The ability of a neuron to detect and enhance a weak periodic flow of information within an ``internal-noise'' background has been studied through the mechanism of stochastic resonance. Two kinds of nonlinear synaptic input, a coherent firing of spikes from a number of coupled neurons and an irregular firing of spikes from a single neuron, are considered as the internal noise for a neuron. The output signal-to-noise ratio (SNR) is found to be finite. This nonzero SNR is able to account for the relevant experiments where the SNR is nonzero when the external noise is switched to zero.