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Questions and Answers

What is the effect of increased sarcomere length on force generation?

Force generated decreases (correct)

Force generated becomes unpredictable

Force remains constant

Force generated increases linearly

What is meant by the optimal length (Lo) for a muscle?

The length that develops the greatest tension (correct)

The length that is usually the shortest

The length where muscle is fully contracted

The length that causes minimal force generation

What happens when the muscle deviates from its optimal length?

Tension remains the same

Tension increases significantly

Tension becomes unpredictable

Tension decreases (correct)

How does the sliding filament mechanism relate to muscle contraction?

It describes the overlap of actin and myosin filaments Signup and view all the answers

What role do motor units play in muscle strength?

They determine the number of contracting myofibers Signup and view all the answers

What does motor unit recruitment refer to?

Increasing the number of contracting myofibers Signup and view all the answers

What is the relationship between muscle length and filament contact points?

Long muscle lengths have reduced contact points Signup and view all the answers

How does passive elasticity contribute to muscle tension during relaxation?

It maintains muscle length at Lo Signup and view all the answers

What directly influences the duration of skeletal muscle contraction?

The amount of calcium released Signup and view all the answers

What does twitch summation refer to in muscle contraction?

Increase in muscle tension from successive action potentials Signup and view all the answers

Which factor is NOT responsible for the latency period during muscle contraction?

The speed of myosin cross-bridge cycling Signup and view all the answers

How does fused tetanus affect muscle contraction?

It results in more myosin-binding sites becoming available Signup and view all the answers

Which statement about fast twitch fibers is incorrect?

Their contraction duration is longer than slow twitch. Signup and view all the answers

In what scenario will muscle maintain peak contraction?

When the frequency of stimulus is high Signup and view all the answers

What is the primary effect of ATPase activity on muscle contraction?

Facilitating the cross-bridge cycle Signup and view all the answers

The onset of contractile response is characterized by a delay known as the:

Latent period Signup and view all the answers

What is the role of the neuromuscular junction (NMJ) in muscle contraction?

It serves as a connection point where motor neurons transmit signals to muscle fibers. Signup and view all the answers

What initiates the influx of sodium ions at the motor end plate?

The release of acetylcholine from the terminal button. Signup and view all the answers

How does the action potential in skeletal muscle compare to that in neurons?

It can spread in both directions from the NMJ. Signup and view all the answers

Which statement accurately describes the motor unit?

Muscle fibers of a motor unit are activated simultaneously by the same motor neuron. Signup and view all the answers

What is the purpose of the excitation-contraction coupling process?

To convert electrical signals into mechanical muscle contractions. Signup and view all the answers

What happens when acetylcholine binds to its receptors at the motor end plate?

Generation of an end-plate potential that can trigger an action potential. Signup and view all the answers

What neurotransmitter is primarily involved in signaling at the neuromuscular junction?

Acetylcholine Signup and view all the answers

What unique characteristic does the end-plate potential (EPP) have in skeletal muscles?

It can generate an action potential in the presence of sufficient local depolarization. Signup and view all the answers

What is the effect of stopping the release of acetylcholine at the NMJ?

Muscle fibers will relax as they are no longer stimulated. Signup and view all the answers

What is a key difference between action potentials in neurons and those in skeletal muscle fibers?

Muscle fiber action potentials occur in both directions, unlike neuronal action potentials. Signup and view all the answers

What is the primary function of the sarcomere?

To generate force Signup and view all the answers

Which component of the sarcomere is responsible for the light bands observed in microscopy?

I-band Signup and view all the answers

What prevents the interaction between myosin and actin during muscle relaxation?

Tropomyosin Signup and view all the answers

What structural feature separates two sarcomeres?

Z-line Signup and view all the answers

What is the arrangement of thick and thin filaments in the compartment labeled as the A-band?

Both thick and thin filaments Signup and view all the answers

What protein stabilizes the interaction between actin and tropomyosin?

Troponin Signup and view all the answers

During muscle contraction, what mechanism occurs to shorten the sarcomere?

Sliding filament theory Signup and view all the answers

Which structure provides elasticity and helps return the muscle to its original structure after contraction?

Titin Signup and view all the answers

What is the role of calcium ions in muscle contraction?

To enable tropomyosin to remove from binding sites Signup and view all the answers

Which statement is true regarding myosin molecules in the thick filament?

They have a golf club-shaped structure Signup and view all the answers

Which component is absent in the H-zone?

Actin filaments Signup and view all the answers

The region where the actin protrusions anchor occurs is called the?

Z-line Signup and view all the answers

What method is primarily used to visualize the detailed structure of sarcomeres?

Electron microscopy Signup and view all the answers

What does the term 'hexagonal arrangement' refer to in muscle fibers?

The organization of filaments Signup and view all the answers

What initiates the release of Ca2+ from the sarcoplasmic reticulum during muscle contraction?

Change in structure of the Ryanodine receptor Signup and view all the answers

What action must occur to terminate muscle contraction and return cytosolic Ca2+ to its basal level?

Activation of Ca2+-ATPase Signup and view all the answers

What is the result of calcium binding to troponin during muscle contraction?

Tropomyosin shifts to expose the myosin-binding site Signup and view all the answers

Which of the following correctly describes the cross-bridge cycle?

Involves attachment, movement, detachment, and energizing of cross-bridges Signup and view all the answers

What happens during rigor mortis after death?

Muscle stiffness occurs due to lack of ATP Signup and view all the answers

What role does ATP play in the cross-bridge cycle?

It energizes the myosin head for stroke power Signup and view all the answers

How does the sliding filament model of muscle contraction work?

Thick and thin filaments slide past each other Signup and view all the answers

What is the function of titin in muscle cells?

It contributes to the elastic properties of muscle Signup and view all the answers

What happens if ATP levels are insufficient during muscle contraction?

Cross-bridges will remain attached Signup and view all the answers

What triggers the conformational change of the troponin complex?

Increase in cytosolic calcium concentration Signup and view all the answers

What occurs immediately after ATP binds to myosin during muscle contraction?

Myosin heads detach from actin Signup and view all the answers

What is the role of the sarcoplasmic reticulum in muscle cells?

To store and release calcium ions Signup and view all the answers

What is the primary function of the protein titin in skeletal muscle?

To provide elasticity and return muscles to their original structure Signup and view all the answers

During the power stroke phase of the cross-bridge cycle, what happens?

ADP and Pi are released from myosin Signup and view all the answers

What is a consequence of excessive calcium in the cytosol during muscle contraction?

Continuous contraction of the muscle Signup and view all the answers

Which connective tissue wraps a single muscle fiber?

Endomysium Signup and view all the answers

How do skeletal muscles achieve coordinated movement?

By working in antagonistic pairs to contract and relax Signup and view all the answers

What is the relationship between muscle fibers and fascicles in skeletal muscle structure?

Fascicles are a collection of muscle cells bound by epimysium. Signup and view all the answers

What happens during muscle relaxation?

Muscles return to their original structure without input Signup and view all the answers

Which muscle action is associated with the flexor muscle group?

Decreasing the angle at a joint Signup and view all the answers

What type of muscle fibers primarily constitute myofibrils?

Skeletal muscle fibers Signup and view all the answers

What describes the unique feature of skeletal muscles when it comes to movement?

Skeletal muscles can only pull on bones, not push. Signup and view all the answers

Which structure within the muscle is primarily responsible for contraction?

Sarcomere Signup and view all the answers

What is the function of sensory neurons in muscle tissues?

To determine the status of muscle contractions Signup and view all the answers

Which muscle group includes the gastrocnemius and quadriceps?

Synergistic muscle group Signup and view all the answers

What happens at the level of muscle fibers during contraction?

Myofibrils shorten and cause overall muscle shortening Signup and view all the answers

What is the primary function of the epimysium in muscle anatomy?

To surround the entire muscle and provide structure Signup and view all the answers

In which scenario would extensibility be most important for a muscle?

During a static stretch when lengthening is required Signup and view all the answers

During muscle contraction, what role do antagonistic muscle groups play?

They ensure that one muscle contracts while the other relaxes to facilitate movement. Signup and view all the answers

Study Notes

Human Physiology BIOL3205 - Muscle System

Course instructor: Prof. Chi Bun Chan
Contact information: [email protected], 39173822
Location: 5N10 Kadoorie Biological Sciences Building

Lecture Outline

Muscle types and their structural characteristics
Mechanism of muscle contraction
Muscle contraction mechanics
Control of muscle contractions
Muscle metabolism and fatigue
Muscle diseases

Classification of Muscles

Structural: Striated vs. smooth (unstriated)
Control Methods: Voluntary or involuntary
Locations: Skeletal muscle, heart (cardiac), hollow organs (e.g., intestinal smooth)
Metabolic Classification: Fast-twitch and slow-twitch muscle

Characteristics of Muscle Types

Skeletal Muscle: Striated, voluntary, multinucleated, long, cylindrical cells. Located attached to bones. Function: body movement.
Cardiac Muscle: Striated, involuntary, uninucleated, branching cells. Located in the heart. Function: pumping blood.
Smooth Muscle: Unstriated, involuntary, uninucleated, spindle-shaped cells. Located in hollow organs. Function: movement of contents within organs.

Properties of Muscle

Excitability: Responds to stimuli, generating action potentials.
Contractility: Shortens in response to stimulation.
Extensibility: Stretches without damage.
Elasticity: Returns to original length after stretching.

Muscular System Functions

Gesture maintenance
Locomotion (movement)
Organ movement (e.g., eye movement)
Propulsion of contents through hollow organs
Breathing
Accomplishing work (pushing, pulling)

Shapes of Muscle

Fusiform, parallel, convergent, unipennate, bipennate, multipennate, circular

Skeletal Muscle Structure

Connective Tissues: Epimysium, perimysium, endomysium
Blood Vessels/Nerves: Supply muscle fibers with nutrients and impulses.
Muscle Fibers: Bundles of myofibrils.
Myofibrils: Protein structures causing muscle contractions.
Sarcomere: Functional unit of myofibrils.

Myofibril Structure

Thick Filaments: Composed primarily of myosin.
Thin Filaments: Primarily composed of actin.
Sarcomere: The repeating unit with A-band, I-band, Z-line, M-line.

Microfilaments Organization

Thick Filaments: Myosin molecules with heads for cross-bridges.
Thin Filaments: Actin filaments, plus tropomyosin and troponin involved in Ca2+ and myosin interaction.
Cross-bridges form between thick and thin filaments initiating muscle contractions.

Sliding Filament Mechanism

Muscle contraction occurs by thin filaments sliding over thick filaments.
Sarcomere shortening results in muscle contraction.

Mechanisms of Skeletal Muscle Contraction

Alpha Motor Neurons: Nerves controlling skeletal muscle.
Motor Unit: Motor neuron and all the muscle fibers it stimulates.
Neuromuscular Junction (NMJ): Where motor neuron communicates with muscle fiber.
Excitation-Contraction Coupling: The sequence turning nerve signal into muscle contraction.

Mechanism of Skeletal Muscle Contraction—Cross-bridge Cycle

Attachment: Myosin head attaches to actin.
Power Stroke: Myosin head bends, moving actin.
Detachment: ATP binds, releasing myosin from actin.
Energization: ATP hydrolysis "resets" the myosin head.

Rigor Mortis

Postmortem muscle stiffening due to ATP depletion.
Muscles remain contracted.

Type of Skeletal Muscle Contraction

Isometric: Muscle tension increases, but muscle length stays the same.
Isotonic: Muscle length changes, but tension remains constant.
Concentric: Muscle shortens when tension exceeds load.
Eccentric: Muscle lengthens when load exceeds tension.
Cross-bridge cycle occurs in all type of skeletal muscle contractions.

Sarcomere Length-Tension Relationship

Optimal sarcomere length provides maximum tension based of overlap and cross-bridge formation between actin and myosin.
Decreased Tension: If length deviates from optimal, tension decreases.

Control of Muscle Tension

Motor Unit Recruitment: Increasing the number of motor units being stimulated to increase muscle tension. Weak contraction -> use less motor units. Stronger contraction -> recruit more motor units.
Number of Myofibers per Motor Unit: Eyes need fine coordinated movements which require small motor units. Legs use larger motor units to perform larger movements.

Muscle Twitch

Mechanical response of a muscle fiber to a single action potential.
Phases: Latent period, contraction period, relaxation period
Fast-twitch vs. slow-twitch fibers: Different myosin types, ATPase (actin-myosin interactions) activity and metabolic properties.
Factors affecting twitch duration include myosin type, cytosolic calcium concentration.

Frequency-Tension Relation

Twitch Summation: Increasing tension by stimulating muscle fiber before complete relaxation.
Tetanus: Continued, sustained contraction caused by rapid stimulation of fiber
• Frequency-tension relationship: Rate of stimulation -> sustained muscle contraction -> muscle fatigue.

Muscle Fatigue

Inability to maintain muscle tension.
Depletion of ATP, leakage of Ca+ in the sarcoplasmic reticulum, centrally induced (in the CNS)
Coordinated asynchronous recruitment of motor units and recruitment of metabolically different fibers prevent fatigue for prolonged periods.

Energy Source During Exercise

ATP and creatine phosphate (short bursts)
Anaerobic glycolysis (medium duration)
Aerobic oxidation (long duration)

Muscle Fiber Types

Slow-oxidative: Myoglobin rich, slow speed of contraction, high fatigue resistance.
Fast-oxidative: Intermediate speed of contraction, intermediate fatigue resistance
Fast-glycolytic: Fast speed of contraction, low fatigue resistance.

Coordination of Muscle Type Usage

Most muscles have a mixture of fiber types.
Different muscle groups (e.g., erector spinae vs. biceps brachii) have different proportions of fiber types.
Fiber type composition can potentially depend on genetics or adaptation.

Muscle Cramping

Involuntary muscle contractions, often caused by electrolyte imbalances or neuromuscular issues.

Tetanus Infection

Caused by bacterial toxin affecting inhibitory neurons.
Results in muscle stiffness, breathing problems, and spasms.

Skeletal Muscle and Smooth Muscle Differences

Cell Shape: Skeletal = cylindrical, smooth = spindle-shaped.
Nucleus: Skeletal = multinucleated, smooth = single nucleus.
Functional Units: Skeletal is sarcomeres; smooth is not.
Microfilament Arrangement: Skeletal is parallel bundles; smooth are often diagonally arranged.
Contractile Response: Skeletal is quick; smooth is slower.
Neuronal Control: Skeletal via NMJ; smooth is diffuse via neurotransmitters

Neuronal control of smooth muscle

Contractile activity controlled by neurotransmitters from autonomic neurons.
Varicosities are swollen regions of the autonomic neurons that release neurotransmitters.
Same neurotransmitter may cause different responses depending on the specific receptor and tissue.

Types of Smooth Muscles

Single-unit smooth muscle - cells contract as a single unit coordinated via gap junctions
Multiunit smooth muscle- each cell contracts autonomously and is controlled by its own nerve supply

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