Muscle Physiology Overview

Muscle Physiology Overview

• Muscle tissue comprises roughly half of the human body's weight (men 40%, women 32%).
• Smooth and cardiac muscle make up approximately 10%.
• Muscles are categorized based on striations (alternating dark and light bands):
  • Striated: skeletal and cardiac muscle.
  • Unstriated: smooth muscle.
• Muscles are also categorized by innervation:
  • Voluntary (somatic nervous system): skeletal muscles.
  • Involuntary (autonomic nervous system): smooth and cardiac muscle.

Skeletal Muscle Structure

• A skeletal muscle cell is a muscle fiber.
• Muscle fibers are large, elongated, and cylinder-shaped.
• Multiple muscle fibers are bundled together by connective tissue.
• Each muscle fiber contains numerous myofibrils.
• Myofibrils are cylindrical and extend the length of the muscle fiber.
• Myofibrils consist of contractile elements (thick and thin filaments).

Skeletal Muscle Detail

• Myofibrils:
  • Contractile elements within muscle fibers.
  • Composed of repeating units called sarcomeres.
  • Arranged in a regular pattern, creating striations (alternating dark and light bands).
• Dark Band (A Band):
  • Composed of thick filaments (myosin) and overlapping thin filaments (actin).
• Light Band (I Band):
  • Contains only thin filaments (actin).
• H Zone (of the A band):
  • Lighter area within the center of the A band where thin filaments do not overlap thick filaments.
• Z Line (or Z disk):
  • Found in the middle of each I band, a dense vertical line. The Z-line acts as the sarcomere border.
• M Line:
  • Proteins that hold thick filaments together vertically.
• Sarcomere:
  • The functional unit of muscle contraction.
  • The region between two successive Z-lines.

Myosin

• Component of thick filament.
• Protein molecule with two identical subunits.
• Subunits resemble a golf club.
• Tail regions intertwine.
• Globular heads project outward at the ends, forming cross-bridges.
• Thick filaments have regularly spaced cross-bridges between thin and thick filaments.
• Cross bridges contain: An actin-binding site and A myosin ATPase (ATP-splitting) site.

Actin

• Primary structural component of thin filaments.
• Spherical shape.
• Thin filaments consist of actin molecules, tropomyosin, and troponin.
• Actin binding sites allow for cross-bridge interaction between actin and myosin.

Tropomyosin and Troponin

• Tropomyosin:
  • Thread-like protein that lies end-to-end along the groove of actin's spiral.
  • When not participating in cross-bridge forming, the tropomyosin covers actin's binding sites, preventing myosin interaction.
• Troponin:
  • Made of three polypeptide units.
  • One binds to tropomyosin; One binds to actin; and One binds with calcium ions (Ca2+).
  • In the absence of Ca2+, troponin stabilizes the tropomyosin/actin blocking position.
  • In the presence of Ca2+, troponin changes shape and pulls tropomyosin away.
  • This exposure of the actin binding site allows for myosin interaction, enabling muscle contraction.

T-tubules

• Run perpendicularly from the surface of the muscle cell membrane into the central portions of the muscle fiber.
• Action potentials travel through T tubules.
• Action potentials stimulate release of calcium.

Sarcoplasmic Reticulum

• Modified endoplasmic reticulum.
• Interconnected compartments surrounding the myofibrils.
• Wraps around each A band and I band.
• Expand to form lateral sacs (terminal cisternae) at the ends of the segments.

Muscle Contraction

• Sliding Filament Mechanism: Thin filaments slide over thick filaments during contraction.
• This mechanism does not shorten the thick or thin filaments themselves. The sarcomere shortens.
• Cross-bridge interaction: Myosin cross-bridges attach to actin, pull thin filaments towards the center of the sarcomere, causing shortening This pulling action is fuelled by ATP.

Excitation–Contraction Coupling

• Stepwise process:
  • Acetylcholine (ACh) release initiates the signal.
  • Action potential propagates through T tubules.
  • Calcium (Ca2+) released from the sarcoplasmic reticulum.
  • Ca2+ binding changes the conformation of troponin, exposing actin binding sites.
  • Myosin binds to actin initiating the power stroke and sliding filaments.

Muscle Relaxation

• Ca2+ is actively taken up into the sarcoplasmic reticulum.
• Tropomyosin returns to its blocking position over actin's binding sites preventing further myosin interaction.
• Muscle contraction ceases.

Muscle Types

• Skeletal muscle: Voluntary muscle attached to bones for movement, striated and multi-nucleated.
• Smooth muscle: Mostly involuntary, found in internal organs and blood vessels; non-striated and single-nucleated. Smooth muscles operate differently from skeletal muscles, regulating through phosphorylation of myosin and calcium binding mechanisms. Different forms of smooth muscle include multi-unit and single-unit subtypes.
• Cardiac muscle: Involuntary muscle of the heart; striated and single-nucleated with intercalated discs (gap junctions). Cardiac muscles exhibit pacemaker activity in the absence of neuronal stimulation.

Explore the fundamentals of muscle physiology, including the different types of muscle tissue and their structures. Understand how muscles are categorized based on striations and innervation. This quiz will enhance your knowledge of skeletal muscle structure and function.