FIGURE 3.

MRGPRD (MAS-related GPR family member D), FFAR3 (a G-protein-coupled receptor), GABAaR (gamma-aminobutyric acid type A receptor), GABBR1-2 (gamma-aminobutyric acid type B receptor subunit 1–2), GLRA1-4 (glycine receptor alpha 1–4), and GAD (glutamate decarboxylase) are all involved in the control of muscle tonicity. GABA is a major neurotransmitter that is generated in the central nervous system (CNS) and spinal cord, and its action controls muscle tonicity both centrally and peripherally. BAIBA is a myokine secreted from skeletal muscles that has direct effects on bone/osteocytes in mice. Exercise promotes the secretion of both myokines and osteokines, which can have autocrine and paracrine effects. It is hypothesized that muscle tonicity could potentially influence the release of myokines, which could in turn affect the levels of BAIBA, and vice versa. The receptors for GABA and BAIBA mediate their functions, and certain SNPs may act as modifiers of these effects. Thus, muscle tonicity may represent a novel mechanism for the regulation of myokine release and its effects on bone and muscle.