Question

Name all of the proteins involved in the sliding filament theory and their respective roles. And explain how muscle excitation and contraction are coupled.

Answer 1

Ans:

• Proteins involved in the sliding filament theory of muscle contraction are:

(i) Actin: Actin is a spherical protein that forms the thin filament in muscle cells. Thin filaments are composed of two long chains of these actin molecules that are twisted around one another. Each actin molecule has a myosin-binding site where a myosin head can bind.

(ii) Myosin: Myosin is a motor protein that generates the force in a muscle contraction. It consists of a head and a tail region. Together, the tails of approximately three hundred myosin molecules form the shaft of the thick filament. The myosin heads of these molecules project outward towards the thin filaments like the oars of a rowboat.

(iii) Troponin: Troponin is the sarcomeric Ca2+ regulator for striated (skeletal and cardiac) muscle contraction. On binding Ca2+ troponin transmits information via structural changes throughout the actin-tropomyosin filaments, activating myosin ATPase activity and muscle contraction.

(iv) Tropomyosin: Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation, which prevents contraction in a muscle without nervous input. The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule.

• Muscle excitation and contraction are coupled to form an Excitation-Contraction Coupling. It is the process by which an electrical stimulus triggers the release of calcium by the sarcoplasmic reticulum, initiating the mechanism of muscle contraction by sarcomere shortening. This coupling involves the sequence of events by which an action potential in the sarcolemma of the muscle cell initiates the sliding of the myofilaments, resulting in contraction. Excitation-contraction coupling can be categorized into three phases:

1. The spread of depolarization

2. The binding of calcium to troponin

3. The generation of force

Excitation-contraction coupling begins with depolarization and spread of an action potential along the sarcolemma and continues with the propagation of the action potential into the T tubules. An action potential in the T tubules causes the release of calcium from the lateral sacs of the sarcoplasmic reticulum. When calcium is released from the sarcoplasmic reticulum (the second phase), it binds to the troponin molecules on the thin filament. The binding of calcium to troponin causes troponin to undergo a configurational change, thereby removing tropomyosin from its blocking position on the actin filament. The third phase of excitation-contraction coupling is the cross-bridging cycle. The cross-bridging cycle describes the cyclic events that are necessary for the generation of force or tension within the myosin heads during muscle contraction.