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Western University
Physiology 2130

Muscle Skeletal muscle structure: - Composed of bundles of multi-nucleated cells - Number of muscle cells in a whole muscle depends on size of whole muscle - Lengths very from 1mm to 12 cm - Myofibril is made of a series of sarcomeres, the basic contractile unit of muscle - Contain glycogen as form of energy store and myoglobin in store O2 Breakdown of muscle: Whole muscle- fascicle- muscle cell/fiber- myofibril- myofilaments (thick and thin) Sarcoplasmic reticulum: forms a sleeve like structure around each myfibril, at the end of each fiber there is a lateral sac or terminal cisternae where calcium is stored. Also wraps around myofibrils Transverse t- tubules: are continuations of the cell membrane that travel into muscle and wrap around myofibrils. They allow the AP to travel inside the muscle cell where contractile proteins are located Thin Myofilament: Made of 3 proteins 1) Actin- a globular protein forming a 2 strand chain, contains myosin binding sites 2) Tropomyosin- rod shaped protein composed of 2 protein chains wrapped to form a super coil. Lies lengthwise on actin strands and each molecule is associated with 6-7 actin globular molecules. In relaxed muscle it is situated to partially cover the myosin binding site 3) Troponin- in relaxed muscle it is attached to trophomyosin and actin so that it holds the trophomyosin molecule over the myosin binding site on the actin Thick Myofilament: made of many individual myosin molecules. Myosin is made of 2 polypeptide chains with an alphahelical head and a globular tail. Head has binding site for actin Sliding Filament Theory: - when myosin binds to actin a cross bridge is formed - myosin then changes shape and a power stroke occurs - actin slides past myosin The Neuromuscular Junction: - the point of contact between the motor nerve and the muscle cell. (similar to a chemical synapse between 2 nerves) 1)AP in motor nerve, depolarization alpha MN terminal 2) VG Ca channels open, influx of Ca down concentration gradient 3) release of Ach, diffusion of ach to muscle cell membrane at endplate 4)
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