Muscle System Reading AS111

Muscle Tissue

  • Muscle cells, or muscle fibers, are specialized for contraction.
    • Composed of proteins: actin and myosin arranged into microfilaments.
    • Contraction occurs via sliding of microfilaments over one another, changing cell shape and length.
    • Muscle contraction allows for:
    • Movement of bones
    • Circulation of blood
    • Movement of food through intestines.

Types of Muscle Tissue

  • Three major types of muscle tissue: Skeletal, Smooth, and Cardiac.
Skeletal Muscle
  • Contains large cells, possibly over a foot in length.
    • High metabolic requirements lead to the presence of multiple nuclei and mitochondria for homeostasis.
  • Responsible for voluntary movements: walking, running, biting, and facial expression.
    • Voluntary Muscle: Controlled consciously, unlike smooth and cardiac muscle.
  • Histological features:
    • Striated appearance (striped) due to alternating bands of light and dark.
    • Muscle fibers clustered into bundles held by loose connective tissue.
    • Collagen fibers merge with tendon fibers to ensure strong attachment to bones.
  • Stimulated by nerve fibers throughout the muscle belly:
    • Nerve damage can lead to muscle paralysis (paretic).
Smooth Muscle
  • Comprised of small, spindle-shaped, non-striated cells.
    • Controlled involuntarily with contractions triggered by nerve actions.
  • Found in:
    • Walls of hollow organs (blood vessels, intestines, bladder, uterus).
    • Responsible for peristalsis and constriction of blood vessels.
  • Requires only one centrally located nucleus per cell.
Cardiac Muscle
  • Exclusive to the heart; contracts involuntarily.
    • Contains specialized pacemaker cells for rhythmic contraction.
    • Forms a complex network: branches connect at intercalated discs.
  • Microscopic features:
    • Striated cells with a single nucleus.

Nervous Tissue

  • Nervous tissue transmits electrical and chemical signals.
    • Found in the brain, spinal cord, peripheral nerves.
  • Composed of:
    • Neurons: long cells with a cell body (perikaryon), dendrites, and an axon.
    • Neurons are sensitive to environmental changes and transmit impulses to other tissues.
    • Neuroglial Cells: support neurons, isolate conductive membranes, direct nutrients, and digest debris.

General Overview of Muscle Functions

  • Muscle is one of the four basic tissues, alongside epithelial, connective, and nervous tissue.
  • Characteristics of muscle cells:
    • Excitability: Respond to stimuli.
    • Contractibility: Shorten when stimulated.
    • Extensibility: Can stretch.
    • Elasticity: Return to original shape after contraction.
  • Muscle functions include:
    • Providing Motion: Movement of limbs and body.
    • Maintaining Posture: Support for the skeleton.
    • Generating Heat: Important for maintaining body temperature.
Skeletal Muscle Structure & Attachment
  • Composed of the epimysium, a connective tissue sheath surrounding muscle fibers.
  • Muscle Attachment Types:
    • Tendons: Fibrous connections to bones.
    • Aponeuroses: Broad sheets of connective tissue (e.g., linea alba).
    • Origin: Stationary attachment site during contraction.
    • Insertion: Site of movement during contraction.
    • Example: The gastrocnemius muscle's origin on the femur and insertion on the calcaneal tuberosity impacts movement efficiency.
Muscle Actions
  • Muscles contract and shorten, producing movement.
  • Group action:
    • Agonist: Primary mover producing desired motion.
    • Antagonist: Opposes action of agonist.
    • Synergist: Assists agonist during movement.
    • Fixator: Stabilizes joints to allow other movements to occur smoothly.

Muscle Naming Conventions

  • Action: Named for function (e.g., flexor for flexing joints).
  • Shape: Reflective of muscle shape (e.g., deltoid = triangular).
  • Location: Indicated by names (e.g., biceps brachii = upper arm).
  • Direction of fibers: Named based on orientation (e.g., rectus = straight).
  • Number of heads: Refers to attachment sites (biceps = two heads).

Selected Skeletal Muscles

  • Focus on clinically important or easily referenced muscles.
  • Cutaneous Muscles: Located in connective tissue beneath skin, control skin twitches.
Muscles of the Head and Neck
  • Control facial expressions and chewing.
    • Major muscles include:
    • Masseter: Powerful muscle for chewing.
    • Splenius & Trapezius: Extend head and neck.
    • Brachiocephalicus: Pulls leg and flexes neck.
Abdominal Muscles
  • Support abdominal organs, assist in flexion, and participate in expulsion activities (defection, urination, parturition).
  • Arranged in layers:
    • External/Internal Abdominal Oblique (oblique fibers), Rectus Abdominis (strap-like), Transversus Abdominis (deepest).
Thoracic and Pelvic Limb Muscles
  • Thoracic Limb: Focus on shoulder and brachial muscles (e.g., Latissimus Dorsi, Pectorals, Deltoid).
  • Pelvic Limb: Major extensor and flexor muscles include gluteal and hamstring groups.

Respiration Muscles

  • Inspiration: Increases thoracic cavity volume using diaphragm and external intercostal muscles.
  • Expiration: Internal intercostal muscles and abdominal muscles decrease cavity volume to push air out.

Microscopic Anatomy of Skeletal Muscle

  • Long and thin cells (up to 80 µm diameter).
  • Multinucleated: up to 100 nuclei per cell.
  • Structure includes numerous myofibrils and organelles like mitochondria and sarcoplasmic reticulum.
  • Sarcomere: Basic unit of contraction, defined by Z lines and contains actin and myosin filaments.
Neuromuscular Junction
  • Connection point between motor nerve fibers and muscle fibers.
    • Acetylcholine released triggers contraction, with the enzyme acetylcholinesterase ending its effect.
    • Motor Unit: One nerve fiber and all innervated muscle fibers.
    • Smaller motor units for fine movements, larger for powerful contractions.
Connective Tissue Layers
  1. Endomysium: Surrounds individual muscle fibers.
  2. Perimysium: Surrounds groups of muscle fibers (fascicles).
  3. Epimysium: Outer layer surrounding entire muscle.
  • Continuous with tendons/attachments.

Physiology of Skeletal Muscle

  • Contraction and Relaxation: Nerve impulse leads to Ca2+ release from the sarcoplasmic reticulum, initiating contraction; Ca2+ is subsequently pumped back, causing relaxation.
  • Contraction Mechanics: Myosin cross-bridges ratchet against actin to shorten the sarcomere.
Characteristics of Muscle Contraction.
  • Based on the All-or-Nothing principle for individual fibers, controlled by the number of recruited fibers for whole muscles.
  • Twitch Contraction Phases:
    1. Latent Phase: Delay before contraction (≈10 ms).
    2. Contracting Phase: Active contraction (≈40 ms).
    3. Relaxation Phase: Muscle returns to resting state (≈50 ms).
Energy Sources for Muscle Contraction
  • ATP: Main energy source for muscle contractions, produced by mitochondria.
  • Creatine Phosphate (CP): Recharge ATP through phosphate donation.
  • Glucose & Oxygen: Body's main energy sources;
    • Stored as glycogen and released via myoglobin.
Anaerobic and Aerobic Metabolism
  • Anaerobic: Occurs during high energy demands with insufficient oxygen, produces lactic acid.
  • Aerobic: Utilizes oxygen, more efficient energy production, and results in less lactic acid.
Heat Production
  • Muscle activity generates heat, crucial for body temperature regulation. Excess heat can lead to mechanisms like sweating or shivering.

Cardiac Muscle

  • Voluntary striated muscle, primarily found in the heart.
  • Gross Anatomy: Forms structure of cardiac chambers, continuous contraction without rest.
  • Microscopic Anatomy: Similar to skeletal muscle, but cells branch and are interconnected via intercalated discs.

Cardiac Muscle Physiology

  • Contracts rhythmically with or without external stimulation, forms wavelike contractions to pump blood.
  • Conducts impulses through an internal conduction system initiated by the SA node, with nerves able to modify action.

Smooth Muscle

  • Non-striated involuntary muscle found in various organ systems.
Anatomy and Physiology of Smooth Muscle
  • Visceral Smooth Muscle: Found in walls of hollow organs, contracts rhythmically.
  • Multi-Unit Smooth Muscle: Small, discrete groups controlling delicate movements.
  • Smooth muscle functions are modified by autonomic nervous system influences.
  • Pregnancy Hormone Regulation: Progesterone inhibits uterine contraction, allowing for fetal development until labor.

Conclusion

  • Muscle tissue plays critical roles in the movement, heat regulation, and physiological functions throughout the body, organized into distinct types with unique features and functions, relying on complex systems for contraction, control, and coordination in action.