Problem 8 - Biology Today and Tomorrow with Physiology

Question

A sarcomere shortens when _____________. a. thick filaments shorten b. thin filaments shorten c. both thick and thin filaments shorten d. none of the above

Step-by-Step Solution

Answer

The correct answer is d. none of the above.

Step 1: Understanding the Sarcomere Structure

A sarcomere is the basic contractile unit of muscle fiber, comprised of thick filaments (myosin) and thin filaments (actin). These filaments interact during muscle contraction but do not shorten themselves.

Step 2: Identifying the Correct Contraction Mechanism

During muscle contraction, the sarcomere shortens as the thick and thin filaments slide past one another, known as the sliding filament theory. The filaments overlap more without a change in length.

Step 3: Evaluating the Options

Given the understanding of sarcomere function, neither thick nor thin filaments change in length (they do not shorten). Instead, they slide past each other, making option d, none of the above, the correct answer.

Key Concepts

Understanding the Sarcomere

A sarcomere is like the building block of muscle tissue, acting as the basic contractile unit within muscle fibers. Imagine it as a tiny segment within each muscle cell that contains the tools, namely the

• thick filaments, primarily made of a protein called myosin
• thin filaments, composed mainly of another protein called actin

These components are carefully aligned to facilitate the process of muscle contraction.

Each sarcomere is flanked by Z-lines, which mark its boundaries. Between these Z-lines, the thick and thin filaments overlap. When looking into a microscope, this creates the striated appearance you see in skeletal muscle.

It’s important to note that while the overall sarcomere shortens during contraction, the thick and thin filaments themselves do not change in length.

Exploring the Sliding Filament Theory

The sliding filament theory is fundamental in explaining how muscles contract. During contraction, the thick and thin filaments within the sarcomere move past each other.

Think of this action as similar to sliding the fingers of one hand between those of the other hand. The fingers themselves do not shorten, but the hand gets smaller as the fingers slide.

This theory states that as the muscle receives a signal to contract, myosin heads on the thick filaments latch onto the actin sites on the thin filaments, forming cross-bridges.

These cross-bridges pull the thin filaments toward the center of the sarcomere, thereby decreasing its length without altering the length of the filaments themselves. The result is a coordinated shortening of the muscle.

Key Roles of Thick and Thin Filaments

In the machinery of muscle contraction, thick and thin filaments play crucial roles. The thick filaments, made of myosin, serve as the "motors." Myosin heads on these filaments act as tiny binding sites that attach to actin on the thin filaments.

Each actin filament in the thin filament contains binding sites that are crucial for myosin to attach during contraction. These interactions are powered by the energy molecule ATP, which provides the energy needed for myosin heads to pull actin filaments toward the center, regardless of the length of the filaments themselves.

It is through the combined operation of these thick and thin filaments sliding past each other that muscles contract and produce motion.