Responses of group IV afferent units from skeletal muscle to stretch, contraction and chemical stimulation

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Abstract
In an attempt to differentiate between nociceptive group IV muscle receptors and “ergoceptive” ones, the discharges of single group IV fibres from skeletal muscle in response to local pressure, sustained stretch, repetitive contraction and intra-arterial injections of bradykinin, 5-hydroxytryptamine (5-HT), potassium, phosphate, and lactate were studied in anaesthetized cats. Of the 75 fibres of the study, 5 units were activated by sustained stretch, the responses occurring with a delay. These stretch-sensitive units could not be activated by local pressure or muscular contraction. Thirteen group IV afferents raised their discharge frequency during repetitive contractions. Some of the units responded immediately with the onset of the contractions, whereas the others showed a pronounced delay. Forty-six units were tested with all or most of the above mechanical and chemical stimuli. In 32 afferents a response to at least one of the stimuli was present. Taking only these units into account, several groups of receptors could be distinguished by their different response combinations. One group was activated by pain-producing substances, but not by muscular activity and thus showed nociceptive properties. Another group showed a raised activity during muscular contractions but did not respond to the algesic agents bradykinin and 5-HT. Units belonging to this group might serve as “ergoceptors”. The borderline between the two groups was not sharp, a considerable number of group IV afferents was found which had both nociceptive and “ergoceptive” properties.