Chapter 20: Problem 52
Electron microscopic studies of the sarcomeres have revealed that during muscle contraction (a) The width of \(\mathrm{A}\) -band remains constant (b) The width of the \(\mathrm{H}\) -zone increases (c) The width of I-band increases (d) The diameter of the fibre increases
Short Answer
Expert verified
The only accurate statement regarding the structural changes in a sarcomere during muscle contraction is option (a): 'The width of A-band remains constant'.
Step by step solution
01
Analyzing the A-band
The A-band corresponds to the length of the myosin (thick) filament in the sarcomere. During muscle contraction, the actin (thin) filaments slide over the myosin filaments, but the length of the myosin filaments remains unchanged. Hence, the width of the A-band remains constant. This aligns perfectly with option (a).
02
Analyzing the H-zone
The H-zone is the region in the center of the A-band that is not overlapped by actin filaments. During muscle contraction, the actin filaments slide towards the center of the sarcomere and actually invade the H-zone. This causes the H-zone to decrease in width, which contradicts option (b), where it says that the width of the H-zone increases.
03
Analyzing the I-band
The I-band is the region of the sarcomere that only contains actin (thin) filaments. During muscle contraction, as the actin filaments slide over the myosin filaments, the I-band shortens. This contradicts option (c) which says that the width of I-band increases.
04
Analyzing the fiber diameter
The fiber diameter corresponds to the overall sarcomere width. During muscle contraction, the sliding filament model suggests no changes to the diameter of the sarcomere (or fiber). Thus, the statement in option (d) is incorrect.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Sliding Filament Theory
The sliding filament theory is a well-established explanation of how muscles contract to produce force. It involves the interaction and movement of actin and myosin filaments within the sarcomere, the basic unit of a muscle fiber.
Imagine two sets of interlacing fingers (filaments) pulling closer together without either set changing length; this is how we envision the actin (thin) filaments sliding over the myosin (thick) filaments. The power strokes of myosin heads attach to the actin filaments and pull them towards the center of the sarcomere. This movement causes the sarcomere to shorten, leading to muscle contraction. Importantly, the filaments themselves do not shorten; they merely slide past each other.
Imagine two sets of interlacing fingers (filaments) pulling closer together without either set changing length; this is how we envision the actin (thin) filaments sliding over the myosin (thick) filaments. The power strokes of myosin heads attach to the actin filaments and pull them towards the center of the sarcomere. This movement causes the sarcomere to shorten, leading to muscle contraction. Importantly, the filaments themselves do not shorten; they merely slide past each other.
Muscle Contraction Mechanism
The muscle contraction mechanism involves complex physiological and biochemical processes, initiated by a signal from the nervous system. Once a signal is received, calcium ions are released inside the muscle fiber, leading to changes in the muscle proteins. These changes allow myosin heads to bind to actin filaments.
ATP and Muscle Contraction
ATP (adenosine triphosphate) plays a key role by providing the energy needed for myosin to perform successive power strokes. This whole process repeats several times during a contraction, with myosin heads detaching and reattaching to new positions on the actin filaments. The collective result is the shortening of the sarcomere and the contraction of the entire muscle.- Calcium ions bind
- Myosin heads attach
- Power strokes occur
- Myosin detaches and reattaches
A-band in Sarcomere
The A-band is part of the sarcomere structure, representing the area where myosin filaments are present. It appears as the dark region under a microscope and maintains a constant width during contraction. This constancy reflects the fact that the length of the myosin filaments does not change during contraction; it is only the relative positioning of actin to myosin that alters.
The A-band includes regions of overlap where actin and myosin filaments lie side by side as well as a central part, called the H-zone, where only myosin filaments are found. The preservation of A-band width during muscle contraction is a key piece of evidence supporting the sliding filament theory.
The A-band includes regions of overlap where actin and myosin filaments lie side by side as well as a central part, called the H-zone, where only myosin filaments are found. The preservation of A-band width during muscle contraction is a key piece of evidence supporting the sliding filament theory.
H-zone in Muscle Contraction
During muscle contraction, the H-zone undergoes a notable transformation. Located at the center of the A-band, the H-zone contains only myosin filaments and is marked by a lighter area visible under a microscope. When contraction occurs, this zone decreases in width because the actin filaments slide inward, reducing the myosin-only region.
As the muscle contracts, the edges of the actin filaments come closer to one another within the A-band, causing the H-zone to nearly disappear at full contraction. The change in the H-zone's width is a visual cue for the extent of sarcomere shortening and thus serves as an indicator of muscle contraction intensity.
As the muscle contracts, the edges of the actin filaments come closer to one another within the A-band, causing the H-zone to nearly disappear at full contraction. The change in the H-zone's width is a visual cue for the extent of sarcomere shortening and thus serves as an indicator of muscle contraction intensity.
I-band in Sarcomere
The I-band is the lighter region of the sarcomere, primarily comprising actin filaments, and it flanks either side of the Z-line (the demarcation between sarcomeres). During muscle contraction, the sliding of actin filaments toward the center of the sarcomere results in a reduced width of the I-band.
A decrease in I-band width is another hallmark of muscle contraction according to the sliding filament theory. Because it contains portions of actin filaments not overlapping with myosin, the I-band shortening visually represents the overlap increase between actin and myosin, thus providing a microscopic view of the muscle contraction process in progress.
A decrease in I-band width is another hallmark of muscle contraction according to the sliding filament theory. Because it contains portions of actin filaments not overlapping with myosin, the I-band shortening visually represents the overlap increase between actin and myosin, thus providing a microscopic view of the muscle contraction process in progress.