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WorksheetsCross Bridge Formation & Sliding Filament Model
Total questions: 129
Worksheet time: 1hrs 5mins
Which immediate chemical event enables a myosin head to bind to actin during contraction?
Calcium binds to troponin C
Pi binds to actin monomers
ATP splits into ADP and Pi
ATP is resynthesized from ADP
When ADP and Pi leave the myosin head, what mechanical action occurs?
Power stroke pulling actin inward
Elongation of the sarcomere
Rotation of tropomyosin outward
Detachment of myosin from actin
Fill in the blank: After the power stroke, binding of a new (a) to myosin causes detachment from actin.
Which statement best describes the sliding filament model outcome for the sarcomere?
Sarcomere shortens generating tension
Sarcomere lengthens reducing tension
Z-lines move apart with no tension
Filaments dissolve to release energy
What drives actin filaments to move toward the center of the sarcomere?
Hydrolysis of actin subunits
Passive recoil of titin strands
Diffusion of calcium ions alone
Repeated myosin power strokes
Which step allows the cross-bridge cycle to repeat along the actin filament?
Pi attaches back to myosin
Calcium detaches from troponin
Actin converts ATP to ADP
Myosin binds a new ATP molecule
A muscle fiber is unable to detach myosin from actin. Which molecular shortage most likely explains this?
High titin stiffness
Insufficient ATP available
Excess Pi accumulation
Low actin polymerization
Sequence the events initiating force production: choose the first event.
Sarcomere shortening completes
Binding of new ATP to myosin
Release of ADP and Pi
ATP splitting on myosin head
During sustained contraction, what repeatedly forms and breaks to produce movement of thin filaments?
Multiple actin–myosin cross-bridges
Continuous sarcomere polymer chains
Permanent myosin head attachments
Stable titin–nebulin complexes
Which statement best defines a motor unit?
A spinal reflex arc linking several interneurons
A single motor neuron and all fibers it innervates
Multiple motor neurons controlling one whole muscle
One muscle fiber plus its sarcomere proteins
In skeletal muscle, how many motor neurons innervate a single muscle fiber?
It varies with movement demand
Several alternating motor neurons
Two cooperating motor neurons
Exactly one motor neuron
Complete the rule: When the motor neuron fires, (a) contract.
Which pattern describes how fibers of one motor unit are arranged within a muscle?
Aligned only near tendinous insertions
Spread out across the muscle
Clustered in one compact region
Grouped by fiber-type compartments
What principle explains that a motor unit contracts fully once threshold is reached?
Size principle of recruitment
All-or-none principle
Length-tension relationship
Henneman adaptation rule
A motor neuron innervating few fibers typically produces which movement feature?
Coarse force production
Greater velocity contraction
High-precision control
Fatigue-resistant endurance
A motor neuron innervating many fibers most likely yields which outcome?
Improved sensory feedback
Fine gradation of force
Coarse, powerful movement
Selective fiber hypertrophy
Which statement correctly contrasts motor units with few versus many fibers?
Few fibers give precision; many fibers give coarse power
Many fibers always decrease total force
Few fibers give coarse control; many fibers give precision
Both give identical movement precision
Which scenario best illustrates the all-or-none rule for a motor unit?
Random contraction of dispersed unit fibers
Partial contraction of half the unit fibers
Graded contraction of selected unit fibers
Simultaneous contraction of every unit fiber
Why are fibers of a single motor unit distributed throughout the muscle rather than clumped?
To minimize metabolic cost only
To spread force evenly across the tissue
To isolate fatigue to one region
To align with tendon collagen orientation
Fill in the precise term: A motor neuron connecting to multiple fibers is said to (a) those fibers.
Which choice correctly states the relationship between motor neurons and muscle fibers?
No fixed relationship exists
Many neurons to one fiber routinely
One neuron to many fibers possible
One neuron to one fiber exclusively
If a task requires fine motor control, which motor unit property is most beneficial?
Intermittent neuron firing thresholds
Clumped fiber distribution within muscle
Low fiber count per motor neuron
High fiber count per motor neuron
Which statement best defines a graded muscle contraction?
A contraction producing fixed force only
A contraction occurring without neural input
A contraction modified to vary generated force
A contraction limited by single motor unit
Temporal summation primarily increases force by changing which variable?
Length of the muscle fibers
Frequency of motor neuron firing
Amplitude of each action potential
Number of recruited motor units
Fill in the blank: Grading by motor unit summation increases force by increasing the number of (a) used.
During temporal summation, when does the second impulse arrive relative to the first contraction?
Before complete relaxation has finished
After complete relaxation has finished
Long after tension has fully dissipated
During the absolute refractory period
Which approach directly grades force by recruiting additional motor units?
Temporal summation mechanism
Motor unit summation mechanism
Isometric tension normalization
Post-tetanic potentiation only
According to the size principle of recruitment, which motor units are activated first?
Units with the largest muscle fibers
Units with moderate fiber diameters
Units with the smallest muscle fibers
Units with fatigued muscle fibers
Which motor units produce the greatest force but are recruited last during graded contractions?
Units with the largest muscle fibers
Units with slow oxidative fibers
Units with the smallest muscle fibers
Units with intermediate fiber sizes
Fill in the blank: In motor unit summation, recruitment follows the (a) principle.
Why are motor units recruited asynchronously during sustained contractions?
To synchronize fiber relaxation phases
To prevent fatigue and smooth force output
To maximize peak twitch amplitude
To increase refractory period duration
Which scenario exemplifies temporal summation rather than motor unit summation?
Adding more active motor units gradually
Increasing firing rate to overlap twitches
Switching from concentric to eccentric
Shortening muscle to optimal length
A lab increases stimulation frequency without changing the number of active motor units. What outcome is most likely?
Greater force via temporal summation
Immediate recruitment of large units
Reduced tetanic tension overall
No change in total muscle tension
Which statement distinguishes the two grading mechanisms most accurately?
Temporal varies firing frequency; motor units vary unit number
Temporal reduces asynchrony; motor units increase refractory
Temporal changes fiber length; motor units alter nerve size
Temporal recruits largest units; motor units vary twitch speed
Fill in the blank: Graded contractions allow muscles to produce varying amounts of (a) .
Which statement best defines muscle tone in resting skeletal muscle?
Sustained maximal fiber contraction at rest
Complete flaccidity without any neural input
Slight, continuous contraction of relaxed fibers
Periodic twitching triggered by voluntary signals
Does muscle tone produce movement under normal conditions?
Yes, continuous small joint motions
Only in smooth muscle tissues
No, it does not produce movement
Only during rapid postural shifts
Fill in the blank: Muscle tone helps (a) by providing low-level, continuous tension.
A patient shows poor posture and frequent joint subluxations but normal strength during voluntary contractions. Which impairment most likely explains these findings?
Excessive recruitment of fast-twitch fibers
Loss of muscle tone reducing stabilization
Hyperactive stretch reflexes increasing movement
Enhanced motor unit synchronization improving power
If muscle tone is lost, what immediate physiological consequence is most likely?
Enhanced endurance during repetitive tasks
Increased voluntary movement precision
Improved flexibility with stable joints
Muscle becomes unresponsive to stimuli
Which statement best defines an isotonic contraction in skeletal muscle?
Tension develops without any sarcomere change
Tension develops and the muscle length changes
Tension remains constant and the load is fixed
Tension drops as the muscle relaxes fully
During a concentric contraction, what happens to muscle length while producing force against a load?
Muscle alternates between short and long
Muscle lengthens under sustained tension
Muscle shortens while doing mechanical work
Muscle length remains exactly unchanged
Identify the primary feature of an eccentric contraction.
Muscle length does not change at all
Muscle fully relaxes and loses tension
Muscle shortens while performing work
Muscle lengthens while still generating tension
Fill in the blank: In an isometric contraction, tension develops but the muscle (a) .
Which scenario most accurately illustrates isometric contraction?
Holding a heavy box without moving it
Swinging the arm freely without load
Curling a dumbbell to your shoulder
Lowering a barbell with control
When performing a slow, controlled lowering phase of a bicep curl, which contraction type predominates in the biceps?
No contraction occurs at all
Isometric contraction only
Eccentric isotonic contraction
Concentric isotonic contraction
Which statement correctly contrasts isotonic and isometric contractions regarding load movement?
Isotonic moves the load; isometric does not
Isotonic never changes length; isometric shortens
Isotonic relaxes the muscle; isometric contracts
Isotonic lengthens only; isometric shortens only
Cross-bridge cycling occurs in isometric contractions. What is the key sarcomere-level outcome in this case?
Sarcomere length remains essentially constant
Sarcomeres shorten and slide markedly
Sarcomeres detach and stop cycling
Sarcomeres lengthen under passive stretch
Which molecule is the only immediate energy source that powers skeletal muscle contraction?
ATP
ADP
CP
Glucose
Skeletal muscle stores which carbohydrate polymer to supply glucose for ATP production?
(a)
Direct phosphorylation uses creatine phosphate to transfer which component directly to ADP?
Electron pair
Carbonyl group
Hydrogen ion
Phosphate group
How many ATP molecules are regenerated per creatine phosphate molecule in direct phosphorylation?
Four ATP
Three ATP
One ATP
Two ATP
Direct phosphorylation for ATP regeneration requires which condition?
Lactate
High oxygen
No oxygen
Mitochondria
During anaerobic glycolysis, glucose is broken down to produce 2 ATP and which intermediate?
Phosphocreatine
Acetyl-CoA
Citric acid
Pyruvic acid
In the absence of oxygen, pyruvic acid is converted into what product?
(a)
Which statement best describes a benefit of anaerobic glycolysis?
Requires oxygen
Eliminates fatigue
High ATP yield
Rapid ATP production
What is a key drawback of anaerobic glycolysis for muscle cells?
Low ATP yield
Slow activation
Consumes CP extensively
Needs oxygen
Compare ATP yields: which pathway provides one ATP per high-energy donor molecule?
Direct phosphorylation
Aerobic respiration
Beta-oxidation
Gluconeogenesis
If oxygen delivery is limited during intense exercise, which ATP-regenerating pathway will predominate early?
Anaerobic glycolysis
Aerobic respiration
Oxidative phosphorylation
Pentose phosphate
A sprinter relies on immediate ATP resynthesis. Which stored compound in muscle enables this rapid transfer?
Myoglobin
Triglycerides
Glycogen granules
Creatine phosphate
Short, high-intensity bouts often cause soreness due to accumulation of which metabolic product?
Lactic acid
Pyruvic acid
Carbon dioxide
Acetyl-CoA
Which statement best describes the ATP yield of aerobic respiration in skeletal muscle?
Generates about 30 to 32 ATP per glucose
Produces fewer than ten ATP per glucose
Yields roughly 60 to 64 ATP per glucose
Creates exactly 24 ATP per glucose
Name one key requirement that must be continuously available for the aerobic pathway to sustain ATP production.
(a)
During light to moderate long-term exercise, why is the aerobic pathway favored over anaerobic processes?
It works fastest but gives minimal ATP
It avoids using glucose during activity
It provides higher ATP yield with steady supply
It requires no oxygen for prolonged output
Identify a drawback of the aerobic pathway in muscle tissue.
It cannot function when glucose is present
It produces more lactic acid than glycolysis
It rapidly depletes oxygen-independent fuels
It is a slow process compared to anaerobic
Fill in the blank with the exact quantitative range: Aerobic respiration typically produces (a) ATP per glucose molecule.
Which statement best defines muscle fatigue in physiological terms?
Sudden spasm caused by lactic acid
Loss of nerve supply to the muscle
Permanent damage to muscle fibers
Temporary inability to generate force
Muscle contractions require ATP. What immediate consequence follows if ATP becomes critically low during exercise?
Sarcomeres permanently shorten
Actin filaments disintegrate
Motor neurons stop firing
Cross-bridge cycling halts quickly
Fill in the blank: Muscle fatigue serves a protective role by preventing (a) when energy availability drops.
During high-intensity exercise, how does the rate and duration of fatigue typically compare to low-intensity exercise?
Faster onset, shorter duration
Slower onset, shorter duration
Faster onset, longer duration
Slower onset, longer duration
A sprinter experiences rapid fatigue after a 200 m sprint. Which reasoning best explains this outcome?
Actin and myosin no longer exist in fast-twitch fibers
Nerve transmission increases energy reserves
Oxygen diffusion becomes unlimited in fast fibers
ATP is consumed rapidly, limiting cross-bridge cycling
You plan a 45-minute low-intensity cycling session. Predict the fatigue pattern and justify the expectation.
No onset, infinite duration
Immediate onset, very brief duration
Gradual onset, longer duration
Abrupt onset, moderate duration
Which statement most accurately captures why muscle fatigue is necessary and important?
It permanently strengthens muscle fibers
It prevents severe energy depletion damage
It increases lactic acid production always
It eliminates the need for ATP entirely
Which statement best explains why more cross-bridges increase contraction force?
They prevent calcium binding to troponin
They allow stronger actin–myosin interaction
They lengthen sarcomeres during pulling
They reduce ATP demand per twitch
Temporal summation primarily increases force by altering which variable?
Frequency of neural stimulation
Diameter of muscle fibers
Resting sarcomere length
Number of motor units active
Motor unit summation refers to progressively recruiting additional motor units to increase force.
(a)
Which factor most directly determines the maximum number of cross-bridges that can form at any instant?
Overlap of actin and myosin
Rate of acetylcholine release
Amount of lactic acid produced
Speed of action potential conduction
A muscle produces less tension when overly stretched because
Actin–myosin overlap decreases markedly
ATP stores are fully depleted
Troponin becomes saturated with calcium
The sarcolemma becomes hyperpolarized
Which scenario best illustrates temporal summation?
Rapid successive stimuli increase tension
Recruiting larger motor units first
Hypertrophy after resistance training
Optimal length yields peak tension
Which change most effectively increases force through hypertrophy?
Greater myofibril number per fiber
Higher motor neuron firing rate
Shorter resting sarcomere length
More mitochondria in sarcoplasm
Hypertrophy is the increase in the size of muscle fibers.
(a)
At optimal muscle length, tension is maximal because
Motor unit firing is minimal
Calcium stores are exhausted
Titin is fully slackened
Cross-bridge overlap is ideal
Which factor does motor unit summation directly manipulate to change force output?
Concentration of lactic acid
Amount of ATP hydrolyzed
Number of fibers activated
Length of the sarcomere
During high-frequency stimulation, individual twitches fuse into sustained tension. Name this phenomenon.
(a)
Why do larger muscle fibers typically generate greater force?
They prevent motor unit fatigue
They contain more parallel myofibrils
They produce longer action potentials
They avoid calcium reuptake entirely
Which factor would most likely reduce tension in an excessively shortened muscle?
Acetylcholine release is blocked
Mitochondrial density increases
Motor units fire asynchronously
Actin filaments overlap improperly
List the four primary factors that influence muscle contraction force.
(a)
Which factor directly influences speed of muscle contraction by determining how fast cross-bridges form and break?
Level of myoglobin within fibers
Number of mitochondria present
Amount of stored glycogen
Rate of ATP splitting by myosin
Fast motor neuron activity typically results in what contraction characteristic?
Lower ATP demand during work
Greater myoglobin accumulation
Slower relaxation after contraction
Faster contraction of innervated fibers
Which pathway is primarily used by oxidative muscle fibers to generate ATP?
Lactic acid fermentation
Anaerobic glycolysis pathways
Aerobic metabolic pathways
Phosphagen-only pathways
Fill in the blank: Glycolytic fibers predominantly use (a) pathways for ATP production.
A sprinter relying on brief, powerful bursts most likely uses which fiber type?
Mixed smooth fibers
Fast glycolytic fibers
Slow oxidative fibers
Fast oxidative fibers
Which profile best matches slow oxidative fibers?
Contract quickly, low myoglobin, low mitochondria
Contract slowly, high glycogen, low blood supply
Contract slowly, high myoglobin, many mitochondria
Contract quickly, some glycogen, lots of myoglobin
Compared with fast glycolytic fibers, fast oxidative fibers typically have which difference?
Greater myoglobin and mitochondria content
Slower contraction velocity always
Lower aerobic capacity overall
Higher glycogen and less blood supply
Fill in the blank: Fast glycolytic fibers characteristically have (a) glycogen content.
Which combination best predicts long-duration endurance performance?
Fast oxidative fibers with low myoglobin
Fast glycolytic fibers with anaerobic pathways
Slow oxidative fibers using aerobic pathways
Glycolytic fibers with limited blood supply
Reason through this scenario: During sustained moderate exercise, which fiber type will maintain force with minimal fatigue and why?
Slow oxidative due to aerobic metabolism
Fast glycolytic due to large glycogen stores
Fast oxidative due to anaerobic glycolysis
Fast glycolytic due to low mitochondria
Which statement best describes where smooth muscle is predominantly found in the human body?
Only in superficial fascia beneath the skin
Exclusively in cardiac chambers like ventricles
Primarily in large skeletal muscles of limbs
Within hollow organs such as gut and vessels
Smooth muscle in hollow organs typically organizes into two layers that do not contract simultaneously in the same region. Which layers are these?
Deep and superficial layers
Oblique and transverse layers
Longitudinal and circular layers
Radial and spiral layers
When the longitudinal layer of smooth muscle contracts, what is the most direct mechanical effect on a segment of a hollow organ?
Segment shortens and narrows
Segment lengthens and narrows
Segment shortens and dilates
Segment lengthens and dilates
When the circular layer of smooth muscle contracts, what is the primary change in the organ’s geometry at that segment?
Luminal diameter increases
Luminal diameter decreases
Wall thickness decreases
Organ length increases
Fill in the blank with the precise term: The smooth muscle layer with fibers running around the circumference of an organ is the (a) layer.
A peristaltic wave requires alternating activity of the two smooth muscle layers. Which pattern best explains forward propulsion?
Both layers contract simultaneously at the same location
Upstream longitudinal relaxes while downstream circular relaxes
Only the longitudinal layer contracts throughout
Upstream circular contracts while downstream longitudinal contracts
In a cross-section of the small intestine, which smooth muscle layer is responsible for shortening the intestinal tube along its length?
Oblique smooth muscle layer spiraling around the lumen
Muscularis mucosae within the villi core
Longitudinal smooth muscle layer running along the tube
Circular smooth muscle layer surrounding the mucosa
Fill in the blank: The circular smooth muscle layer has fibers oriented (a) to the longitudinal layer, producing constriction of the lumen.
A segment of intestine must both narrow and propel contents forward. Using the diagram, which coordinated action best achieves this?
Circular layer contracts while longitudinal layer relaxes
Both layers relax simultaneously then contract together
Circular and longitudinal layers contract sequentially in waves
Longitudinal layer contracts while circular layer relaxes
Which feature replaces neuromuscular junctions in smooth muscle innervation, allowing transmitter release to multiple fibers?
Motor end plates with narrow clefts
Varicosities with wide clefts
Synaptic boutons with tight clefts
Terminal cisternae with narrow gaps
Smooth muscle cells coordinate electrical activity primarily through what intercellular connection?
Hemidesmosomes for basal anchoring
Tight junctions for barrier sealing
Gap junctions for ionic coupling
Desmosomes for mechanical coupling
Spontaneous depolarizations in many smooth muscles are best explained by:
Pacemaker-like activity spreading via gap junctions
Troponin-mediated Ca2+ sensing on thin filaments
T-tubule mediated rapid action potentials
Large SR stores releasing massive Ca2+ bursts
Compared with skeletal muscle, smooth muscle lacks which membrane specialization for rapid conduction into the fiber?
Dense bodies anchoring filaments
Caveolae invaginations of sarcolemma
Gap junctions intercellular channels
T-tubules transverse conduits
In smooth muscle, the sarcoplasmic reticulum typically releases:
Calcium only during fatigue
A large sustained Ca2+ load
No measurable Ca2+ at rest
A small amount of Ca2+
Caveolae in smooth muscle are best described as:
Cytosolic calcium-binding proteins
Invaginations of sarcolemma with Ca2+ channels
Transverse tubules forming triads with SR
Extracellular matrix anchoring plaques
Most Ca2+ that triggers smooth muscle contraction enters from the:
(a)
Which structural organization is absent in smooth muscle but present in skeletal muscle?
Sarcomeres with repeating striations
Actin and myosin filaments
Basal lamina surrounding fibers
Mitochondria near ATP demand
Despite lacking sarcomeres, smooth muscle still contains:
Actin and myosin filaments
Tropomyosin only without actin
Troponin complex on thin filaments
Z-lines with titin scaffolds
Which regulatory protein is absent in smooth muscle, changing how Ca2+ triggers contraction?
Calmodulin Ca2+-binding protein
Troponin thin-filament sensor
Dystrophin membrane stabilizer
Nebulin thin-filament ruler
In smooth muscle, Ca2+ binds directly to (a) to initiate contraction.
Select the most accurate sequence for initiating smooth muscle contraction.
Gap junctions break, cells contract independently
T-tubules depolarize, SR releases large Ca2+
Ca2+ enters via caveolae, activates calmodulin
Ca2+ binds troponin, exposes myosin sites
Varicosities facilitate transmitter delivery to:
Only cardiac intercalated discs
Only skeletal motor end plates
Multiple smooth muscle fibers
A single isolated muscle fiber
Which pair correctly matches structure with function in smooth muscle?
Troponin—primary Ca2+ sensor
T-tubules—large SR triad formation
Caveolae—membrane Ca2+ entry sites
Gap junctions—mechanical adhesion only
Reasoning: A drug blocks Ca2+ channels in caveolae. Predict the immediate effect on contraction strength.
Increase from enhanced SR release
No change due to SR compensation
Oscillation from spontaneous depolarization
Decrease due to reduced extracellular influx
Reasoning: A mutation removes gap junctions between smooth muscle cells. What pattern of activity is most likely?
Enhanced T-tubule conduction speed
Hyper-striated sarcomere formation
Independent, poorly coordinated contractions
Synchronized waves of depolarization
Which feature best distinguishes multi-unit smooth muscle from unitary smooth muscle?
Formation of discrete motor units
Location limited to hollow visceral organs
Presence of abundant gap junctions
Automatic rhythmic pacemaker activity
In multi-unit smooth muscle, fibers are structurally independent because there are no (a) .
Which tissue location most likely contains multi-unit smooth muscle?
Uterine myometrium
Intestinal wall of the jejunum
Urinary bladder detrusor
Arrector pili in the skin
A laboratory preparation shows graded contractions achieved by recruiting additional fibers. What type of smooth muscle is most consistent with this observation?
Unitary smooth muscle
Skeletal muscle
Multi-unit smooth muscle
Cardiac muscle
Which statement about unitary smooth muscle is most accurate?
Less common and found in arrector pili
Much more common and found in hollow organs
Contracts via recruitment into motor units
Composed of structurally independent fibers
Explain why multi-unit smooth muscle can produce finely graded force changes compared with unitary smooth muscle.
Each fiber is pacemaker-driven and oscillatory
Independent fibers allow recruitment of motor units
It receives fewer autonomic nerve inputs overall
It contains gap junctions enabling synchronous firing
Which pair correctly matches the smooth muscle type with a hallmark property?
Multi-unit smooth muscle — graded contractions
Unitary smooth muscle — forms motor units
Unitary smooth muscle — no gap junctions
Multi-unit smooth muscle — common in hollow organs
