unit 3 week 3 pt 1 Flashcards
What are skeletal muscles and how are they controlled?
Skeletal muscles are muscles anchored to bones and are under voluntary control, meaning they can be consciously commanded to contract.
How is the structure of a skeletal muscle cell (muscle fiber) different from other cells?
A skeletal muscle cell, or muscle fiber, is cylindrical, typically 10–100 µm thick and over 100 mm long, and contains hundreds of nuclei due to the fusion of myoblasts (premuscle cells) in the embryo.
What is the organization of skeletal muscle fibers?
Muscle fibers have a highly ordered structure, composed of myofibrils. Each myofibril contains repeating contractile units called sarcomeres, which are responsible for the striated appearance of the muscle fiber.
What are sarcomeres and what is their banding pattern?
Sarcomeres are the contractile units of muscle fibers. They display a banding pattern due to the partial overlap of thin and thick filaments, giving skeletal muscle fibers their striated appearance.
What are the key structural components of a sarcomere?
A sarcomere includes:
* Z lines: The boundaries of each sarcomere.
* I bands: Light bands that contain only thin filaments.
* A band: A darker band that contains both thick and thin filaments.
* H zone: A light zone in the middle of the A band, containing only thick filaments.
* M line: A dark line in the center of the H zone, providing structural support.
How are the thin and thick filaments arranged within a sarcomere?
Thin filaments are arranged in a hexagonal pattern around thick filaments in the region of overlap, with each thin filament situated between two thick filaments.
What are cross-bridges in muscle contraction?
Cross-bridges are projections from the thick filaments in the sarcomere that can form attachments with neighboring thin filaments, facilitating muscle contraction.
How do skeletal muscles perform work during contraction?
Skeletal muscles perform work by shortening. The shortening of the sarcomeres, which are the contractile units, causes the entire muscle to shorten.
What changes occur in the sarcomere during muscle contraction?
During contraction, the A band remains the same length, while the H and I bands decrease in width and may disappear. The Z lines move inward until they contact the edges of the A band.
What is the Sliding Filament Model of Muscle Contraction?
The Sliding Filament Model proposes that muscle contraction occurs not by the shortening of filaments but by the sliding of thin filaments over thick filaments, increasing the overlap of the filaments and decreasing the width of the I and H bands.
What proteins make up the thin filaments in skeletal muscle?
The thin filaments are made up of actin, tropomyosin, and troponin. Tropomyosin fits into grooves within the actin filament, and troponin is a complex of three subunits that interact with both actin and tropomyosin.
How are the thick filaments in skeletal muscles organized?
Thick filaments are composed of several hundred myosin II molecules. The myosin heads project along the length of the filament, and the center is devoid of heads. The polarity of the thick filament is reversed at the center of the sarcomere.
What role does titin play in skeletal muscle?
Titin is the largest protein in organisms and is highly elastic. It helps prevent the sarcomere from being pulled apart during muscle stretching and maintains the position of myosin filaments within the center of the sarcomere during contraction. Titin also extends from the Z line to the myosin filament.
What is the role of myosin heads in muscle contraction?
The myosin heads extend outward and bind tightly to a thin filament, forming cross-bridges. During contraction, the myosin head undergoes a conformational change that moves the thin filament approximately 10 nm toward the center of the sarcomere.
How do multiple myosin heads work together during contraction?
A single myosin filament interacts with six surrounding actin filaments. The myosin heads beat out of synchrony, causing continuous movement of the thin filament during each contractile cycle, allowing the filament to move several hundred nanometers in 50 milliseconds.
How does the myosin molecule move the actin filament during contraction?
The ATP hydrolysis energy induces a small conformational change in the myosin head. This change is amplified by the swinging movement of the myosin’s neck, acting as a rigid lever arm, which causes the actin filament to slide a greater distance.
What experiment supported the lever-arm hypothesis of myosin movement?
Experiments by James Spudich and colleagues showed that the length of the myosin II neck is proportional to the length of the power stroke. Myosin molecules with longer necks generated greater displacements of the actin filament.
How does ATP contribute to the myosin cross-bridge cycle?
ATP binding causes myosin to dissociate from the actin filament. ATP is then hydrolyzed, energizing the myosin, which reattaches to the actin filament. This releases phosphate, triggering a conformational change that moves the actin filament. The release of ADP and binding of a new ATP molecule completes the cycle.
What happens in the absence of ATP?
Without ATP, the myosin head remains tightly bound to the actin filament, leading to rigor mortis, where muscles stiffen after death due to the inability of myosin to detach from actin.
What are motor units, and how do they work?
Motor units are groups of muscle fibers innervated by branches from a single motor neuron. All fibers in a motor unit contract simultaneously when stimulated by an impulse transmitted along the motor neuron.
What is a neuromuscular junction, and what role does it play in muscle contraction?
A neuromuscular junction is the point of contact between the terminus of an axon and a muscle fiber. It transmits the nerve impulse across a synaptic cleft to the muscle fiber, leading to an action potential in the muscle that triggers contraction.
What is excitation-contraction coupling?
Excitation–contraction coupling is the process linking the arrival of a nerve impulse at the muscle plasma membrane to the shortening of sarcomeres deep within the muscle fiber.
How does an action potential travel through a muscle fiber?
The action potential generated in the muscle fiber propagates into the cell along the transverse (T) tubules, which are membranous folds that reach deep into the muscle fiber and are close to the sarcoplasmic reticulum (SR).
What is the role of calcium in muscle contraction?
Calcium ions are essential for muscle contraction. When an action potential arrives via T tubules, calcium channels in the SR open, releasing calcium into the cytoplasm, which increases intracellular calcium levels and triggers muscle contraction.
How does calcium trigger contraction in skeletal muscle fibers?
Calcium binds to troponin C, causing a conformational change in the troponin molecule. This movement shifts the tropomyosin, exposing the myosin-binding sites on actin molecules, allowing the myosin heads to bind and initiate contraction.
What happens during muscle relaxation?
After the motor nerve stimulation ceases, calcium channels in the SR close, and calcium is actively pumped out of the cytosol by the Ca2+-ATPase pump. As calcium levels decrease, it dissociates from troponin, causing tropomyosin to block the actin-myosin interaction, leading to muscle relaxation.
How is calcium removed from the cytosol during relaxation?
Calcium is removed by the Ca2+-ATPase pump in the SR membrane, which has a higher affinity for calcium, preferentially removing it from the cytosol and causing muscle relaxation.
