Muscle Physiology and Adrenergic Receptors Quiz

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Questions and Answers

What is the primary function of myosin phosphatase in smooth muscle contraction?

Inhibition of acetylcholine release

Activation of adrenergic receptors

Phosphorylation of myosin light chains

Dephosphorylation of S19 in myosin light chains (correct)

Which adrenergic receptor type is primarily coupled to Gs-type G-proteins and is involved in increasing heart rate?

α2

β1 (correct)

β3

α1

Which adrenergic receptor subtype is known for its role in vasoconstriction and is distributed widely in the central nervous system?

α2C (correct)

β2

β1

α1B

Which adrenergic receptor subtype is abundant in brown adipose tissue and regulates lipolysis?

β3 Signup and view all the answers

What effect do adrenergic receptors have on gastric acid secretion?

Inhibition of secretion via α2 receptors Signup and view all the answers

What primarily distinguishes skeletal muscle from cardiac and smooth muscle?

Presence of striations Signup and view all the answers

Which muscle type is classified as voluntary?

Skeletal muscle Signup and view all the answers

Which proteins are primarily involved in muscle contraction?

Actin and myosin Signup and view all the answers

What role does the sarcolemma play in muscle fibers?

Serves as an electrically excitable membrane Signup and view all the answers

What is a major characteristic of smooth muscle?

Non-striated appearance Signup and view all the answers

Which function is NOT achieved by controlled muscle contraction?

Structural support of bones Signup and view all the answers

Which of the following describes the skeletal muscle fiber structure?

Fibers are elongated, multi-nucleated, and cylindrical Signup and view all the answers

What role does troponin I play in muscle contraction?

Inhibits the binding of actin to myosin Signup and view all the answers

What is the primary function of troponin C in muscle fibers?

To bind calcium and promote actin-myosin interactions Signup and view all the answers

How many calcium ions can troponin C bind?

4 Signup and view all the answers

What constitutes the thick filament in muscle tissue?

Comprised of myosin molecules forming a spiral structure Signup and view all the answers

What percentage of muscle protein by weight do thick filaments contribute?

55% Signup and view all the answers

Which structure of the myosin molecule is critical for actin interaction?

Globular head Signup and view all the answers

What is the significance of the cross bridge formed by myosin?

It contains ATPase activity important for energy generation Signup and view all the answers

What is the structural orientation of the myosin tails within the thick filament?

Intertwined toward the filament's center Signup and view all the answers

What is the primary structural component of thin filaments?

Actin Signup and view all the answers

What happens to the A band during muscle contraction?

It remains unchanged. Signup and view all the answers

Which of the following correctly describes the role of calcium ions in muscle contraction?

They cause a conformational change in troponin. Signup and view all the answers

How many myosin heads are typically found on each thick filament?

500 Signup and view all the answers

In the sliding filament model, which action results from the interaction of actin and myosin?

Sliding of actin past myosin. Signup and view all the answers

What is required for the interaction between actin and myosin to occur?

Presence of ATP. Signup and view all the answers

What structural change occurs in the sarcomere during contraction?

The sarcomere decreases in size. Signup and view all the answers

What is the primary function of tropomyosin in muscle contraction?

To cover the myosin binding sites on actin. Signup and view all the answers

What is the frequency at which each myosin head cycles during rapid contraction?

5 times per second. Signup and view all the answers

Which of the following ions is crucial for muscle contraction regulation?

Calcium. Signup and view all the answers

Which of the following statements about the sliding filament model is TRUE?

Filaments slide past each other without changing their length. Signup and view all the answers

What leads to the increased levels of active calmodulin (CaCM) in smooth muscle activation?

Increased intracellular calcium levels Signup and view all the answers

Which phosphorylation site on myosin light chains is affected during smooth muscle activation?

Serine 19 Signup and view all the answers

What is the role of caldesmon in smooth muscle contraction?

It replaces the troponin complex as a calcium-dependent regulator. Signup and view all the answers

What is the impact of β2-adrenergic receptor activation on smooth muscle relaxation?

Increase in cAMP production Signup and view all the answers

How does PKA influence MLCK in smooth muscle relaxation?

It phosphorylates MLCK, inhibiting its activity. Signup and view all the answers

What happens to the activity of MLCK when intracellular calcium levels decline?

MLCK activity is reduced. Signup and view all the answers

What physiological change occurs via activation of potassium channels mediated by PKA?

Closure of plasma membrane Ca2+ channels Signup and view all the answers

What is the consequence of activating α2-adrenergic receptors in the context of β2 receptor activation?

Inhibition of adenylate cyclase activity Signup and view all the answers

What change occurs to tropomyosin during smooth muscle contraction?

Its location shifts within the helical grooves of F-actin. Signup and view all the answers

What initiates smooth muscle contraction when calcium is elevated?

Phosphorylation of myosin light chains by MLCK Signup and view all the answers

Signup and view all the answers

Study Notes

Biochemistry of the Muscle

Muscle is an aggregate of proteins involved in contraction.
The musculature (system of muscles) enables body movement.
Muscle is a biochemical transducer, changing chemical energy to kinetic (mechanical) energy.
Muscles consist of the largest group of tissues in the body.
There are three types of muscle: skeletal, cardiac, and smooth.

Classification of Muscles

Muscles are classified into three basic types: skeletal, cardiac, and smooth.
Muscles are also categorized into two based on microscopic appearance: striated (cardiac and skeletal) and non-striated (smooth).
Skeletal muscles are voluntary, controlled by the central nervous system (CNS).
Cardiac and smooth muscles are involuntary, not directly controlled by conscious thought.

Muscle Action

Controlled muscle contractions cause purposeful body movements and manipulation of external objects.
Muscles propel contents through internal organs.
Muscles also empty organs, releasing contents into the external environment.

Muscle Contraction

Muscle contraction is based on the interplay of actin (thin filament) and myosin (thick filament) protein filaments.
Skeletal muscle is striated and comprised of parallel bundles of muscle fibers, which attach to tendons.

Organization of Skeletal Muscle

Skeletal muscle consists of parallel bundles of muscle fibers connected to tendons at both ends.
The muscle fasciculus is made up of muscle fibers.
Muscle fibers are multinucleated, large, elongated, and cylindrical.
Muscle fibers contain myofibrils, which are surrounded by the sarcolemma.
Sarcoplasm (cytosol) is rich in glycogen, ATP, creatine phosphate, and glycolytic enzymes.
Mitochondria are found in high quantities in active muscles.
Myofibrils exhibit sarcomere structure, a repeating unit.

Sarcomere Structure

Sarcomeres are functional units of myofibrils, exhibiting repeating bands.
The I-band is less optically dense; the A-band is denser.
The Z-line or disc appears in the center of the I-band, and the M-line is in the center of the A-band.
The H-zone is adjacent to the M-line in the A-band and is more dense.
Interdigitation of thin (actin) and thick (myosin) filaments creates the bands.

Myofilaments

Myofilaments are filamentous protein aggregates within a sarcomere.
Thick myofilaments are made of hundreds of myosin molecules.
Thin myofilaments are made of two strands of actin polymers, often interwound, and helical in structure.
Thin filaments contain accessory proteins, such as tropomyosin and troponin.

Proteins of the Myofibril

Thin filaments are primarily composed of actin, along with tropomyosin and troponin.
Actin is a major constituent of thin filaments (comprising 25% of muscle protein).
The monomer of actin is G-actin (globular).
G-actin polymerizes into F-actin (fibrous) as ionic strength increases to physiological levels.
F-actin looks like two strands of beads wound around each other, helical.
Tropomyosin is a filamentous protein that lies on either side of the F-actin filament and blocks myosin binding sites on actin in a relaxed muscle cell.
Troponin is a three-subunit complex (troponin I, troponin T, and troponin C). Troponin I inhibits myosin interaction with actin. Troponin C binds calcium.

Myosin

Thick filaments consist of myosin molecules, which have a fibrous tail, a connecting neck (often called the hinge), and a globular head region.
Myosin heads contain ATPase activity and bind to actin.
Myosin is split by enzymes into light meromyosin (LMM) and heavy meromyosin (HMM.)

Muscle Contraction Summary

Myosin heads generate force through a sliding filament mechanism that causes actin filaments to slide towards each other, shortening the sarcomere.
Calcium is key in regulating muscle contraction by enabling myosin's interaction with actin.

Regulation of Muscle Contraction

Troponin and tropomyosin regulate muscle contraction by determining myosin binding to actin.
Calcium ion concentrations regulate troponin and tropomyosin positioning.
Ion depolarization leads to higher calcium concentrations and stimulates muscle contraction.
After stimuli ceases and calcium levels fall, tropomyosin returns to its blocking position and contraction terminates.
The movement of the thick filaments pulls the thin filaments towards each other, and the sarcomere length shortens (called sliding filament theory).

Control of Intracellular Calcium

Depolarisation of the sarcolemma is relayed to the sarcoplasmic reticulum, increasing calcium levels.
Calcium is pumped back into the sarcoplasmic reticulum (by a specific ATPase) once stimulation is over

Muscle Metabolism

Muscle uses glucose, fatty acids, and ketone bodies for energy.
Muscle stores glycogen.
Muscle contraction is highly dependent on ATP production.
The rates of glycolysis and TCA cycle differ in white and red muscle fibers based on the duration and complexity of the effort.

Muscle-Specific Auxiliary Reactions

Creatine phosphate acts as an energy buffer.
Adenylate kinase disproportionates ADP into ATP and AMP.
AMP deaminase deaminates AMP into IMP.

Cori Cycle

The Cori cycle describes the glucose transport between muscle and liver during significant effort and relaxation.
Lactic acid produced in muscle is converted to glucose in the liver (gluconeogenesis), and then delivered back to the muscles.

Protein and Amino Acid Metabolism

Skeletal muscle degrades branched-chain amino acids (valine, leucine, isoleucine).
Amino acids are converted to glutamate and pyruvate, then transported to the liver or kidneys as glutamine following protein degradation to supply nitrogen.
Muscle serves as an energy store during periods of hunger and starvation.

Oxygen Debt

Oxygen consumption continues during recovery after maximal effort.
Extra oxygen is used for lactate oxidation in maximal effort.

Tetany and Rigor Mortis

Tetany is caused by muscle over-simulation (with high sustained calcium) and the depletion of ATP and high energy phosphate.
Rigor mortis develops after death due to the inability to produce ATP to remove cross-linked myosin-actin complexes.

Adrenergic Receptors

Catecholamines (epinephrine and norepinephrine) exert effects on muscle cells through adrenergic receptors (A1, A2, B1, B2, B3.).
Cholinergic receptors influence muscle function.

Muscular Dystrophies (DMD and BMD)

DMD and BMD are X-linked recessive disorders that cause loss of muscle function, resulting in atrophy.
Mutations in the dystrophin gene cause these disorders.
BMD (Becker Muscular Dystrophy) is less severe than DMD due to minimal functional dystrophin protein
Symptoms of DMD include muscle weakness (especially in proximal muscles, legs, and pelvis.)
Muscle loss in DMD is characteristic and is progressively more pronounced leading to eventually death.

Smooth Muscle Contraction

Smooth muscle differs from skeletal muscle.
Smooth muscle contains actin and myosin but not in the organized filaments seen in striated muscle.
Smooth muscle contractile proteins are influenced by Ca2+ ions and regulated by caldesmon and calmodulin.
Smooth muscle contraction is influenced by nerve activation and the activation of specific G-protein coupled receptors.
Smooth muscle relaxation is influenced by cAMP and its effectors.

Actin-Binding Drugs

Drugs like cytochalasin B and phalloidin affect actin filaments and muscle function.

Accessory Proteins of the Myofibril (summary)

Titin and nebulin help maintain sarcomere structure, stability, and elasticity.
α-actinin and desmin secure the sarcomere and integrate it with other components of the myofibril.

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Description

Test your knowledge on muscle physiology and the role of adrenergic receptors in the human body. This quiz covers various aspects of muscle types, contraction mechanisms, and the effects of adrenergic signaling. Perfect for students in advanced biology or physiology courses.