Mechanism of Muscle Contraction
The sliding filament theory provides an explanation for muscle contraction, stating that thin filaments slide over thick filaments. Muscle contraction is initiated by a signal from the central nervous system (CNS) transmitted through a motor neuron. The motor neuron and the muscle fibers connect to form a motor unit. At the neuromuscular junction, the point of contact between the motor neuron and the muscle fiber’s sarcolemma, acetylcholine is released as a neurotransmitter, generating an action potential in the sarcolemma.
The action potential spreads through the muscle fiber, causing the release of calcium ions into the sarcoplasm. These calcium ions bind to a subunit of actins on actin filaments, uncovering active sites for myosin. Utilizing ATP hydrolysis, the myosin head binds to the exposed active sites on actin, forming a cross-bridge. The cross-bridge formation results in the pulling of attached actin filaments toward the center of the A-band. As a consequence, the Z-lines attached to these actins are also drawn inward, shortening the sarcomere and leading to muscle contraction.
During muscle contraction, the I-bands decrease in length, while the A-bands maintain their length. The myosin releases ADP and P1, returning to its relaxed state. A new ATP molecule binds, breaking the cross-bridge. ATP is hydrolyzed by the myosin head, initiating the cycle of cross-bridge formation and breakage, causing further sliding of the filaments. This process continues until calcium ions are pumped back to the sarcoplasmic reticulum, resulting in the masking of actin filaments and the return of Z-lines to their original position, leading to muscle relaxation.
The reaction time of muscle fibers can vary across different muscles. Repeated muscle activation can result in the accumulation of lactic acid due to the anaerobic breakdown of glycogen, leading to fatigue. Myoglobin, a red-colored oxygen-storing pigment, is found in muscles and is particularly high in red fibers. Red fibers appear reddish and contain numerous mitochondria for ATP production. On the other hand, white fibers have lower myoglobin content, a pale or whitish appearance, and fewer mitochondria, but a high amount of sarcoplasmic reticulum. White fibers rely on anaerobic processes for energy production.
NCERT Notes for Class 11 Biology Chapter 17: Locomotion and Movement
NCERT and CBSE Notes for Class 11 Biology Chapter 17: Locomotion and Movement: The given article is a detailed introduction to the class 11 chapter “Locomotion and Movement”. The article discusses all the important topics and their related sub-topics which include muscles, types of muscles, muscle contraction, skeletal system, and a brief illustration of different types of bones present in the human body.
Notes on Locomotion and Movement of NCERT Class 11 The article also looks into the structure of contractile protein and its functioning. A bunch of frequently asked questions are also included in this article for better understanding.
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