MUSCLE PHYSIOLOGY

• Cessation of interaction between actin and myosin is represented by a diagram of contraction/relaxation.
• Skeletal muscles constitute the great mass of the somatic musculature and have a well-developed cross striations.
• Skeletal muscles do not normally contract in the absence of nerve supply and lack anatomic and functional connections between individual muscle fibers.
• Skeletal muscles are generally under voluntary control.
• Cardiac muscle has cross-striations but functionally syncytial in character and contracts rhythmically in the absence of external innervation because of the presence of pacemaker cells that discharge impulses spontaneously.
• Smooth muscle lacks cross-striations, those found in the hollow viscera are functionally syncytial in character and contain pacemakers that discharge impulses irregularly, while those found in the eye and some other locations are not spontaneously active and resemble skeletal muscle in function.
• Skeletal muscle is composed of individual muscle fibers, begins and ends in tendons, and the force of contraction of the unit is additive.
• Each muscle fiber is a single cell, multinucleated, long and cylindric in shape with no syncytial bridges between cells.
• The muscle fibers are made up of fibrils divided into individual filaments which are in turn made up of the contractile proteins.
• Muscle cells, like neurons, can be excited to produce an action potential that is transmitted along their cell membrane.
• Unlike neurons, muscles have a contractile mechanism that is activated by the action potential.
• There are three types of muscles: skeletal, cardiac, and smooth.
• During muscle contraction, there is shortening of the contractile elements in the muscle brought about by the sliding of the thin filaments over the thick filaments.
• This sliding will move the Z lines closer together while the width of the A band remains the same.
• At the initial stage of contraction, the action potential is transmitted to all the fibrils in the fiber via the T system triggering the release of Ca++ from the sarcoplasmic reticulum.
• This Ca++ release will initiate contraction by binding to troponin C.
• The duration of the muscle twitch varies with the type of muscle being tested: "Fast" muscle fibers are concerned with fine, rapid, precise movement, while "Slow" muscle fibers are concerned with strong, gross, sustained movements.
• Shortly after releasing Ca++, the sarcoplasmic reticulum begins to reaccumulate Ca++ and store it up to the time Ca++ concentration outside of the sarcoplasmic reticulum has been lowered sufficiently then chemical interaction between myosin and actin ceases and the muscle relaxes.
• If the active transport of Ca++ is inhibited, relaxation does not occur even though there are no more action potentials resulting to a sustained contraction.
• The T system consists of transverse tubules which are continuous with the membrane of the muscle fiber and facilitates rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle.
• The sarcolemma forms an irregular curtain around each fibril between its contact with the T system and is concerned with Ca++ movement and cell metabolism.
• There are two kinds of muscle response: electrical and mechanical.
• The sarcolemmal reticulum forms an irregular curtain around each fibril between its contact with the T system, and is concerned with Ca++ movement and cell metabolism.
• Troponin C in skeletal muscle contains the binding sites for the Ca++ that initiates contraction.
• A single action potential causes a brief contraction followed by relaxation, this is called MUSCLE TWITCH.
• The T system in skeletal muscle consists of transverse tubules which are continuous with the membrane of the muscle fiber, facilitating rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle.
• In the resting muscle, troponin I is tightly bound to actin and tropomyosin covers the sites where myosin binds to actin, therefore, the troponin - tropomyosin complex constitutes a "relaxing protein" that inhibits the interaction between actin and myosin.
• The steps in skeletal muscle contraction include the discharge of motor neuron, release of transmitter substance (acetylcholine) at the motor end plate, generation of end-plate potential, generation of action potential in muscle fibers, inward spread of depolarization along T tubules, release of Ca++ from lateral sacs of sarcoplasmic reticulum and diffusion to thick and thin filaments, binding of Ca++ to troponin C, uncovering myosin binding sites on actin, formation of cross-linkages between actin and myosin and sliding of thin on thick filaments, and shortening.
• When the Ca++ released by the action potential binds to troponin C, the binding of troponin I to actin is weakened and this will permit the tropomyosin to move thereby uncovering the binding sites for myosin so that ATP is released - contraction occurs.
• Isometric contraction is a type of contraction without appreciable decrease in the length of the whole muscle; no joint movement.
• The steps in relaxation in skeletal muscle include the pumping back of Ca++, the release of Ca++, and the cessation of interaction between actin and myosin.
• MUSCLE FIBRILS are surrounded by structures made up of a membrane that appear as vesicles and tubules forming the sarcotubular system made up of the T system and the sarcoplasmic reticulum.
• In skeletal muscle, the steps in relaxation include the pumping back of Ca++, the release of Ca++, and the cessation of interaction between actin and myosin.
• Repolarization in skeletal muscle is caused by K+ efflux.
• Troponin I inhibits the interaction of myosin with actin, while Troponin T binds the other two to tropomyosin.
• The electrical events in skeletal muscle and the ionic distribution are similar to those in the nerve resting membrane potential, with a resting potential of -90 mV.
• Isotonic contraction is a contraction against a constant load with approximation of the ends of the muscle; there is joint movement.
• The contractile proteins in skeletal muscle include Myosin (MW 460,000, thick filament), Actin (MW 43,000, thin filament), Tropomyosin (MW 70,000, thin), and Troponin (MW 18 - 35,000, thin).
• The functional unit of the muscle is the sarcomere.
• Tetanus is a response to rapidly repeated stimulation occurring before any relaxation has occurred; responses fuse into one continuous contraction.