Axonal projections of auditory cells with short and long response latencies in the medial geniculate nucleus: distinct topographies in the connection with the thalamic reticular nucleus

Akihisa Kimura, Hiroki Imbe, Tomohiro Donishi
European Journal of Neuroscience 2009, 30 (5): 783-99
The thalamic reticular nucleus (TRN) is a crucial anatomical node of thalamocortical connectivity for sensory processing. In the rat auditory system, we determined features of thalamic projections to the TRN, using juxtacellular recording and labeling techniques. Two types of auditory cells (short latency, SL, and long latency, LL), exhibiting unit discharges to noise burst stimuli (duration, 100 ms) with short (< 50 ms) and long (> 100 ms) response latencies, were obtained from the ventral division of the medial geniculate nucleus (MGV). Both SL and LL cells had a propensity to exhibit reverberatory discharges in response to sound stimuli. The primary discharges of SL cells were mostly single spikes while the non-primary discharges of SL cells and the whole discharges of LL cells were mostly burst spikes. SL cells sent topographic projections to the TRN along the dorsoventral and rostrocaudal neural axes while LL cells only along the rostrocaudal axis. As tonotopy-related cortical projections to the TRN are topographic primarily along the dorsoventral extent of the TRN and the MGV is tonotopically organized along the dorsoventral axis, SL cells, directly activated by ascending auditory inputs, may be closely involved in tonotopic thalamocortical connectivity. On the other hand, LL cells, which are suppressed by ascending inputs and could be driven to discharge by corticofugal inputs, are assumed to activate the TRN in a manner less related to tonotopic organization. There may exist heterogeneous projections from the MGV to the TRN, which, in conjunction with corticofugal connections, could constitute distinct channels of auditory processing.

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