Question

4\. Read the given statements and select the correct option. Statement \(1:\) A primary myofilament is composed of a bundle of rod-like molecules of a protein myosin. Statement \(2:\) Myosin and actin together form a contractile apparatus. (a) Both statements 1 and 2 are correct and statement 2 is the correct explanation of statement \(1 .\) (b) Both statements 1 and 2 are correct but statement 2 is not the correct explanation of statement 1 . (c) Statement 1 is correct and statement 2 is incorrect. (d) Both statements 1 and 2 are incorrect.

Short answer

(b) Both statements 1 and 2 are correct but statement 2 is not the correct explanation of statement 1.

Step-by-step solution

Understanding the Statements

Assess each statement independently to determine whether it's correct based on knowledge of muscle structure.

Evaluating the Relationship Between Statements

Analyze if statement 2 is the correct explanation for statement 1 by understanding their scientific connection.

Selecting the Correct Option

After determining the veracity and relationship of the statements, select the option that correctly reflects these conclusions.

Key concepts

Myofilaments Structure

Myofilaments are the essential building blocks of muscle fibers and are crucial for muscle contraction. They are primarily of two types: thick and thin filaments.

Thick filaments are composed of myosin protein molecules, which aggregate in a way that the tails form the central part of the filament and the heads project outward at regular intervals along the filament. These heads are important for muscle contraction as they form cross-bridges with the thin filaments during the process.

Thin filaments, on the other hand, principally consist of actin molecules. They are structured as two strands that twist around one another, and they also contain regulatory proteins called tropomyosin and troponin. These regulatory proteins play key roles in allowing or inhibiting interactions between actin and myosin, depending on whether the muscle is contracting or at rest.

When discussing the myofilaments idiomatically as a 'contractile apparatus', it is referring to the role they play in the physiology of muscle contraction, whereby the sliding of these myofilaments past one another shortens the muscle fiber, leading to contraction.

Myosin Protein

Myosin is an essential motor protein found in muscle cells and performs a pivotal role in the process of muscle contraction. It is the predominant protein forming the thick filaments of the myofilaments structure.

The myosin molecule is shaped like a golf club, with a tail and two protruding heads; it looks somewhat like two golf clubs twisted together. The heads are known as the 'cross-bridges' because they are the parts of the myosin molecules that bind to actin, causing muscle contraction. Myosin heads have binding sites for adenosine triphosphate (ATP), the energy molecule, and actin.

The ATPase activity of myosin heads hydrolyzes ATP to release the energy necessary for muscle contraction. The cycle of myosin heads binding to actin, the power stroke, and detachment powered by ATP is the fundamental mechanism behind muscle movement.

Actin-Myosin Interaction

The interaction between actin and myosin, also known as the cross-bridge cycling, is the heart of muscle contraction. When a muscle fiber receives a signal to contract, calcium ions are released, binding to the troponin on thin filaments. This causes a conformational change that moves tropomyosin away from the myosin binding sites on actin.

With the binding sites exposed, the heads of the myosin molecules attach to the actin forming a cross-bridge. The release of stored energy by the hydrolysis of ATP on the myosin heads then drives a power stroke. During this stroke, the myosin heads pivot, pulling the actin filaments towards the centre of the sarcomere. The heads then release the actin, bind to a new ATP molecule, and detach from the actin, resetting to their original position to start the cycle again.

This cycle repeats countless times during a muscle contraction, with each cycle drawing the actin filaments closer to the center of the sarcomere, thus shortening the muscle fiber and resulting in contraction.

Muscle Anatomy

To understand muscle contraction fully, one must take a step back and consider the overall muscle anatomy. Muscles are made up of muscle fibers, which in turn contain many myofibrils. These myofibrils are the basic unit of contraction in the muscle and are composed of repeating units called sarcomeres.

A sarcomere is the functional unit of the muscle fiber and is delineated by Z-lines. The area between two Z-lines is packed with actin and myosin filaments, which interact to cause muscle contraction. Muscles contract following the all-or-none principle - a muscle fiber will contract to its fullest extent when stimulated adequately.

Moreover, muscles are grouped into bundles surrounded by connective tissues which help transmit the force generated by the contracting muscle to the bones they are attached to, resulting in the movement of limbs and other parts of the body. Understanding the higher-level organization of muscles gives context to the molecular events that cause muscle contraction.