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Your muscles make hope molecules
Muscle as a pharmacy
Scientists a couple of decades ago would have described muscles as machinery for moving bones around and stabilizing the skeleton. The picture now is closer to a pharmacy. Muscles manufacture chemicals that fight cancer cells, boost the immune system, protect the heart, regulate blood sugar, and help build strength.
Signals reaching the brain
Muscle contraction pushes these chemicals into the bloodstream, and some of them travel to the brain. There they help the body recover from stress, make a person more resilient, and boost motivation, which is why they get called hope molecules. The research comes out of exercise science, but the body does not need a gym session to respond. A walk, lifting heavy things around the house, or a dance party in the living room all register as the body engaging with life.
A body that is less active releases less of this, which makes going from nothing to anything the biggest step available.
How do muscles send chemical signals to the brain?
Contracting muscle fibers release proteins called myokines into the bloodstream, and researchers have catalogued hundreds of them. Danish researchers led by Bente Klarlund Pedersen reported around the turn of the century that the signaling protein interleukin-6 rose sharply in the blood during exercise and was coming from the working muscle itself. A 2016 study in Cell Metabolism by Hyo Youl Moon and colleagues found that in mice, an enzyme released by muscle called cathepsin B reached the brain and raised levels of BDNF, a protein that supports the growth of neurons.
Why was muscle long seen as only machinery?
For most of the twentieth century, signaling molecules were thought to come mainly from glands and immune cells, while muscle was studied for force and fuel use. The shift began when Pedersen's group showed that muscle itself secretes messengers into the blood. In 2003 the group proposed the name myokines for muscle-derived signaling proteins. Muscle is now classed as an endocrine organ in much of the exercise science literature.
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