All questions of Muscular System for MCAT Exam

Role of Acetylcholine in Calcium Release

Acetylcholine plays a crucial role in stimulating cellular contraction by opening voltage-gated ion channels. This process is essential for the release of calcium ions from the sarcoplasmic reticulum in muscle cells.

Stimulates Cellular Contraction
Acetylcholine acts as a neurotransmitter that binds to postsynaptic acetylcholine receptors located on the muscle cell membrane. This binding triggers a series of events that ultimately lead to the opening of voltage-gated ion channels, particularly calcium channels.

Calcium Release from Sarcoplasmic Reticulum
Once the voltage-gated calcium channels are opened, calcium ions flow into the cytoplasm of the muscle cell. This influx of calcium then triggers further release of calcium from the sarcoplasmic reticulum, a specialized organelle in muscle cells that stores and releases calcium ions during muscle contraction.

Importance of Calcium Release
The release of calcium from the sarcoplasmic reticulum is crucial for muscle contraction. Calcium ions bind to proteins like troponin, which ultimately leads to the exposure of actin-binding sites on the thin filaments of muscle cells. This interaction between actin and myosin filaments is necessary for the generation of force and movement in muscle cells.

In summary, acetylcholine plays a pivotal role in stimulating cellular contraction by opening voltage-gated ion channels, leading to the release of calcium ions from the sarcoplasmic reticulum and ultimately enabling muscle contraction.

Myosin is primarily responsible for the contraction of skeletal muscle fibers.

Myosin is a protein found in muscle fibers that plays a crucial role in muscle contraction. It is one of the key components of the thick filaments in the sarcomere, which is the basic unit of contraction in skeletal muscles. Here is a detailed explanation of why myosin is primarily responsible for muscle fiber contraction:

1. Structure of Myosin:
Myosin is a large, complex protein made up of several subunits. It consists of a long tail region and a globular head region. The tail region is responsible for anchoring myosin to the thick filaments, while the head region contains the ATP-binding site and the actin-binding site.

2. Interaction with Actin:
During muscle contraction, myosin interacts with another protein called actin. Actin is the main component of the thin filaments in the sarcomere. The myosin heads bind to specific binding sites on actin, forming cross-bridges.

3. Sliding Filament Theory:
The sliding filament theory explains how muscle contraction occurs. According to this theory, during contraction, the thin filaments slide past the thick filaments, causing the sarcomere to shorten. This shortening of sarcomeres leads to the contraction of the entire muscle fiber.

4. Cross-Bridge Cycling:
The interaction between myosin and actin is dynamic and cyclic. It involves a series of steps known as cross-bridge cycling. The steps of cross-bridge cycling include:

- Myosin heads bind to actin, forming cross-bridges.
- This binding triggers the release of inorganic phosphate (Pi) from the myosin head, causing a conformational change in the myosin molecule.
- The conformational change leads to the power stroke, where the myosin head pulls the actin filament towards the center of the sarcomere.
- ADP is then released from the myosin head, resulting in detachment from actin.
- The myosin head returns to its original position by binding to a new ATP molecule. This detachment allows the myosin head to repeat the cycle and bind to actin again, creating a continuous contraction.

5. ATP as an Energy Source:
ATP is required for the myosin heads to detach from actin and reset for the next cycle of cross-bridge formation. ATP binds to the myosin head, causing it to detach from actin. The ATP molecule is then hydrolyzed into ADP and inorganic phosphate (Pi), releasing energy that allows the myosin head to return to its original position.

Overall, myosin is primarily responsible for the contraction of skeletal muscle fibers due to its ability to bind to actin, generate force through cross-bridge cycling, and utilize ATP as an energy source. Without myosin, the sliding filament mechanism and muscle contraction would not occur.

Understanding Muscle Types
Muscles in the human body can be classified into three main types: skeletal, smooth, and cardiac muscle. Each type has distinct characteristics and functions.
Skeletal Muscle: The Voluntary Muscle
- Skeletal muscles are primarily responsible for movement and are attached to bones.
- These muscles are under voluntary control, meaning you can consciously decide to contract or relax them.
- They allow for actions like walking, running, and lifting, demonstrating their importance in daily activities.
Smooth Muscle: The Involuntary Muscle
- Smooth muscle is found in the walls of hollow organs such as the intestines, blood vessels, and the bladder.
- Unlike skeletal muscle, smooth muscle operates involuntarily, meaning it functions without conscious control.
- This type of muscle helps regulate functions such as digestion and blood flow.
Cardiac Muscle: The Heart's Control
- Cardiac muscle is found only in the heart and is responsible for pumping blood throughout the body.
- It is also involuntary, operating autonomously to maintain a consistent heartbeat.
- Cardiac muscle cells have unique properties that allow them to contract rhythmically and continuously without fatigue.
Conclusion: Why Skeletal Muscle is the Correct Answer
- Among the three types, only skeletal muscle is under voluntary control.
- Smooth and cardiac muscles operate involuntarily, responding automatically to physiological demands.
- Therefore, the correct answer to the question is option 'C', as only skeletal muscle allows for conscious control and movement.