Muscle Phys 1 AY2024 (Combined)


Part I: Muscle Function & Organisation

Muscle Functions

  • Muscles develop tension and shorten (contraction) to produce movements.

  • Purposeful Movements:

    • Propel contents through internal organs

    • Emptying contents to the external environment

  • Other Functions:

    • Maintain posture and body position

    • Stabilize joints

    • Generate heat

    • Assist in circulation, digestion, and breathing

    • Protect internal organs

Types of Muscles

  1. Skeletal Muscles:

    • Voluntary, Striated

    • Bundles of long, thick, cylindrical, multinucleate cells

  2. Cardiac Muscles:

    • Involuntary, Striated

    • Interlinked network of short, branched cells with intercalated discs

  3. Smooth Muscles:

    • Involuntary, Unstriated

    • Loose network of short, spindle-shaped cells arranged in sheets

Skeletal Muscle Organisation

  • Hierarchy of Muscle Structure:

    • Whole Muscle

    • Muscle Fascicle

    • Muscle Fiber (cell)

    • Sarcomere

    • Myofibril

Sarcomere: Functional Unit

  • Components:

    • Nucleus, Mitochondrion, Sarcolemma

    • Thin (actin) and Thick (myosin) filaments

    • Bands: I band, A band, H zone, Z disc

Molecular Basis of Skeletal Muscle Contraction

Sliding Filament Theory

  • Sarcomere contracts as thick and thin filaments slide past each other.

  • Calcium ions regulate actin-myosin binding to allow cross-bridge formation.

Power Stroke Mechanism

  • Myosin head pulls thin filaments toward the center of the sarcomere.

  • Power strokes repeat through cycles of binding, bending, and detachment driven by ATP.

Role of ATP

  1. Energy Currency:

    • ATP provides energy for muscle contraction processes

  2. Cocking of Myosin Head:

    • Prepares for the next power stroke and detachment from actin

Rigor Mortis (Post-mortem Muscle Stiffness)

  • Caused by lack of ATP, leading to persistent contraction and asynchronous cycling of cross-bridges.

Smooth and Cardiac Muscle Function

Smooth Muscle Characteristics

  • Involuntary, unstriated, and capable of slow, prolonged contractions.

  • Unique features include:

    • Myogenic contraction via pacemaker cells

    • Regulation by the autonomic nervous system

    • Calcium-dependent phosphorylation of myosin light chain

Cardiac Muscle Characteristics

  • Found only in the heart, involves:

    • Involuntary contractions

    • Myogenic properties with intercalated discs for cell connectivity

    • Functional syncytium capabilities allowing synchronized heartbeats

Excitation-Contraction Coupling

Mechanism of Muscle Excitation

  • Arrival of action potential at T-tubules induces calcium release from sarcoplasmic reticulum (SR).

  • Calcium is crucial for initiating contraction and enabling actin-myosin interaction.

Calcium Reuptake and Muscle Relaxation

  • Active reuptake of calcium into SR by SERCAs allows for muscle relaxation.