Spike trains and signaling modes of neurons in the ferret lateral geniculate nucleus

Intracellular recordings were used to examine the action potential firing modes of cells in the ferret lateral geniculate nucleus (LGN) in vitro. We compared the effects of altered membrane voltage on patterns of action potential trains evoked by direct current injection and by retinal afferent (synaptic) stimulation. The results confirm that LGN cells in the ferret can fire action potentials in the "burst" and "tonic" modes that have been described previously for other species. At depolarized membrane potentials, LGN neurons respond to sustained depolarization with short-latency trains of action potentials whose frequency is directly proportional to the amount of current injected. At hyperpolarized membrane potentials, LGN cells enter burst mode, in which depolarizing inputs are differentiated into brief high-frequency discharges whose latency varies with membrane potential. We also observed a "mixed" mode, in which LGN cell responses to synaptic or injected currents within a narrow range of membrane potentials reflect aspects of both burst and tonic firing simultaneously. Thus a striking consequence of the interplay among voltage-dependent membrane conductances in thalamic cells is wide variability in length, duration, and latency of spike discharges elicited by identical stimuli. These results also suggest that the concept that LGN cells display only two active response modes must be expanded to include varying amounts of delay and the possibility of mixed discharges.