Muscle Types and Physiology
Three Muscle Types
Skeletal Muscle
Characteristics:
Striated appearance
Long cylindrical fibers
Voluntary control
Functions and Special Properties:
Body movement
Posture maintenance
Heat generation
Rapid contraction, but tires easily
Cardiac Muscle
Characteristics:
Striated appearance
Branched cells with intercalated discs
Involuntary control
Functions and Special Properties:
Pumps blood continuously
Auto rhythmic
Maintains steady contraction rate with some adjustability
Smooth Muscle
Characteristics:
Non-striated appearance
Spindle-shaped cells
Involuntary control
Functions and Special Properties:
Moves substances within hollow organs
Capable of slow, sustained contractions
Resistant to fatigue
Skeletal Muscle Tissue
Connective Tissue Components
Epimysium: Surrounds entire muscle.
Perimysium: Surrounds fascicles (bundles of muscle fibers).
Endomysium: Surrounds individual muscle fibers.
Function of Connective Tissue Layers:
Provides support and protection for muscle tissues.
Supplies pathways for blood vessels and nerves.
Microscopic Anatomy
Muscle Fiber: The basic unit of skeletal muscle.
Sarcolemma: The cell membrane surrounding a muscle fiber.
T Tubules: Extensions of the sarcolemma that penetrate into the muscle cell.
Sarcoplasm: Cytoplasm of a muscle fiber containing organelles and myofibrils.
Myofibrils: Filamentous structures containing the contractile elements of muscle.
Sarcoplasmic Reticulum: Smooth endoplasmic reticulum associated with myofibrils, regulating calcium ions.
Filaments
Thick Filaments: Composed primarily of myosin.
Thin Filaments: Comprised of actin, tropomyosin, and troponin.
Protein Functions
Myosin: Acts as a motor protein essential for muscle contraction.
Actin: Provides a binding site for myosin.
Tropomyosin and Troponin: Regulatory proteins that control the binding of myosin to actin.
Titin: A protein that stabilizes thick filaments and helps maintain structural integrity.
Dystrophin: Links muscle fibers to connective tissue, important for muscular integrity.
Contraction and Relaxation
Sliding Filament Mechanism
Myosin binds to actin.
The power stroke pulls thin filaments inward, shortening the muscle.
Myosin detaches from actin when ATP binds to it.
ATP hydrolysis resets myosin to its high-energy conformation.
Excitation-Contraction Coupling
Action potential travels along the sarcolemma and T tubules.
Calcium ions (Ca2+) are released from the sarcoplasmic reticulum.
Calcium ions bind to troponin, exposing binding sites on actin for myosin.
Length-Tension Relationship
Optimal overlap of thick and thin filaments produces the strongest contraction.
Neuromuscular Junction
Action potentials reach the axon terminal of a motor neuron.
Acetylcholine (ACh) is released into the synaptic cleft.
ACh binds to receptors on the sarcolemma, generating an action potential in the muscle fiber.
Muscle Metabolism
ATP Production Pathways
Creatine Phosphate:
Provides quick ATP for short-duration activities.
Anaerobic Glycolysis:
Operates without oxygen, produces lactic acid, suitable for short bursts of energy.
Aerobic Respiration:
Utilizes oxygen and produces the majority of ATP, supports extended activities.
Causes of Muscle Fatigue
Low levels of calcium ions (Ca2+).
Low levels of acetylcholine (ACh).
Depletion of glycogen stores.
Buildup of lactic acid.
Low oxygen availability.
Imbalance of ions.
Control of Muscle Tension
Motor Unit
Defined as one motor neuron and all muscle fibers it innervates.
Motor Unit Recruitment
Increasing the number of active motor units raises the overall muscle tension.
Twitch Phases
Latent Period: Time between stimulation and initiation of contraction.
Contraction Period: Period of active muscle shortening.
Relaxation Period: Period when the muscle returns to its resting state.
Frequency Effects
More frequent stimulation leads to greater muscle tension.
Terms:
Summation: Increased tension due to combined effects of multiple stimuli.
Unfused Tetanus: Partial relaxation occurs between stimuli.
Fused Tetanus: No relaxation between stimuli, maximal tension achieved.
Muscle Tone
Constant, small tension resulting from partial activation of muscle fibers, critical for posture and readiness.
Contractions
Isotonic Contraction: Muscle changes length, producing movement.
Isometric Contraction: Muscle remains the same length, tension increases without movement.
Skeletal Muscle Fiber Types
Slow Oxidative Fibers:
High endurance due to numerous mitochondria, primarily used for posture maintenance.
Fast Oxidative Glycolytic Fibers:
Moderate power and endurance, capable of using both aerobic and anaerobic pathways.
Fast Glycolytic Fibers:
High power output but fatigue quickly, mainly used for rapid, powerful movements.
Exercise Effects
Endurance Training: Increases the number of mitochondria and capillaries in muscle fibers.
Strength Training: Leads to an increase in muscle fiber size and force generation.
Different muscle fiber types adapt based on the type of training they undergo.
Cardiac Muscle Tissue
Composed of branched cells with intercalated discs and central nuclei.
Auto rhythmic and exhibits contractions that last longer than skeletal muscle contractions.
Primarily relies on aerobic respiration for energy production.
Smooth Muscle Tissue
Composed of non-striated, spindle-shaped cells.
Located within the walls of organs and blood vessels.
Exhibits slow, sustained contractions and is regulated by the autonomic nervous system and various hormones.
Aging and Disorders
Aging Effects
Associated with loss of muscle mass, decreased reflexes, and reduced muscle strength.
Common Disorders
Muscular Dystrophy: Genetic disorders causing progressive weakness and degeneration of skeletal muscles.
Myasthenia Gravis: Autoimmune disorder leading to weakness in voluntary muscles.
Muscle Cramps: Involuntary contractions causing pain and discomfort.
Fibrosis: Excessive fibrous connective tissue in muscle that can impair function.