Skeletal Muscle Lecture Notes

Chapter 09 Lecture Outline: Skeletal Muscle

9.1 Muscles

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

9.2 Structure of a Skeletal Muscle

• Characteristics of Skeletal Muscles:
  • Over 600 skeletal muscles in the body
  • Attached to bones and skin of the face
  • Under conscious control (voluntary)
  • Organs of the muscular system
• Composition of Skeletal Muscles:
  • Skeletal muscle tissue
  • Nervous tissue
  • Blood
  • Connective tissues

Connective Tissue of Muscles

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

9.3 Compartments and Fascia

• Compartment:
  • Space containing a group of muscles, blood vessels, and nerves enclosed by fascia
• Compartment Syndrome:
  • Fluid accumulation within a compartment, leading to an increase in pressure in the compartment
  • Results in deficiency of oxygen and nutrients causing severe pain
• Fascia Network:
  • Deep fascia surrounds muscles and connects to the subcutaneous fascia under the skin; connects to subserous fascia of serous membranes

9.4 Skeletal Muscle Fibers

• Structure of Skeletal Muscle Fiber:
  • Multinucleated
  • Sarcolemma: Cell membrane of muscle fiber
  • Sarcoplasm: Cytoplasm of muscle fiber
  • Myofibrils: Long, parallel structures running down fiber, consisting of:
  • Thin actin filaments
  • Thick myosin filaments
  • Sarcomeres: Units that connect end-to-end to make up myofibrils
  • Sarcoplasmic Reticulum (SR): Endoplasmic reticulum of muscle; stores calcium
  • Transverse (“T”) Tubule: Relays electrical impulses to the SR
  • Triad: Unit consisting of one T tubule and two SR cisternae

Sarcomere Components

• Striation Pattern:
  • Made by arrangement of myofilaments in myofibrils
• Structure of Sarcomere:
  • I Band: Light band; composed of thin actin filaments
  • A Band: Dark band; composed of thick myosin filaments with portions overlapping with thin actin filaments
  • H Zone: Center of A band; contains thick myosin filaments
  • Z Line (Z Disc): Sarcomere boundary; in the center of the I band, anchors filaments in place
  • M Line: Center of the sarcomere and A band; anchors thick filaments

Thin and Thick Filaments

• Thick Filaments:
  • Composed of myosin protein, with heads forming cross-bridges with thin filaments
• Thin Filaments:
  • Composed of actin protein, associated with troponin and tropomyosin, which prevent cross-bridge formation when muscle is not contracting

9.5 Skeletal Muscle Contraction

• Contraction Process:
  • Requires interaction from several chemical and cellular components
  • Results from movement within the myofibrils, where actin and myosin filaments slide past each other, shortening the sarcomeres
  • Muscle fiber shortens and pulls on attachment points

Neuromuscular Junction (NMJ)

• Definition:
  • A type of synapse where an axon of a motor neuron and skeletal muscle fiber interact
  • Skeletal muscle fibers contract only when stimulated by a motor neuron
• Parts of NMJ:
  • Motor Neuron: Neuron that controls skeletal muscle fiber
  • Motor End Plate: Specialized folded portion where fiber binds to neurotransmitter
  • 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 to deliver a message to muscle fiber

Stimulus for Contraction

• Acetylcholine (ACh):
  • Neurotransmitter involved in muscle contraction
• Mechanism:
  • Nerve impulse causes the release of ACh from synaptic vesicles
  • ACh binds to receptors on motor end plate, changing membrane permeability to sodium and potassium ions, generating a muscle impulse (action potential)
  • Impulse causes calcium ions to be released from SR, leading to muscle contraction

9.6 Clinical Application: Muscular System Disorders

• Myasthenia Gravis (MG):
  • Autoimmune disorder where antibodies attack ACh receptors at NMJs
  • Symptoms include widespread muscle weakness and fatigue
  • Treatment options:
  • Drugs that inhibit acetylcholinesterase
  • Immunosuppressant drugs
  • Antibodies that inactivate harmful antibodies
  • Plasma exchange
• Muscular Dystrophy:
  • Caused by deficiency or mutation in the muscle protein dystrophin
  • Dystrophin maintains muscle cell integrity during contraction
  • Absence of dystrophin leads to cell degeneration
• Clostridium Botulinum:
  • Anaerobic bacterium producing a toxin causing food poisoning
  • Symptoms include digestive and muscular disturbances
  • Treatment involves antitoxin and uses of Botox to treat wrinkles and migraines

9.7 Excitation-Contraction Coupling

• Definition:
  • Connection between muscle fiber stimulation and muscle contraction
• During Muscle Relaxation:
  • Calcium ions are stored in the SR
  • Troponin-tropomyosin complexes cover binding sites on actin filaments
• Upon Muscle Stimulation:
  • Muscle impulses cause SR to release calcium ions
  • Calcium ions bind to troponin, changing its shape and exposing binding sites
  • Myosin heads bind to actin, forming cross-bridges

9.8 The Sliding Filament Model

• Definition:
  • Describes muscle contraction by the sliding of thick and thin filaments past each other within sarcomeres
• Key Points:
  • Sarcomeres shorten as H zones and I bands narrow; Z lines move closer together
  • Thin and thick filaments do not change length; overlap increases

Cross-Bridge Cycling

• Order of Events:
  • Myosin head attaches to actin binding site, forming cross-bridge
  • Myosin cross-bridge pulls thin filament toward the center of the sarcomere
  • ADP and phosphate are released from myosin
  • New ATP binds to myosin, breaking linkage between actin and myosin
  • ATP is split, reverting myosin head to original position for another binding

9.9 Relaxation

• Process of Relaxation:
  • ACh is decomposed by acetylcholinesterase, stopping muscle impulse
  • Stimulus to sarcolemma and muscle fiber ceases
  • Calcium is pumped back into the SR
  • Troponin-tropomyosin complex covers binding sites on actin, preventing further binding and allowing muscle fiber to relax

9.10 Energy Sources for Contraction

• ATP Reserves:
  • First source of energy for muscle contraction, but muscle cells store only a small amount
• Creatine Phosphate:
  • Initial source of energy to regenerate ATP from ADP and phosphate; fuels ~10 seconds of intense activity
• Cellular Respiration:
  • Breaks down glucose to produce ATP for longer muscle contractions
  • Glucose is stored as glycogen in muscle cells

Phases of Cellular Respiration

• Anaerobic Phase:
  • Glycolysis occurring in the cytoplasm, producing little ATP
• Aerobic Phase:
  • Citric acid cycle and electron transport system occur in mitochondria, producing the most ATP
• Myoglobin:
  • Stores extra oxygen in muscles

Oxygen Debt

• Anaerobic Threshold:
  • Shift from aerobic to anaerobic metabolism during strenuous activity causing lactic acid production
• Oxygen Debt:
  • Amount of oxygen needed post-exercise to convert lactic acid to glucose and restore ATP and creatine phosphate levels

9.11 Muscle Fatigue

• Definition:
  • Inability to contract muscle
• Common Causes of Muscle Fatigue:
  • Decreased blood flow
  • Ion imbalances across the sarcolemma
  • Loss of desire to continue exercising
  • Accumulation of lactic acid (controversial)
• Muscle Cramp:
  • Sustained, involuntary muscle contraction, possibly linked to electrolyte imbalances

9.12 Heat Production

• By-Product of Cellular Respiration:
  • Muscles are a major source of body heat; more than half of energy released becomes heat, less than half is used for ATP production
• Blood Transports Heat:
  • Heat diffuses from muscles into blood and is distributed throughout the body

9.13 Muscular Responses

• Observation of Muscle Contraction:
  • Can be measured using electrical stimulators to observe changes in length of muscle fibers

Threshold Stimulus

• Definition:
  • Minimum strength of stimulation to cause muscle contraction; generates action potential in muscle fibers
  • Action potentials from motor neurons release sufficient ACh, reaching threshold for muscle impulse

Recording of a Muscle Contraction

• Twitch:
  • Contractile response of a single muscle fiber to a single impulse
  • Periods associated with a twitch:
  • Latent Period: Delay between stimulation and start of contraction
  • Period of Contraction: Fiber pulls at attachments
  • Period of Relaxation: Pulling force decreases

Length-Tension Relationship

• Influence of Initial Length on Force Development:
  • Optimum resting length allows the greatest force development; stretched or shortened fibers develop less force

Summation and Tetanus

• Summation:
  • Process where the force of individual muscle twitches combine with increased stimulation frequency, leading to sustained contractions
• Tetanus:
  • Partial Tetany: Higher stimulation frequencies leading to brief relaxation
  • Complete Tetany: Very high frequencies producing sustained contractions without relaxation

9.14 Recruitment of Motor Units

• Motor Unit:
  • A motor neuron plus all muscle fibers it controls; multiple motor units comprise a whole muscle
• Recruitment:
  • Increase in the number of motor units activated to produce more force; activated in a sequence of increasing intensity

Types of Contractions

• Isotonic Contractions:
  • Muscle changes length while contracting; includes:
  • Concentric: Shortening contraction, e.g., lifting an object
  • Eccentric: Lengthening contraction, e.g., when force is insufficient to lift an object
• Isometric Contractions:
  • Muscle contracts without changing length; tension develops, but attachments do not move

Fast- and Slow-Twitch Muscle Fibers

• Muscle Fiber Types:
  • Slow-Twitch Fibers (Type I):
  • Oxidative, resistant to fatigue, containing abundant myoglobin and mitochondria
  • Fast-Twitch Fatigue-Resistant Fibers (Type IIa):
  • Intermediate oxidative capacity, resistant to fatigue
  • Fast-Twitch Glycolytic Fibers (Type IIb):
  • Anaerobic, more susceptible to fatigue, rapid ATPase activity
• Adaptation to Exercise:
  • Hypertrophy: Enlargement of exercised muscles
  • Atrophy: Decrease in muscle size and strength due to disuse

9.15 Smooth Muscle

• Characteristics Compared to Skeletal Muscle:
  • Shorter fibers with single, centrally located nucleus
  • Myofilaments are randomly organized, lack striations and T-tubules, and have a poorly developed SR

Types of Smooth Muscle

• Multi-Unit Smooth Muscle:
  • Cells function independently, e.g., in irises and blood vessels
• Visceral Smooth Muscle:
  • Cells function as a unit, exhibit rhythmic activity, and conduct peristalsis in hollow organs

Smooth Muscle Contraction

• Similarities to Skeletal Muscle:
  • Interaction between actin and myosin with calcium and ATP involvement
• Differences:
  • Lacks troponin, uses calmodulin; affected by neurotransmitters (ACh, NE) and hormones; slower, more fatigue-resistant; can change length without tautness

9.16 Cardiac Muscle

• Location:
  • Found only in the heart, striated muscle cells joined by intercalated discs
• Characteristics:
  • Single nucleus per fiber, syncytial contraction, self-exciting, and rhythmic with a longer refractory period compared to skeletal muscle

9.17 Comparison of Muscle Tissues

9.18 Skeletal Muscle Actions

• Factors Influencing Muscle Action:
  • Type of joint associated with muscle
  • Attachment of muscle on both sides of joint

Body Movement Mechanics

• Levers System:
  • Bones and muscles act as levers comprising four components:
  • Rigid bar (bones)
  • Fulcrum (joint)
  • Resistance (weight)
  • Force (muscles)
• Example:
  • Bending and straightening at the elbow acts as a third-class lever

Origin and Insertion

• Definitions:
  • Origin: Less movable end of a muscle
  • Insertion: More movable end of a muscle
  • When a muscle contracts, the insertion is pulled toward the origin, and a muscle may have multiple origins and insertions

Interaction of Skeletal Muscles

• Roles in Muscle Function:
  • Agonist: Muscle that causes an action
  • Prime Mover: Agonist primarily responsible for movement
  • Synergists: Muscles that assist agonist/prime mover
  • Antagonist: Muscles whose contraction causes movement in the opposite direction of the prime mover

Major Skeletal Muscles

• Anatomical Overview:
  • Figures illustrating major skeletal muscles, including anterior and posterior views