Question

* Describe the relationship between Ca++, troponin, tropomyosin, and the ability of myosin to bind actin. * Describe how myosin and actin interact for a muscle to contract.

Answer 1

Skeletal muscles process muscle bundle containing number of muscle fibres each muscle fibre is lined by the plasma membrane called Sarcolemma in enclosing the sarcoplasm.

muscle fibre is a syncitium as the sarcoplasm contain many nuclei the endoplasmic reticulum that is sarcoplasmic reticulum of the muscle fibre is the store house of calcium ion . A characteristic feature of the muscle fibre is the presence of large number of parallel arrangement of elements in the sarcoplasm called myofilaments aur myofibrils. Each myofibril has alternate dark and light bands on it , that is too important proteins actin and myosin the light to band contain actin and is called I or isotropic band where as dark band called A or anisotropic band contain myosin . Both the proteins are arranged as rod like structure parallel to each other. actin filaments are thinner as compared to the myosin filaments hence are commonly called thin and thick filaments respectively in the centre of I band is an elastic fibres called is z line which bisect it . The thin filaments are firmly attached to the z line. the thick filament in the a band are also held together in the middle of this band by thin fibrous membrane called M line . The A and I band are arranged alternatively throughout the length of myofibrils . The portion of the myofibril between two successive is a line is considered as the functional unit of contraction and is called sarcomere.

structure of contractile protein .

Each actin filament is made of two filamentous actin for or F actin helically wound to each other . Each F actin is a polymer of monomeric globular actin or G actin.

to filaments of another protein tropomyosin also run close to F actins throughout its length . A complex protein troponin is distributed at regular intervals on the tropomyosin. Inthe resting state a subunit of troponin mask reactive binding sites for myosin on the actin filaments. Each myosin is a monomeric protein of Mero myosin constitutive and thick filament . Each meromyosin has two important parts a globular head with the short arm and a tail . The globular head is an active ATP is enzyme and has binding site for ATPase and active site for actin.the head and short arm project outwards at regular distance and the angle from each other from the surface of polymerized myosin filament and is known as cross arm.

mechanism of muscle contraction.

mechanism of muscle contraction is best explained by the sliding filament theory which state that contraction of a muscle fibre takes place by the sliding of thin filaments over the thick filament. Muscle contraction is initiated by a signal send by the central nervous system through a motor neuron and motor neuron along with the muscle fibre connected to it constituted motor unit . The junction between a motor neuron and the sarcolemma of the muscle fibre is called neuromuscular junction or motor end plate. neural signals reaching this junction release a neurotransmitter acetyl choline which generate an action potential in the sarcolemma . This spread through the muscle fibre and causes the release of calcium ions into the sarcoplasm . Increasing calcium level lead to the binding of calcium with a subunit of troponin on actin filaments. This will remove the masking of active sites for myosin. In utilising the energy from ATP hydrolysis the myosin head bind to the exposed active site on actin to form a cross bridge. this police the attached acting filaments towards the centre of a band the sideline attached to this actings are also called in words there by causing a shortening of sarcomere which causes contraction. So during shortening of muscle contraction the I band get reduced whereas the A band retain the length. The myosin releasing the ADP and phosphate goes back to its relaxed state. A new ATP binds and the cross bridge is broken. The ATP is again hydrolysed by the myosin in head and cycle of crossbridge formation and breakage is repeated causing further sliding.the process continues till the calcium ions are pump back to the sarcoplasmic cisternae resulting in the masking of actin filaments this causes the return of the Z lines back to their original position which is called relaxation.