Chapter 12: Muscle Physiology

Oxidative fibers contain high concentrations of the oxygen-binding protein myoglobin. Myoglobin is responsible for storing and transporting oxygen within muscle cells. These fibers rely on aerobic metabolism, which requires a constant supply of oxygen, to generate energy. The presence of myoglobin allows oxidative fibers to efficiently extract oxygen from the bloodstream and use it for sustained muscle contractions. This enables them to perform endurance activities that require a steady supply of energy for prolonged periods of time.

During an isometric muscle contraction, the muscle develops contractile force but does not change in length. This means that the muscle is exerting force without actually shortening or lengthening. Isometric contractions are commonly seen in activities such as holding a heavy object in a fixed position or maintaining a static posture. In these situations, the muscle is generating force to stabilize or resist an external force, but there is no movement occurring at the joint.

When calcium is released from the sarcoplasmic reticulum in skeletal muscle, it binds to troponin. This binding of calcium to troponin causes a conformational change in the troponin-tropomyosin complex, exposing the active sites on the actin filament. This allows for the crossbridge cycle to be initiated, where myosin heads bind to the exposed active sites on actin, leading to muscle contraction.

During muscle contraction, ATP hydrolysis is catalyzed by myosin head groups. Myosin is a protein found in muscle cells that interacts with actin to generate force and movement. The myosin head groups contain an ATPase enzyme that breaks down ATP into ADP and inorganic phosphate. This hydrolysis of ATP provides the energy needed for the myosin heads to bind to actin, undergo a conformational change, and generate force, resulting in muscle contraction. Therefore, the myosin head groups play a crucial role in catalyzing ATP hydrolysis during muscle contraction.

Oxidative muscle fibers are more resistant to fatigue because they rely on aerobic metabolism, which produces energy by utilizing oxygen. This allows them to sustain prolonged contractions and maintain a steady supply of energy. In contrast, glycolytic muscle fibers rely on anaerobic metabolism, which produces energy without oxygen but is less efficient and leads to the accumulation of lactic acid. This buildup of lactic acid causes fatigue to set in more quickly, making glycolytic muscle fibers less resistant to fatigue compared to oxidative muscle fibers.

Glycolytic fibers generate more force than oxidative fibers because they are larger in diameter. The size of the fiber plays a significant role in determining its force-generating capacity. Glycolytic fibers, which primarily rely on anaerobic metabolism, are larger in diameter compared to oxidative fibers that primarily rely on aerobic metabolism. The larger diameter allows glycolytic fibers to have a greater number of contractile elements, resulting in more force production. This is why glycolytic fibers are capable of generating more force than oxidative fibers.

When a muscle fiber is significantly longer than its optimum length, there is a reduction in the number of active crossbridges. Crossbridges are responsible for generating force in the muscle fiber during contraction. Therefore, when there are fewer active crossbridges, the force-generating capacity of the muscle fiber decreases. This explains why the correct answer is "longer" in this context.

The plasma membrane of a muscle cell is referred to as the sarcolemma. This term is commonly used to describe the specialized membrane that surrounds muscle fibers and is responsible for maintaining the integrity of the cell. The sarcolemma plays a crucial role in muscle contraction by regulating the movement of ions and nutrients into and out of the cell. Additionally, it serves as a site for various signaling molecules and receptors that are involved in muscle function.

Increasing the number of mitochondria in the cell would tend to reduce the concentration of lactic acid that accumulates in a muscle cell as a result of contractile activity. Mitochondria are responsible for aerobic respiration, which produces energy in the form of ATP without the accumulation of lactic acid. By increasing the number of mitochondria, the muscle cell can generate more ATP through aerobic respiration, reducing the reliance on anaerobic glycolysis and therefore decreasing the production of lactic acid.

Muscles that have high glycolytic capacity and large glycogen stores are not more resistant to fatigue. In fact, muscles with high oxidative capacity and a rich blood supply are more resistant to fatigue. Glycolytic capacity and glycogen stores are associated with anaerobic metabolism, which is not as efficient in providing sustained energy for muscle contractions.

When a muscle cell is relaxed and intracellular ATP levels are normal, a crossbridge will remain in the high-energy form, with ADP and Pi bound to it. This is because ATP is hydrolyzed to ADP and Pi to provide energy for muscle contraction. In the relaxed state, the crossbridge is still in the high-energy form as it awaits the binding of ATP to detach from actin and reset for the next contraction.

Smooth muscle and cardiac muscle contain gap junctions. Gap junctions are specialized protein channels that allow for direct electrical and chemical communication between adjacent cells. These junctions play a crucial role in coordinating the contraction of smooth and cardiac muscle cells, allowing them to function as a synchronized unit. Skeletal muscle, on the other hand, does not contain gap junctions and relies on other mechanisms, such as neuromuscular junctions, for communication between muscle fibers.

Smooth muscle cells have the ability to respond to more than one type of neurotransmitter. This means that a single smooth muscle cell can be influenced by different neurotransmitters, which can either excite or inhibit the contraction of the muscle. The response of the smooth muscle cell to a specific neurotransmitter is not dependent on the location of the muscle. Therefore, regardless of where the muscle is located, a given neurotransmitter can have either an excitatory or inhibitory effect on the smooth muscle cell.

In an isotonic contraction, all of the above statements are correct. In this type of contraction, the muscle length shortens, the muscle tension exceeds the force of the load, and the load is moved. Isotonic contractions are characterized by the muscle changing in length and generating enough force to move a load. Therefore, all of the statements mentioned in the options are true for an isotonic contraction.