Segmental and supraspinal actions on dorsal horn neurons responding to noxious and non-noxious skin stimuli

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Abstract
Single dorsal horn neurons were recorded extracellularly in 8 anesthetized (sodium pentobarbital) cats and in 1 unanesthetized decerebrated cat. The animals were either spinalized by transection of the cord at L1 (6 experiments) or a cold block was used for reversible spinalization at L1 (3 experiments). Sixty-five units were recorded in the dorsal horn and in the dorsolateral funiculus which could be excited by electrical stimulation of the ipsilateral plantar nerves and by natural stimulation of the skin in the foot region. The recording position of the microelectrode was verified histologically. According to their excitability by electrical stimulation of the afferent nerve fibers and by natural stimulation of the receptive fields, 2 major classes of units could be distinguished in the spinalized cat. Class 1 cells were excited by electrical stimulation of myelinated axons (group II) in the plantar nerves. Four of 9 could be excited by low threshold cutaneous mechanoreceptors; 5 had input probably from deep receptors. Class 2 cells, which were more than twice as common as class 1 cells, could, like the latter, be excited by electrical stimulation of group II myelinated afferent fibers in the plantar nerves, but in addition, were excited by electrical stimulation of C-fibers. When stimulated naturally, virtually all of the class 2 cells received an excitatory input from low threshold cutaneous mechanoreceptors and also from receptors excited by noxious radiant heat stimulation in their receptive fields. They responded to noxious heating in a quantitatively similar manner as the primary C-heat nociceptors. The discharges of the class 2 cells evoked by heating the skin could be suppressed by electrical stimulation of the collaterals of myelinated afferent fibers in the dorsal columns. Using reversible cold block of the spinal cord, the excitatory influence of electrical C-fiber stimulation and of noxious radiant heat stimulation was diminished in all class 2 cells, and in some of them it was completely suppressed when the spinal cord was intact. This tonic descending inhibition did not depend on the presence or absence of activity in large myelinated fibers.