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.