Myalgia and Fatigue: Translation from Mouse Sensory Neurons to Fibromyalgia and Chronic Fatigue Syndromes

Muscle fatigue and pain are among the most common complaints at emergency rooms and clinics across the country. Fatigue and pain are often acute, remitting spontaneously or appearing to be attenuated by a variety of drugs and treatment modalities. In spite of these remissions, popular magazines (e.g., estimate that each year Americans spend over $30 billion on herbal remedies and $50 billion on alternative therapies to treat symptoms that include muscle pain and fatigue. These statistics indicate that even acute muscle pain and fatigue are serious health problems that are not adequately addressed by current medical practice. Occasionally, muscle pain and fatigue take on a chronic nature, leading to syndromes including chronic fatigue syndrome (CFS) and fibromyalgia syndrome (FMS)—devastating conditions characterized by continuing, debilitating fatigue, which is made worse by even mild exercise in the case of CFS and by chronic widespread pain (CWP) with a particular emphasis in the muscles, which can prevent most or all activities in the case of FMS. Both of these conditions are frequently associated with each other and with a variety of other illnesses, such as temporomandibular disorder (TMD), irritable bowel syndrome (IBS), and multiple chemical sensitivity. These syndromes destroy lives, respond poorly to current treatment strategies, and can lead to exhaustion of the financial resources of afflicted patients. Together, these disorders affect 7 to 20 million people in the United States each year, as reported by various authorities (Reeves et al. 2007). Clearly, patients with these syndromes deserve a concerted research effort to understand, treat, and eventually cure these illnesses. In contrast to cutaneous pain, which has been thoroughly studied and is comparatively well understood, the molecular mechanisms for muscle pain are still unknown. Even more enigmatic is the symptom of debilitating fatigue. Mosso, in his compendious volume on the subject a century ago, remarked that all cultures seem to have just one word for (Mosso 1904). Yet describes many conditions, including failure of muscle fibers to shorten normally, deficient motor command signals, feelings of tiredness, heaviness, pressure, and weakness from muscles, and a feeling of mental fatigue that impedes concentration and performance of conceptual tasks. The subject of most physiological investigations of fatigue has been muscle contraction. Decreased function causing failure of voluntary muscle contraction can occur at all levels of the neuromuscular system, including the motor cortex, signaling to motoneurons, motoneuron signals to the muscle, excitation-contraction coupling in the muscle, and actin-myosin filament interactions. However, the most common failure is a decrease in the motor command signal from the motor cortex (see recent reports and reviews by Bellinger et al. 2008; Gibson et al. 2003; Noakes et al. 2005; St Clair and Noakes 2004). A recent review suggests that failures in voluntary muscle contraction are most often caused by a central comparator that integrates homeostatic inputs from many physiological systems and shuts down motor commands when energy resources are threatened (Noakes 2007). One of the homeostatic inputs is suggested to “originate from a difference between subconscious representations of baseline physiological homeostatic state and the state of physiological activity induced by physical activity, which creates a second order representation which is perceived by consciousnessproducing structures as the sensation of fatigue” (Gibson et al. 2003, page 174). We suggest that there is a simpler sensation of fatigue that is triggered by inputs from specific receptors that are sensitive to metabolites produced by muscle contraction. We further propose that this elementary sensation is transduced, conducted, and perceived within a unique sensory system with properties analogous to other sensory modalities such as pain. We call it the “sensation of muscle fatigue.”