Muscle Anatomy and Function

Muscle Contractions

• Muscle contractions are the mechanisms of animal movement
• Amoeboid movement: Cells change shape to move forward (e.g., eosinophils in immune cells)
• Ciliary and flagellar bending: Movement from cilia and flagella

Muscle Tissue Types

• Skeletal: Striated, voluntary, responsible for movement, heat production, and increased manoeuvrability
• Cardiac: Striated, involuntary, pumps blood
• Smooth: Non-striated, involuntary, controls organ size and movement

Muscle Lever Systems

• Muscle lever systems work at a mechanical disadvantage
• Muscle fibers are wrapped in connective tissue (epimysium) for mechanical support

Skeletal Muscle Architecture

• Muscle fibers are wrapped in connective tissue (epimysium, perimysium, endomysium)
• Arrangement: Muscle belly, tendon, single muscle fiber, fasciculus, capillary, sarcolemma, sarcoplasm, nuclei

Specialized Membranes of Skeletal Muscle

• Myofibrils: Long, thread-like structures containing myosin and actin. Organized into sarcomeres, which causes striations
• Sarcolemma: Cell membrane, has openings for T-tubules to reach inside the cell
• T-tubules: Extensions of the cell membrane that allow action potentials to reach muscle fibers and release calcium ions
• Sarcoplasmic Reticulum (SR): Stores calcium ions. Contains terminal cisternae on either side of T tubules (collectively called a triad).
• Triad: Important for excitation-contraction coupling

Myofibril Structure

• Sarcomere: Basic contractile unit of a myofibril
  • I-band (isotropic): Light band, only actin filaments
  • A-band (anisotropic): Dark band, overlapping actin and myosin filaments
  • Z-line: Located in the center of the I-band; the distance between adjacent Z-lines defines a sarcomere
  • H-zone: Center of the A-band, only myosin filaments
  • M-line: Middle of the sarcomere
• Myosin: Thick filament
• Actin: Thin filament
• Titin, Nebulin: Accessory proteins important for maintaining the architecture of myofibrils

The Sliding Filament Mechanism

• Each cross-bridge generates force independently of other cross-bridges
• Force is determined by the number of cross-bridges formed between myosin and actin filaments; force is also determined by the amount of overlap between myosin and actin filaments

Sarcomere Length-Tension Relationship

• Optimal range: 2.0-2.2 μm; allows actin and myosin overlap and produces maximal force efficiently
• Too short or too long causes insufficient overlap for optimal force generation

The Cross-Bridge Cycle

• ATP hydrolysis energizes the cross-bridge
• Cross-bridge binds to actin, causing a power stroke
• ATP binds to myosin, causing the cross-bridge to detach.

ATP as an Allosteric Regulator

• ATP is essential, it's energy source for cross-bridge movement
• Binding of ATP to myosin regulates the cycle by breaking the link between actin and myosin

Calcium-Ion Regulation

• Calcium ions initiate contraction by binding to troponin.
• This binding moves tropomyosin, uncovering the myosin binding sites on actin to initiate contraction

Twitch and Tetanus

• Twitch: Brief contraction in response to a single action potential
• Tetanus: Sustained contraction from repeated stimulation; can be unfused (incomplete) or fused (complete)

Membrane Receptors in Triads

• DHP receptor: Voltage sensor
• Ryanodine receptor: Calcium release channel

Mechanism of Calcium Release from the Sarcoplasmic Reticulum

• Action potentials cause calcium release from the SR into the cytoplasm
• Calcium binding to troponin initiates contraction

Isometric and Isotonic Contraction

• Isometric: Muscle tension without length change
• Isotonic: Muscle shortens while maintaining constant load. Eccentric contraction involves muscle lengthening under load

Heart Muscle Structure

• Contains intercalated disks (specialized cell junctions)
• Desmosomes hold cells together.
• Gap junctions allow for rapid transmission of electrical impulses between cells

EC Coupling in Cardiac Muscle

• Action potentials cause Ca2+ release from the sarcoplasmic reticulum
• Activation of ryanodine receptors to initiate calcium release
• Na+/Ca2+ exchangers pump Ca2+ out of the muscle to relax.

Smooth Muscle

• Unitary: Muscle fibers contract together
• Multiunit: Muscle fibers contract independently
• Has Dense bodies
• Ca2+ binding to calmodulin activates myosin light chain kinase, initiates contraction

Regulation of Contraction in Different Muscle Types

• The mechanisms for regulating contraction is based on different types of muscles
• Calcium binding to troponin in skeletal and cardiac muscle
• Calcium binding to calmodulin in smooth muscle