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Questions and Answers
Which layers of connective tissue surround individual muscle fibers?
Which layers of connective tissue surround individual muscle fibers?
What is the functional unit of skeletal muscle?
What is the functional unit of skeletal muscle?
Sarcomere
Muscle fibers can only contract if they receive a stimulus from the motor neuron.
Muscle fibers can only contract if they receive a stimulus from the motor neuron.
True
What is the name of the specialized synapse between the motor neuron and muscle fiber?
What is the name of the specialized synapse between the motor neuron and muscle fiber?
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What happens to the strength of muscle contraction as more motor units are recruited?
What happens to the strength of muscle contraction as more motor units are recruited?
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The motor unit consists of a motor neuron and all the muscle fibers that it ______.
The motor unit consists of a motor neuron and all the muscle fibers that it ______.
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Match the following muscle fiber properties with their definitions:
Match the following muscle fiber properties with their definitions:
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Which type of motor unit would likely be recruited at lower stimulus strengths?
Which type of motor unit would likely be recruited at lower stimulus strengths?
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Which of the following layers surrounds an individual muscle fiber?
Which of the following layers surrounds an individual muscle fiber?
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What are the functional units of skeletal muscle called?
What are the functional units of skeletal muscle called?
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What is the ability of skeletal muscle to shorten in response to electrical stimulation called?
What is the ability of skeletal muscle to shorten in response to electrical stimulation called?
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A motor unit consists of a single motor neuron and all the muscle fibers innervated by it.
A motor unit consists of a single motor neuron and all the muscle fibers innervated by it.
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The specialized synapse between the motor neuron and muscle fiber is called the ______.
The specialized synapse between the motor neuron and muscle fiber is called the ______.
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What structures extend from the sarcolemma deep into the muscle fiber?
What structures extend from the sarcolemma deep into the muscle fiber?
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What type of filaments are involved in the formation of sarcomeres?
What type of filaments are involved in the formation of sarcomeres?
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Study Notes
Skeletal Muscle Structure
- A single skeletal muscle consists of many muscle fibers surrounded by layers of connective tissue: endomysium, perimysium, and epimysium.
- Each muscle fiber is enveloped by endomysium, grouped into bundles called fascicles surrounded by perimysium, and the entire muscle is enclosed by epimysium.
- Muscle fibers are long and striated due to the repetition of sarcomeres, the functional units of the muscle, found within myofibrils.
Muscle Fiber Properties
- Contractile: Muscle fibers can shorten in response to electrical stimulation.
- Elastic: They can recoil and return to their resting length after stretching.
- Excitable: They can receive and respond to stimuli.
- Extensible: They can stretch when not contracted.
Motor Units
- Motor neurons innervate groups of muscle fibers, forming a motor unit.
- An action potential in a single motor neuron activates all the fibers it innervates.
- Motor unit size varies: a single motor neuron can innervate a few (small motor unit) or thousands (large motor unit) of muscle fibers.
- Muscles often contain motor units of varying sizes, working together for coordinated movement.
Neuromuscular Transmission
- The motor neuron and muscle fiber are separated by a synaptic cleft.
- Signals cross the cleft at the neuromuscular junction (NMJ).
- The NMJ comprises the axon terminal, synaptic cleft, and motor endplate of the muscle fiber.
Muscle Fibers and Myofibrils
- Muscle fibers are enveloped by a sarcolemma (muscle membrane).
- The sarcolemma forms t-tubules, extending into the fiber's center.
- Depolarization of the motor endplate triggers an action potential that propagates along the sarcolemma and into the t-tubules.
Sarcomeres
- Sarcomeres, joined at their ends by Z lines, house actin (thin filaments) and myosin (thick filaments), which are tethered to the Z lines.
- Myosin overlaps with actin within the sarcomere.
Recruitment
- Progressive activation of motor units is called recruitment.
- Muscle contraction often involves hundreds of motor axons firing.
- Motor units are recruited based on their size: smaller units at lower stimulus strengths, larger units at higher strengths.
- Increasing recruitment enhances muscle contraction strength.
Frequency
- Frequency refers to the number of action potentials delivered to a muscle per unit time.
- At low frequencies, tension decreases to resting levels between stimulations due to Ca2+ release and uptake.
- Ca2+ is released with each stimulation, and the muscle relaxes when nerve stimulation ceases and Ca2+ is pumped back into the sarcoplasmic reticulum.
- The duration of an action potential is short compared to Ca2+ release and uptake, allowing for multiple action potentials during a contraction.
- Increasing frequency increases the muscle's tension, leading to tetanus.
Tetanus
- Tetanus is a state of sustained muscle contraction, caused by high stimulation frequency.
- When the frequency of stimulation is high enough, the tension in the muscle remains at its maximum level and the muscle does not relax.
- This is because the action potentials arrive so quickly that the muscle fibers do not have enough time to fully relax between contractions.
- Tetanus is important in sustaining muscle contractions needed for activities like holding a heavy object.
Muscle Fatigue
- Muscle fatigue is a decline in muscle force or ability to maintain force during sustained activity.
- The exact cause of fatigue is not fully understood, but it is likely due to a combination of factors, including:
- Depletion of energy stores (e.g., glycogen)
- Accumulation of metabolic byproducts (e.g., lactic acid)
- Alterations in nerve conduction or muscle fiber excitability
- Fatigue can lead to a decrease in force production, reduced speed of contraction, and a slower rate of relaxation.
- It can also lead to pain and discomfort.
Muscle Adaptation
- Muscle fibers can adapt to changes in workload and activity levels.
- Regular exercise can lead to:
- Increased muscle size (hypertrophy)
- Improved muscle strength
- Greater endurance
- Conversely, inactivity or reduced workload can lead to muscle atrophy, a decrease in muscle size and strength.
- Adaptations are due to changes in muscle protein synthesis, muscle fiber types, and energy generation capacity.
Skeletal Muscle Structure
- A single skeletal muscle contains many muscle fibers.
- Each muscle fiber is surrounded by endomysium (a layer of connective tissue).
- Muscle fibers are grouped into bundles called fascicles, each surrounded by perimysium.
- The entire muscle is surrounded by epimysium.
- Each muscle fiber is striated due to the presence of sarcomeres, which are the functional units of the muscle.
- Sarcomeres are found in myofibrils.
Muscle Fiber Properties
- Contractile: Muscle fibers can shorten in response to electrical stimulation.
- Elastic: Muscle fibers can recoil and return to their resting length after stretching.
- Excitable: Muscle fibers can receive and respond to stimuli.
- Extensible: Muscle fibers can stretch when not contracted.
Motor Units
- Muscle fibers are innervated by motor neurons.
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
- An action potential in a motor neuron activates all muscle fibers in the motor unit.
- Motor units vary in size, with some innervating only a few fibers (small motor unit) and others innervating thousands (large motor unit).
Neuromuscular Transmission
- The synapse between a motor neuron and a muscle fiber is called the neuromuscular junction (NMJ).
- The NMJ consists of the axon terminal, synaptic cleft, and motor endplate of the muscle fiber.
- Signals from the motor neuron are transmitted across the synaptic cleft to the muscle fiber, causing muscle contraction.
Muscle Fibers and Myofibrils
- Muscle fibers are surrounded by a sarcolemma (muscle membrane).
- T-tubules are invaginations of the sarcolemma that extend deep into the muscle fiber.
- Depolarization of the motor endplate initiates an action potential that travels along the sarcolemma and down the t-tubules.
Sarcomeres
- Sarcomeres are joined at their ends by Z lines, to which actin (thin filaments) and myosin (thick filaments) are attached.
- Myosin filaments overlap with actin filaments within the sarcomere.
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Description
This quiz explores the structure and properties of skeletal muscle, including muscle fibers, connective tissues, and motor units. Dive into the functional aspects such as contractility, elasticity, excitability, and extensibility of muscle fibers. Test your knowledge on how these components work together for muscle function.
