Muscle Physiology Quiz

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Questions and Answers

What is the primary function of titin within the sarcomere?

To regulate the binding of myosin to actin.

To facilitate the conversion of ATP into mechanical energy.

To stabilize the thick filament and provide elasticity. (correct)

To block the active sites on actin during muscle relaxation.

Which of the following best describes the arrangement of myosin heads on a thick filament?

They are directed inward towards the center of the filament.

They are randomly oriented with no specific pattern.

They are angled to the left on one half of the filament and to the right on the other half, with a bare zone in the middle. (correct)

They are uniformly angled in the same direction around the filament.

What is the structural relationship between troponin and tropomyosin?

Troponin is bound to tropomyosin, which in turn is bound to the actin filament. (correct)

Tropomyosin is bound to the actin filament and troponin is an independent molecule.

Troponin and tropomyosin are subunits of the same protein complex.

Troponin blocks the active site of actin and tropomyosin binds to myosin.

How does tropomyosin contribute to muscle relaxation?

By blocking the active sites on actin and preventing myosin from binding. Signup and view all the answers

Which of these proteins directly uses ATP to power muscle contraction?

Myosin Signup and view all the answers

What are the two primary proteins that are considered contractile proteins?

Actin and myosin Signup and view all the answers

What is the function of the 'bare zone' in a thick filament?

It creates a region where there are no myosin heads. Signup and view all the answers

According to the sliding filament theory, what is the direct result of the thin filaments sliding over the thick ones?

The sarcomere shortens as the muscle contracts. Signup and view all the answers

If one cycle of power and recovery strokes shortens a muscle fibre by 1%, how can a fibre shorten up to 40%?

Through repetitive cycles of the power and recovery strokes. Signup and view all the answers

What happens to the level of free calcium in the cytosol when a nerve fibre stops firing?

The free calcium level sharply declines due to reabsorption into the SR. Signup and view all the answers

Which event directly prevents myosin from binding to actin, causing muscle relaxation?

Tropomyosin blocking the active sites on actin. Signup and view all the answers

What is the role of Acetylcholinesterase (AChE) in muscle relaxation?

To break down ACh into fragments that can no longer stimulate the muscle. Signup and view all the answers

What is NOT directly involved in returning a muscle to its resting length after relaxation?

The movement of tropomyosin to block active sites on actin. Signup and view all the answers

According to the length-tension relationship, what is the main determinant of the force of a muscle contraction?

The overlap of actin and myosin filaments at the start of contraction. Signup and view all the answers

What best describes the process of muscle relaxation according to the text?

Caused by the cessation of nerve signals which results in a decrease in calcium, and the assistance of an external force to achieve resting length. Signup and view all the answers

During a single cycle of power and recovery strokes, approximately how much does a muscle fibre shorten, and how many times does a myosin head typically repeat this in a single second?

1%, repeating fives times per second Signup and view all the answers

What happens to the force of muscle contraction if the thick filaments butt against the Z discs?

The contraction will be weak and brief. Signup and view all the answers

What is the primary reason that a muscle in situ doesn't achieve the extreme states of contraction or stretching observed in a laboratory setting?

The attachments of muscles to bones, limitations of bone movement and central nervous system adjustments restrict the range of muscle contraction. Signup and view all the answers

If a muscle has sarcomeres that are less than 60% of their optimal length, what is the expected tension developed in response to a stimulus?

The muscle will develop no tension at all. Signup and view all the answers

What is the role of muscle tone in maintaining muscle function?

It maintains optimum sarcomere length to prepare for efficient contraction. Signup and view all the answers

What is the main initial source of oxygen for aerobic respiration during a short, intense muscle exercise?

Myoglobin within the muscle fiber. Signup and view all the answers

What determines the optimum resting length for a muscle to respond with the greatest force?

The degree of overlap between the thick and thin filaments. Signup and view all the answers

Why is ATP required for muscle relaxation as well as contraction?

ATP is used to pump Ca²⁺ into the sarcoplasmic reticulum (SR). Signup and view all the answers

What is the sarcomere length range that allows a muscle to respond with the greatest force?

Between 2.0 and 2.25 micrometers. Signup and view all the answers

What would most likely happen if a muscle was extremely stretched according to the provided content?

The contraction would be weak due to a lack of overlap between thick and thin filaments. Signup and view all the answers

Which of the following is NOT a way for muscles to produce ATP?

Directly from the breakdown of myoglobin. Signup and view all the answers

What characterizes incomplete tetanus in muscle physiology?

It produces a fluttering contraction. Signup and view all the answers

Which mechanism prevents complete tetanus in human muscles?

Spinal cord inhibition. Signup and view all the answers

What is the primary difference between isometric and isotonic contraction?

Isometric contraction maintains muscle length against resistance. Signup and view all the answers

What is a potential consequence of complete tetanus in the body?

Injury to muscle and associated soft tissues. Signup and view all the answers

How do motor units contribute to the smooth contraction of a muscle?

Motor units contract asynchronously. Signup and view all the answers

What is the phenomenon called when a muscle is stimulated at a high frequency and each new stimulus arrives before the previous twitch is over, resulting in a stronger contraction?

Temporal summation Signup and view all the answers

Which of the following is NOT a factor contributing to stronger muscle contractions?

Decreasing the stimulus frequency Signup and view all the answers

What is the principle that dictates that smaller, less powerful motor units with slower nerve fibres are activated first, followed by larger, more powerful motor units with faster nerve fibres as more strength is needed?

Size principle Signup and view all the answers

What does the term 'recruitment' refer to in the context of muscle contraction?

The activation of additional motor units in response to increased stimulus intensity Signup and view all the answers

Which of the following accurately describes the relationship between stimulus intensity and muscle contraction strength?

Higher stimulus intensity excites more nerve fibres, leading to more motor units being activated, resulting in stronger contractions. Signup and view all the answers

Why is it important that smaller motor units are activated first, followed by larger motor units as more power is needed for a task?

All of the above. Signup and view all the answers

Which of the following correctly describes the relationship between stimulus frequency and the strength of muscle contractions?

Lower stimulus frequencies result in individual, isolated twitches, while higher frequencies lead to a summation of twitches, resulting in stronger contractions. Signup and view all the answers

Which of the following accurately describes the phenomenon of 'wave summation' in muscle contractions?

The increase in muscle tension that occurs when multiple stimuli arrive close together in time, before the previous twitch has fully subsided Signup and view all the answers

What is the primary difference between 'recruitment' and 'temporal summation' regarding muscle contraction strength?

Recruitment involves activating more motor units, while temporal summation involves increasing the frequency of stimuli. Signup and view all the answers

How does the neuromuscular system ensure that delicate movements can be performed with precision?

By activating smaller motor units first and gradually activating larger ones as more power is needed, allowing for precise control. Signup and view all the answers

Study Notes

Histology of Muscle

Muscle tissue is composed of specialized elongated cells arranged in parallel
Functions include movement, stability, communication, control of body openings and passages, heat production, and glycaemic control
Key characteristics include excitability (responsiveness), conductivity, contractility, extensibility, and elasticity

Excitability

All living cells have excitability, but muscle and nerve cells have it to a high degree
Excitation involves electrical changes across the plasma membrane in response to stimuli
Stimuli can be chemical signals, stretch, or other stimuli

Conductivity

Muscle cell excitation produces a wave of excitation travelling rapidly along the cell
This initiates processes necessary for contraction

Contractility

Muscle cells have the unique ability to shorten substantially when stimulated
This enables them to pull on bones and other organs, causing movement

Extensibility

Muscle cells can stretch between contractions
Skeletal muscle cells have a greater extensibility than other cell types

Elasticity

Muscle cells recoils to a shorter length after being stretched
This elastic recoil is crucial for maintaining resting muscle length

Myofilaments

Two types of myofilaments are necessary for contraction:
- Thin filaments (6-8nm diameter)
  - Composed of actin (a polymer of globular actin)
- Thick filaments (15nm diameter)
  - Composed of myosin-II protein

Muscle Classification

Striated muscle:
- Exhibits cross-striations under a light microscope (LM)
  - Skeletal muscle: attached to bones, produces skeletal movement, and maintains posture
  - Cardiac muscle: located in the heart
Smooth muscle:
- Lacks cross-striations under a light microscope (LM)

Skeletal Muscle

Composed of multinucleated syncytia (fusions of multiple individual myoblast cells)
Often referred to as muscle fibers or myofibers.
Surrounded by connective tissues: endomysium, perimysium, and epimysium
These tissues are continuous with collagen fibers in tendons and bone matrix, allowing for muscle fiber pull on these structures during contraction

Muscle Contraction

Sliding filament theory: thin filaments slide over thick filaments to shorten the sarcomere
Myosin heads bind to actin, causing a power stroke and sliding
ATP is necessary for the detachment of myosin heads from actin, allowing for further cyclical contraction

Muscle Fiber Types

Slow-twitch (type I) fibers: resistant to fatigue, high oxidative capacity
- High myoglobin content (red color)
- Rich capillary supply, numerous mitochondria, and aerobic respiratory enzymes
Fast-twitch (type IIa and IIb) fibers:
- Type IIa: Fast oxidative glycolytic fibers (intermediate color, mix), higher resistance to fatigue
- Type IIb: Fast glycolytic fibers (pale color, less resistance to fatigue), high glycolytic capacity

Muscle Metabolism

Muscle fibres generate ATP through:
- Creatine phosphate
- Anaerobic respiration
- Aerobic respiration

Muscle Fatigue

Progressive inability of muscle to maintain force of contraction after prolonged use
Factors contributing to fatigue: inadequate release of calcium, depletion of creatine phosphate, insufficient oxygen, depletion of glycogen, buildup of lactic acid, failure of motor neuron acetylcholine release

Muscle Tone

State of partial contraction
Maintains optimum sarcomere length

Muscle Contractions

Isometric: length remains constant
Isotonic: creates movement;
- Concentric: shortens, creates movement against resistance (lifting a weight)
- Eccentric: lengthens, creates movement with resistance (setting a weight down)

Cardiac Muscle

Limited to the heart, responsible for pumping blood
Striated but shorter, thicker, and branched with intercalated discs
Electrical synapses (intercalated discs) allow for synchronized contractions
Highly resistant to fatigue due to aerobic respiration emphasis

Smooth Muscle

Found in the walls of internal organs
Non-striated, fusiform cells
Involuntary muscle contractions
Can exhibit spontaneous activity and maintain sustained contraction

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Description

Test your knowledge on muscle physiology concepts, including the roles of key proteins in muscle contraction and relaxation. This quiz focuses on sarcomere structure, the sliding filament theory, and the biochemical processes that govern muscle functioning.