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Questions and Answers
The triad of a skeletal muscle cell is formed by what three structures?
The triad of a skeletal muscle cell is formed by what three structures?
In a relaxed myocyte, where along a sarcomere will you NOT find the regulatory proteins?
In a relaxed myocyte, where along a sarcomere will you NOT find the regulatory proteins?
Which of the following structures run parallel to the long axis of a skeletal muscle cell?
Which of the following structures run parallel to the long axis of a skeletal muscle cell?
Why is the middle of the thick myofilament called the 'bare zone'?
Why is the middle of the thick myofilament called the 'bare zone'?
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Which of the following is a structural protein that extends along each thick filament from M line to Z line?
Which of the following is a structural protein that extends along each thick filament from M line to Z line?
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Which of the following stores calcium to be released for muscle contraction?
Which of the following stores calcium to be released for muscle contraction?
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Which of the following transmits action potentials to the interior of the muscle cell to trigger calcium release?
Which of the following transmits action potentials to the interior of the muscle cell to trigger calcium release?
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In its energized form, myosin __________.
In its energized form, myosin __________.
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The sequence of events that links an end plate potential to the activation of crossbridge cycling is referred to as __________.
The sequence of events that links an end plate potential to the activation of crossbridge cycling is referred to as __________.
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Which of the following statements is true regarding intracellular calcium regulation in a skeletal muscle cell?
Which of the following statements is true regarding intracellular calcium regulation in a skeletal muscle cell?
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What is the regulatory protein component of the thin filament that binds to calcium, thereby initiating skeletal muscle contraction?
What is the regulatory protein component of the thin filament that binds to calcium, thereby initiating skeletal muscle contraction?
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During skeletal muscle contraction, as the muscle shortens, the thick and thin filaments __________.
During skeletal muscle contraction, as the muscle shortens, the thick and thin filaments __________.
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Study Notes
The Triad
- Three structures comprise the triad of a skeletal muscle cell: two terminal cisternae and one transverse tubule.
- The terminal cisternae are enlargements of the sarcoplasmic reticulum, which stores calcium.
- The T-tubule is a deep invagination of the sarcolemma, the cell membrane, which allows action potentials to penetrate into the muscle fiber.
Regulatory Proteins
- Regulatory proteins are responsible for controlling muscle contraction and relaxation.
- In a relaxed myocyte, regulatory proteins are found in the I-band and A-band of the sarcomere.
- The I-band (Isotropic) is the region of the sarcomere that contains only thin filaments (actin).
- The A-band (Anisotropic) is the region of the sarcomere that contains both thin and thick filaments (myosin).
Muscle Structure
- Myofibrils are the basic contractile units of a muscle cell and are made up of repeating units called sarcomeres.
- Z-lines define the boundaries of individual sarcomeres.
- The M-line is the center of the sarcomere and serves as an anchoring point for thick filaments.
- T-tubules run perpendicular to the long axis of the muscle fibers, forming a network that ensures the rapid spread of action potentials.
- Actin and myosin are the two primary contractile proteins.
- Actin comprises the thin filaments, while myosin comprises the thick filaments.
Bare Zone
- The middle of the thick myofilament is called the "bare zone" because it is devoid of myosin heads, also known as cross-bridges.
- This region is responsible for the attachment of the thick filaments to the M-line.
Titin
- Titin is a structural protein that extends along each thick filament from the M-line to the Z-line.
- It functions as a molecular spring, providing elasticity to the muscle fiber and preventing overstretching.
Calcium Storage and Release
- The sarcoplasmic reticulum (SR) is a network of membrane-enclosed tubules that surrounds each myofibril.
- The SR stores calcium ions.
- T-tubules transmit action potentials from the sarcolemma to the SR.
- The arrival of an action potential at the T-tubule triggers the release of calcium ions from the SR.
Myosin
- Myosin is a motor protein that converts chemical energy (ATP) into mechanical energy to drive muscle contraction.
- In its energized state (bound to ADP and Pi), myosin has a high affinity for actin.
- The binding of ATP to myosin results in a power stroke, causing the thin filaments to slide past the thick filaments.
Excitation-Contraction Coupling
- This refers to the series of events that link an action potential at the neuromuscular junction to the activation of cross-bridge cycling.
- The process begins with the release of acetylcholine from the motor neuron at the neuromuscular junction.
- Acetylcholine binds to receptors on the muscle fiber membrane, triggering an action potential that spreads along the sarcolemma and into the T-tubules.
Intracellular Calcium Regulation
- At rest, calcium is stored within the SR, maintaining a low concentration in the cytosol.
- When an action potential reaches the SR, calcium is released from the SR into the cytosol.
- This influx of calcium binds to troponin, causing a conformational change that exposes the myosin binding sites on actin.
- After contraction, calcium is actively pumped back into the SR via the calcium ATPase pump, restoring the muscle to a relaxed state.
Troponin
- Troponin, a regulatory protein, binds to calcium ions.
- This binding triggers a conformational change that moves tropomyosin away from the myosin binding sites on actin, allowing contraction to occur.
Sliding Filament Mechanism
- Muscle contraction occurs as the thin filaments slide past the thick filaments.
- The length of the thin and thick filaments themselves does not change during contraction.
- The Z-lines move closer together, shortening the sarcomere.
- The H-zone, which is the region of the sarcomere containing only thick filaments, also shortens.
- The I-band, which is the region of the sarcomere containing only thin filaments, also shortens.
Muscle Fatigue
- Muscle fatigue is a physiological state where the muscle can no longer maintain its force of contraction.
- Several factors can contribute to muscle fatigue, including:
- Depletion of ATP
- Accumulation of lactic acid
- Depletion of glycogen stores
- Changes in the concentration of ions (e.g., Ca2+, K+)
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Description
Explore the intricate structures of skeletal muscle cells, focusing on the triad, regulatory proteins, and overall muscle architecture. Learn about the role of calcium storage in contraction and the organization of myofibrils and sarcomeres. Test your understanding of muscle cell functionality.
