6. Describe the changes to a muscle fiber as a result of physical training and with disuse, with regards to the diameter of the fiber, the number of myofibrils, blood flow to the fiber, and numbers of mitochondria. Use the terms hypertrophy and atrophy in your description.

Answer 1

Physical training and disuse have significant physiological effects on muscle fibers, influencing their structure and function.

Changes Due to Physical Training:

1. Diameter of the Fiber: With consistent resistance training and physical activity, muscle fibers experience hypertrophy, leading to an increase in the diameter of the fibers. This enlargement is primarily due to an increase in the synthesis of proteins within the muscle, which enhances overall muscle mass.

2. Number of Myofibrils: As muscle fibers undergo hypertrophy, there is also an increase in the number of myofibrils within each fiber. Myofibrils are the contractile units of muscle, and their proliferation allows for greater force production during muscle contractions.

3. Blood Flow to the Fiber: Physical training enhances capillarization, increasing the blood flow to the muscle fibers. This improved blood supply allows for more efficient delivery of oxygen and nutrients while facilitating the removal of metabolic waste products, which supports endurance and recovery.

4. Numbers of Mitochondria: Regular exercise, particularly aerobic training, leads to an increase in the number of mitochondria within muscle fibers. Mitochondria are the powerhouses of the cell, responsible for energy production through oxidative phosphorylation. An increased mitochondrial density enhances the muscle’s ability to produce energy aerobically, improving endurance and overall muscular performance.

Changes Due to Disuse:

1. Diameter of the Fiber: In contrast, disuse or inactivity results in atrophy, which is the reduction in the diameter of muscle fibers. This occurs as the muscle degenerates due to a lack of mechanical stress and stimulus that promote muscle maintenance and growth.

2. Number of Myofibrils: During disuse, there is also a decrease in the number of myofibrils within each muscle fiber. This reduction limits the muscle's ability to generate force, making it weaker and less capable of performing physical tasks.

3. Blood Flow to the Fiber: With decreased physical activity, blood flow to the muscle fibers may decrease as a response to the lowered demand for oxygen and nutrient delivery. This can further exacerbate atrophy and negatively affect recovery from exercise.

4. Numbers of Mitochondria: Disuse is associated with a reduction in mitochondrial density within muscle fibers. A decrease in mitochondria hampers the muscle's aerobic capacity and ability to efficiently produce energy, which can lead to fatigue and decreased overall athletic performance.

In summary, regular physical training promotes hypertrophy, enhances myofibril and mitochondrial content, and improves blood flow to muscle fibers, while disuse causes atrophy, reduces myofibril and mitochondrial quantities, and diminishes blood supply, leading to weaker and less efficient muscles.