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Questions and Answers
Force exerted by a muscle is solely dependent on the mechanical properties of the muscle itself.
Force exerted by a muscle is solely dependent on the mechanical properties of the muscle itself.
False (B)
A single muscle fiber can be innervated by multiple motor neurons.
A single muscle fiber can be innervated by multiple motor neurons.
False (B)
Myofibrils are made up of sarcomeres connected in parallel.
Myofibrils are made up of sarcomeres connected in parallel.
False (B)
Terminal cisternae are smaller structures along the T tubules.
Terminal cisternae are smaller structures along the T tubules.
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T tubules directly connect the myofibrils and the Z disks.
T tubules directly connect the myofibrils and the Z disks.
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Actin filaments extend all the way to the midline of the sarcomere, overlapping with the myosin.
Actin filaments extend all the way to the midline of the sarcomere, overlapping with the myosin.
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The sarcoplasmic reticulum is arranged perpendicular in relation to the sarcomere.
The sarcoplasmic reticulum is arranged perpendicular in relation to the sarcomere.
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The Z disk serves as the location where the myosin filaments attach.
The Z disk serves as the location where the myosin filaments attach.
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Calcium concentration in the cytoplasm is typically high, facilitating immediate muscle contraction.
Calcium concentration in the cytoplasm is typically high, facilitating immediate muscle contraction.
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The interaction between troponin and tropomyosin directly causes the binding of myosin to actin.
The interaction between troponin and tropomyosin directly causes the binding of myosin to actin.
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DHPR is located on the sarcoplasmic reticulum and directly facilitates calcium release upon action potential arrival.
DHPR is located on the sarcoplasmic reticulum and directly facilitates calcium release upon action potential arrival.
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The velocity of calcium pumps in the sarcoplasmic reticulum is faster than the diffusion of calcium out of it.
The velocity of calcium pumps in the sarcoplasmic reticulum is faster than the diffusion of calcium out of it.
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The closure of calcium channels in the sarcoplasmic reticulum is induced by muscle fiber contraction.
The closure of calcium channels in the sarcoplasmic reticulum is induced by muscle fiber contraction.
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RyR channels are found on the T tubules and are directly activated by dihydropyridine receptors.
RyR channels are found on the T tubules and are directly activated by dihydropyridine receptors.
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During muscle relaxation, tropomyosin uncovers the myosin binding sites on actin.
During muscle relaxation, tropomyosin uncovers the myosin binding sites on actin.
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The strength of muscle contraction is independent of the free calcium concentration in the cytoplasm.
The strength of muscle contraction is independent of the free calcium concentration in the cytoplasm.
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T tubules are responsible for evenly distributing action potentials to all myofibrils, including those located deeply within the muscle fiber.
T tubules are responsible for evenly distributing action potentials to all myofibrils, including those located deeply within the muscle fiber.
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A decrease in cytoplasmic calcium concentration leads to the binding of myosin to actin.
A decrease in cytoplasmic calcium concentration leads to the binding of myosin to actin.
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The central zone of the sarcomere is characterized by the presence of both actin and myosin filaments.
The central zone of the sarcomere is characterized by the presence of both actin and myosin filaments.
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The Z disk, which delineates the boundaries of a sarcomere, is a linear structure with a very small surface area.
The Z disk, which delineates the boundaries of a sarcomere, is a linear structure with a very small surface area.
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Titin and nebulin filaments function to disrupt the structure of the sarcomere.
Titin and nebulin filaments function to disrupt the structure of the sarcomere.
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An electrical event, known as a voltage potential, is required to initiate a mechanical reaction in muscle cells.
An electrical event, known as a voltage potential, is required to initiate a mechanical reaction in muscle cells.
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Motor neurons transmit action potentials to muscle fibers via their axons, which directly originate from the brain.
Motor neurons transmit action potentials to muscle fibers via their axons, which directly originate from the brain.
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The trigeminal (V) motor nucleus innervates the muscles responsible for facial expression.
The trigeminal (V) motor nucleus innervates the muscles responsible for facial expression.
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The extensive synaptic contact at the endplate is paired with a sparse organization of nicotinic receptors.
The extensive synaptic contact at the endplate is paired with a sparse organization of nicotinic receptors.
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Calcium voltage-gated channels facilitate the efflux of calcium ions during muscle activation.
Calcium voltage-gated channels facilitate the efflux of calcium ions during muscle activation.
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The endplate potential is solely dependent on the movement of K+ ions through nicotinic receptor channels.
The endplate potential is solely dependent on the movement of K+ ions through nicotinic receptor channels.
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The action potential is generated within the sarcomere, linking it directly to the mechanical event inside the cell.
The action potential is generated within the sarcomere, linking it directly to the mechanical event inside the cell.
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In an isometric contraction, muscle length changes while tension remains constant.
In an isometric contraction, muscle length changes while tension remains constant.
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During the latent period of a single muscle twitch, the sarcomeres begin to shorten.
During the latent period of a single muscle twitch, the sarcomeres begin to shorten.
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The contraction phase of a muscle twitch is characterized by a decrease in tension.
The contraction phase of a muscle twitch is characterized by a decrease in tension.
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During the relaxation phase, calcium ions are pumped into the sarcoplasmic reticulum leading to a decrease in muscle tension.
During the relaxation phase, calcium ions are pumped into the sarcoplasmic reticulum leading to a decrease in muscle tension.
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A single muscle twitch typically lasts 70 seconds.
A single muscle twitch typically lasts 70 seconds.
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During muscle relaxation, calcium is actively transported back into the sarcoplasmic reticulum by pumps to reduce the likelihood of actin-myosin bridge formation.
During muscle relaxation, calcium is actively transported back into the sarcoplasmic reticulum by pumps to reduce the likelihood of actin-myosin bridge formation.
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Dihydropyridine receptors and ryanodine receptors remain unchanged in their conformation after the action potential has terminated.
Dihydropyridine receptors and ryanodine receptors remain unchanged in their conformation after the action potential has terminated.
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A muscle twitch is considered a variable process because the amount of released sodium is not always the same.
A muscle twitch is considered a variable process because the amount of released sodium is not always the same.
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In a graph of muscle force, with higher peaks, where the second action potential occurs before the first contraction is finished, the third line should have a lower peak of force than the second.
In a graph of muscle force, with higher peaks, where the second action potential occurs before the first contraction is finished, the third line should have a lower peak of force than the second.
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Tetanic summation occurs when the stimulation frequency is low, allowing separate muscle twitches to be observed.
Tetanic summation occurs when the stimulation frequency is low, allowing separate muscle twitches to be observed.
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The maximum force exerted by a muscle during tetanic summation occurs when the second action potential triggers before calcium levels decline after the first action potential.
The maximum force exerted by a muscle during tetanic summation occurs when the second action potential triggers before calcium levels decline after the first action potential.
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Tetanic force is a mechanism by which muscle force can be decreased due to frequent action potentials.
Tetanic force is a mechanism by which muscle force can be decreased due to frequent action potentials.
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During tetanic summation, calcium is effectively sequestered by the pumps in between frequent action potentials.
During tetanic summation, calcium is effectively sequestered by the pumps in between frequent action potentials.
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The duration of an action potential in muscle is longer than the mechanical event it triggers and takes about 70ms.
The duration of an action potential in muscle is longer than the mechanical event it triggers and takes about 70ms.
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During tetanic summation, the progressive summation of calcium concentration leads to a reduction of actin myosin cross-bridges.
During tetanic summation, the progressive summation of calcium concentration leads to a reduction of actin myosin cross-bridges.
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Study Notes
Neural Control of Force - Different Force Requests
- Force exerted by muscles acts on tendons, which act on bones, resulting in joint movement.
- The body must adjust muscle force to the task.
- Muscle force depends on two factors:
- Mechanical properties of the muscle.
- Nervous system control of muscle motor neurons.
- The brain needs to be aware of muscle mechanical properties to accurately control force.
- This lecture discusses nervous system control of muscle force recruitment, also considering muscle structure.
The Motor Unit
- The fundamental unit of the motor system.
- Formed by a spinal motor neuron and the muscle fibers it innervates.
- A single motor axon may branch to innervate multiple muscle fibers.
- Each muscle fiber is innervated by a single motor neuron.
- An entire muscle may receive input from hundreds of different motor neurons.
The Structure of a Motor Unit
- Myofibrils are tubular structures formed by connected sarcomeres in series.
- The arrangement of sarcoplasmic reticulum cisternae is parallel to sarcomeres and T tubules.
- T tubules are invaginations of the sarcoplasm, coinciding with Z disks.
- The sarcoplasmic reticulum is highly developed in muscle cells.
- Terminal cisternae are enlargements of the sarcoplasmic reticulum alongside T tubules.
The Sarcomere
- The functional unit of a myofibril.
- Defined by Z disks, where actin filaments attach.
- Actin filaments extend towards the sarcomere midline but never meet.
- Myosin filaments are in the sarcomere midline and go in opposite direction to the actin filaments.
- Sarcomere characterized by three zones:
- Peripheral zone with only actin.
- Central zone with only myosin.
- Intermediate zone where actin and myosin overlap.
- Titin and nebulin filaments maintain the correct sarcomere geometry.
The Neuromuscular Junction
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Electrical (action potential) signals initiate muscle contraction.
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Motor neuron axons originated in the spinal cord (lamina IX of anterior horn) innervate muscles below the neck.
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Cranial nerve nuclei (trigeminal and facial) innervate muscles above the neck.
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The endplate has extensive synaptic contacts with nicotinic receptors, enabling action potential transduction to the muscle.
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Action potential steps:
- Voltage-gated calcium channels open.
- Acetylcholine (ACh) vesicle exocytosis.
- ACh binds to nicotinic receptors, opening Na+ and K+ channels, causing depolarization.
Mechanism of Contraction and Importance of Ca2+
- Action potential generated by the membrane, but the sarcomere is inside the cytoplasm.
- Calcium stored in the sarcoplasmic reticulum (SR) is crucial for linking the action potential to the mechanical contraction.
- DHPR (dihydropyridine receptor) is a transmembrane protein on the T tubule.
- RyR (ryanodine receptor) is a Ca2+ channel on the SR.
- Arrival of the action potential at the T tubule changes the DHPR conformation, opening RyR channels and releasing Ca2+ into the sarcoplasm.
- Calcium binding to troponin causes tropomyosin to move, allowing myosin to bind to actin, triggering contraction.
- Muscle relaxation occurs when Ca2+ is pumped back into the SR, and tropomyosin recovers the actin-binding sites.
Single Twitch and Tetanic Contraction
- A single twitch involves a latent period, contraction phase, and relaxation phase.
- During latent period, the action potential propagates.
- The contraction phase is characterized by Ca2+ binding, cross-bridge formation, and sarcomere shortening.
- During relaxation phase, Ca2+ ions are pumped back into SR and cross-bridge cycling stops.
- The tetanic force is the force generated by a muscle fiber stimulated with trains of action potentials.
- Tetanic force depends on stimulus frequency and is greater than a single twitch.
Tetanic Summation
- Increased discharge frequency leads to summation of contractions.
- Calcium concentration progressively increases, leading to maximal possible bridge formation.
- Tetanic summation is possible due to the shorter action potential duration compared to a single twitch duration.
Motor Neuron Location and Motor Nuclei
- Lower motor neurons are located in the anterior horn of the spinal cord (Lamina IX).
- Motor pools (nuclei) are distributed along neuromeres, innervating specific muscles.
Innervation Ratio
- Innervation ratio refers to the number of muscle fibers innervated by a single motor neuron.
- High ratio (e.g., large muscles) results in less precise movements.
- Low ratio (e.g., fine motor control) enables more precise movements.
- Muscle fibers innervated by a single motor unit are distributed homogeneously within the muscle, enabling synchronized contraction.
Functional and Biochemical Properties of Muscle Fibers
- Velocity, maximal force, and fatiguability of contraction are key properties.
- Slow and fast twitch fibers have differing properties.
- Key characteristics for classification, like ATPase activity, oxidative phosphorylation capacity, and glycolytic enzyme levels are examined.
Functional and Biochemical Properties of Motor Neurons
- Two main motor neuron types: α (skeletal muscle) and γ (intrafusal fibers).
- Tetanic summation is dependent on discharge frequency and directly correlates with muscle force.
Dynamic Sensitivity of Motor Neurons
- Muscle force is very dependent on motor neuron firing and tetanic summation.
- Motor neurons exhibit both static and dynamic sensitivity.
- Muscles have inertia which means initial fast firing required.
- Initial high-frequency firing overcomes inertia of the muscle, eventually leading to constant-frequency firing.
Clinical Point – Amyotrophic Lateral Sclerosis (ALS)
- ALS (Lou Gehrig's disease) is a progressive neurodegenerative disease.
- ALS results in the death of motor neurons, eventually leading to paralysis, difficulty swallowing, breathing problems.etc.
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Description
Test your knowledge on muscle physiology with this quiz, focusing on the mechanical properties of muscles, muscle fiber innervation, and the roles of various structures within the sarcomere. Explore concepts such as calcium dynamics and the interactions between actin and myosin filaments.
