Response properties and descending control of rat dorsal horn neurons with deep receptive fields

The study was designed to obtain information on the spinal processing of input from receptors in deep somatic tissues (muscle, tendon, joint). In anaesthetized rats, the impulse activity of single dorsal horn cells was recorded extracellularly. In a pilot series, the proportion of neurons responding to mechanical stimulation of deep tissues was determined: 46.7% had receptive fields in the skin only, 35.5% could only be driven from deep tissues (deep cells), and 17.7% possessed a convergent input from both skin and deep tissues (cutaneous-deep cells). In each category, neurons with low and high mechanical thresholds were encountered. Experiments employing a reversible cold block of the spinal cord showed that deep cells with high threshold were subject to a stronger descending inhibition than low-threshold deep cells. In cutaneous-deep neurons the input combination high-threshold cutaneous and high-threshold deep was the most frequent one (48.7% of the cutaneous-deep cells). In these presumably nociceptive cells the descending inhibition had a differential action in that the input from deep tissues was more strongly affected than was the cutaneous input to the same neuron. The recording sites of the neurons with deep input were located in the superficial dorsal horn and in and around lamina V. The results suggest that in the rat a considerable proportion of dorsal horn cells receives input from deep nociceptors and that this input is controlled by descending pathways in a rather selective way.