Muscle Contraction Types and Functions Quiz

Muscle & Nerve Tissues (Excitable Tissues)

• Excitable tissues include muscles and nerves, which respond to stimuli.
• Nerves respond by impulse propagation and neurotransmitter release.
• Muscles respond by contraction.

Contents

• Excitable tissues are discussed.
• Types of muscles are detailed.
• Skeletal muscle structure is explained.
• Changes during muscle contraction include electrical, mechanical, metabolic/chemical, thermal, and excitability changes.

Properties of Excitable Cell Membranes

• Membranes exhibit electrical excitability, responding to depolarization above a threshold voltage to transmit impulses.
• Ion channels (pores) within the membranes regulate ion flow.

Types of Muscles

• Three muscle types exist: skeletal, cardiac, and smooth.
• Skeletal: striated, found in the skeleton, voluntary, mechanical work.
• Cardiac: striated, found in the heart, involuntary, pumps blood.
• Smooth: non-striated, found in visceral organs, involuntary, motility.

General Properties of Skeletal Muscles

• Excitability: the ability to respond to a stimulus.
• Conductivity: the ability to transmit an excitatory state along muscle fibers.
• Contractility: the ability to shorten without changing external shape, increasing tension.
• Recoverability: the ability to return to original form after stimulation.

Skeletal Muscles

• Skeletal muscles constitute 40-45% of body weight.
• They are attached to the skeleton.
• Functions of skeletal muscles include:
  • Locomotion
  • Respiration
  • Posture
  • Eating
• Generating heat through muscle tone and shivering.
• Storage sites for fat, protein, glycogen, minerals, and vitamins.
• Source of meat for human consumption.

Structure of Skeletal Muscles

• Composed of muscle fibers, blood vessels, nerve fibers, and connective tissue (CT).
• Muscle fibers (20-40) are grouped into muscle bundles.
• Each muscle is enclosed by epimysium, perimysium, and endomysium (CT sheaths).

Muscle CT Sheaths

• Epimysium: surrounds the entire muscle.
• Perimysium: surrounds muscle fiber groups (bundles or fasciculi).
• Endomysium: surrounds individual muscle fibers.

Muscle Fiber Structure

• Sarcolemma (plasma membrane) contains:
  • Nuclei: located peripherally
  • Sarcoplasm: cytoplasm containing myofibrils, mitochondria, sarcoplasmic reticulum, myoglobin, glycogen granules, and fat droplets.
  • Myofibrils: contractile elements within muscle fibers.

Myofibrils

• Muscle fibers contain hundreds to thousands of myofibrils.
• Myofibrils are densely packed, rod-like contractile elements.
• Composed of actin (thin) and myosin (thick) filaments, arranged in sarcomeres.

Myofibril Structure (Banding Pattern)

• Repeating series of dark A bands and light I bands.
• A bands contain both actin and myosin, exhibiting a complex response to polarized light (anisotropic).
• I bands contain only actin, showing a simple response to polarized light (isotropic).
• H-zone: light zone in the middle of A band with no actin-myosin overlap.
• M-line: center of H-zone (desmin protein, creatine kinase).
• Z-line/disk: dark zone in the middle of I band (zwischenschelbe).

Sarcomere

• The region between two successive Z-lines represents a sarcomere, the functional unit of muscle.

Molecular Structure of Myofibrils

• Thin filaments (actin): 2 µm, extend across I band and into the A band.
• Thick filaments (myosin): 1.5 µm, extend entire length of A band.

Thick Filament

• Composed of the protein myosin.
• Each myosin filament has 100 pairs of myosin molecules, (50 pairs in each direction).
• Myosin molecule shape resembles a golf club with two heads.

Myosin Molecule Parts

• Tail: two polypeptide chains in a double helix.
• Head: two parts
  • Cross bridge/S1/subfragment 1 (globular heads), characterized by actin-binding and ATP-binding sites. ATP binding site has ATPase activity to transform ATP energy into cross bridge energy for muscle contraction.
  • Double helix/S2, characterized by two hinges, which create vertical movement so the cross bridge can bind to actin. The cross bridge to double helix connection allows back and forth movement called the power stroke.

Thin Filament

• Composed of:
  • Actin: major component, forming a double helix with G-actin (globular) subunits.
  • Tropomyosin: regulatory protein covering actin-binding sites at rest, binds to troponin.
  • Troponin: complex of three globular subunits
    • Troponin T (TnT): binds to tropomyosin.
    • Troponin C (TnC): binds to calcium ions.
    • Troponin I (TnI): binds to actin, inhibiting myosin-actin binding.

Sarcoplasmic Reticulum (SR)

• Smooth endoplasmic reticulum surrounding myofibrils.
• Regulates intracellular calcium level via calcium ATP pump.
• Composed of paired terminal cisternae and longitudinal tubules.

Transverse Tubules (T-tubules)

• Invaginations of sarcolemma extending into the muscle cell at right angles.
• Transmit action potentials from the sarcolemma to the sarcoplasmic reticulum to trigger calcium release and muscle contraction.

Muscle Contraction

• Muscle tension can occur without muscle shortening (e.g., holding a heavy book).
• Classified as isometric (muscle length constant) or isotonic (muscle tension constant). Isotonic can be concentric (muscle shortens) or eccentric (muscle lengthens)

Types of Muscle Contraction

• Isotonic contraction:
  • Concentric: muscle shortens.
  • Eccentric: muscle lengthens.
• Isometric contraction: muscle length remains constant.

Changes During Muscle Contraction

• Electrical changes (membrane potential, action potential).
• Mechanical changes (sliding filament theory).
• Metabolic/chemical changes (energy source).
• Thermal changes (heat production).
• Excitability changes (refractory periods).

Electrical Changes

• Membrane potential: difference in charge between inside and outside a cell due to ion distribution.
• Causes of membrane potential include electrogenic and diffusion potential.

Resting Membrane Potential

• Cellular ions (Na+, K+, Cl-, organic compounds) have different concentrations inside and outside cells.

Electrogenic Membrane Potential

• Sodium pump actively transports 3 Na+ out and 2 K+ in per ATP molecule, creating a positive charge outside the cell.

Diffusion Membrane Potential

• Negatively charged ions attract positive ions resulting in a potential difference.

Action Potential

• Rapid changes in membrane potential, transmitting signals between cell parts.
• Factors increasing membrane permeability to Na⁺ cause action potentials.

Stages of Action Potential

• Depolarization: inflow of Na⁺.
• Repolarization: outflow of K⁺.

Nerve Supply

• Skeletal muscles are stimulated by motor neurons of the somatic nervous system.
• Each skeletal muscle fiber is supplied by one motor nerve ending.
• An axon branch forms a neuromuscular junction with a single muscle fiber.

Neuromuscular Junction

• Formed by a motor nerve ending, motor endplate, and synaptic cleft.
• Motor nerve endings contain synaptic vesicles with acetylcholine (ACh).
• Motor endplate contains ACh receptors on the sarcolemma.
• Synaptic cleft contains ACh esterase to destroy ACh.

Excitation-Contraction Coupling

• Links electrical (action potential) and mechanical (contraction) changes.
• Excitation-calcium coupling (neural stimulation).
• Calcium-contraction coupling (action on myofibrils).

Excitation-Calcium Coupling

• Neural stimulation initiates action potentials that reach the neuromuscular junction.
• Depolarization occurs, opening voltage-gated calcium channels, allowing Ca²⁺ inflow to trigger ACh release into the synaptic cleft.
• ACh receptor activation triggers muscle fiber depolarization and action potential spread along the sarcolemma (motor end plate).
• Action potentials travel along T-tubules.

Calcium-Contraction Coupling

• Calcium release from sarcoplasmic reticulum.
• Calcium initiates actin and myosin filament linking; this moves actin towards the center of each sarcomere (shortening the muscle). This process drives muscle contraction.
• Muscle contraction continues as long as there's excess calcium in the sarcoplasm.

Mechanical Changes (Sliding Filament Theory)

• Thin filaments slide past thick filaments.
• Sarcoplasm shortens, and thin and thick filaments overlap.

Molecular Participants in Sliding Filament Theory

• Myosin, actin, troponin, tropomyosin, ATP, and calcium ions are involved in the sliding filament process.

Single Cross Bridge Cycle

• Series of steps for muscle contraction.
  • Step I: Exposure of actin binding sites (Ca²⁺ release, troponin/tropomyosin conformational change).
  • Step II: Binding of myosin and actin.
  • Step III: Power stroke (myosin head flexes, pulling thin filament). Release ADP and Pi.
  • Step IV: Disconnecting the cross bridge (ATP binds to myosin, disconnecting it from actin).
  • Step V: Re-energizing and repositioning the cross bridge (ATP hydrolysis, returning myosin to high-energy configuration).
  • Step VI: Removal of calcium ions (Ca²⁺ actively pumped back into the sarcoplasmic reticulum). Return troponin/tropomyosin to initial state.

Metabolic or Chemical Changes

• Energy derives from ATP hydrolysis.
• ADP phosphorylation via creatine phosphate (CrP).
• CrP phosphorylation by energy from food (muscle glycogen, fats, proteins).

Energy Systems

• ATP serves as the direct energy source for muscle contraction.
  • ATP-CrP system: high-energy phosphate reservoir, regenerates ATP from ADP.
  • Anaerobic glycolysis system: glucose breakdown to pyruvate, lactic acid (muscle fatigue).
  • Aerobic glycolytic system: Carbohydrates + O₂ → 38 ATP + CO₂ + H₂O.
  • Aerobic lipolytic system: Fats + O₂ → 138-456 ATP + by-products.

Thermal Changes

• Energy from chemical changes drives mechanical work and generates heat.
  • Initial heat (45%): heat produced during contraction.
  • Relaxation heat (15%): heat produced during relaxation.
  • Recovery heat (55%): heat liberated after contraction.

Excitability Changes (Refractory Periods)

• Ability of skeletal muscle to respond to stimuli.
• Phases:
  • Absolute refractory period (ARP): no response to any stimulus, coincides with depolarization.
  • Relative refractory period (RRP): stronger stimulus needed for response, coincides with repolarization.
  • Supranormal state of excitability: Increased excitability, response to stimulation.
  • Subnormal state of excitability: reduced excitability after complete relaxation.

Factors Affecting Simple Muscle Twitch

• Muscle fiber type
• Initial muscle length
• Temperature
• Staircase phenomena (treppe)

Types of Muscle Fibers

• Type I fibers (slow-twitch):
  • Characteristics: red, high myoglobin, high mitochondria, aerobic metabolism, slow contraction, prolonged contraction, prolonged aerobic energy source, less easily fatigued.
  • Examples: extensors (gluteal, thigh)
• Type II fibers (fast-twitch):
  • Characteristics: white/pale, low myoglobin, low mitochondria, anaerobic metabolism, fast contraction, rapid/powerful contraction, easily fatigued.
  • Examples: flexors (eye muscles)

Temperature

• Warmer temperatures increase enzyme activity, accelerating muscle contraction speed and strength but decrease duration.
• Warmer temperatures reduce muscle viscosity, facilitating contraction processes.

Fatigue

• Reduced strength and prolonged duration of contraction; relaxation becomes incomplete due to decreased energy sources.

Staircase Phenomena (Trepp)

• Repeated muscle stimulation at a rate not causing fatigue results in progressively stronger first few contractions.
  • Factors: Reduced potassium ions inside the fiber and slight increase of extracellular, increase calcium concentration inside the muscle, Initial contraction generates heat, stimulating heat and more calcium inflow, which, in turn, stimulates more cross bridge and a stronger contraction.

Test your knowledge on different types of muscle contractions including isometric and isotonic, as well as the unique characteristics of skeletal, smooth, and cardiac muscles. Explore the physiological changes that occur during muscle activity and their role in body temperature regulation and movement.