Differential control of blood flow in masseter and biceps brachii muscles during stress

Objective: The present study aimed to compare sympathetic hemodynamic effects in masticatory and limb muscles in response to different stressors. Design: Twelve healthy participants were subjected to a randomized series of stressors, including cold pressor test (CPT), mental arithmetic test, apnea, isometric handgrip (IHG) and post-handgrip muscle ischemia (PHGMI), while in the supine position. Spatially-resolved near-infrared spectroscopy was used to measure relative changes in blood volume and oxygenation (TOI) of the resting masseter and biceps muscles. Cardiac output, heart rate, and arterial blood pressure (ABP) were also monitored. Results: Except apnea, all tests increased ABP. Different response patterns were observed in the 2 muscles: TOI significantly increased during contralateral IHG (1.24 & PLUSMN; 1.17%) but markedly decreased during CPT (-4.84 & PLUSMN; 4.09%) and PHGMI (-6.65 & PLUSMN; 5.31%) in the biceps muscle, while exhibiting consistent increases in the masseter (1.88 +/- 1.85%; 1.60 +/- 1.75%; 1.06 +/- 3.29%, respectively) (p < 0.05). Conclusions: The results allow us to infer differential control of blood flow in head and limb muscles. In general, the masseter appears more prone to dilatation than the biceps, exhibiting opposite changes in response to painful stimuli (CPT and PHGMI). Several mechanisms may mediate this effect, including reduced sympathetic outflow to the extracranial vasculature of the head, generally exposed to lower hydrostatic loads than the rest of the body.