Muscle Physiology: Thick Filaments Quiz

Muscular System Overview

• Muscles comprise nearly half of the body's mass.
• Muscles transform chemical energy (ATP) into directed mechanical energy, creating force.
• Muscles are classified into three types: skeletal, cardiac, and smooth.
• Prefixes like "myo," "mys," and "sarco" are frequently used in muscle-related terminology.

Muscle Tissue Types

Skeletal Muscle

• Skeletal muscles are attached to bones and skin.
• Their cells are elongated, called muscle fibers, and are striated (striped).
• Skeletal muscle contractions are voluntary (consciously controlled).
• These muscles contract quickly but tire easily.
• They require nervous system stimulation for contraction.

Cardiac Muscle

• Found only in the heart, forming the bulk of its walls.
• Cardiac muscle cells are striated.
• These muscles can contract without nervous system stimulation.
• Contraction of cardiac muscle is involuntary.

Smooth Muscle

• Found in the walls of hollow organs (e.g., stomach, urinary bladder, airways).
• Smooth muscle cells lack striations.
• Smooth muscle contractions are involuntary.
• Smooth muscle can contract without nervous system stimulation.

Muscle Tissue Comparisons

• A table comparing characteristics of skeletal, cardiac, and smooth muscle is provided.
  • Key distinctions include body location, cell shape/appearance, myofibrils/sarcomeres, and presence of T tubules and/or gap junctions.
  • Different types also have various contraction regulation mechanisms.

Special Characteristics of Muscle Tissue

• Excitability (responsiveness): The ability to receive and respond to stimuli.
• Contractility: The ability to shorten forcibly when stimulated.
• Extensibility: The ability to be stretched.
• Elasticity: The ability to recoil to resting length.

Muscle Functions

• Movement: Movement of bones or fluids (e.g., blood).
• Maintaining posture and body position.
• Stabilizing joints.
• Heat generation: Primarily skeletal muscles.
• Additional functions: Protecting organs, forming valves, controlling pupil and lumen size, and causing "goosebumps."

Skeletal Muscle Structure and Function

• Each skeletal muscle is served by one nerve, one artery, and one or more veins.
• The connective tissue sheaths (epimysium, perimysium, and endomysium) surround and support muscle fibers.
• Muscles attach in at least two places:
  • Insertion (movable bone)
  • Origin (immovable or less movable bone)
  • Some attachments are direct, others indirect (tendons or aponeuroses).

Skeletal Muscle Fiber Structure

• Skeletal muscle fibers are long, cylindrical, multinucleate cells with peripheral nuclei.
• Sarcolemma: The plasma membrane of the muscle fiber.
• Sarcoplasm: The cytoplasm of the muscle fiber containing glycosomes and myoglobin (oxygen-binding protein).
• Myofibrils: Contractile organelles within the muscle fiber, containing sarcomeres (the functional units of contraction).
• Sarcoplasmic reticulum: Specialized smooth endoplasmic reticulum that regulates calcium ion levels within the muscle fiber.
• T tubules: Tubular infoldings of the sarcolemma that penetrate through the muscle fiber, bringing the action potential from the surface membrane into the interior of the cell.

Myofibrils and Sarcomeres

• Myofibrils are densely packed, rod-like elements that compose roughly 80% of the muscle cell volume.
• They are composed of repeating subunits called sarcomeres.
• Sarcomeres exhibit striations (alternating light and dark bands).
• The structure of thick and thin filaments within sarcomeres accounts for the banding patterns.

Sliding Filament Model of Contraction

• In a relaxed muscle, thin and thick filaments overlap only at the ends of the A band.
• During contraction, thin filaments slide past thick filaments, causing the sarcomere to shorten.
• Myosin heads bind to actin, forming cross bridges, pulling the thin filaments toward the center of the sarcomere.
• ATP is crucial for cross-bridge detachment and myosin head recocking.

Physiology of Skeletal Muscle Fibers

• Excitation: A nervous system signal is required before contraction can occur.
• Excitation-contraction coupling: The events that connect the nerve signal to the muscle contraction.
• AP propagated along the sarcolemma and down into T tubules, which eventually causes SR to release calcium ions.

Neuromuscular Junction (NMJ)

• A specialized area where a motor neuron synaptic terminal meets a muscle fiber.
• Synaptic vesicles contain acetylcholine, a neurotransmitter involved in signal transmission across the synapse.

Events at the Neuromuscular Junction

• Nerve impulse arrives at the axon terminal.
• ACh is released into the synaptic cleft.
• ACh diffuses across the cleft and binds to receptors on the sarcolemma.
• This binding triggers a local electrical event (end-plate potential).
• The end-plate potential triggers an action potential in the muscle fiber.

Destruction of Acetylcholine

• Acetylcholinesterase breaks down ACh in the neuromuscular junction.
• This termination of ACh activity prevents continuous muscle fiber contraction.

Channels in Muscle Contraction

• Voltage-gated channels are central to the process.
• ACh binds to receptors, opening ligand-gated channels allowing Na+ and K+ flow.
• Voltage-gated Na+ channels open leading to depolarization (action potential).
• Voltage-sensitive proteins in T tubules help trigger Ca2+ release.

Role of Calcium in Contraction

• At low Ca2+ levels, tropomyosin blocks active sites on actin, preventing myosin from binding.
• At higher Ca2+ levels, Ca2+ binds to troponin, moving tropomyosin away, exposing active sites, and allowing myosin binding.

Cross Bridge Cycle

• The cycle of cross-bridge formation, working stroke, and detachment continually pulls thin filaments inward.
• Requires energy from ATP.
• Crucial to muscle contraction.

Muscle Mechanics

• Isometric contractions: Muscle tension increases but does not exceed the load, no shortening occurs.
• Isotonic contractions: Muscle shortens because muscle tension exceeds the load.
• Force and duration of contraction vary: Based on stimulus frequency and intensity.
• Motor units: A motor neuron and all the muscle fibers it supplies. Smaller motor units enable fine control

Homeostatic Imbalances

• Myasthenia gravis: Autoimmune disease where antibodies block ACh receptors leading to progressive muscle weakness.
• Rigor mortis: When death occurs, muscle fibers run out of ATP causing cross-bridge detachment failure, resulting in muscle stiffening.

Muscular Dystrophies

• Duchenne muscular dystrophy (DMD): Inherited sex-linked disorder, characterized by a deficiency or absence of dystrophin that supports the sarcolemma.

Polio

• Polio is a viral infection that destroys motor neurons.

Muscle Action Types

• Agonists: Main movers in joint actions
• Antagonists: Muscles that oppose or reverse agonist actions
• Synergists: Aid agonists in a movement
• Fixators: Stabilize bones involved in the movement.

Muscle Names

• Location: Reflecting the muscles' location (e.g., frontalis, pectoralis).
• Size: Indicating muscle size (e.g., maximus, minimus).
• Shape: Based on muscle shape (e.g., deltoid, trapezius).
• Direction of fibers: Muscle fibers' orientation (e.g., rectus, oblique).
• Number of origins: Based on the number of attachments to the bone (e.g., bicep, tricep).
• Attachments: The location and attachment points on bones.
• Movement actions: The type of movement the muscle performs (e.g., flexors, extensors).

Muscle Fiber Types

• Speed of contraction (slow/fast twitch): Classified based on the speed of myosin ATPase activity.
• Metabolic pathways: Classified based on how they generate ATP (aerobic vs. anaerobic).
• Oxidative fibers: Use aerobic pathways to generate ATP, suited for endurance activities.
• Glycolytic fibers: Use anaerobic glycolysis to generate ATP, more suited for short, powerful bursts of activity.

Exercise Types

• Isotonic: Muscles change in length during contraction (e.g., lifting weights).
• Isometric: Muscle tension increases but no change in length (e.g., holding a heavy object).
• Anaerobic: Respiration without oxygen, can result in lactic acid buildup.
• Aerobic: Respiration using oxygen, a longer term energy source.