Muscle Tissue and Contraction

General Characteristics of Muscle Tissue

  • Myology: The study of muscles.
  • Energy Conversion: When muscles work, chemical energy is transformed into mechanical energy.
  • Structural Composition: Skeletal muscle is a distinct organ consisting of individual muscle cells known as myofibers, along with connective tissue, blood vessels, and nerves.

Properties of Muscles

  • Excitability: Ability of muscle to respond to stimuli.
  • Conductivity: Propagation of electrical signals over the muscle membrane.
  • Contractility: Muscle's capacity to shorten or exert force.
  • Extensibility: Ability to stretch beyond normal resting length.
  • Elasticity: Muscle's ability to recoil after being stretched.

Functions of the Muscular System

  • Movement of the body.
  • Maintenance of posture.
  • Facilitation of respiration.
  • Production of body heat.
  • Communication (e.g., facial expressions).
  • Constriction of various organs and vessels, including sphincters.
  • Movement of substances through the body: blood, lymph, air, food, and digestive fluids.

Types of Muscle Tissue

  • Skeletal Muscle: Striated, multi-nucleated, and under voluntary control; attached to bones.
  • Cardiac Muscle: Striated, single nucleus, involuntary, found in the heart; characterized by intercalated discs.
  • Smooth Muscle: Non-striated, single nucleus, involuntary, found in walls of hollow organs.

Connective Tissue Components of Skeletal Muscle

  • Superficial Fascia: Areolar connective tissue and fat beneath the dermis.
  • Deep Fascia: Dense irregular connective tissue wrapping around muscles and lining body walls.
  • Endomysium: Surrounds each muscle fiber.
  • Perimysium: Surrounds fascicles (groups of fibers).
  • Epimysium: Surrounds the entire muscle.

Muscle Contraction at the Molecular Level

Anatomy of Myofibers

  • Sarcolemma: Cell membrane of muscle fibers, with transverse tubules (T-tubules) for action potential transmission.
  • Myofibrils: Packed structures of contractile proteins.
  • Myofilaments: Thin (actin) and thick (myosin) filaments involved in contraction.
  • Sarcoplasmic Reticulum (SR): Stores calcium ions; involved in excitation-contraction coupling.

Sarcomeres

  • The functional unit of muscle; organized into repeating units containing myofilaments, separated by Z discs.

Action at the Neuromuscular Junction (NMJ)

  • Motor Neuron: Releases acetylcholine (ACh) at NMJ, inducing muscle contraction by depolarizing the sarcolemma.
  • Mechanism: The action potential in motor neuron leads to calcium influx and ACh release, which binds to muscle receptor sites.

Contraction Cycle

  1. Myosin Activation: Myosin heads become energized and bind to actin.
  2. Power Stroke: Myosin heads swivel, pulling actin past myosin.
  3. Detachment: Myosin heads bind ATP and detach from actin.
  4. Cycle continues if ATP and Ca²⁺ are present.

Muscle Relaxation

  • Ca²⁺ is pumped back into the SR, and ACh is broken down, preventing further contraction.

Muscle Metabolism

  • ATP Sources:
    • Creatine Phosphate: Rapid ATP regeneration; provides energy for ~15 seconds.
    • Anaerobic Cellular Respiration: Produces ATP without oxygen; lasts 30-40 seconds.
    • Aerobic Cellular Respiration: Most efficient; provides ATP for exercise lasting longer than 30 seconds.

Muscle Fatigue

  • Inability to contract; due to factors like depletion of ACh, creatine phosphate, oxygen, and the accumulation of lactic acid.

Muscle Unit and Twitch

  • Motor Unit: Motor neuron and all fibers it innervates; controls the strength of contraction.
  • Muscle Twitch: A brief contraction in response to a single action potential, consisting of latent, contraction, and relaxation periods.

Types of Muscle Contractions

  • Isotonic Contraction: Muscle produces tension to move a load.
  • Isometric Contraction: Muscle generates force without movement; important for posture and stabilization.

Cardiac and Smooth Muscle Differences

  • Cardiac Muscle: Striated, autorhythmic, and linked by intercalated discs; relies on calcium from extracellular fluid.
  • Smooth Muscle: Non-striated; uses calmodulin for calcium binding; slower contraction rates.

Muscle Aging

  • Begins at age 30 with a gradual replacement of muscle by fat, decreased reflexes, and strength.

Muscle Disorders

  1. Myasthenia Gravis: Autoimmune disorder affecting ACh receptors.
  2. Muscular Dystrophies: Genetic disorders causing muscle degeneration.

Rigor Mortis

  • Post-mortem muscle stiffness due to calcium influx and inability of myosin heads to detach from actin due to lack of ATP.