Describe the EC and the players involved in the electrical a

Describe the E-C and the players involved in the electrical and mechanical events leading to the skeletal muscle contraction and relaxation. Use the following terms in your description: chemically-gated ion channels, Na+ voltage-gated, K+-voltage-gated channels, threshold, calcium, end plate potential, motor unit, muscle fiber (myocyte) (muscle cell), Acetylcholine, acetyl choline esterase, sarcolemma, T tubules, latent period, contraction period, relaxation period, refractory period, SR, cross bridges, ATP, troponin, tropomyosin, myosin, actin, sarcomere, z discs, sarcomere, contraction, relaxation.

Solution

A rapid and large change in membrane potential of excitable cells (such as nerve cells) during which the potential actually reverses so that the inside of the excitable cell transiently becomes more positive than the outside is termed as action potential.

Action potential allows communication over long distances within the body. An action potential in a muscle fibre is called muscle action potential, when an action potential occurs in a nerve cell, it is called a nerve action potential (nerve impulse). Skeletal muscle consists of numerous bundles called fascicles. Each fasciculus is composed of numerous muscle fibers (or muscle cells) and each individual muscle cells is surrounded by connective tissue called the endomysium. The cell membrane of the muscle fibre is called the sarcolemma. the sacoplasm is the specialized cytoplasm os a muscle fibre that contains a modified ER known as the sarcoplasmic reticulum. Transverse (T) tubules invaginate the sarcolemma, allowing impulses to penetrate the cell and activate the sarcoplasmic reticulum. Sarcoplasm contains fine thread like contractile structure called myofibrils. In each myofibrils actin thin filaments and myosin thick filaments are organized into a linear chain of highly ordered structure called sacomeres, the basic functional unit of a myofibril. One end of the thin filaments is attached at their plus ends to the Z disc. Thin filaments contain the proteins actin, nebulin, tropomodulin, tropomyosin and troponin. Tropomyosin is a fibrous molecule that consists of two chains alpha and beta, that attach to F-actin in the groove its filaments. Troponin a complex of three polypeptide troponins T, I and C ( named foe their tropomysin binding, inhibitory and calcium binding activities respectively). Four molecules of calcium ion bind per molecule of troponin.

Generation of an Action Potential Across the Sarcolemma : Movements of ions occur through ion channels, ion channels may be either leaky channels or gated channels. Action potential are the direct consequence of the voltage gated cation channels.

Steps of E-C coupling:

1. Action potential generated and propagated along sarcomere to T-tubules

2. Action potential triggers Ca2+ release

3. Ca++ bind to troponin; blocking action of tropomyosin released

4. contraction via crossbridge formation; ATP hyrdolysis

5. Removal of Ca+2 by active transport

6. tropomyosin blockage restored; contraction ends.

The period of time after an action potential begins during which an excitable cell cannot genetrate another action potential in response to a normal threshold stimulus is called the refractory period.

Time taken for the excitation-contraction coupling to occur is known as latent period. The contraction period is the time during which tension develops in the muscle, and the relaxation period is the time during which tension is removed from the muscle.

Latent period: the time following stimulation, first few msec after stimulus; EC coupling taking place.

Period of contraction: time when cross bridges are active, cross bridges form; muscle shortens.

Period of relaxation: time when cross bridges are inactive, Ca2+ reabsorbed; muscle tension goes to zero