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Questions and Answers
What happens to muscle tension at low stimulation frequencies?
What happens to muscle tension at low stimulation frequencies?
- Muscle tension only changes when nerve stimulation stops.
- Muscle tension continuously increases with each stimulus.
- Muscle tension decreases back to a resting level between stimulations. (correct)
- Muscle tension remains constant without fluctuations.
How does increasing the frequency of action potentials affect Ca2+ levels?
How does increasing the frequency of action potentials affect Ca2+ levels?
- Ca2+ levels remain unchanged regardless of frequency.
- Ca2+ is completely eliminated from the sarcoplasm when frequency increases.
- Ca2+ levels decrease, leading to weaker muscle contractions.
- Ca2+ levels rise, allowing for more crossbridge formation. (correct)
What is the primary function of the sarcoplasmic reticulum (SR) during muscle relaxation?
What is the primary function of the sarcoplasmic reticulum (SR) during muscle relaxation?
- To transport Ca2+ ions into the sarcoplasm for contraction.
- To generate action potentials that sustain muscle contraction.
- To release neurotransmitters into the muscle.
- To pump Ca2+ back into the SR and clear it from the sarcoplasm. (correct)
What occurs when many action potentials are fired in quick succession?
What occurs when many action potentials are fired in quick succession?
What role does frequency of stimulation play in muscular contraction?
What role does frequency of stimulation play in muscular contraction?
What do large motor units primarily consist of?
What do large motor units primarily consist of?
What is the space between the motor neuron and muscle fiber called?
What is the space between the motor neuron and muscle fiber called?
During neuromuscular transmission, what is required to elicit a muscle response?
During neuromuscular transmission, what is required to elicit a muscle response?
What initiates an action potential in a muscle fiber?
What initiates an action potential in a muscle fiber?
What structure stores calcium ions (Ca2+) necessary for muscle contraction?
What structure stores calcium ions (Ca2+) necessary for muscle contraction?
What role do T-tubules play in muscle fibers?
What role do T-tubules play in muscle fibers?
How do motor units in a muscle coordinate movement?
How do motor units in a muscle coordinate movement?
What is a characteristic of small motor units?
What is a characteristic of small motor units?
What immediate result follows muscle depolarization?
What immediate result follows muscle depolarization?
Which molecule does Ca2+ bind to during muscle contraction?
Which molecule does Ca2+ bind to during muscle contraction?
What surrounds each individual muscle fiber in skeletal muscle?
What surrounds each individual muscle fiber in skeletal muscle?
What is the purpose of sarcomeres in skeletal muscle fibers?
What is the purpose of sarcomeres in skeletal muscle fibers?
What is the role of ATP in the muscle contraction cycle?
What is the role of ATP in the muscle contraction cycle?
What happens during the powerstroke phase of muscle contraction?
What happens during the powerstroke phase of muscle contraction?
What is a motor unit comprised of?
What is a motor unit comprised of?
What occurs after ADP dissociates from the myosin head?
What occurs after ADP dissociates from the myosin head?
Which characteristic describes skeletal muscle's ability to stretch?
Which characteristic describes skeletal muscle's ability to stretch?
What describes excitation-contraction coupling?
What describes excitation-contraction coupling?
What best defines the structure surrounding an entire skeletal muscle?
What best defines the structure surrounding an entire skeletal muscle?
Which of the following is NOT a property of skeletal muscle cells?
Which of the following is NOT a property of skeletal muscle cells?
Which step follows the hydrolysis of ATP in the contraction cycle?
Which step follows the hydrolysis of ATP in the contraction cycle?
What is the function of Pi dissociating from myosin?
What is the function of Pi dissociating from myosin?
What is the primary role of electrical stimulation in muscle contraction?
What is the primary role of electrical stimulation in muscle contraction?
How long can an individual skeletal muscle fiber potentially be?
How long can an individual skeletal muscle fiber potentially be?
What is required to initiate another crossbridge cycle between actin and myosin?
What is required to initiate another crossbridge cycle between actin and myosin?
What happens to muscle contraction when the troponin-tropomyosin complex is activated?
What happens to muscle contraction when the troponin-tropomyosin complex is activated?
During recruitment of motor units, which type of motor units is activated at lower stimulus strengths?
During recruitment of motor units, which type of motor units is activated at lower stimulus strengths?
What occurs when stimulus strength is increased during electrical stimulation of motor nerves?
What occurs when stimulus strength is increased during electrical stimulation of motor nerves?
What is created when all muscle fibers in a motor unit are activated?
What is created when all muscle fibers in a motor unit are activated?
What happens as more motor nerve fibers are excited?
What happens as more motor nerve fibers are excited?
What is it called when the maximum response is reached in muscle contraction?
What is it called when the maximum response is reached in muscle contraction?
What happens when calcium ions are reuptaken by the sarcoplasmic reticulum?
What happens when calcium ions are reuptaken by the sarcoplasmic reticulum?
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Study Notes
Connective Tissue Layers
- Endomysium: Surrounds each individual muscle fiber.
- Perimysium: Surrounds bundles of muscle fibers called fascicles.
- Epimysium: Surrounds the entire muscle.
Muscle Fiber Structure
- Muscle fibers are up to 30 cm long and striated.
- Striations arise from repeating units called sarcomeres, which are the functional units of the muscle.
- Sarcomeres are found in myofibrils.
Muscle Cell Properties
- Contractile: Muscle fibers can shorten in response to electrical stimulation.
- Elastic: Can recoil and regain their resting length after stretching.
- Excitable: Can receive and respond to stimuli.
- Extensible: Can be stretched when not contracted.
Motor Units
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
- A single motor neuron can innervate a few muscle fibers (small motor unit) or thousands of muscle fibers (large motor unit).
- Muscles often contain motor units of varying sizes, working together to coordinate movement.
Neuromuscular Transmission
- The motor neuron and muscle fibers are separated by the synaptic cleft.
- Signals travel across the cleft to elicit a muscle response.
- Neuromuscular junction (NMJ): The specialized synapse between a motor neuron and a muscle fiber.
- Comprises the axon terminal, synaptic cleft, and motor endplate.
Muscle Fiber and Myofibril Structure
- Sarcolemma: Muscle membrane surrounding each muscle fiber.
- T-tubules: Invaginations of the sarcolemma that extend deep into the muscle fiber.
- Sarcoplasmic reticulum (SR): Membrane-bound structure that stores Ca2+ in close association with t-tubules.
Excitation-Contraction Coupling
- Depolarization of the motor endplate travels along the sarcolemma and down t-tubules, triggering the release of Ca2+ from the SR.
- Ca2+ binds to troponin, causing a conformational change and shifting the troponin-tropomyosin complex to expose actin binding sites.
- This allows actin and myosin to bind, forming crossbridges and initiating contraction.
Crossbridge Cycle
- 1. ATP binds: An ATP molecule attaches to the myosin head, detaching it from actin.
- 2. ATP hydrolysis: ATP is broken down into ADP and Pi, which remain attached to the myosin head. The myosin head rotates to a "cocked" position, aligning with a new actin binding site.
- 3. Pi dissociates: Pi leaves the myosin head, allowing it to bind with high affinity to actin and form a new crossbridge.
- 4. Powerstroke: The myosin neck rotates around the myosin head, pulling actin closer to the center of the sarcomere. The muscle shortens, generating force.
- 5. ADP dissociates: ADP leaves the myosin head, leaving it waiting for another ATP molecule to start the cycle again.
Sliding Filament Theory
- The formation and breakdown of crossbridges allows actin filaments to slide over myosin, resulting in muscle shortening.
- When nerve impulses stop, Ca2+ is reabsorbed by the SR, preventing actin and myosin interaction. Crossbridges detach, and the muscle relaxes.
Recruitment
- Recruitment: The progressive activation of motor units during muscle contraction.
- Smaller motor units are recruited first with weaker stimuli.
- Larger motor units are recruited with stronger stimuli, increasing the strength of muscle contraction.
Frequency
- Frequency: The number of action potentials delivered to a muscle within a set time period.
- At low frequencies, the muscle relaxes between stimulations. Ca2+ is released with each stimulation.
- Increasing frequency:
- More action potentials are fired during a muscle contraction.
- The level of Ca2+ in the sarcoplasm increases.
- More crossbridges form, leading to increased force and duration of muscle contraction.
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