Wide-field neurons in somatosensory thalamus of domestic cats under barbiturate anesthesia

Abstract

A sample of 392 somatosensory thalamic neurons from barbiturate-anesthetized cats, all responsive to stimulation of the contralateral forepaw (CFP), was compared with a previously described sample from chloralose-anesthetized animals with respect to (i) the functional properties of CFP-responsive neurons isolated within nucleus ventralis posterolateralis (VPL), (ii) the spatial distributions of distinguishable neuronal subsets within VPL, and (iii) the distribution of CFP-evoked single-unit discharge along the mediolateral dimension of the nucleus and over time. Eighty-two percent of the barbiturate neurons were responsive only to CFP; their receptive fields were small, usually confined to one or two digits or an area on the paw dorsum. Members of this sa subset were predominantly excited by light touch or deflection of hairs. The other 18% responded to stimulation of at least one “off-focus” limb in addition to CFP. These ∼ sa neurons possessed larger receptive fields (though not as large as those observed under chloralose), which were bilaterally disposed in 32% of this subset. Fifty-eight percent of the ∼ sa neurons were excited by light touch or hair deflection. The proportion of the barbiturate sample subject to corticofugal influences (62%) was smaller than that under chloralose (74%), with inhibitory influences predominating on both sa and ∼ sa neurons in contrast to an excitatory effect on ∼ sa neurons under chloralose. Compared with chloralose data, impulse discharge reached peak intensity earlier and overall duration of response was reduced for all subsets. Although the spike density distribution for ∼ sa neurons was particularly abbreviated, this subset was well represented among VPL neurons responding to stimulation of the forepaw. As they are seen in barbiturate preparations also, wide-field somatosensory thalamic neurons are not an artifact of chloralose anesthesia.