During high level of motor activity, the amount of serotonin released increases and a spillover occurs. Serotonin binds to extrasynaptic receptors located on the axon initial segment of motoneurons with the result that nerve impulse initiation and thereby muscle contraction are inhibited. Peripheral muscle fatigue
Peripheral muscle fatigue during physical work is an inability for the body to supply sufficient energy or other metabolites to the contracting muscles to meet the increased energy demand. This is the most common case of physical fatigue—affecting a national average of 72% of adults in the work force in 2002. This causes contractile dysfunction that manifests in the eventual reduction or lack of ability of a single muscle or local group of muscles to do work. The insufficiency of energy, i.e. sub-optimal aerobic metabolism, generally results in the accumulation of lactic acid and other acidic anaerobic metabolic by-products in the muscle, causing the stereotypical burning sensation of local muscle fatigue, though recent studies have indicated otherwise, actually finding that lactic acid is a source of energy.The fundamental difference between the peripheral and central theories of muscle fatigue is that the peripheral model of muscle fatigue assumes failure at one or more sites in the chain that initiates muscle contraction.
Psychogenic pruritus, also known as psychogenic itch or functional itch disorder is pruritus not associated with a dermatologic or systemic cause. More often than not, it is attributed to a psychiatric cause. Psychogenic pruritus is not the same as neuropathic itch though both are conditions which require more research. This condition is not explained well in DSM-V and is typically considered a diagnosis of exclusion. This condition is not well-studied and it is difficult to ascertain as it is seen by both dermatologists and psychiatrists. In order to provide some consensus to this condition, The French Psychodermatology Group have created diagnostic criteria for this condition. Symptoms
Psychogenic pruritus typically appears as itching on the face and on the extensor surfaces of the body. This includes the back side of the arms, the abdomen, the side of the legs and the upper back and shoulders. These areas are more frequent because they are within hands reach. Diagnosis
When considering the diagnosis of psychogenic pruritus, it is important to rule out medical, dermatologic and neuropathic causes. A strong history should be obtained and appropriate labwork should be drawn. One should consider drawing for a CBC (Complete Blood Count), ESR (Erythrocyte Sedimentation Rate), liver, renal and thyroid panel as abnormalities with any can contribute to pruritus. Pruritus can be seen with hepatic and renal disease such as cholestasis, alcoholic liver disease, primary biliary cholangitis, hepatitis B and C, and chronic kidney disease.