EW

lecture 2Muscle Physiology part 1

Lecture Context & Scope

  • Muscle physiology will be the core of the upcoming lecture-exam.
  • Instructor divides the material into multiple segments so students can keep track and avoid information overload.
  • Current talk covers
    • Gross & microscopic structure of all 3 muscle tissues.
    • Development of skeletal muscle fibers.
    • Intro to molecular/ionic mechanisms that underlie contraction (sets up next unit on the nervous system).

Big-Picture Preview of Molecular Physiology

  • Emphasis on how molecules (proteins) and ions move/interact.
    • Main ions: \mathrm{Ca}^{2+} (calcium) and \mathrm{Na}^{+} (sodium).
    • Main energy source: ATP.
  • Neuron–muscle interface introduced (motor neuron terminal pictured in slide).
  • Knowledge of muscle physiology provides bridge to understanding nervous system signaling next unit.

Three Types of Muscle Tissue (Review)

  • Skeletal Muscle
    • Voluntary, striated.
    • Very long cylindrical cells (muscle fibers); each fiber extends the length of the whole muscle.
    • Multinucleated (fusion of embryonic cells).
  • Cardiac Muscle
    • Involuntary, striated.
    • Shorter, sometimes branched cells.
    • One nucleus per cell (uninucleated / mononucleated).
    • Intercalated discs present; darker bands created by dense protein complexes.
    • Discs contain gap junctions → allow direct cytoplasmic continuity and electrical coupling between cells.
  • Smooth Muscle
    • Involuntary, non-striated.
    • Spindle-shaped cells; single centrally located nucleus.
    • Lines visceral organs and tubes (blood vessels, digestive tract, respiratory passages, urinary & reproductive ducts).

Schematic/Whiteboard Summaries (verbal drawings)

  • Skeletal: long striped cylinders, multiple nuclei lined along periphery.
  • Cardiac: short, striated, branched rectangles; single nuclei; intercalated discs between cells.
  • Smooth: elongated football-shaped cells, no stripes, single nucleus.

Functional Roles of Each Muscle Type

  • Skeletal → move bones & facial skin; maintain posture (tonic contractions keep us upright unknowingly).
  • Cardiac → pump blood; cell-to-cell communication via intercalated discs unifies contractions into single heartbeat.
  • Smooth → transports or regulates flow of materials inside body:
    • Blood pressure control (vascular smooth muscle).
    • Urine propulsion (ureters, bladder neck).
    • Gamete/zygote movement (reproductive tracts).
    • Food propulsion & mixing (GI tract peristalsis).
    • Airway diameter adjustment (bronchioles).

General Functions of Muscle Tissue (All Types)

  • Produce body movements.
  • Stabilize body positions (posture & joint stabilization).
  • Move substances internally (peristalsis = rhythmic, wave-like contractions that push content through a tube).
  • Thermogenesis (heat production) – e.g., shivering when cold.

Fundamental Physiological Properties

  • Contractility – ability to shorten (generate pulling force).
  • Extensibility – ability to be stretched/lengthened without damage.
  • Elasticity – ability to recoil and return to original shape (loss implicated in rigor mortis when ATP absent after death).
  • Electrical Excitability – ability to respond to electrical/ionic imbalance.
    • Driven primarily by \mathrm{Ca}^{2+} and \mathrm{Na}^{+} fluxes.
    • Most conceptually challenging part; principle overlaps heavily with nervous system physiology.

Striations & Contractile Proteins

  • Striations result from orderly overlap of actin (thin) and myosin (thick) filaments.
  • Present in skeletal & cardiac muscle; absent visually in smooth, but actin and myosin still exist (randomly arranged → no stripes).

Development of Skeletal Muscle Fibers

  • Myoblasts (embryonic muscle progenitors) differentiate into satellite cells.
  • Satellite cells fuse → create long multinucleated immature muscle fiber → continues fusing/elongating to full length.
  • Remaining satellite cells persist along periphery of mature fiber → serve as adult muscle stem cells.
    • Participate in repair & hypertrophy after exercise.
    • Their population & proliferative capacity decline with age → harder to build/maintain muscle later in life.
    • Decline influenced by growth hormone and sex hormones.

Hierarchical Organization of Skeletal Muscle (macroscopic → microscopic)

  • Muscle (organ) → composed of multiple fascicles.
  • Each fascicle → bundle of muscle fibers (cells).
  • Each muscle fiber → packed with myofibrils (contractile rods).
  • Each myofibril → linear series of sarcomeres (smallest functional contractile unit).

Connective-Tissue Sheaths (continuous, merge into tendon)

  • Endomysium – wraps each individual muscle fiber.
  • Perimysium – wraps each fascicle.
  • Epimysium – wraps entire muscle.
  • All three layers converge & extend beyond muscle belly forming a tendon.
    • Tendon = dense regular connective tissue; fibers interweave with bone’s periosteum via Sharpey’s fibers.
    • Because tendon is continuous with internal sheaths, ripping tendon often damages muscle tissue as well.

Microscopic Anatomy of a Muscle Fiber (Cell)

  • Plasma membrane = sarcolemma ("sarco" or "myo" roots always indicate muscle).
  • Cytoplasm = sarcoplasm (not explicitly named in transcript but implicit).
  • Myofibrils – cylindrical contractile elements; dozens to hundreds per cell; run entire length.
  • Nuclei – multiple, located just beneath sarcolemma (peripheral).
  • Mitochondria – scattered throughout; sites of ATP synthesis.
  • Sarcoplasmic Reticulum (SR)
    • Modified smooth ER; extensive network enveloping each myofibril.
    • Special function: Ca^{2+} storage.
    • Enlarged end-sacs = terminal cisterns – even higher \mathrm{Ca}^{2+} concentration.
  • Transverse (T) Tubules
    • Invaginations of sarcolemma that penetrate cell interior at regular intervals.
    • Surround myofibrils alongside paired terminal cisterns → form triads (not explicitly named, but implied).
    • Rapidly conduct action potentials & distribute \mathrm{Na}^{+} into depths of fiber.
  • Dystrophin
    • Cytoskeletal protein anchoring sarcomeres to sarcolemma.
    • Mutation/absence → muscular dystrophy (progressive weakness because forces not transmitted properly to membrane & connective tissue).

Sarcomere – Structural Details

  • Region between two successive Z-lines (middle of light band → middle of next light band).
  • Contains precise actin–myosin arrangement that produces dark (A) and light (I) bands viewed as striations.
  • Hundreds to thousands stacked end-to-end inside each myofibril → entire myofibril shortens when all sarcomeres shorten.

Key Ions & Their Roles (Electrical Excitability Primer)

  • \mathrm{Ca}^{2+}
    • Stored in SR; released upon electrical stimulation.
    • Triggers interaction between actin & myosin (binds troponin in later lecture).
  • \mathrm{Na}^{+}
    • Main extracellular cation; depolarizes sarcolemma when entering via ion channels.
    • Propagated internally through T-tubules.
  • Muscle & nerve cells share this ionic language → explains upcoming integration with nervous system lectures.

Real-World/Clinical Connections & Examples

  • Shivering – involuntary skeletal muscle contractions raise body temperature via ATP breakdown (heat by-product).
  • Rigor mortis – post-mortem depletion of ATP; lack of energy prevents detachment of cross-bridges → muscles become stiff until proteins degrade.
  • Muscular dystrophy – genetic defects in dystrophin or related proteins; sarcomeres cannot anchor → progressive muscle wasting.
  • Peristalsis – smooth muscle example of extensibility + contractility working rhythmically to move food, urine, etc.
  • Exercise & Aging
    • Youth: abundant satellite cells enable hypertrophy and repair.
    • Aging: hormone shifts → fewer satellite cells → slower recovery, sarcopenia risk.

Vocabulary & Etymology Reminders

  • Prefix myo- or my- → muscle.
  • Prefix sarco- → flesh/muscle.
  • Peristalsis = rhythmic contraction for propulsion.
  • Viscera = internal organs → hence "visceral muscle" synonym for smooth muscle.
  • Blasts = immature progenitor cells (e.g., myoblast).
  • -lemma = sheath or membrane (e.g., sarcolemma).

Numbers, Symbols & Formulas Recap

  • Principal ions: \mathrm{Ca}^{2+}, \mathrm{Na}^{+}.
  • ATP hydrolysis (implied): \text{ATP} \; \rightarrow \; \text{ADP} + \text{P}_i + \text{Energy (Heat + Work)}.
  • Sarcomere length conceptually from Z-line to Z-line; thousands per myofibril.

Where the Course Goes Next

  • Detailed mechanism of excitation–contraction coupling (how neuron signals open ion channels, SR releases \mathrm{Ca}^{2+}, actin–myosin cycle, ATP usage).
  • Comparisons/variations in cardiac & smooth muscle physiology (to be covered at end of PowerPoint).

These notes consolidate every major and minor point from the transcript, clarify definitions, relate structures to function, and tie content to broader physiological and clinical contexts. They are formatted for efficient study while preserving depth equivalent to the original lecture.