Muscle Physiology and Adrenergic Receptors Quiz

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.

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.