The purpose of the present study was to determine the relative importance of peripheral feedback from mechanically (mechanoreflex) and metabolically (metaboreflex) sensitive muscle afferents and central signals arising from higher centers (central command) to the exercise-induced increases in regional cerebral perfusion. To accomplish this, anterior cerebral artery (ACA) mean blood velocity ( Vmean) responses were assessed during sustained and rhythmic passive calf muscle stretch (mechanoreflex), volitional calf exercise (mechanoreflex, metaboreflex, and central command), and electrically stimulated calf exercise (mechanoreflex and metaboreflex but no central command) at 35% of maximum voluntary contraction ( n = 16). In addition, a period of postexercise muscle ischemia (PEMI) was used to isolate the metaboreflex. Blood pressure, cardiac output, and the end-tidal partial pressure of carbon dioxide (PetCO2) were also measured. ACA Vmean was unchanged from rest during either sustained or rhythmic calf muscle stretch ( P > 0.05). However, ACA Vmean was increased from rest during both isometric (+15 ± 1%) and rhythmic (+15 ± 2%, voluntary exercise P < 0.05) but remained unchanged during stimulated exercise ( P > 0.05). Isometric and rhythmic exercise-induced increases in blood pressure and cardiac output were similar during voluntary and stimulated exercise ( P > 0.05 between conditions). Blood pressure remained elevated during PEMI after all exercise conditions ( P < 0.05 vs. rest), whereas cardiac output and ACA Vmean were not different from rest ( P > 0.05). PetCO2 was unchanged from rest throughout. These data suggest that selective activation of skeletal muscle afferents (i.e., stretch, PEMI, or stimulated exercise) does not increase ACA Vmean and that increases in ACA Vmean during volitional contractions of an exercising calf muscle are dependent on the presence of central command.
Contribution of muscle afferent activation to the anterior cerebral artery blood velocity response to calf exercise in humans
