Skeletal muscle fibers, fatigue

Zebrafish larvae possess two types of skeletal muscle fibers. Slow (red) muscle fibers, a superficial monolayer on the siuface of the myotome, are equipped for oxidative phosphorylation, can generate relatively large stores of energy, and are most resistant to fatigue. Fast (white) muscle fibers, in the deep portion of the myotome, are least resistant to fatigue because they rely on anaerobic glycolysis for... 

Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiber is the most common. Intrafusal fibers, which make up the central region of the muscle, are weaker than extrafusal fibers. Skeletal muscles fibers are additionally characterized as fast or slow based on their activity patterns. Fast, also called white, muscle fibers contract rapidly, have poor blood supply, operate anaerobically, and fatigue rapidly. Slow, also called red, muscle fibers contract more slowly, have better blood supplies, operate aerobically, and do not fatigue as easily. Slow muscle fibers are used in movements that are sustained, such as maintaining posture. 

Myasthenia gravis is a disease tiiat involves rapid fatigue of skeletal muscles because of die lack of ACh released at die nerve endings of parasympathetic nerve fibers. Drugs used to treat this disorder include ambeno-nium (Mytelase) and pyridostigmine (Mestinon). 

Caffeine has been shown to have a bronchial and smooth muscle relaxant effect and to improve skeletal muscle contractility. Significant increases in hand tremor and forearm extensor electromyogram were observed in human subjects after the ingestion of 6mgkg of caffeine. This effect is more likely due to a CNS stimulatory effect than to direct action on the muscle fibers. Skeletal muscle fatigue can be reversed by high concentrations of caffeine obtained only in vitro but not in vivo. 

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