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Questions and Answers
What happens during the transient rigor state in muscle contraction?
Which molecule is essential for the detachment of myosin from actin?
What is the role of calcium in the contraction cycle?
During the power stroke, what happens to myosin?
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What occurs after hydrolysis of ATP in the muscle contraction process?
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What is primarily responsible for the detachment of cross bridges during muscle contraction?
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During a muscle twitch, which phase comes immediately after the latent period?
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Which motor unit type is most likely to be activated during endurance activities?
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What happens to calcium ions during muscle relaxation?
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Which of the following best describes the sliding filament model?
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In which scenario would you expect to see increased tension production?
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What role does myoglobin play in muscle cells?
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What happens during the isometric contraction phase?
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How does the nervous system regulate muscle force?
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Which type of contraction involves a change in muscle length?
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What is a characteristic of fast glycolytic fibers?
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Which of the following correctly describes the relationship between force and velocity in muscle contractions?
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What is the primary energy source for muscle contraction during sustained activity?
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Study Notes
Contraction Cycle
- Myosin binds to actin in the rigor state
- ATP binds to myosin allowing detachment and returns myosin to a low energy state
- ATP hydrolysis causes myosin to assume a high energy state and form a weak bond with actin
- Pi is lost, myosin forms a strong bond with actin and performs the power stroke
- ADP is lost and the cross-bridge returns to the rigor state
- Calcium is needed for the cycle to occur
Regulation
- Calcium release initiates contraction
- Loss of calcium causes loss of tension
- Calcium is needed for the contraction cycle
Review
- The neuromuscular junction connects motor neurons and muscle cells
- Excitation-contraction coupling converts an electrical signal in the sarcolemma to a calcium signal in the cytoplasm
- Calcium release from the SR is a critical regulatory step in contraction
- ATP is needed for detachment of cross bridges and by calcium pumps needed to move calcium into the SR
Length-Tension Relationship
- Isometric contractions determine the muscle's ability to generate force at a given length
- The most overlap between myosin and actin occurs at intermediate lengths, and this is where the most force can be generated
- This is an intrinsic property of the muscle
Force-Velocity Relationship
- Isotonic contractions occur when a muscle shortens at a constant load
- The tradeoff between speed and force is driven by the time required for cross-bridge detachment
- The inability to detach quickly enough causes cross bridges to oppose force production
Neural Control of Muscle Force
- The nervous system regulates muscle force through changes in action potential frequency and the number of muscle cells activated
- The nervous system regulates muscle force by activating motor units
Motor Unit Recruitment
- A motor unit consists of a motor neuron and all the muscle fibers it innervates
- Motor units are recruited in order, generally slow twitch fibers first, then intermediate, then fast twitch
Fiber Types
- There are three major fiber types:
- Slow twitch: slow fatigue, high oxidative capacity, low glycolytic capacity
- Intermediate twitch: moderate fatigue, moderate oxidative capacity, moderate glycolytic capacity
- Fast twitch: fast fatigue, low oxidative capacity, high glycolytic capacity
- Slow twitch fibers have high oxidative capacity, with high mitochondria and capillary density to support aerobic metabolism
- Fast twitch fibers have low oxidative capacity, with low mitochondria and capillary density to support anaerobic metabolism
Cellular Metabolism
- Slow oxidative fibers have a high mitochondrial content and a high density of capillaries which is important for aerobic metabolism. These fibers are resistant to fatigue.
- Fast oxidative fibers have a moderate mitochondrial content and a moderate capillary density, and these fibers are also resistant to fatigue.
- Fast glycolytic fibers have a low mitochondrial content and a low capillary density. These fibers are easily fatigued.
Fiber Type Comparison
- Slow oxidative fibers have slow twitch speed, slow myosin ATPase activity, small fiber diameter, few calcium pumps in the SR, and they rely on oxidative metabolism. They have high capillary density, high mitochondria, and myoglobin.
- Fast oxidative fibers have intermediate twitch speed, intermediate ATPase activity, moderate fiber diameter, many calcium pumps in the SR, and they are resistant to fatigue. They have moderate capillary density and mitochondria content.
- Fast glycolytic fibers have fast twitch speed, fast ATPase activity, large fiber diameter, many calcium pumps in the SR, and they are easily fatigued. They have low capillary density, low mitochondria content, and no myoglobin.
Twitch Summation
- Muscle tension can be increased by increasing the frequency of stimulation.
- Fused tetanus is when the muscle is stimulated at a high enough frequency that it doesn't relax between contractions.
- Unfused tetanus is when there is partial relaxation between stimulations.
Muscle Twitch
- A muscle twitch is a single contraction and relaxation cycle in response to a single action potential.
- The latent period is the time between the stimulus and the start of the contraction.
- The contraction phase is the time during which the muscle develops tension.
- The relaxation phase is the time during which the muscle relaxes.
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Description
Test your understanding of the contraction cycle and its regulation in muscle physiology. This quiz covers the roles of myosin, ATP, and calcium in muscle contraction. Additionally, explore the neuromuscular junction and the length-tension relationship.
