Muscle Tissue Characteristics Quiz

Muscle Physiology Lecture Notes

• Muscle physiology lecture covers skeletal muscle.
• Learning outcomes include listing skeletal muscle characteristics, describing muscle fiber structures, identifying and explaining the functions of major muscle proteins, explaining stimulation and contraction processes in skeletal muscle fibers, and describing muscle twitch stages.
• Universal characteristics of muscle include excitability (responding to stimuli), conductivity (transmitting electrical signals), contractility (shortening), extensibility (stretching), and elasticity (returning to resting length).
• Skeletal muscle is attached to bone by tendons via a continuous external connective tissue layer. Contractions cause movement at joints. Collagen is extensible and elastic, stretching under tension and recoiling. It resists overstretching and protects muscle from injury, returning the muscle to its resting length. It contributes to power output and muscle efficiency.
• Skeletal muscle is voluntary and striated.
• Voluntary - under conscious control.
• Striated - alternating light & dark bands due to internal contractile proteins.
• A muscle cell is a muscle fiber (myofibre) up to 30 cm in length.
• A muscle fiber has many nuclei, mitochondria positioned between myofibrils, glycogen (carbohydrate energy storage), and myoglobin (red pigment for oxygen delivery).

Structural Hierarchy of Skeletal Muscle

• A muscle is a contractile organ composed of bundles of muscle fibers (fascicles).
• Each fascicle is a bundle of muscle fibers enclosed by perimysium.
• A muscle fiber is a single muscle cell, elongated enclosed in a specialized plasma membrane (sarcolemma). It contains bundles of myofibrils.
• Myofibrils contain myofilaments – thin filaments (actin) and thick filaments (myosin). Interactions of thick and thin filaments lead to muscle contraction.
• Myofibrils are organized into repeating functional units called sarcomeres.

Myofilaments

• Myofilaments are the protein filaments found within myofibrils.
• Actin, arranged in thin filaments:
  • fibrous (F) actin: two intertwined strands
  • globular (G) actin subunits with active sites for myosin binding.
  • tropomyosin - an actin-binding protein that blocks active sites on G actin
  • troponin - calcium-binding protein on tropomyosin.
• Myosin, arranged in thick filaments:
  • Two headed club-shaped myosin molecules.
  • A myosin molecule has a tail and globular head.
  • Heads are arranged in a helix around the myosin bundle.
• Dystrophin: important protein linking actin in myofilaments to endomysium, transfers contractile force to connective tissue. Defects can cause muscular dystrophy.
• Titin or connectin: large protein, runs through myosin filament, stabilizing and positioning the thick filament. Prevents overstretching. Prevents overstretching and provides recoil).

Sarcomeres: Contractile Units

• Sarcomeres are repeating subunits within myofibrils.
• Sarcomere sections are demarcated by Z lines.
• Thick filaments are found in the A band, with the M line in the center.
• Thin filaments, called I bands, are on both sides of the A band.
• The H band contains only thick filaments.
• The zone of overlap is where thin and thick filaments overlap

The Neuromuscular Junction

• Synapse: where a nerve fiber meets a target cell, or in a motor neuron meeting a muscle fiber.
• Neuromuscular junction (NMJ): the synapse where a motor neuron stimulates a muscle fiber.
• Acetylcholine (ACh): neurotransmitter releases at the NMJ, stimulating muscle contraction.
• The NMJ includes terminal branches of nerve fibers, synaptic cleft, and acetylcholine receptors.
• The axon terminal has synaptic vesicles containing ACh.
• Basal lamina - thin layer of collagen and glycoproteins separating nerve and muscle tissue.

Electrically Excitable Cells

• Muscle fibers and neurons are electrically excitable.
• Membrane voltage changes in response to stimulation
• Voltage or electrical potential: difference in electrical charge between points.
• Resting membrane potential: ± -90 mV in skeletal muscles.
• Maintained by sodium-potassium pump.

Excitation-Contraction Coupling

• Action potential travels along the T tubules.
• Calcium released from terminal cisterns of the sarcoplasmic reticulum.
• Calcium binds to troponin, causing tropomyosin to move, revealing active sites on actin.

Contraction Cycle

• Energized myosin heads bind to active sites on F-actin.
• Myosin head pivots toward the M line (power stroke).
• ATP binds to myosin and breaks the link to actin.
• Active site exposed again ready for new cycle/cross-bridge.
• Myosin head pivots toward the M line, pulling the actin filament.

Muscle Relaxation

• Cessation of nervous stimulation
• ACh breakdown by acetylcholinesterase (AChE).
• Calcium ions are reabsorbed into the sarcoplasmic reticulum.
• Tropomyosin returns to its blocking position, actin and myosin detach.

The Length-Tension Relationship

• Amount of tension generated depends on how stretched or shortened the muscle was before stimulation.
• Optimal resting length produces the greatest force during contraction.
• Nervous system maintains resting muscle tone near optimal length.

Rigor Mortis

• Happens after death.
• Deterioration of sarcoplasmic reticulum releases calcium ions.
• Muscle activation leads to contraction but unable to relax.

Threshold, Latent Period, and Twitch

• Threshold: minimum voltage necessary to generate an action potential in muscle cells for contraction.
• Twitch: single contraction-relaxation cycle.
• Latent period: delay between stimulation and contraction (excitation, excitation-contraction coupling, and tensing).
• Contraction phase: tension generation.
• Relaxation phase: tension declines (calcium reabsorption).
• Contraction strength of twitches varies due to: muscle starting length, fatigue, temperature, and hydration.

Stimulation of Muscle

• Treppe: increase in tension in successive stimuli.
• Wave summation: increased tension with successive stimuli.
• Incomplete tetanus: only partial relaxation between stimuli.
• Complete tetanus: no relaxation between stimuli, maximum tension.