@article{9de89118ae574351946110c430590b22,
title = "Effects of static and rhythmic twitch contractions on the discharge of group III and IV muscle afferents",
abstract = "Summary: Although both static and rhythmic twitch contractions of the hindlimb muscles of anaesthetised cats have been shown to reflexly evoke pressor responses, the increase in arterial pressure evoked by the former type of contraction has been shown to be substantially larger than that evoked by the latter. We have therefore recorded the impulse activity of single group III and IV muscle afferents, whose activation reflexly increases arterial pressure, while we both statically and rhythmically twitch-contracted the triceps surae muscles of anaesthetised cats. We found that group III afferents (n=17) discharged significantly more impulses in response to static contraction than in response to rhythmic contraction. By contrast, group IV afferents (n=18) fired approximately the same number of impulses in response to the two types of contraction. In addition, we found that many of the group III but only a few of the group IV afferents displayed discharge properties suggestive that these afferents were mechanoreceptors. We conclude that the discharge of group III afferents are likely to be responsible for the difference in the magnitudes of the reflex pressor responses evoked by static and rhythmic contraction.",
keywords = "Cats, Exercise, Mechanoreceptors, Somato-sympathetic reflexes, Thin-fibre muscle afferents",
author = "Kaufman, {Marc P.} and Waldrop, {Tony G.} and Rybicki, {Kenneth J.} and Ordway, {George A.} and Mitchell, {Jere H.}",
note = "Funding Information: tension and spent more time working during static than during rhythmic contraction. The reflex cardiovascular effects evoked by rhythmic twitch contractions have been shown to differ across species. For example, in both anaesthetised rabbitsI3 and dogsI4, rhythmic contractions have been shown to reflexly decrease arterial pressure, whereas in cats, our results have shown that this ty e of contraction reflexly increases arterial pressure.' In each of these three species, however, static contraction reflexly increases arterial pressure. l3 I4 The d ifferences in the direction of the reflex responses evoked by rhythmic twitch contractions between rabbits, dogs and cats might be explained by a spinal or supraspinal mechanism which in rabbits and dogs translates the afferent input arising from this type of contraction into a depressor response, whereas in cats this mechanism translates the afferent input into a pressor response. Based on our findings, it is interesting to speculate that a part of the reflex cardiovascular increases evoked by muscular contraction arise from the stimulation of mechanoreceptors, many of which are supplied by group I11 afferents. These mechanoreceptors displayed discharge properties that would be ideally suited to reflexly evoke the contraction-induced rapid increases in heart rate that have been reported to be caused by a withdrawal of cholinergic input to the S-A node of both humans{"} and cats.I6 Furthermore, the discharge rate of these mechanoreceptors may signal the force of contraction, while the number of mechanoreceptors activated by the contraction may signal the number of working motor units. We must emphasise, however, that there is substantial evidence suggesting that at least part of the reflex cardiovascular increases evoked by contraction arise from some as of yet undefined chemical which stimulates metaboreceptors. l7 These metaboreceptors are likely to be supplied mostly but not exclusively by group IV afferent~l.1~ Supported by NIH Grants HM6296, HL30710, HL07360 and NS20165, an American Heart Association Grant-in-Aid (83-1 179) and the Lawson and Rogers Lacy Research Fund in Cardiovascular Diseases.",
year = "1984",
month = nov,
doi = "10.1093/cvr/18.11.663",
language = "English (US)",
volume = "18",
pages = "663--668",
journal = "Cardiovascular Research",
issn = "0008-6363",
publisher = "Oxford University Press",
number = "11",
}