Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans

We determined the effects of stimulation of the mesencephalic locomotor region (MLR) and the muscle reflex, each evoked separately, on the discharge of cutaneous sympathetic fibers innervating the hairy skin of decerebrate cats. Electrical stimulation of the MLR was performed while the cats were paralyzed with vecuronium bromide. The muscle reflex was evoked while the cats were not paralyzed by electrical stimulation of the tibial nerve at current intensities that did not activate directly group III and IV muscle afferents. MLR stimulation increased, on average, the discharge of the 23 cutaneous sympathetic fibers tested ( P < 0.05). The muscle reflex, in contrast, had no overall effect on the discharge of 21 sympathetic fibers tested ( P > 0.05). Both maneuvers markedly increased mean arterial pressure and heart rate ( P < 0.05). Prevention of the baroreceptor reflex with the α-adrenergic blocking agent phentolamine did not reveal a stimulatory effect of the muscle reflex on cutaneous sympathetic discharge. We conclude that the MLR is a more important mechanism than is the muscle reflex in controlling sympathetic discharge to hairy skin during dynamic exercise.

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Reflex cardiovascular responses evoked by selective activation of skeletal muscle ergoreceptors

Journal of Applied Physiology ◽

10.1152/jappl.2001.90.1.308 ◽

2001 ◽

Vol 90 (1) ◽

pp. 308-316 ◽

Cited By ~ 18

Author(s):

B. G. Leshnower ◽

J. T. Potts ◽

M. G. Garry ◽

J. H. Mitchell

Keyword(s):

Skeletal Muscle ◽

Muscle Contraction ◽

Cardiovascular Response ◽

Afferent Fiber ◽

Cardiovascular Responses ◽

Muscle Afferents ◽

Triceps Surae ◽

Muscle Stretch ◽

Group Iii ◽

Passive Muscle

It is well known that the exercise pressor reflex (EPR) is mediated by group III and IV skeletal muscle afferent fibers, which exhibit unique discharge responses to mechanical and chemical stimuli. Based on the difference in discharge patterns of group III and IV muscle afferents, we hypothesized that activation of mechanically sensitive (MS) fibers would evoke a different pattern of cardiovascular responses compared with activation of both MS and chemosensitive (CS) fibers. Experiments were conducted in chloralose-urethane-anesthetized cats ( n = 10). Passive muscle stretch was used to activate MS afferents, and electrically evoked contraction of the triceps surae was used to activate both MS and CS muscle afferents. No significant differences were shown in reflex heart rate and mean arterial pressure (MAP) responses between passive muscle stretch and evoked muscle contraction. However, when the reflex responses were matched according to tension-time index (TTI), the peak MAP response (67 ± 4 vs. 56 ± 4 mmHg, P < 0.05) was significantly greater at higher TTI (427 ± 18 vs. 304 ± 13 kg · s, high vs. low TTI, P < 0.05), despite different modes of afferent fiber activation. When the same mode of afferent fiber activation was compared, the peak MAP response (65 ± 7 vs. 55 ± 5 mmHg, P < 0.05) was again predicted by the magnitude of TTI (422 ± 24 vs. 298 ± 19 kg · s, high vs. low TTI, P < 0.05). Total sensory input from skeletal muscle ergoreceptors, as predicted by TTI and not the modality of afferent fiber activation (muscle contraction vs. passive stretch), is suggested to be the primary determinant of the magnitude of the EPR-evoked cardiovascular response.

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Gadolinium attenuates exercise pressor reflex in cats

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.2001.280.5.h2153 ◽

2001 ◽

Vol 280 (5) ◽

pp. H2153-H2161 ◽

Cited By ~ 61

Author(s):

Shawn G. Hayes ◽

Marc P. Kaufman

Keyword(s):

Pressor Response ◽

Oxygen Demand ◽

Muscle Afferents ◽

Triceps Surae ◽

Mechanical Stimuli ◽

Group Iii ◽

Exercise Pressor Reflex ◽

Static Contraction ◽

Pressor Reflex ◽

Triceps Surae Muscles

The exercise pressor reflex, which arises from the contraction-induced stimulation of group III and IV muscle afferents, is widely believed to be evoked by metabolic stimuli signaling a mismatch between blood/oxygen demand and supply in the working muscles. Nevertheless, mechanical stimuli may also play a role in evoking the exercise pressor reflex. To determine this role, we examined the effect of gadolinium, which blocks mechanosensitive channels, on the exercise pressor reflex in both decerebrate and α-chloralose-anesthetized cats. We found that gadolinium (10 mM; 1 ml) injected into the femoral artery significantly attenuated the reflex pressor responses to static contraction of the triceps surae muscles and to stretch of the calcaneal (Achilles) tendon. In contrast, gadolinium had no effect on the reflex pressor response to femoral arterial injection of capsaicin (5 μg). In addition, gadolinium significantly attenuated the responses of group III muscle afferents, many of which are mechanically sensitive, to both static contraction and to tendon stretch. Gadolinium, however, had no effect on the responses of group IV muscle afferents, many of which are metabolically sensitive, to either static contraction or to capsaicin injection. We conclude that mechanical stimuli arising in contracting skeletal muscles contribute to the elicitation of the exercise pressor reflex.

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Group III/IV muscle afferents limit the intramuscular metabolic perturbation during whole body exercise in humans

The Journal of Physiology ◽

10.1113/jp272283 ◽

2016 ◽

Vol 594 (18) ◽

pp. 5303-5315 ◽

Cited By ~ 68

Author(s):

Gregory M. Blain ◽

Tyler S. Mangum ◽

Simranjit K. Sidhu ◽

Joshua C. Weavil ◽

Thomas J. Hureau ◽

...

Keyword(s):

Muscle Afferents ◽

Whole Body ◽

Group Iii ◽

Metabolic Perturbation ◽

Exercise In Humans

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