Neurones responding to noxious stimulation in VB complex and caudal adjacent regions in the thalamus of the rat

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Abstract
Cells (163) responding to mechanical cutaneous stimulation were recorded in VB [ventrobasal] complex and caudal adjacent region in rats anesthetized with a mixture of 2/3 N2O-1/3 O2 and 0.5% halothane. Some cells (51) were exclusively activated by non-noxious cutaneous stimuli applied to restricted and contralateral receptive fields (RF) and had the classical characteristics of lemniscal responses. Some cells (93) responded only to noxious mechanical stimuli (N cells) and had either uni- or bilateral receptive fields. Other cells (19) responded both to noxious and non-noxious stimuli (NnN cells). When tested with intense electrical stimuli applied transcutaneously or on the sural nerve, N and NnN cells responded with a late irregular discharge. Poststimulus histograms obtained in 1/3 of these units revealed that the late component was consistent with a C fiber input; some responses were consistent with a C fiber input; some responses were consistent with A.delta. fiber input. NnN cells also had a short latency discharge probably due to A.alpha. fiber involvement. When tested with other intense stimuli such as noxious radiant heat or noxious visceral stimulation induced by i.p. injection of bradykinin, N and NnN cells were strongly activated. The different kinds of cells were scattered in VB and PO [caudal adjacent region] and no significant differences were found between cells recorded in VB and PO; a rostrocaudal organization of the cells, according to the location of their RF on the caudal or rostral part of the body, was clear for the Nn cells and for N and NnN cells.