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Questions and Answers
Within the sarcomere structure, if a myocyte were treated with a compound that selectively and completely disrupted the function of titin, what immediate effect would be observed?
Within the sarcomere structure, if a myocyte were treated with a compound that selectively and completely disrupted the function of titin, what immediate effect would be observed?
- Selective paralysis of fast-twitch muscle fibers, while slow-twitch fibers remain unaffected.
- Uncontrolled and irreversible shortening of the sarcomere, leading to hypercontraction.
- Loss of structural integrity, leading to disorganization of actin and myosin filaments, and impaired force generation. (correct)
- Complete cessation of muscle contraction due to the inability of actin and myosin filaments to interact.
Imagine a scenario where a novel drug selectively inhibits the release of calcium ions from the sarcoplasmic reticulum, yet the action potential propagation along the sarcolemma remains unaffected. What immediate consequence would this have on skeletal muscle function?
Imagine a scenario where a novel drug selectively inhibits the release of calcium ions from the sarcoplasmic reticulum, yet the action potential propagation along the sarcolemma remains unaffected. What immediate consequence would this have on skeletal muscle function?
- Increased muscle fatigue due to enhanced reliance on anaerobic metabolism.
- Complete and irreversible cessation of muscle contraction due to the absence of calcium-mediated events. (correct)
- Spastic paralysis due to continuous, uncontrolled muscle fiber stimulation.
- Selective impairment of fast-twitch muscle fibers, while slow-twitch fibers remain functional.
If a researcher were to selectively cleave the light chains associated with the myosin head domain, thereby decoupling their regulatory function, what immediate effect would be observed during muscle contraction?
If a researcher were to selectively cleave the light chains associated with the myosin head domain, thereby decoupling their regulatory function, what immediate effect would be observed during muscle contraction?
- Impaired relaxation, leading to prolonged contraction cycles and eventual muscle fatigue.
- Complete cessation of muscle contraction due to the inability of myosin to bind actin.
- Increased force generation but decreased contraction velocity (correct)
- Uncontrolled and continuous muscle contraction due to unrestrained ATPase activity.
Consider a hypothetical scenario where the acetylcholine-gated cation channels at the neuromuscular junction are genetically modified to exhibit a significantly reduced permeability to sodium ions, while potassium and calcium ion permeability remain unchanged. What immediate effect would this have on muscle fiber excitability?
Consider a hypothetical scenario where the acetylcholine-gated cation channels at the neuromuscular junction are genetically modified to exhibit a significantly reduced permeability to sodium ions, while potassium and calcium ion permeability remain unchanged. What immediate effect would this have on muscle fiber excitability?
In a carefully controlled experiment, a researcher introduces a non-hydrolyzable analog of ATP (adenosine triphosphate) into the sarcoplasm of a skeletal muscle fiber. What immediate effect would this have on the cross-bridge cycle during muscle contraction?
In a carefully controlled experiment, a researcher introduces a non-hydrolyzable analog of ATP (adenosine triphosphate) into the sarcoplasm of a skeletal muscle fiber. What immediate effect would this have on the cross-bridge cycle during muscle contraction?
If a mutation occurred that prevented the binding of calcium ions to troponin C, what direct effect would this have on the actin-myosin interaction in skeletal muscle?
If a mutation occurred that prevented the binding of calcium ions to troponin C, what direct effect would this have on the actin-myosin interaction in skeletal muscle?
What immediate effect would result from selectively blocking the function of the calcium ATPase (SERCA) pump in the sarcoplasmic reticulum of skeletal muscle fibers?
What immediate effect would result from selectively blocking the function of the calcium ATPase (SERCA) pump in the sarcoplasmic reticulum of skeletal muscle fibers?
In a scenario where the M-line proteins within the sarcomere are selectively disrupted, what immediate consequence would be observed during muscle contraction?
In a scenario where the M-line proteins within the sarcomere are selectively disrupted, what immediate consequence would be observed during muscle contraction?
In a scenario where a muscle cell's sarcolemma is rendered completely impermeable to sodium ions, what immediate impact would this have on the cell's ability to contract in response to neural stimulation?
In a scenario where a muscle cell's sarcolemma is rendered completely impermeable to sodium ions, what immediate impact would this have on the cell's ability to contract in response to neural stimulation?
Imagine a muscle fiber where the connection between the transverse tubules (T-tubules) and the sarcoplasmic reticulum (SR) is selectively disrupted. What immediate functional consequence would you expect to observe?
Imagine a muscle fiber where the connection between the transverse tubules (T-tubules) and the sarcoplasmic reticulum (SR) is selectively disrupted. What immediate functional consequence would you expect to observe?
Flashcards
Sarcolemma
Sarcolemma
A muscle fiber's true cell membrane, also known as the plasmalemma.
Sarcoplasm
Sarcoplasm
Intracellular fluid found between myofibrils, containing potassium, magnesium, phosphate, and mitochondria.
Sarcoplasmic reticulum
Sarcoplasmic reticulum
Specialized endoplasmic reticulum of skeletal muscle that stores calcium ions.
Sarcomere
Sarcomere
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Z disc
Z disc
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M line
M line
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Myofibrils
Myofibrils
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Titin
Titin
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Acetylcholine
Acetylcholine
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Sliding filament theory
Sliding filament theory
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Study Notes
Skeletal Muscle Contraction
- Skeletal muscle makes up approximately 40% of the body's mass
- Cardiac and smooth muscle account for 10% of body muscle
- Basic principles of contraction are applicable to all three muscle types
Physiologic Anatomy of Skeletal Muscle
- Skeletal muscles are composed of numerous fibers ranging from 10 to 80 micrometers in diameter
- Each fiber contains successively smaller subunits
- Muscle fibers typically extend the entire length of the muscle
Sarcomere
- The M line is at the center of the sarcomere
- The Z disk is where two actin filaments connect
- H bands contain thick filaments not overlapped by thin filaments
Sarcolemma
- The sarcolemma consists of a true cell membrane, also known as the plasmalemma
- It includes an outer coat of polysaccharide material containing numerous collagen fibrils
- At the end of a muscle fiber, the sarcolemma surface layer fuses with a tendon fiber
- Tendon fibers bundle together to form muscle tendons that insert into bones
Myofibrils
- Myofibrils are composed of actin and myosin filaments
- Each muscle fiber contains hundreds to thousands of myofibrils
- A myofibril consists of approximately 1,500 myosin and 3,000 actin filaments
- Thick filaments are comprised of myosin
- Thin filaments are comprised of myosin
- Myosin and actin interdigitate, creating alternating light and dark bands
- Light bands are actin (I bands), which are isotropic to polarized light
- Thick bands are myosin (A bands), which are anisotropic to polarized light
- Striated appearance in skeletal and cardiac muscle comes from the arrangement of actin and myosin
- A sarcomere represents the portion of a myofibril between two successive Z disks
Titin Filamentous Molecule
- It maintains the side-by-side relationship between actin and myosin
- Each titin molecule has a large molecular weight of 3 million
- Titin is the largest protein molecule in the body
- Titin is filamentous, providing springiness
- It serves as a framework holding actin and myosin filaments in place
- One end is attached to the Z disk
- The other end is tethered to the M filament
Sarcoplasm
- The intracellular fluid between myofibrils is called sarcoplasm
- Sarcoplasm contains substantial amounts of potassium, magnesium, and phosphate
- It also contains mitochondria
Sarcoplasmic Reticulum
- The sarcoplasmic reticulum is a specialized type of endoplasmic reticulum (ER) found in skeletal muscle
Muscle Contraction
- An action potential travels along a motor nerve to its endings on muscle fibers
- The nerve endings secrete a small amount of acetylcholine (a neurotransmitter)
- Acetylcholine acts on the muscle fiber, opening acetylcholine-gated cation channels
- The opening of these channels allows a large influx of sodium ions, causing membrane depolarization
- Voltage-gated sodium channels open, initiating another action potential at the membrane
- Action potential travels along the muscle fiber membrane similar to nerve fiber membranes
- This action potential causes the sarcoplasmic reticulum to release large amounts of stored calcium ions
- Calcium ions initiate attractive forces between actin and myosin filaments, which causes them to slide alongside each other, resulting in contraction
- After a fraction of a second, calcium ions are pumped back into the SR by a calcium membrane pump and stored until another action potential arrives
- Ca ions from the myofibrils are removed, which causes the muscles contraction to cease
Contractile Filaments
- Myosin Filaments are composed of multiple myosin molecules
- A myosin molecule consists of 6 polypeptide chains
- There are 2 heavy chains that wrap spirally to form the tail
- One end folds bilaterally into a globular polypeptide forming the myosin head
- There are 4 light chains controlling the head’s function during muscle contraction
Actin Filaments
- The backbone is a double-stranded F-actin molecule
- Two strands are wound in a helix, similar to myosin
- Each strand comprises G-actin molecules
- One molecule of ADP is attached to each G-actin molecule
- ADP molecules are active sites on actin, which interact with myosin during muscle contraction
Tropomyosin
- It is wrapped around the sides of the f-actin helix
- During rest, tropomyosin covers active sites, thus attraction can not occur (between actin and myosin)
- The above results in no contraction
Troponin
- Troponin are small proteins attached along the sides of tropomyosin
- It is composed of 3 loosely bound protein subunits with roles in muscle contraction
- Troponin I has a high affinity for actin
- Troponin T is for tropomyosin
- Troponin C is for calcium ions
Muscle Contraction - Role of Troponin-Tropomyosin (T-T) Complex
- Pure actin filament without the presence of troponin-tropomyosin complex binds instantly with heads of myosin filament
- If the T-T complex is added to the actin filament, binding between myosin and actin does not take place
- Active sites on relaxed muscles are inhibited or physically covered by T-T complex
Muscle Contraction - Role of Calcium Ions
- Large amounts of Ca ions inhibit the T-T complex
- Calcium ions bind with Troponin-C, the troponin complex changes and tugs on the tropomyosin molecule
- This uncovers the active sites of actin which allows attraction of myosin cross-bridge heads, causing contraction
Walk Along (Ratchet) Theory of Contraction
- The heads of cross-bridges attach to and engage from the active sites of actin
- A head attaches to an active site, and then tilts to drag the actin filament
- The tilt of the head is known as the power stroke
- Immediately after tilting, the head breaks away
- The head binds to the next active site along the actin filament
- The head tilts again to produce a power stroke
Assignment Topics
- Characteristics of whole muscle contraction
- Isometric vs Isotonic
- Fast vs Slow Muscle Fibers
- Summation
- Tetanization
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Description
Overview of skeletal muscle contraction, its physiological anatomy, and the role of the sarcomere and sarcolemma. Includes muscle fiber composition, the structure of the sarcomere with its M line, Z disk, and H bands, and the function of the sarcolemma in muscle contraction.
