1. A set of 11 cutaneous stimuli defined previously to differentiate among different types of cutaneous sensory receptors in the rat hindpaw was also effective in differentially activating second-order sensory neurons in the dorsal horn and the gracile nucleus of rats. 2. All sampled units were responsive to more than 1 of the 11 stimuli. However, none responded to innocuous warming or cooling stimuli. Therefore further analysis was restricted to responses to nine of the selected stimuli. 3. Cluster analysis of the responses to nine selected innocuous and noxious mechanical stimuli and noxious thermal stimuli yielded seven classes that seemed functionally distinct from each other: a class of high-threshold neurons, three classes of convergent (wide dynamic range) neurons, a class of a mixture of poorly responsive neurons and neurons receiving Pacinian inputs, and two classes of low-threshold neurons. 4. High-threshold neurons responded predominantly to noxious mechanical and thermal stimuli and presumably received an input from both mechanically and thermally sensitive nociceptors. These cells were located in the dorsal horn, and some were spinothalamic tract cells. Wide dynamic range neurons were excited by innocuous and noxious stimuli, but better by noxious stimuli. These classes of cells were either in the dorsal horn (some were spinothalamic tract cells) or in the nucleus gracilis. 5. One class contained poorly responsive neurons that did not respond well to any of the stimuli and low-threshold neurons that were entrained by 300-Hz mechanical sinusoids and so were presumably activated by Pacinian corpuscle afferents. All of the neurons receiving Pacinian corpuscle inputs were located in the nucleus gracilis. Another class of low-threshold neurons responded both to 30-Hz sinusoids and to maintained mechanical stimuli. Most of these neurons were in the nucleus gracilis, but a few were in the dorsal horn. The other class of low-threshold neurons responded chiefly to 30-Hz sinusoids, and most were located in the nucleus gracilis. 6. The results of the present study indicate that the clustering procedures based on the response profiles to a variety of somatosensory stimuli used previously for the functional classification of cutaneous sensory receptors are also effective in functionally grouping second-order neurons of the rat somatosensory system. Possible sensory modalities that could be subserved by each class are discussed. These include nociception, vibratory sense, flutter, and touch-pressure.
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