with the data. We apply the model to a neural circuit for chemotaxis and find that the neuronal activity measured by the fluorescence intensity shows quantitatively different behavior from that measured by the membrane potential in some neurons. The difference is discussed from the viewpoint of neuronal mechanisms.