Noxious and tactile input to medial structures of midbrain and pons in the rat

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Abstract
Response patterns of single units in the medial parts of the pons and midbrain were studied extracellularly in chloralose-anaesthetized rats, paralyzed with oancuronium. The units were tested with natural skin stimuli including slight touch and hair-brushing in order to provide input from sensitive mechanoreceptors supplied by myelinated afferent fibres. Noxious pinch and controlled noxious radiant heat were also used to provide nociceptive input. The influence of these stimuli on unit activity was compared with that evoked by electrical stimulation of the coccygeal nerve, which excited either the A-fibres only or, with stronger stimuli A- and C-fibres. It has been shown that about half the units responding to electrical stimulation of the A-fibres exhibited a second peak of excitation with a latency of 100–300 msec when the stimulus strength surpassed C-fibre threshold. A high and statistically significant correlation has been found to exist between increased excitability induced by noxious heating and/or squeezing the skin on the one hand and the occurrence of a C-fibre response on the other hand. From this evidence it is concluded that a large proportion of the units in the medial pons and midbrain receive nociceptive input from the skin and that this input is mediated predominantly by C-fibres. When applying controlled radiant heat it has been found that the discharge frequencies of units responding to noxious input do not code the stimulus strength exactly, rather the units usually switch from a lower to a higher discharge frequency when noxious levels of stimulation are reached. The functional implications of this are discussed.