Physiology

• Enzymes lower the activation energy required for a reaction to occur, increasing the rate of the reaction.
• There are three basic types of muscle tissue: skeletal, cardiac, and smooth.
• Muscle tissue makes up nearly half the body’s mass.
• Muscles have the ability to transform chemical energy (ATP) into mechanical energy, becoming capable of exerting force.
• The three types of muscle tissue are skeletal, cardiac, and smooth.
• Skeletal and smooth muscle cells (but not cardiac muscle cells) are elongated, and for this reason, they are called muscle fibers.
• The prefixes myo or mys or sarco is meaning “ muscle ”.
• The plasma membrane of muscle cells is called the sarcolemma and muscle cell cytoplasm is called sarcoplasm.
• Muscles begin to stiffen 3 to 4 hours after death.
• Dying cells are unable to exclude calcium and the calcium influx into muscle cells promotes formation of myosin cross bridges.
• Cross bridge detachment is ATP driven.
• Sustained high calcium activates apoptosis, leading to cell death.
• Actin and myosin become irreversibly cross-linked, producing the stiffness of rigor mortis, which gradually disappears as muscle proteins break down after death.
• Shortly after breathing stops, ATP synthesis ceases and cross bridge detachment is impossible, this is called rigor mortis.
• Peak rigidity occurs at 12 hours and then gradually dissipates over the next 48 to 60 hours.
• Skeletal muscles are attached to and cover the bony skeleton.
• Skeletal muscle fibers are the longest muscle cells and have obvious stripes called striations.
• Skeletal muscle is called voluntary muscle because it can be activated by reflexes.
• Skeletal muscle is responsible for overall body mobility, contracting rapidly but tiring easily and must rest after short periods of activity.
• Skeletal muscle exerts tremendous power, for example, to lift a car.
• Skeletal muscle is adaptable, for example, to pick up a paper clip and to pick up a book.
• Each skeletal muscle is a discrete organ, made up of several kinds of tissues, with skeletal muscle fibers predominating, but blood vessels, nerve fibers, and substantial amounts of connective tissue are also present.
• Cardiac muscle constitutes the bulk of the heart walls, is striated, but is not voluntary, and can contract without being stimulated by the nervous system.
• Cardiac muscle contracts at a steady rate set by the heart’s pacemaker, but neural controls allow the heart to speed up for brief periods, as when you race across the tennis court to make that overhead smash.
• Smooth muscle tissue is found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages, and its role is to force fluids and other substances through internal body channels.
• Like skeletal muscle, smooth muscle consists of elongated cells, but smooth muscle has no striations.
• Smooth muscle is not subject to voluntary control, its contractions are slow and sustained.
• When intracellular calcium levels are low, the muscle cell is relaxed, and tropomyosin molecules physically block the active (myosin-binding) sites on actin.
• When the cycle is ready to start again, the myosin head is in its upright high-energy configuration, ready to take another “step” and attach to an actin site farther along the thin filament.
• Cross bridge formation (attachment of myosin heads to actin) requires Ca 2+, and calcium ions promote muscle cell contraction.
• Contracting muscles routinely shorten by 30–35% of their total resting length, so each myosin cross bridge attaches and detaches many times during a single contraction.
• As the Ca 2+ pumps of the SR reclaim calcium ions from the cytosol and troponin again changes shape, tropomyosin again blocks actin’s myosin-binding sites, and the contraction ends, and the muscle fiber relaxes.
• Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an action potential along the sarcolemma causes myofilaments to slide.
• The action potential causes the rise in intracellular levels of calcium ions, which allows the filaments to slide.
• When nerve impulses arrive rapidly, intracellular Ca 2+ levels soar due to successive “puffs” of Ca 2+ released from the SR, and the muscle cells do not completely relax between successive stimuli.
• As Ca 2+ levels rise, the ions bind to Tn C, tropomyosin is rolled into the actin helix, away from the myosin-binding sites, and the tropomyosin “blockade” is removed.
• Except for the brief period following muscle cell excitation, calcium ion concentrations in the cytosol are kept almost undetectably low.
• The thin filaments continue to slide as long as the calcium signal and adequate ATP are present.
• Although the action potential itself lasts only a few ms, the contraction phase of a muscle fiber may persist for 100 ms or more.
• To excite a muscle cell, four sets of ion channels are activated: the nerve impulse reaches the axon terminal and opens voltage-gated calcium channels in the axonal membrane, ACh binds to ACh receptors in the sarcolemma, opening chemically (ligand) gated Na-K channels, local depolarization opens voltage-gated sodium channels in the neighboring region of the sarcolemma, and transmission of the action potential along the T tubules stimulates SR calcium release channels to release Ca 2+ into the cytosol.