Year 13 Exam > Year 13 Notes > Physical Education for A Level > Skeletal and Muscular Systems: Skeletal Muscle Contraction
Skeletal and Muscular Systems: Skeletal Muscle Contraction | Physical Education for A Level - Year 13 PDF Download
| Table of contents |
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| Skeletal Muscle Structure |
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| The Sliding Filament Theory |
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| The Neuromuscular Junction and Muscle Stimulation |
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| Role of Calcium Ions in Muscle Contraction |
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| ATP and Muscle Contraction |
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Skeletal Muscle Structure
Skeletal muscles enable voluntary movements by connecting to the skeletal system.- Each muscle consists of numerous fibers called myofibrils, which are made up of two types of filamentous proteins: actin (thin filaments) and myosin (thick filaments).
- Within a myofibril, the repeating units of actin and myosin form a sarcomere, the fundamental functional unit of a muscle.
- The sarcolemma, the outer membrane of a muscle fiber, conducts electrical signals to initiate muscle contraction.
- Inside the muscle fibers, the sarcoplasmic reticulum stores calcium ions, which are critical for the contraction process.
The Sliding Filament Theory
Muscle fiber contraction occurs through a mechanism known as the Sliding Filament Theory.- During contraction, myosin heads, or “cross bridges,” pull the actin filaments toward the center, resulting in the shortening of the sarcomere.
- The simultaneous shortening of multiple sarcomeres within a myofibril, and across multiple myofibrils, leads to the overall contraction of the muscle.
The Neuromuscular Junction and Muscle Stimulation
An electrical signal, or action potential, originates in the brain or spinal cord and travels along a motor neuron to reach a skeletal muscle.- At the motor neuron’s end, this electrical signal triggers the release of a neurotransmitter called acetylcholine at the neuromuscular junction, a specialized synapse between the neuron and the muscle fiber.
- Acetylcholine binds to receptors on the sarcolemma, generating an action potential that spreads through the muscle fiber, prompting the release of calcium ions from the sarcoplasmic reticulum.
Role of Calcium Ions in Muscle Contraction
Calcium ions released from the sarcoplasmic reticulum bind to troponin, a protein on the actin filaments.- This binding causes tropomyosin, another protein, to shift, exposing binding sites for myosin heads on the actin filament.
- With these sites exposed, myosin heads can attach to the actin filament, forming cross bridges, which marks the start of muscle contraction.
ATP and Muscle Contraction
Muscle contraction depends on energy supplied by adenosine triphosphate (ATP).- ATP binds to myosin heads, providing the energy needed for them to flex and pull the actin filaments inward during the power stroke.
- After the power stroke, ATP is required again to detach the myosin head from the actin filament, completing the cycle of contraction and relaxation.
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FAQs on Skeletal and Muscular Systems: Skeletal Muscle Contraction - Physical Education for A Level - Year 13
| 1. What is the structure of skeletal muscle and how does it contribute to muscle function? |  |
Ans. Skeletal muscle is composed of long, cylindrical fibers known as muscle fibers, which contain multiple nuclei. These fibers are organized into bundles called fascicles, surrounded by connective tissue. Each muscle fiber contains myofibrils, which are further divided into sarcomeres—the basic units of contraction. The arrangement of actin and myosin filaments within the sarcomeres allows for the sliding filament mechanism, enabling muscle contraction and movement.
| 2. Can you explain the sliding filament theory of muscle contraction? | |
Ans. The sliding filament theory states that muscle contraction occurs when the thick (myosin) and thin (actin) filaments within the sarcomeres slide past each other. During contraction, myosin heads bind to actin, forming cross-bridges, and pull the actin filaments inward, shortening the sarcomere. This process requires ATP for energy and is regulated by the presence of calcium ions, which facilitate the interaction between actin and myosin.
| 3. What is the role of the neuromuscular junction in muscle stimulation? | |
Ans. The neuromuscular junction is the synapse between a motor neuron and a muscle fiber. When a nerve impulse reaches the junction, it triggers the release of the neurotransmitter acetylcholine (ACh) from the motor neuron. ACh binds to receptors on the muscle fiber's membrane, leading to depolarization and the generation of an action potential. This action potential travels along the muscle fiber, initiating muscle contraction via the sliding filament mechanism.
| 4. How do calcium ions contribute to muscle contraction? | |
Ans. Calcium ions (Ca²⁺) play a crucial role in muscle contraction by binding to troponin, a protein associated with actin filaments. When Ca²⁺ levels increase in the muscle fiber cytoplasm, it causes a conformational change in troponin, which moves tropomyosin away from the actin binding sites. This exposure allows myosin heads to attach to actin and initiate contraction. After contraction, Ca²⁺ is pumped back into the sarcoplasmic reticulum, leading to muscle relaxation.
| 5. What is the importance of ATP in muscle contraction? | |
Ans. ATP is essential for muscle contraction as it provides the energy needed for several processes. First, ATP is required for the formation of cross-bridges between myosin and actin. Second, it powers the myosin heads to detach from actin after a power stroke, allowing for subsequent contractions. Additionally, ATP is necessary for the active transport of Ca²⁺ back into the sarcoplasmic reticulum during relaxation. Without sufficient ATP, muscle contractions cannot occur effectively.
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Sep 06, 2025
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