Lumbar dorsal horn neurons responding to electrical hindlimb nerve stimulation at A-fiber strength were recorded in cats anesthetized with sodium pentobarbital and N2O. Twenty-six of 52 units gave additional responses to nerve stimulation at C-fiber strength and responded to a range of tactile and noxious heat stimuli (class 2), while 22 units receiving only A-fiber input responded only to weak mechanical stimuli (class 1). All units selected responded to brushing of hairs in the receptive field. To investigate descending effects on dorsal horn neurons produced by midbrain stimulation, velocity-controlled brushing of hairs, and temperature-controlled noxious heating of glabrous skin of the hindlimb were used. The brush-evoked responses of most class 1 and 2 units were inhibited during stimulation (mean, 30 Hz; 450 or 600 μA) in midbrain periaqueductal gray (PAG) and lateral reticular formation (LRF). Inhibition during stimulation in LRF (to 72% of control) was stronger than that during stimulation in PAG (to 81%) in the mean of all class 1 and 2 units' responses to brush. Stimulation in both PAG and LRF inhibited noxious heat-evoked (50°C, 10 s) responses of class 2 units to a mean of about 55%, whereas the brush-evoked responses of the same units were inhibited to 81 and 72% from PAG and LRF, respectively. Both class 1 and 2 units gave graded responses to brush stimuli applied at different velocities, such that the velocity-response relationship was monotically increasing over most of the velocity range. Discharge rates in class 2 units were linearly related to the temperature of noxious skin heating. PAG and LRF stimulation resulted in a variety of alterations in the brush velocity- and noxious heat-response curves, including reductions in slope of the curves and/or increases in unit response thresholds. The results indicate that there is relative, but not absolute, differential midbrain control of nociceptive versus nonnociceptive spinal transmission. The possibility of differential controls from PAG and LRF, synaptic mechanisms, and functional implications are discussed.
|Original language||English (US)|
|Number of pages||14|
|Journal||Journal of Neurophysiology|
|State||Published - 1981|
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