Capitolo 6 Flashcards
(31 cards)
What are the main types of muscle tissue found in the human body, and what percentage do they each comprise?
• About 40% of the body is made up of skeletal muscle, and another 10% is composed of smooth and cardiac muscle.
What is the sarcolemma, and what is its role in skeletal muscle?
The sarcolemma is a thin membrane that encloses a skeletal muscle fiber. It consists of the plasma membrane and an outer layer of polysaccharide material with collagen fibrils. It fuses with tendon fibers to connect muscles to bones.
Describe the structure of myofibrils and their role in muscle contraction.
• Myofibrils are made up of actin and myosin filaments, with each muscle fiber containing hundreds to thousands of these. Myosin and actin filaments interdigitate to create alternating light (I bands) and dark (A bands), which gives skeletal muscle its striated appearance. The interaction between myosin cross-bridges and actin filaments enables muscle contraction.
What is the function of titin molecules within the sarcomere?
Titin molecules help maintain the alignment of actin and myosin filaments, holding them in place. Titin is elastic, acting as a spring that attaches to the Z disk, and it stabilizes the structure as the sarcomere contracts and relaxes.
What is the role of the sarcoplasmic reticulum in skeletal muscle fibers?
• The sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum surrounding myofibrils. It regulates calcium storage, release, and reuptake, which is crucial for muscle contraction.
How does the sarcoplasm contribute to muscle function?
• The sarcoplasm, the fluid between myofibrils, contains potassium, magnesium, phosphate, protein enzymes, and numerous mitochondria that provide ATP to fuel muscle contractions.
What happens to the sarcomere length during muscle contraction, and why is this significant?
During contraction, the sarcomere shortens to about 2 micrometers, allowing actin filaments to overlap with myosin filaments. At this length, the muscle can generate maximum contraction force.
What triggers the beginning of the muscle contraction process?
Muscle contraction is initiated when an action potential travels along a motor nerve to its endings on muscle fibers.
How does acetylcholine contribute to muscle contraction?
• Acetylcholine is released at the nerve endings on the muscle fibers and binds to receptors, opening acetylcholine-gated cation channels, which allows sodium ions to flow into the muscle fiber, causing depolarization.
What role do sodium ions play in initiating muscle contraction?
Sodium ions enter the muscle fiber through acetylcholine-gated channels, leading to local depolarization. This depolarization opens voltage-gated sodium channels, which triggers an action potential along the muscle fiber membrane.
Describe the sequence of events that occur after the action potential reaches the muscle fiber membrane.
The action potential travels along the muscle fiber membrane, depolarizes it, and causes the sarcoplasmic reticulum to release stored calcium ions, which are essential for initiating the contraction process.
How do calcium ions facilitate the contraction of muscle fibers?
• Calcium ions trigger the attraction between actin and myosin filaments, allowing them to slide past each other, which is the fundamental mechanism of muscle contraction.
What happens to calcium ions after the muscle contraction is completed?
After contraction, calcium ions are pumped back into the sarcoplasmic reticulum by a Ca2+ membrane pump, where they are stored until the next muscle action potential arrives. This removal of calcium stops the contraction.
Why is the action potential important for muscle contraction?
• The action potential depolarizes the muscle membrane, activating the sarcoplasmic reticulum to release calcium ions, which are crucial for enabling the actin-myosin interaction and muscle contraction.
How does the sliding filament mechanism work in muscle contraction?
The sliding filament mechanism works by sliding actin filaments inward among myosin filaments, which pulls the Z disks closer together, shortening the sarcomere and contracting the muscle.
What happens to the actin filaments and Z disks during the transition from the relaxed to the contracted state?
In the relaxed state, the actin filaments barely overlap. During contraction, they are pulled inward so that they overlap maximally, and the Z disks are drawn closer to the ends of the myosin filaments.
What role do myosin cross-bridges play in muscle contraction?
Myosin cross-bridges interact with actin filaments to generate the force necessary for sliding the actin filaments inward, leading to muscle contraction.
What events trigger the release of calcium ions from the sarcoplasmic reticulum?
An action potential traveling along the muscle fiber triggers the sarcoplasmic reticulum to release large amounts of calcium ions.
How do calcium ions contribute to the activation of forces between myosin and actin filaments?
Calcium ions surround the myofibrils and activate the forces between myosin and actin filaments, allowing the cross-bridges to engage and initiate contraction.
What is the function of ATP in the muscle contraction process?
ATP provides the energy needed for muscle contraction by breaking down into ADP, releasing the energy stored in its high-energy bonds.
Why is energy necessary for the contraction process to proceed?
Energy is required to power the interactions between myosin and actin filaments, enabling the sliding action that causes muscle contraction.
What is the composition of a myosin molecule, and how do the heavy and light chains contribute to its function?
A myosin molecule consists of six polypeptide chains: two heavy chains, which form a double-helix tail, and four light chains located on the myosin head. The heavy chains provide structural support, while the light chains assist in muscle contraction.
How are myosin molecules arranged within a myosin filament?
In a myosin filament, 200 or more myosin molecules bundle together. The tails of the molecules form the filament body, while the arms and heads protrude outward to create cross-bridges.
What are cross-bridges in a myosin filament, and what role do the hinges play?
Cross-bridges are structures formed by the arms and heads of myosin molecules extending from the filament body. Each cross-bridge has two flexible hinges: one allows the arm to extend from the body, and the other lets the head attach to the arm. These hinges provide the flexibility needed for movement during contraction.
