The external electric field (EF) has shown to have great influence on neurons. On account of the effects of applied fields on neural activity through the induced EF in the extracellular medium, the electrical characteristics of the extracellular medium should not be ignored for accurately reflecting the biological field effect. Here considering the electrical characteristic of extracellular medium i.e. ephaptic transmission, neuronal firing is studied based on a PR neuron model in the presence of the external (EF), including direct current (DC) and alternating current (AC) EF. Moreover, three key parameters (the inner conductance g c, the inverted potential of potassium ion V K and the extracellular resistance between dendrite and soma R out DS), which are crucial for the dynamics of neurons, are introduced to develop the neuronal firing. Different firing rhythms of neuron imply different stimulus information, and in some particular stimulus frequency the synchronization between the firing rhythm and EF signal is observed.