Functions & Structural Organization
Which function of skeletal muscle maintains body position when sitting or standing still?
→ Answer: Posture maintenance.
Which connective tissue layer surrounds the entire skeletal muscle?
→ Answer: Epimysium.
Why are skeletal muscle fibers multinucleated?
→ Answer: Because they form from the fusion of multiple myoblasts during development.
What proteins make up the thick and thin filaments of skeletal muscle?
→ Answer: Thick filaments are composed of myosin; thin filaments consist of actin, tropomyosin, and troponin.
Which inherited disorder leads to progressive weakening of skeletal muscles due to defective dystrophin protein?
→ Answer: Muscular dystrophy.
Compared to skeletal muscle, do smooth muscle fibers have striations?
→ Answer: No; smooth muscle lacks sarcomeres and appears non-striated.
What skeletal muscle function helps regulate body temperature?
→ Answer: Heat production from muscle metabolism.
Which connective tissue layer bundles muscle fibers into fascicles?
→ Answer: Perimysium.
What is the function of the sarcolemma in a muscle fiber?
→ Answer: It is the cell membrane that conducts electrical impulses and maintains the internal environment of the fiber.
Which sarcomere structure anchors the thin filaments?
→ Answer: The Z-disc.
How does the absence of dystrophin destabilize the sarcolemma during contraction?
→ Answer: It weakens the link between the cytoskeleton and extracellular matrix, causing membrane tears and cell damage.
Which contractile proteins are found in both skeletal and smooth muscle?
→ Answer: Actin and myosin.
Which characteristic of skeletal muscle describes its ability to shorten when stimulated?
→ Answer: Contractility.
Which connective tissue layer surrounds individual muscle fibers?
→ Answer: Endomysium.
What is the purpose of the transverse (T-) tubules?
→ Answer: They carry action potentials deep into the muscle fiber to ensure coordinated contraction.
When relaxed, which region of the sarcomere contains only thick filaments and no thin filaments?
→ Answer: The H-zone.
Why might tendon injuries take longer to heal than muscle strains?
→ Answer: Tendons have limited blood supply, slowing nutrient delivery and repair.
Which microscopic structures replace Z discs in smooth muscle?
→ Answer: Dense bodies.
What property allows skeletal muscle fibers to return to their original shape after contraction or extension?
→ Answer: Elasticity.
What is the difference between a tendon and an aponeurosis (and what's another name for an aponeurosis)?
→ Answer: A tendon is a cordlike structure connecting muscle to bone, while an aponeurosis (ligament) is a broad, flat sheet that connects muscle to muscle or bone.
What is the role of the sarcoplasmic reticulum (SR) in muscle fibers?
→ Answer: It stores and releases calcium ions needed for contraction.
What happens to the H-zone and I-band during contraction?
→ Answer: They both shorten as thin filaments slide toward the M-line.
A patient presents with inflammation of connective tissue around a muscle. What is this condition called?
→ Answer: Myositis.
Why does smooth muscle contract more slowly than skeletal muscle?
→ Answer: It uses a different calcium-regulated mechanism and slower ATP turnover.
Why must skeletal muscle fibers be excitable?
→ Answer: To respond to neural stimulation and generate action potentials for contraction.
Why is it important that skeletal muscle is well vascularized?
→ Answer: Blood vessels supply oxygen and nutrients for ATP production and remove metabolic waste to sustain contraction.
What structure is formed by a T-tubule and two terminal cisternae of the sarcoplasmic reticulum?
→ Answer: A triad.
Why are skeletal muscles striated under a microscope?
→ Answer: Because of the repeating arrangement of light (I-band) and dark (A-band) regions formed by organized filaments.
Why can repeated micro-tears in connective tissue lead to long-term weakness or fibrosis?
→ Answer: Scar tissue replaces functional fibers, reducing elasticity and contractile efficiency.
Which ion still plays the central role in initiating smooth muscle contraction?
→ Answer: Calcium (Ca²⁺).
Explain why muscle extensibility is critical for smooth, coordinated movements.
→ Answer: It allows muscles to stretch as opposing muscles contract, preventing damage and enabling full range of motion.
How does the arrangement of blood vessels and nerves within the perimysium support coordinated muscle activity?
→ Answer: It ensures every fascicle receives oxygen, nutrients, and rapid neural signals simultaneously, allowing uniform contraction and responsiveness across the muscle.
Why does the large number of mitochondria in a muscle fiber matter?
→ Answer: It provides the ATP needed for sustained contraction and relaxation cycles.
Explain the functional significance of the M-line in a sarcomere.
→ Answer: It stabilizes and aligns thick filaments to maintain tension symmetry during contraction.
How does collagen content influence the mechanical strength of tendons and aponeuroses?
→ Answer: High collagen density provides tensile strength and resists overstretching.
Why does smooth muscle contraction persist longer than skeletal muscle contraction with less ATP use?
→ Answer: The latch-bridge mechanism maintains tension efficiently.
How do the five characteristics of skeletal muscle (excitability, conductivity, contractility, elasticity, and extensibility) collectively support its function in movement?
→ Answer: Together, they enable muscle fibers to receive signals, generate and conduct impulses, produce force, return to resting length, and coordinate joint motion.
Why would injury to the perimysium cause both localized pain and a decline in muscle strength, even if the fibers themselves are uninjured?
→ Answer: The perimysium carries sensory nerves and capillaries; damage disrupts nutrient delivery, nerve signaling, and force transmission between fascicles, reducing power output and causing inflammation.
How does the arrangement of T-tubules and the sarcoplasmic reticulum ensure rapid and uniform contraction?
→ Answer: T-tubules deliver the action potential quickly across the fiber, triggering calcium release from adjacent SR regions simultaneously.
How does the arrangement of sarcomeres in series within a myofibril determine overall muscle shortening?
→ Answer: Sarcomeres shorten simultaneously, and their combined contraction produces total muscle shortening proportional to the number of sarcomeres in series.
Explain how damage to the endomysium might impair muscle performance even if fibers remain intact.
→ Answer: It disrupts capillary supply and neural communication, reducing nutrient delivery and coordination between fibers.
Which system besides the nervous system can regulate smooth muscle activity?
→ Answer: The endocrine system (hormonal control).
Compare the functions of skeletal muscle that support movement with those that contribute to homeostasis.
→ Answer: Movement functions include locomotion and manipulation, while homeostatic functions include heat generation, nutrient storage, and posture maintenance.
Compare how structural differences between a tendon and its parent muscle influence their respective functions during movement.
→ Answer: Muscles generate force through contractile fibers and elasticity for motion, while tendons’ dense collagen bundles efficiently transmit that force to bones, maximizing leverage and minimizing energy loss.
Compare the roles of the sarcolemma, T-tubules, and sarcoplasmic reticulum in excitation-contraction coupling.
→ Answer: The sarcolemma conducts impulses, T-tubules transmit them inward, and the SR releases calcium to initiate contraction.
Compare how the sliding filament model explains both shortening and the constant length of filaments themselves.
→ Answer: Actin and myosin filaments don’t shorten; they slide past each other, decreasing sarcomere length without altering filament size.
Compare the effects of damage to tendons versus damage to the epimysium on overall muscle function.
→ Answer: Tendon injury limits force transmission to bones, while epimysium damage compromises protection and alignment of fibers.
Why is single-unit smooth muscle often called “visceral smooth muscle”?
→ Answer: Because it’s found in visceral organs like the digestive tract and contracts as a coordinated unit.
A patient with severe muscle atrophy loses both strength and posture stability. Which functions and properties of skeletal muscle have been compromised?
→ Answer: Contractility (force production) and elasticity (structural integrity).
A deep muscle tear damages both perimysium and endomysium. How would this affect muscle repair and contraction?
→ Answer: It would disrupt fascicle organization, nerve/blood supply, and alignment of fibers, leading to weak or uncoordinated contractions.
A patient’s biopsy reveals damage to the T-tubules. What effect would this have on muscle contraction?
→ Answer: It would delay or prevent the action potential from reaching deep myofibrils, resulting in weak or uncoordinated contractions.
A biopsy shows disorganized Z-discs in a patient’s muscle fibers. What functional problem might this cause?
→ Answer: Poor alignment of filaments would weaken contraction and reduce force efficiency.
Why would muscular dystrophy primarily affect large, weight-bearing muscles before smaller ones?
→ Answer: Larger muscles undergo greater mechanical stress, accelerating membrane damage and degeneration.
A patient is unable to dilate their pupil in low light. Which type of smooth muscle is malfunctioning?
→ Answer: Multiunit smooth muscle (precisely regulated in the iris).
Why does the loss of muscle excitability in certain neuromuscular disorders lead to paralysis even when muscle fibers are intact?
→ Answer: Without excitability, muscle fibers can’t respond to neural signals, preventing contraction despite normal structure.
Why might surgical repair of a ruptured tendon take longer to heal than damage to the muscle belly itself?
→ Answer: Tendons have limited blood supply, slowing nutrient delivery and collagen repair compared to vascularized muscle tissue.
Explain how the close relationship between the SR and T-tubules supports both rapid contraction and efficient relaxation.
→ Answer: Calcium release from SR triggers contraction, while quick reuptake via ATP-dependent pumps allows fast relaxation.
Why is precise alignment of sarcomeres critical for producing smooth, coordinated movement?
→ Answer: Synchronization of filament sliding across fibers ensures uniform tension and prevents jerky or uneven motion.
How does prolonged disuse (e.g., bed rest or immobilization) alter muscle structure at both macroscopic and microscopic levels?
→ Answer: It causes fiber atrophy, reduced myofibril density, and weakened connective support, decreasing overall muscle mass and strength.
Why does single-unit smooth muscle produce rhythmic contractions, while multiunit does not?
→ Answer: Single-unit muscle has gap junctions that allow synchronized depolarization.
Compare how cardiac, smooth, and skeletal muscles differ in excitability and contractility.
→ Answer: Skeletal muscle contracts voluntarily and rapidly, cardiac muscle is involuntary with rhythmic excitability, and smooth muscle contracts slowly and automatically in response to stimuli.
Compare the mechanical roles of epimysium and aponeuroses in maintaining muscle stability.
→ Answer: Epimysium resists overstretching and maintains muscle integrity, while aponeuroses distribute tension across a wider area for stabilization.
How might mitochondrial density differ between postural muscles and fast-twitch muscles, and why?
→ Answer: Postural (slow-twitch) muscles have more mitochondria for endurance, while fast-twitch muscles rely more on glycolytic enzymes for brief, powerful contractions.
Compare sarcomere organization in skeletal versus cardiac muscle.
→ Answer: Both are striated with sarcomeres, but cardiac cells are branched, connected by intercalated discs, and contract rhythmically rather than voluntarily.
In an athlete recovering from a tendon repair, why might rehabilitation emphasize gradual load increases rather than immediate stretching?
→ Answer: Controlled loading promotes collagen alignment and strength without risking re-tearing of immature repair tissue.
In blood vessels, how does smooth muscle regulation differ from skeletal muscle regulation?
→ Answer: Smooth muscle responds to autonomic input, hormones, and local factors, while skeletal muscle requires somatic motor neurons.
A researcher studying a toxin that interferes with skeletal muscle elasticity observes joint stiffness and poor recovery after movement. Predict the long-term effects on posture and locomotion.
→ Answer: Decreased elasticity would impair muscle recoil and coordination, leading to fatigue, postural collapse, and limited mobility.
A patient with Ehlers-Danlos syndrome has abnormally flexible joints and fragile connective tissue. Predict how this affects muscle performance.
→ Answer: Weak connective support decreases force transmission and increases injury risk, leading to inefficient movement and fatigue.
A genetic mutation impairs calcium pumps in the SR. Predict the impact on muscle function.
→ Answer: Calcium would remain in the cytosol, preventing full relaxation and causing prolonged or involuntary contractions.
A mutation prevents troponin from binding calcium. Predict the immediate consequence for muscle function.
→ Answer: Tropomyosin would remain blocking actin’s binding sites, preventing cross-bridge formation and muscle contraction.
A researcher discovers a mutation that reduces collagen cross-linking in tendons. Predict the primary symptom in affected individuals.
→ Answer: Tendons would be overly flexible and prone to rupture because of decreased tensile strength and structural stability.
Imagine a defect eliminates gap junctions in single-unit smooth muscle. How would this affect organ function?
→ Answer: Contractions would lose coordination, impairing peristalsis and other visceral movements.