Comprehensive Muscle Tissue Notes

Muscle Tissue

Muscle Tissue Overview

  • A primary tissue type consisting of three types:

    • Skeletal muscle (voluntary)

    • Cardiac muscle (involuntary)

    • Smooth muscle (involuntary)

Medical Terminology

  • my/o (e.g., myosatellite cells, which are stem cells for muscle tissue)

  • sarc/o (e.g., sarcomere, the basic functional unit of muscle fibers)

Exam Notes

  • Know the shape of each muscle type.

  • Note whether they are uninucleate or multinucleate.

  • Understand the role of intercalated discs (in cardiac muscle).

  • Identify whether they are striated or non-striated.

Muscle Tissue Function

  • Cells are specialized for contraction via myofilaments (proteins for contraction).

  • Skeletal muscles move the body by pulling on bones.

  • Cardiac and smooth muscles control internal movements (involuntary).

  • Proteins of contraction include troponin and tropomyosin.

Common Properties of Muscle Tissue

  • Excitability (responsiveness): Ability to respond to a stimulus, generating an action potential.

  • Contractility: Ability of cells to shorten when stimulated.

  • Extensibility: Ability to stretch.

  • Elasticity: Ability to recoil to the original shape.

Skeletal Muscle

  • Voluntary, controlled by the somatic nervous system.

  • Made of fibers containing actin and myosin for contraction.

  • Actin and myosin form cross-bridges during contraction.

  • Contains connective tissue (tendons, aponeurosis), blood vessels (O2 supply), and nerves.

Connective Tissue Layers in Skeletal Muscle

  • Epimysium: Outermost layer.

  • Perimysium: Middle layer containing fascicles (bundles of muscle fibers).

  • Endomysium: Innermost layer.

Muscle Fiber Development and Structure

  • Myoblasts fuse with myosatellite cells to form mature muscle fibers.

  • Some myosatellite cells remain for muscle growth and repair.

Major Components of Muscle Fibers

  • Sarcolemma

  • Transverse tubules (T-tubules)

  • Terminal cisternae

  • Sarcoplasmic reticulum (SR)

  • Sarcoplasm

Respiration

  • Aerobic respiration: Involves O2 (cellular respiration).

  • Anaerobic respiration: Does not involve O2 (lactic acid fermentation, causing cramps).

Muscle Attachments

  • Tendons (bundles)

  • Aponeurosis (sheets) that attach bones to other muscles. Stabilizes forces distributed across muscles.

  • Sprain: Overstretched or torn tendon or ligament.

Characteristics of Skeletal Muscle

  • Shape: Cylindrical or tubular

  • Appearance: Striated ("stripes")

  • Control: Voluntary

  • Nuclearity: Multinucleate (several nuclei per cell)

  • Cell Division: No mitosis; myosatellite cells facilitate growth via protein synthesis.

Stem Cells in Muscle

  • Myoblasts: Formed during embryonic development.

  • Combine with myosatellite cells to form mature muscle fibers.

Muscle Cancer

  • Sarcoma or myosarcoma

Sarcomeres

  • Smallest functional unit of a muscle fiber.

  • Interactions between filaments (actin and myosin) produce contraction.

  • A bands: Dark bands, thick filaments (myosin).

  • I bands: Light bands, thin filaments (actin).

  • Contraction occurs when actin and myosin interact, forming cross-bridges.

Sarcolemma

  • Plasma membrane of a muscle fiber.

  • Separates the outside from the inside of the cell.

  • Surrounds the sarcoplasm (cytoplasm of a muscle fiber).

  • Changes in membrane potential initiate contractions.

    • Action potentials trigger contractions.

Membrane Potential

  • Resting membrane potential: Natural state with no contraction.

    • -70 MV (millivolts)

    • Due to differences in sodium and potassium ion concentrations.

  • Contraction: −70 MV resting to +30 MV

  • Relaxation: Returns to -70 MV.

Transverse Tubules (T-Tubules)

  • Transmit action potentials from the sarcolemma to the inside of the cell.

  • Reach the sarcoplasmic reticulum (SR).

  • Action potentials trigger contraction via calcium release.

    • Contraction = Release of calcium.

Sarcoplasmic Reticulum (SR)

  • Similar to smooth endoplasmic reticulum.

  • Two terminal cisternae plus a T-tubule form a triad.

  • Triad role: Coordination of calcium release and storage.

  • Contraction releases calcium, relaxation involves reuptake of calcium.

Myofilaments

  • Thin filaments: Composed primarily of actin.

  • Thick filaments: Composed primarily of myosin.

    • Form cross-bridges during contraction.

    • Part of the sliding filament theory.

  • Titin: Allows muscles to expand and contract; contributes to elasticity and recoil.

Filament Composition

  • Thin filaments made of actin: Active sites are exposed during contraction for myosin heads to bind.

  • Thick filaments made of myosin: Heads bind to actin.

  • Core of titin recoils after stretching.

Excitable Membranes and Neuromuscular Junction

  • Action potentials (electrical impulses).

  • Skeletal muscle fibers contract due to stimulation by motor neurons at neuromuscular junctions (NMJ).

  • They release acetylcholine (ACh), a neurotransmitter that stimulates contractions.

Membrane Polarization

  • Depolarization: Cell becomes more positive.

  • Repolarization: Cell becomes more negative, returning to the resting membrane potential.

  • Hyperpolarization: Cell temporarily becomes more negative than the resting potential.

Neuromuscular Junction (NMJ)

  • All neurons connected to a single muscle fiber.

  • Neurotransmitter: Acetylcholine (ACh).

Summary of Contraction

  • Signal initiation.

  • ACh binding and sodium influx.

  • Action potential generation.

  • Calcium release (contraction).

  • Signal ends (relaxation).

Rigor Mortis

  • Fixed muscular contraction after death.

    • Rigor = stiff

    • Mortis = death

  • Muscles stiffen after death due to a lack of ATP, preventing detachment of myosin from actin.

  • Onset: 2-6 hours after death, peaks around 12 hours, resolves in 1-3 days.

Forensic Medicine

  • Used to estimate time of death.

  • Affected by temperature and body mass.

Tetanus

  • Maximum tension.

    • Incomplete tetanus: More common; occurs in voluntary muscle activities requiring sustained force.

    • Complete tetanus: Uncommon; occurs in high-intensity exercise or infection.

      • Infection = Tetanus (bacterial infection).

Thermogenesis

  • Muscles generate heat by using ATP.

Isotonic vs. Isometric Contraction

  • Isotonic: Muscle changes length (e.g., lifting, pushing, pulling).

  • Isometric: Muscle length remains the same (e.g., posture, maintaining balance).

ATP Generation

  • Myosin heads require ATP for contraction.

  • Three ways ATP is generated:

    • Creatine phosphate: For short bursts of energy (10-15 seconds).

    • Anaerobic metabolism: Moderate intensity, short duration activities (30 seconds to 2 minutes).

    • Aerobic metabolism: More common; supports everyday activities and metabolism.

Types of Skeletal Muscle Fibers

  • Fast fibers (Type II or Fast-Twitch fibers, white):

    • Ideal for short bursts of power.

    • Primarily rely on anaerobic metabolism.

  • Slow fibers (Type I or Slow-Twitch fibers, red):

    • Designed for endurance.

    • Primarily rely on aerobic metabolism.

  • Intermediate fibers (Type IIa or Fast Oxidative fibers, pink in humans):

    • Intermediate speed; faster than slow fibers but not as fast as fast fibers.

    • Use both aerobic and anaerobic pathways.

Muscle Adaptations

  • Muscle hypertrophy: Muscle growth (e.g., weight lifting).

  • Muscle atrophy: Muscle wasting (e.g., paralysis, coma, cast, aging, sedentary lifestyle).

    • Examples of paralysis-inducing conditions: Muscular dystrophy and multiple sclerosis.

Cardiac Muscle Tissue

  • Found only in the heart.

  • Have excitable membranes.

  • Striated like skeletal muscle.

  • Uninucleate.

  • Intercalated discs.

  • Do not regenerate, but the heart itself can recover after a myocardial infarction (MI or heart attack).

    • Shape: Branched

    • Also called cardiocytes or cardiomyocytes.

  • Automaticity: Cardiac muscle can generate its own action potentials.

    • Due to pacemaker cells (SA node, AV node, Bundle of His, and Purkinje fibers).

Smooth Muscle Tissue

  • Non-striated.

  • Involuntary.

    • Integumentary system: Arrector pili muscles cause hairs to erect, leading to goosebumps (vestigial trait).

  • Spindle-shaped.

  • Single central nucleus.

  • Non-striated.

  • T-tubule role: Transmits action potentials to the sarcoplasmic reticulum.