Muscular System I - BIO-110

Chapter 9 - Muscular System I

9.1 Muscles

• Types of Muscle Tissue
    - Skeletal Muscle:
      - Attached to bones of the skeleton
      - Voluntary (consciously controlled)
    - Cardiac Muscle:
      - Makes up most of the wall of the heart
      - Involuntary (non-consciously controlled)
      - Responsible for pumping action of the heart
    - Smooth Muscle:
      - Found in walls of internal organs, including those of the digestive tract
      - Involuntary (non-consciously controlled)

9.2 Structure of a Skeletal Muscle

• Skeletal Muscles Characteristics:
    - Over 600 skeletal muscles in the human body
    - Attached to bones, and skin of the face
    - Under conscious control (voluntary movement)
    - Organ composition:
      - Skeletal Muscle Tissue
      - Nervous Tissue
      - Blood
      - Connective Tissues

Connective Tissue Coverings over Muscles

• Connective Tissue Coverings:
    - Fascia: Thin covering of connective tissue around a muscle
    - Tendon: Cord-like mass of connective tissue that connects muscle to a bone
    - Aponeurosis: Sheet-like mass of connective tissue that connects a muscle to bone, skin, or another muscle
• Connective Tissue Surrounding Muscle:
    - Epimysium: Surrounds whole muscle; lies beneath fascia
    - Perimysium: Surrounds fascicles (bundles of skeletal muscle fibers) within a muscle
    - Endomysium: Surrounds individual muscle fibers (cells) within a fascicle

9.3 Skeletal Muscle Contraction

• Contraction Process:
    - Definition: Shortening of a muscle fiber as it pulls on its attachment points
    - Involves interaction of various chemical and cellular components
    - Results from the movement within the myofibrils where actin and myosin filaments slide past each other, leading to shortening of the sarcomeres

Neuromuscular Junction

• Definition:
    - A type of synapse, also known as myoneural junction
    - Site of interaction between an axon of a motor neuron and a skeletal muscle fiber
    - Skeletal muscle fibers contract only when stimulated by a motor neuron
• Components of Neuromuscular Junction:
    - Motor Neuron: Neuron controlling the skeletal muscle fiber
    - Motor End Plate: Specialized folded portion of skeletal muscle fiber where neurotransmitter binds
    - Synaptic Cleft: Space between neuron and muscle fiber, across which neurotransmitter travels
    - Synaptic Vesicles: Membrane-bound sacs containing neurotransmitters
    - Neurotransmitters: Chemicals released by motor neuron delivering signals to muscle fiber

Stimulus for Contraction

• Neurotransmitter Used: Acetylcholine (ACh)
• Process of Initiating Contraction:
    - Nerve impulse causes release of ACh from synaptic vesicles
    - ACh binds to ACh receptors on motor end plate of muscle fibers
    - Changes in membrane permeability to sodium and potassium ions generate a muscle impulse (action potential)
    - Impulse causes release of calcium ions from the sarcoplasmic reticulum (SR) leading to muscle contraction

Clinical Application 9.1: Muscular System Disorders

• Myasthenia Gravis (MG):
    - Autoimmune disorder where antibodies attack ACh receptors on muscle fibers
    - Often shows a decrease in ACh receptors leading to muscle weakness and fatigue
    - Treatments:
      - Drugs inhibiting acetylcholinesterase
      - Immunosuppressant drugs
      - Antibodies that inactivate harmful antibodies
      - Plasma exchange
• Muscular Dystrophy:
    - Deficiency or abnormality in the protein dystrophin, crucial for muscle cell structure
    - Leads to deterioration of muscle fibers
• Clostridium Botulinum:
    - Bacteria causing food poisoning; toxin affects muscle contraction
    - Has therapeutic uses such as Botox for wrinkle reduction and treatment of migraines

Excitation-Contraction Coupling

• Process Description:
    - Connection between muscle fiber stimulation and contraction
    - During Relaxation: Calcium ions are stored in the sarcoplasmic reticulum, and the troponin-tropomyosin complex covers binding sites on actin
    - Upon Stimulation:
      - Calcium ions are released; they bind to troponin altering its shape, which allows tropomyosin to move and expose binding sites on actin
      - Myosin heads bind to actin, forming cross-bridges

The Sliding Filament Model

• Overview:
    - Explains muscle contraction as sliding of thick and thin filaments across each other
    - Effects of Contraction:
      - H zones and I bands narrow
      - Z lines move closer together
      - Filaments themselves do not change length; the overlap between filaments increases
  

Cross-Bridge Cycling

• Events of Cross-Bridge Cycling:
    - Myosin head attaches to actin forming a cross-bridge
    - ADP and phosphate are released, pulling the thin filament toward the center of the sarcomere (power stroke)
    - New ATP molecule binds to myosin, breaking the linkage to actin
    - ATP is split, providing energy to recock the myosin head for the next cycle

Muscle Relaxation

• After Neural Stimulation Stops:
    - Acetylcholinesterase decomposes remaining ACh, which halts the muscle impulse
    - Calcium ions are pumped back into the sarcoplasmic reticulum
    - Troponin-tropomyosin complex covers actin binding sites again, preventing binding of myosin, leading to muscle relaxation

Major Events of Muscle Contraction and Relaxation

Energy Sources for Contraction

• ATP Reserves:
    - First source of energy for muscle contraction, stored in small amounts
• Creatine Phosphate:
    - Initial source of energy to regenerate ATP from ADP and phosphate, lasts for approximately 10 seconds
• Cellular Respiration:
    - Breakdown of glucose to produce ATP, necessary for prolonged contraction
    - Glucose stored as glycogen in muscle cells

Oxygen Supply and Cellular Respiration

• Phases:
    - Anaerobic Phase: Glycolysis occurring in the cytoplasm, producing little ATP
    - Aerobic Phase: Citric acid cycle and electron transport system in mitochondria, producing the most ATP
    - Myoglobin stores extra oxygen within muscles for aerobic respiration

Oxygen Debt

• Concept:
    - Occurs when aerobic respiration cannot meet the oxygen demands during strenuous activity, leading to lactic acid production
    - Oxygen debt refers to the amount of oxygen needed by liver cells to convert lactic acid to glucose and restore muscle ATP and creatine phosphate concentrations

Muscle Fatigue

• Definition: Inability to contract a muscle
• Common Causes:
    - Decreased blood flow
    - Ion imbalances across the sarcolemma
    - Loss of desire to continue exercise
    - Accumulation of lactic acid (controversial)
• Muscle Cramp: Sustained, involuntary muscle contraction due to electrolyte concentration changes

9.4 Muscular Responses

• Observation of Contraction:
    - Can be measured in a laboratory by observing a single skeletal muscle fiber response to stimulus

Threshold Stimulus

• Definition: Minimum strength of stimulation required to cause contraction
• When the strength of the stimulus reaches the threshold, an action potential is generated, leading to muscle contraction through a series of actions involving calcium ions

Recording of a Muscle Contraction

• Twitch Definition: Contractile response to a single impulse
• Twitch Periods:
    - Latent Period: Delay between stimulation and contraction
    - Period of Contraction: When the fiber pulls at attachments
    - Period of Relaxation: Decrease of pulling force

Length-Tension Relationship

• Description:
    - The optimal length of muscle fiber at rest allows for the greatest force development
    - Stretched or overly shortened muscle fibers develop less force due to inability of some myosin heads to reach binding sites on actin.

Summation and Tetanus

• Summation:
    - Process by which individual muscle fiber twitches combine under increased frequency of stimulation, leading to sustained contractions
    - Tetany Types:
      - Partial Tetany: Happen at higher frequencies with brief relaxation
      - Complete Tetany: Occurs at very high stimulation frequencies leading to sustained contraction with no relaxation

Recruitment of Motor Units

• Motor Unit Definition: Combination of a motor neuron and all muscle fibers it controls
• Recruitment increases the number of motor units activated to produce more force. The recruitment pattern varies based on required movement precision, with smaller units activated first and larger units activated under greater demands

Types of Contractions

• Isotonic: Muscle contracts and changes length
    - Concentric: Shortening contraction lifting an object
    - Eccentric: Lengthening contraction when force is insufficient to lift the object
• Isometric: Muscle tension develops, but length does not change

Types of Muscle Fibers

• Variations include contraction speed, ATP production method, fatigue resistance, and force generation
• Three Types of Skeletal Muscle Fibers:
    - Slow-Twitch Fibers (Type I, SO):
      - Always oxidative, resistant to fatigue, red due to myoglobin, good blood supply, smaller in diameter
      - Examples: Postural muscles
    - Fast-Twitch Fibers (Type IIb, FG):
      - Anaerobic, susceptible to fatigue, white due to less myoglobin, larger in diameter
      - Examples: Muscles of the hands and eyes
    - Fast-Twitch Fibers (Type IIa, FO):
      - Intermediate characteristics between SO and FG fibers, pink in color, somewhat resistant to fatigue

Clinical Application 9.2: Use and Disuse of Skeletal Muscles

• Hypertrophy: Enlargement from exercise
• Atrophy: Decrease in size from disuse
• Aerobic Exercise: Stimulates slow-twitch fibers leading to increased mitochondria
• Forceful Exercise: Stimulates fast-twitch fibers leading to increased actin and myosin filament production, enlarging muscle fibers.