Question

What is sarcomere? (a) Part between two H-lines (b) Part between two A-lines (c) Part between two l-bands (d) Part between two Z-lines

Short answer

The sarcomere is the part between two Z-lines, so the answer is (d) Part between two Z-lines.

Step-by-step solution

Understanding sarcomere structure

A sarcomere is the fundamental functional unit of striated muscle fibers, such as skeletal and cardiac muscles. It is defined by the region between two adjacent Z-lines (discs), and it is the segment of the myofibril where muscle contraction occurs.

Identifying the correct option

With the definition of a sarcomere in mind, look for the option that correctly identifies the sarcomere as the part between two entities within the muscle fiber. Only one option corresponds to the definition of a sarcomere as the region from one Z-line to the next.

Choosing the correct answer

Review each given option and compare it to the definition identified in Step 1. The correct answer is the one that matches the structure of a sarcomere as it is commonly defined in anatomy and physiology textbooks.

Key concepts

Muscle Contraction

Understanding how muscles contract involves a look at the microscopic level of a muscle fiber. Muscle contraction occurs when the sarcomere, the basic functional unit of the muscle fiber, shortens. This happens through a complex process known as the sliding filament theory.

During this process, proteins within the sarcomere, mainly myosin and actin, interact to produce movement. Myosin, the thick filament, forms cross-bridges with actin, the thin filament. When energy from ATP is used, myosin heads pivot, pulling the actin filaments toward the center of the sarcomere. This pivot causes the sarcomere to shorten, hence, contracting the muscle.

Several sarcomeres are arranged end to end along the length of myofibrils, and their coordinated contraction leads to the shortening of the entire muscle fiber, and ultimately, the muscle itself. Apart from the ATP required for the process, calcium ions and troponin-tropomyosin complex play a crucial role in the regulation of the contraction.

Striated Muscle Fibers

Striated muscle fibers are distinguishable by their characteristic striped appearance under the microscope, which is due to the organized arrangement of sarcomeres within their myofibrils. These fibers make up skeletal and cardiac muscles.

The striations come from the repeating series of dark and light bands: A-bands and I-bands respectively. A-bands correspond to the regions of overlapping actin and myosin filaments while I-bands contain only actin filaments. The dark A-bands and light I-bands are aligned in a precise pattern, giving the fibers their striped appearance.

The exceptional organization of these filaments within the sarcomeres is essential for the controlled and powerful muscle contractions that are characteristic of these muscle types. These fibers are under voluntary control in skeletal muscles, meaning they contract in response to signals from the nervous system.

Myofibril Anatomy

Delving deeper into the anatomy of myofibrils reveals that they are long, thread-like structures within muscle cells that contain a series of sarcomeres in series. Each myofibril is made up of repeating segments of sarcomeres, and this repetition is what gives the muscle its ability to contract uniformly and efficiently.

Within the myofibril, sarcomeres are demarcated by Z-lines, to which actin filaments are anchored. The M-line runs down the center of the sarcomere and holds myosin filaments in place. The region that spans the entire length of the myosin filaments is known as the H-zone, which is bisected by the M-line.

This complex structure is essential for the muscle's ability to contract and also provides the elastic property necessary to retain the muscle structure upon relaxation. Proper functioning of myofibrils is fundamental to muscle performance and any disruption in their structure can lead to muscle disorders.