Functional significance of the A-current.

This work considers the response to simulated synaptic inputs of an excitable membrane model. The model is essentially of the Hodgkin-Huxley type, but contains an A-current in addition to sodium and delayed-rectifier potassium channels. The results were compared with previous simulations in which the stimulus was an injected current. These two types of stimuli give somewhat different results because synaptic stimuli directly change the membrane resistance, whereas injected current does not. The results of synaptic stimulation were similar to injected current in that very low frequencies of action potentials were elicited only where the stimulus was slightly above threshold. For most of the range of synaptic inputs that produced oscillatory behavior, the A-current had little effect on oscillation frequency. With synaptic stimuli as with injected current, the model membrane's spiking behavior does not begin immediately when an excitatory stimulus is imposed on a quiescent state. The delay before spiking is closely related to the inactivation time of the A-current. The synaptic results were different from the injected current results in that when substantial inhibition was present, the ability to produce very-low-frequency spiking was absent, even just above the excitatory threshold. The higher the degree of inhibition, the narrower the range of spike frequencies that could be elicited by excitation. At very high inhibition, no degree of excitation could elicit spiking.