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Action potential/Resting potential/Depolarization/Repolarization The inside of the membrane is usually around -60 to -90 mV, relative to the outside. An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. In general, there is a high concentration of sodium in the extracellular fluid and a low concentration of sodium in the intracellular fluid or cytosol. This difference in ion distribution contributes to the resting membrane potential of a cell and is maintained by the sodium potassium pump. You will learn more about the sodium potassium pump when we study the nerve system chapter in Chapter 12. Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to depolarize - meaning that the membrane potential of the muscle fiber becomes less negative (closer to zero) As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open Sodium ions enter the muscle fiber, and an action potential rapidly spreads (or “fires”) along the entire membrane to initiate excitation-contraction coupling. Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential.
Muscle Contraction and Relaxation Muscle Contraction – A cross-bridge forms between actin and the myosin heads triggering contraction. As long as Ca++^ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten. Muscle Relaxation - Ca++^ ions are pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle may stop contracting when it runs out of ATP and becomes fatigued.
The Sliding Filament Model of Contraction When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap completely.
Sources of ATP ATP supplies the energy for muscle contraction to take place. In addition to its direct role in the cross-bridge cycle, ATP also provides the energy for the active-transport Ca++^ pumps in the SR. The amount of ATP stored in muscle is very low, only sufficient to power a few seconds worth of contractions. There are three mechanisms by which ATP can be regenerated:
- Creatine phosphate metabolism,
- Anaerobic glycolysis, and fermentation and
- Aerobic respiration 1. Creatine phosphate metabolism In a resting muscle, excess ATP transfers its energy to creatine, producing ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly create more ATP. Creatine phosphate-derived ATP powers the first few seconds of muscle contraction (about 15 seconds)
Muscle Strength and Disorder Muscle strength is directly related to the number of myofibrils and sarcomeres within each fiber but the number of skeletal muscle fibers stays the same. Hypertrophy - an increase the production of sarcomeres and myofibrils within the muscle fibers (and artificial anabolic steroids) , which results in the increased mass and bulk in a skeletal muscle. Atrophy - decreased use of a skeletal muscle results, the number of sarcomeres and myofibrils disappeared (e.g., Polio). Disorder - Muscular Dystrophy (Duchenne muscular dystrophy (DMD) is a progressive weakening of the skeletal muscles. DMD is caused by a lack of the protein dystrophin, which helps the thin filaments of myofibrils bind to the sarcolemma. DMD is an inherited disorder caused by an abnormal X chromosome. It primarily affects males, and it is usually diagnosed in early childhood. Types of skeletal muscle contractions There are two main types of skeletal muscle contractions:
- load – is an object to be moved, muscle tension - force generated. 1. Isotonic contractions - the tension in the muscle stays constant, a load is moved as the length of the muscle changes (shortens). There are two types of isotonic contractions: a. Concentric contraction involves the muscle shortening to move a load (biceps brachii muscle) b. Eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens - Eccentric contractions are also used for movement and balance of the body.
- Isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint.
- Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load. In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability.
