The ability of signal detection and transduction of the Hodgkin-Huxley neuronal systems, associated with rhythmic oscillations in the presence of external modulations, is studied. Both inhibitory and excitatory modulations, regarded as the total effects of the environment in which the neurons are located, are able to modulate the frequencies of the rhythmic oscillations of the neurons. Either subthreshold or suprathreshold rhythmic oscillations can provide the neural system with an effect of frequency selection in processing external signal. Resonance among the noise, the noise-induced oscillation, and the signal enhances intensively the capability of the neurons in processing the weak signal, especially when frequency of the signal is around that of the noise-induced rhythmic oscillation. Thus, the neuronal system can be adjusted to an optimal sensitive state for signal processing through the environmental modulations.