Ch. 9

9.1 Muscles

  • Three types of muscle tissue:

    • Skeletal muscle:

      • Attached to skeleton

      • Voluntary (consciously controlled)

    • Cardiac muscle:

      • Majority of heart wall

      • Involuntary (non-consciously controlled)

      • Responsible for heart's pumping action

    • Smooth muscle:

      • Found in walls of internal organs (e.g., digestive tract)

      • Involuntary (non-consciously controlled)

9.2 Structure of a Skeletal Muscle

  • Skeletal muscles:

    • Over 600 in the body

    • Attached to bones and facial skin

    • Under conscious control (voluntary)

    • Composed of:

      • Skeletal muscle tissue

      • Nervous tissue

      • Blood

      • Connective tissues

9.3 Connective Tissue Coverings over Muscles

  • Connective tissue coverings over skeletal muscles include:

    • Fascia:

      • Thin covering of connective tissue around a muscle

    • Tendon:

      • Cord-like connective tissue that connects muscle to bone

    • Aponeurosis:

      • Sheet-like connective tissue connecting muscles to bones, skin, or other muscles

  • Specific connective tissues surrounding muscles:

    • Epimysium:

      • Surrounds the whole muscle; beneath fascia

    • Perimysium:

      • Surrounds fascicles within a muscle

    • Endomysium:

      • Surrounds individual muscle fibers within a fascicle

9.4 Skeletal Muscle Fibers

  • Skeletal muscle fiber (cell):

    • Multinucleated

    • Components:

      • Sarcolemma: Cell membrane

      • Sarcoplasm: Cytoplasm

      • Myofibrils: Long, parallel structures consisting of actin and myosin filaments

    • Sarcomeres: Units connecting myofibrils

    • Sarcoplasmic reticulum (SR): Stores calcium

    • Transverse (T) tubule: Relays electrical impulses to SR

    • Triad:

      • One T tubule and two SR cisternae

9.5 Skeletal Muscle Fiber Structure

  • Myofibrils: Composed of sarcomeres connected end-to-end

  • Striation pattern: Arranged by myofilaments in myofibrils

  • Sarcomere structures:

    • I band: Light band with thin actin filaments

    • A band: Dark band with thick myosin filaments; some overlap with actin

    • H zone: Center of A band with thick myosin filaments

    • Z line: Sarcomere boundary; anchors filaments

    • M line: Center of sarcomere and A band; anchors thick filaments

9.6 Skeletal Muscle Contraction

  • Contraction of a skeletal muscle fiber:

    • Shortening of muscle fiber pulling on attachment points

    • Requires chemical and cellular interaction

    • Involves actin and myosin filaments sliding past each other, shortening sarcomeres

9.7 Neuromuscular Junction

  • Neuromuscular Junction (NMJ):

    • Synapse between a motor neuron and skeletal muscle fiber

  • Components of NMJ:

    • Motor neuron: Controls skeletal muscle fiber

    • Motor end plate: Specialized portion of muscle fiber for neurotransmitter binding

    • Synaptic cleft: Space between neuron and muscle fiber

    • Synaptic vesicles: Contain neurotransmitters

    • Neurotransmitters: Chemicals released by motor neuron

9.8 Stimulus for Contraction

  • Acetylcholine (ACh): Neurotransmitter stimulating skeletal muscle fibers

  • Action process:

    • Nerve impulse releases ACh

    • ACh binds to receptors causing sodium and potassium ion influx, generating muscle impulses

    • Leads to calcium release from SR, causing contraction

9.9 Clinical Application 9.1: Muscular System Disorders

  • Myasthenia Gravis (MG):

    • Autoimmune disorder affecting ACh receptors, leading to muscle weakness

  • Muscular Dystrophy:

    • Deficiency in dystrophin causing muscle degeneration

  • Clostridium Botulinum:

    • Anaerobic bacteria producing a toxin causing muscle paralysis

9.10 Excitation-Contraction Coupling

  • Excitation-Contraction Coupling: Connection between muscle stimulation and contraction

  • Relaxation phase: Calcium ions stored in SR, troponin-tropomyosin complexes cover binding sites

  • Upon stimulation, calcium ions bind to troponin, exposing binding sites for myosin

9.11 The Sliding Filament Model

  • Sliding Filament Model of Muscle Contraction:

    • Sarcomere shortening occurs as thick and thin filaments slide past one another

    • H zones and I bands narrow while Z lines move closer

    • Filament lengths remain constant, increasing overlap

9.12 Cross-Bridge Cycling

  • Order of events:

    • Myosin head attaches to actin, pulling thin filament towards center of sarcomere

    • ADP and phosphate released from myosin, new ATP binds

    • Linkage between actin and myosin breaks, ATP splits, myosin heads "cock" for next cycle

9.13 Relaxation

  • Relaxation phase:

    • Acetylcholinesterase decomposes ACh, stopping muscle impulse

    • Calcium pumps move calcium back into SR, covering actin binding sites

    • Muscle fiber relaxes

9.14 Major Events of Muscle Contraction and Relaxation

  • Contraction events:

    1. Action potential conducted down motor neuron axon

    2. ACh released, binds to receptors

    3. Muscle impulse generated

    4. Calcium released from SR

    5. Cross-bridges form, muscle fiber shortens

  • Relaxation events:

    1. ACh decomposed

    2. Calcium transported back into SR

    3. Myosin binding sites covered, muscle fiber remains relaxed

9.15 Energy Sources for Contraction

  • Energy sources for muscle contraction:

    • ATP reserves: First source but limited to 10 seconds of activity

    • Creatine phosphate: Regenerates ATP from ADP

    • Cellular respiration: Provides ATP for prolonged contractions by breaking down glucose

9.16 Oxygen Supply and Cellular Respiration

  • Cellular Respiration Phases:

    • Anaerobic Phase: Glycolysis in cytoplasm, produces limited ATP

    • Aerobic Phase: Citric acid cycle in mitochondria, produces most ATP

    • Myoglobin: Stores oxygen in muscles

9.17 Oxygen Debt

  • Oxygen Debt: Amount of oxygen needed to convert lactic acid back to glucose post-exercise

  • Importance of recovery: Allows restoration of ATP and creatine phosphate levels in muscle

9.18 Muscle Fatigue

  • Muscle Fatigue Causes:

    • Decreased blood flow

    • Ion imbalances

    • Accumulation of lactic acid, variable impact

9.19 Heat Production

  • Heat as a by-product of cellular respiration:

    • Major source of body heat

    • More than half of energy from respiration becomes heat

9.20 Muscular Responses

  • Twitch: Contractile response of a single muscle fiber to an impulse

    • Three phases:

      • Latent period

      • Contraction period

      • Relaxation period

9.21 Length-Tension Relationship

  • Length of muscle fiber before contraction:

    • Optimal length allows greatest force

    • Overly stretched or overly shortened fibers generate less force

9.22 Summation and Tetanus

  • Summation: Increased force from combining muscle twitches when stimulated frequently

  • Tetanus: Continuous contraction with no relaxation between twitches, leading to maximum tension

9.23 Recruitment of Motor Units

  • Motor Unit Defined:

    • Motor neuron plus all associated muscle fibers

  • Recruitment process: More motor units activated for increased force

9.24 Types of Contractions

  • Isotonic contractions:

    • Concentric: Muscle shortens while contracting

    • Eccentric: Muscle lengthens while contracting

  • Isometric contractions: Muscle contracts without changing length

9.25 Types of Muscle Fibers

  • Three skeletal muscle fiber types:

    • Slow-Twitch (Type I):

      • Oxidative, resistant to fatigue

    • Fast-Twitch (Type IIb):

      • Glycolytic, more fatigable

    • Intermediate (Type IIa):

      • Combination of characteristics

9.26 Clinical Applications: Use and Disuse

  • Hypertrophy vs. Atrophy:

    • Hypertrophy from exercised muscles

    • Atrophy from disuse

9.27 Smooth Muscle Comparison

  • Smooth muscle fibers:

    • Shorter, single nucleus, lack striations

    • Myofilaments organized differently, responding to neural and hormonal stimuli

9.28 Cardiac Muscle Characteristics

  • Cardiac Muscle:

    • Striated, connected by intercalated discs, contracts rhythmically and self-exciting

9.29 Skeletal Muscle Actions

  • Muscle actions depend on:

    • Type of joint involved and muscle attachment relative to the joint

9.30 Body Movement as Leverage

  • Levers in body movement:

    • Bones and muscles act as levers with rigid bars, pivots, and forces

9.31 Origin and Insertion

  • Definitions:

    • Origin: Less movable end of the muscle

    • Insertion: More movable end

9.32 Interaction of Skeletal Muscles

  • Muscle roles:

    • Agonist: Causes action

    • Antagonist: Opposes action

    • Synergists: Assist agonist

9.33 Major Skeletal Muscles

  • Over 600 skeletal muscles: Controlled by nerves, named based on various characteristics