Test: Muscular System - 1 - MCAT MCQ

Tropomyosin is a protein that plays a regulatory role in muscle contraction. It is positioned along the actin filament in a relaxed muscle fiber, covering the myosin-binding sites on the actin filament. This prevents the myosin heads from binding to actin and inhibits muscle contraction. When calcium ions bind to troponin, a complex associated with tropomyosin, it causes a conformational change in the troponin-tropomyosin complex, uncovering the myosin-binding sites on the actin filament. This allows myosin heads to bind to actin, leading to muscle contraction. Therefore, the primary role of tropomyosin is to regulate the interaction between myosin and actin by blocking or exposing the binding sites on the actin filament.

Remember that acetylcholine functions at the neuromuscular junction, not within the muscle cell - so it would not bind to structural elements like sarcoplasmic reticulum or T-tubules.
Acetylcholine receptors do not release calcium.
Acetylcholine indirectly causes calcium release by opening voltage-gated ion channels.

The organization of muscle components from largest to smallest is as follows:

• Fasciculus: A fasciculus is a bundle of muscle fibers surrounded by connective tissue.
• Muscle fiber: Also known as a muscle cell, a muscle fiber is a single elongated cell that makes up the muscle tissue.
• Myofibril: A myofibril is a cylindrical structure within the muscle fiber composed of contractile proteins called actin and myosin. It is responsible for muscle contraction.

Therefore, the correct order is fasciculus, muscle fiber, and myofibril.

The space between two Z lines in a muscle fiber constitutes a sarcomere. A sarcomere is the functional unit of a muscle fiber and is responsible for muscle contraction. It is defined as the segment between two Z lines, which are structures that anchor the actin filaments. Within a sarcomere, there are various components, including actin and myosin filaments, as well as regulatory proteins like tropomyosin and troponin. Contraction of the sarcomere occurs when the actin and myosin filaments slide past each other, resulting in the shortening of the sarcomere and overall muscle contraction.

An aponeurosis is a sheet-like structure composed of dense connective tissue that serves as a tendon-like structure to anchor muscles to bones or other muscles. It is a broad and flat structure that can withstand and transmit forces generated by muscle contractions. Aponeuroses are commonly found in various regions of the body, such as the abdomen, back, and head, where they provide attachment sites for muscles and help distribute the forces generated during muscle contraction over a larger area. Unlike tendons, which are cord-like structures attaching muscle to bone, aponeuroses have a broad and flattened appearance.

Skeletal muscle is the only type of muscle that is under voluntary control. It is responsible for voluntary movements and is attached to bones by tendons.

Myosin is the main contractile protein in skeletal muscle. It interacts with actin during muscle contraction, resulting in the sliding of filaments and muscle shortening.

Cardiac muscle is autorhythmic, meaning it can generate its own electrical impulses and contract spontaneously. This allows the heart to beat without neural stimulation.

Smooth muscle is found in the walls of the digestive tract and is responsible for peristalsis, which is the coordinated contraction and relaxation of muscles that propels food through the digestive system.

 Smooth muscle is not found in myocardial tissue. Myocardial tissue, also known as cardiac muscle tissue, is a specialized type of muscle found only in the heart. It is composed of striated muscle fibers, similar to skeletal muscle, but with unique characteristics that allow for coordinated and rhythmic contractions to pump blood. Smooth muscle, on the other hand, is found in the walls of various organs and structures throughout the body, including the pulmonary circulatory tissue, esophagus, and small intestine. It is responsible for involuntary movements and contractions in these organs, such as peristalsis in the digestive system and regulation of blood vessel diameter in the circulatory system.