Activity evoked by A- and C-afferent fibers in rat dorsal horn neurons and its relation to a flexion reflex.

The responses of 56 neurons recorded in the lumbosacral spinal cord of halothane-anesthetized rats were studied following the application of mechanical stimuli to the skin on the lateral aspect of the paw or electrical stimulation of the sural nerve. Only neurons driven by A- and C-fiber stimulation were considered. The evoked activity in a nerve supplying flexor muscles, the common peroneal nerve, was also recorded to evaluate possible relations between neuronal events and reflex discharges. To quantify the reflex output we also recorded the activity of 12 motoneurons. Four different populations of dorsal horn neurons activated by C-fibers could be distinguished. The neurons were classified on the basis of their responses to mechanical stimuli and of their location in the dorsal horn. Class 1 neurons were driven by nonnoxious stimulation only. Neurons driven by nonnoxious stimuli and noxious stimuli were denoted class 2S (superficial to the location of the maximal A-beta-fiber-evoked field potentials) or class 2D (deep to the same potential). Class 3 neurons were driven by noxious stimuli only. The functional characteristics of these four classes of neurons differed in many respects. The latency for the A-beta-fiber-evoked discharge was, on average, 2 ms longer in class 2S than in class 2D neurons, indicating a polysynaptic A-beta input to the former class of neurons. The C-fiber-evoked neuronal discharge often showed time-locked peaks of activity during the interval 120-170 ms. Such peaks of activity occurred, in general, later in class 2D neurons (mean, 157 ms) than in class 2S (mean, 137 ms) or in class 3 (mean, 140 ms), suggesting that the different classes received C-fiber input via partially different routes. The responses to repeated C-fiber stimulation also differed markedly among the four classes. After 16 single electrical stimulations (100 T (T = threshold strength for activating A-beta-afferents), 1 Hz), the C-fiber-evoked discharge in class 2D neurons was increased by 196%, whereas the corresponding value for those in classes 2S, 3, and 1 was 41, 24, and 38%, respectively. Ten of 14 class 2D neurons showed a simultaneous increase of the A-fiber-evoked discharge, indicating an increased excitability of these neurons after repeated impulses in C-fiber afferents. An early reflex discharge (latency, 6-10 ms) was evoked in the common peroneal nerve by electrical stimulation of the sural nerve.(ABSTRACT TRUNCATED AT 400 WORDS)