Question

When a muscle contracts, the A. transverse tubules shorten, drawing the myofibrils and sarcoplasmic reticulum closer. B. thin filaments and thick filaments of the sarcomere shorten. C. light chains dissociate from the heavy chains of myosin. D. \(\mathrm{H}\) bands and I bands of the sarcomere shorten because thin filaments and thick filaments slide past each other. E. cross-linking of proteins in the heavy filaments increases.

Short answer

A short answer: According to the sliding filament theory, when a muscle contracts, the H bands and I bands of the sarcomere shorten as a result of thin filaments (actin) sliding past the thick filaments (myosin), causing muscle contraction.

Step-by-step solution

Understand muscle structure and sarcomere

In order to answer this question, we must first understand the basic structure of a muscle. A muscle is made up of myofibrils, which are, in turn, made up of sarcomeres. A sarcomere is the functional unit of muscle and contains two main types of filaments - thick filaments (composed of myosin) and thin filaments (composed of actin). The sarcomere can be divided into several regions, including the I band (made up of thin filaments) and the H band (made up of thick filaments).

Understand the sliding filament theory

The sliding filament theory explains the mechanism of muscle contraction. According to this theory, muscle contraction occurs when the thin filaments (actin) slide past the thick filaments (myosin), resulting in the shortening or overlapping of these filaments. The sliding happens due to the interaction between the myosin heads (part of the thick filaments) and the actin-binding sites (part of the thin filaments). This interaction leads to the formation of cross-bridges, which generate force, and consequently, the muscle shortens.

Evaluate the given answer choices

Now, let's evaluate each answer choice to find the one that best describes the events during muscle contraction: A. This option describes the events taking place during muscle contraction but mixes certain processes. The transverse tubules' shortening is not what draws the myofibrils and sarcoplasmic reticulum closer. B. This option states that the thin and thick filaments shorten, which is not accurate. It's the sliding of these filaments past each other that causes contraction, not their actual shortening. C. The light chains dissociating from the heavy chains of myosin is not relevant to muscle contraction. The interaction between myosin and actin is the key here, not the dissociation of the chains. D. This option correctly describes the H bands and I bands shortening as a result of the thin filaments and thick filaments sliding past each other, which causes muscle contraction - according to the sliding filament theory. E. Cross-linking of proteins in the heavy filaments (myosin) is not what leads to muscle contraction. It's again the sliding of the thin and thick filaments that cause the muscle to contract.

Choose the correct answer

Based on our analysis in Step 3, the correct answer is D. The H bands and I bands of the sarcomere shorten because thin filaments and thick filaments slide past each other during muscle contraction, which is consistent with the sliding filament theory.