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Questions and Answers
What phenomenon describes the increase in muscle tension due to repeated stimulation?
Which type of muscle contraction allows for work to be performed while the muscle shortens?
In which phase of muscle contraction does calcium uptake occur?
What occurs first in the recruitment of motor units according to Henneman's size principle?
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What is the latency period in muscle contraction?
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Which method is NOT typically used for measuring muscle strength?
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What characterizes an isometric contraction?
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Which statement regarding twitch contractions is correct?
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What initiates the action potential at the neuromuscular junction?
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Which component does NOT play a role in the contraction of muscle fibers?
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During the crossbridge cycle, what occurs immediately after the binding of myosin to actin?
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What is the role of the sarcoplasmic reticulum in muscle contraction?
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Which muscle tissue characteristic is primarily responsible for visible striations?
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What happens to the I-band during muscle contraction?
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What does the All or Nothing Principle refer to in muscle contraction?
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Which of the following correctly outlines the order of events in excitation-contraction coupling?
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Study Notes
Neuromuscular Junction (NMJ)
- Action potential arrives at the axon terminal of a motor neuron, causing calcium channels to open.
- Acetylcholine (ACh), a neurotransmitter, is released from the motor neuron.
- ACh binds to receptors on the motor end plate, opening ligand-gated sodium (Na+) channels.
- When the threshold is reached, an action potential is generated, following the "All or Nothing Principle."
- The action potential propagates along the sarcolemma.
Muscle Structure
- Sarcolemma: Plasma membrane of a muscle fiber.
- Sarcoplasm: Cytoplasm of a muscle fiber.
- Myofibrils: Highly organized intracellular structures responsible for muscle contraction.
- Mitochondria: Powerhouse of the cell, produce ATP for energy.
- Sarcoplasmic reticulum: A network of smooth endoplasmic reticulum responsible for calcium storage and release.
- Transverse tubules (T tubules): Extensions of the sarcolemma that conduct action potentials deep into the muscle fiber.
Levels of Muscle Organization
- Epimysium: Connective tissue that surrounds the entire muscle.
- Perimysium: Connective tissue that surrounds bundles of muscle fibers called fascicles (approximately 150 fibers per fascicle).
- Endomysium: Connective tissue that surrounds each individual muscle fiber.
Muscle Striations
- Longitudinal views of skeletal muscle reveal striations due to the highly organized arrangement of myofibrils.
Myofilaments
- Actin: A contractile protein that forms thin filaments.
- Troponin: A regulatory protein that binds to calcium and initiates muscle contraction.
- Tropomyosin: A regulatory protein that covers the myosin binding sites on actin in the absence of calcium.
The Crossbridge Cycle
- Binding: Myosin heads bind to actin.
- Power Stroke: Myosin heads pull actin filaments towards the center of the sarcomere, shortening the muscle fiber.
- Rigor: Myosin heads remain bound to actin in a low-energy state.
- Unbinding: ATP binds to the myosin head, causing it to detach from actin.
- Cocking: The myosin head hydrolyzes ATP, releasing energy to return to its high-energy state, ready to bind again.
Sliding Filaments Theory
- Actin and myosin filaments do not shorten; they slide past each other.
- The A band remains the same length.
- The I band shortens.
- The H band shortens.
Length-Tension Relationship
- There is an optimal length at which a muscle can generate maximum force.
- This relationship has important implications in physiotherapy as it can help determine optimal exercise strategies for preventing muscle injury and maximizing performance.
Twitch Contraction
- A single muscle contraction in response to a single action potential.
- "All or nothing" response for a given muscle fiber.
- Can be induced in individual muscle fibers, motor units, or entire muscles.
- Latency: Time delay between stimulus and muscle fiber contraction due to E-C coupling.
- Contraction Phase: Cross-bridge cycling.
- Relaxation Phase: Calcium uptake, removing calcium from the sarcoplasm, and muscle relaxation.
Types of Muscle Contraction
-
Isometric Contraction:
- Muscle does not shorten.
- No external work is done.
- Load is greater than the tension generated by the muscle.
-
Isotonic Contraction:
- Constant force.
- Muscle does shorten and external work is done.
- Load is less than the tension generated by the muscle.
Concentric vs. Eccentric Contraction
- Concentric Contraction: Muscle generates force as it shortens.
- Eccentric Contraction: Muscle generates force as it lengthens (absorbs energy).
Motor Units
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
- All muscle fibers within a motor unit contract simultaneously when the motor neuron fires.
Recruitment
- Henneman's Size Principle: Motor units are recruited in a specific order based on their size.
- Smallest motor units (type I) are recruited first.
- Largest motor units (type II) are recruited last.
Frequency and Summation
- Muscle contractions can summate due to the relatively longer duration of contraction compared to the short duration of an action potential.
- Summation: Increased frequency of stimulation leads to increased calcium levels in the sarcoplasm, increasing the duration of the contraction.
- Tetanus: When the frequency of stimulation is very high, the muscle fiber reaches a state of sustained contraction, due to maximal calcium levels maintained in the sarcoplasm.
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Description
This quiz covers the fundamentals of the neuromuscular junction (NMJ) and the basic structure of muscles. It explores the processes involved in muscle contraction, including action potentials and the roles of various muscle cell components. Test your understanding of how these systems work together for movement.
