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Chapter 9Muscular System

Three Types of Muscle Tissues

Skeletal Muscle• usually attached to bones, skin, deep fascia• voluntary• striated

Smooth Muscle• walls of most viscera, blood vessels, skin• involuntary• not striated

Cardiac Muscle• wall of heart• involuntary• striated

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• Excitability– Similar to nervous tissue– Stimulus initiates action potential (impulse)

• Contractility– Shorten and thicken– Actively do work when stimulus is received

Muscle Tissue Characteristics

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• Extensibility– Ability to be stretched– Paired muscle groups

• Elasticity– Ability to return to its original shape after

contracting or extending

Muscle Tissue Characteristics

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• Motion

• Maintenance of posture

• Heat production – 85% of heat generated from muscles

Muscle Tissue Functions

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Structure of a Skeletal Muscle

Skeletal Muscle• organ of the muscular system - skeletal muscle tissue - nervous tissue - blood - connective tissues• fascia• tendons• aponeuroses

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• Superficial Fascia– Subcutaneous layer

– Immediately deep to the skin

– Stores fat, insulates, protects, provides pathway for nerves and blood vessels

Connective Tissue Coverings

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• Deep Fascia– Lines body walls, extremities, and

holds muscles together

– Splits muscles into functional groups• Pectoralis major/pectoralis minor

Connective Tissue Coverings

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• Deep Fascia– Functions

• Allows free movement of muscles• Fills space• Carries nerve and vascular supply• Sometimes provides origin for muscles

Connective Tissue Coverings

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Connective Tissue Coverings

• Epimysium•Wraps the entire muscle bundle

• Perimysium•Covers muscle fiber bundles (fascicles)

• Endomysium•Covers individual fibers within the fascicles

•“-mysiums” may extend to become tendon

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• Epimysium• Covers entire muscle

• Muscles are bundles of fascicles

• Perimysium• Covers individual

fascicles

Connective Tissue Coverings

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• Fascicle• Many bundles of

fibers

• Fiber• Many myofibrils

• Myofibril• Many myofilaments

Connective Tissue Coverings

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• Myofilaments• Smallest part of a

muscle

• DO NOT extend entire length of muscle

• Fit into compartments called sarcomeres

Connective Tissue Coverings

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• Sarcomeres• Contraction of

muscles occurs here

• Lie end to end within myofibril

Connective Tissue Coverings

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Skeletal Muscle Fibers

• Sarcolemma•Muscle cell membrane

• Sarcoplasm•Muscle cell cytoplasm

• Sarcoplasmic reticulum•Muscle cell ER

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• 2 types of myofilaments• myosin-

• THICK contractile protein

• actin-• THIN contractile

protein

• The ability for actin and myosin to change shape allows for the 2 myofilaments to be pulled (slide) over each other

Sarcomere Structure

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• Myofilaments form patterns in the sarcomeres• “I” band consist of the disc (stationary) and actin filaments

• “A” band consist of myosin overlapping with the ends of the actin filaments• “H” zone —is within “A” band and contains only myosin

• Another “I” band

Sarcomere Structure

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• Structures between the disc make up the sarcomere

• Heads or hooks on the myosin bind with actin to pull the myosin filament over the actin

• Tropomyosin blocks the binding site on the actin when the muscle is relaxed

Sarcomere Structure

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Myofilaments

Thick Filaments • composed of myosin• cross-bridges

Thin Filaments• composed of actin• associated with troponin and tropomyosin

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• The myofibril shortens because the myofilaments in each sarcomere slide over each other

Sliding Filament Theory

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Neuromuscular Junction• also known as myoneural junction

• site where an axon and muscle fiber meet

• motor neuron

• motor end plate• synapse

• synaptic cleft

• synaptic vesicles

• neurotransmitters

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Motor Unit

• single motor neuron• all muscle fibers controlled by motor neuron

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Stimulus for Contraction

• acetylcholine (ACh)• nerve impulse causes release of ACh from synaptic vesicles• ACh binds to ACh receptors on motor end plate• generates a muscle impulse• muscle impulse eventually reaches sarcoplasmic reticulum

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Excitation Contraction Coupling

• muscle impulses cause sarcoplasmic reticulum to release calcium ions into cytosol• calcium binds to troponin to change its shape• position of tropomyosin is altered• binding sites on actin are exposed• actin and myosin molecules bind

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Sliding Filament Model of Muscle Contraction

• When sarcromeres shorten, thick and thin filaments slide past one another• H zones and I bands narrow• Z lines move closer together

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Cross-bridge Cycling

• myosin cross-bridge attaches to actin binding site

• myosin cross-bridge pulls thin filament

•ADP and phosphate released from myosin

• new ATP binds to myosin

• linkage between actin and myosin cross-bridge break

•ATP splits

•myosin cross-bridge goes back to original position

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Relaxation

• Acetylcholinesterase •rapidly decomposes Ach remaining in the synapse

• Muscle impulse stops

• Stimulus to sarcolemma and muscle fiber membrane ceases

• Calcium moves back into sarcoplasmic reticulum

• Myosin and actin binding prevented•Tropomyosin slides over binding sites

• Muscle fiber relaxes•Sarcomeres return to original length

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Major Events of Muscle Contraction and Relaxation

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Energy Sources for Contraction

• creatine phosphate – stores energy that quickly converts ADP to ATP

1) Creatine phosphate 2) Cellular respiration

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Oxygen Supply and Cellular Respiration

• Anaerobic Phase• glycolysis• Occurs in cytoplasm• Produces little ATP

• Aerobic Phase• Citric acid cycle• Electron transport chain• Occurs in mitochondria• Produces most ATP • Myoglobin

•Pigment that stores extra oxygen

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Oxygen Debt

• Oxygen not available• Glycolysis continues• Pyruvic acid converted to lactic acid• Liver converts lactic acid to glucose

Oxygen debt – amount of oxygen needed by liver cells to use the accumulated lactic acid to produce glucose

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Muscle Fatigue

• Inability to contract

• Commonly caused from • decreased blood flow• ion imbalances across the sarcolemma• accumulation of lactic acid

• Cramp – sustained, involuntary muscle contraction

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Heat Production

• By-product of cellular respiration

• Muscle cells are major source of body heat

• Blood transports heat throughout body

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Muscular Responses

Threshold Stimulus• minimal strength required to cause contraction

Recording a Muscle Contraction

• twitch•Single muscle fiber response to an impulse

• latent period•Delay between impulse and contraction

• period of contraction

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• Period of relaxation• Refractory period

• Time when neuron will not respond to stimulus

• All-or-none response• Each twitch generates

the same force

Muscular Responses

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Length-Tension Relationship

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Summation

• process by which individual twitches combine• produces sustained contractions• can lead to tetanic contractions

•Lacks partial relaxation

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Recruitment of Motor Units

• Recruitment• Increase in the number of motor units activated

• Whole muscle composed of many motor units

• More precise movements are produced with fewer muscle fibers within a motor unit

•Eye has fewer than 10 muscle fibers per motor unit

• As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

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Sustained Contractions

• Smaller motor units (smaller diameter axons) •Recruited first

• Larger motor units (larger diameter axons) •Recruited later

• Produce smooth movements•Spinal cord stimulates contractions in different sets of motor units at different times

• Muscle tone – continuous state of partial contraction•Maintains posture•Completely lost with loss of consciousness

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Types of Contractions

• Isotonic – muscle contracts and changes length

• Concentric – shortening contraction

• Eccentric – lengthening contraction

• Isometric – muscle contracts but does not change length

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Fast and Slow Twitch Muscle Fibers

Slow-twitch fibers (type I)• Always oxidative• Resistant to fatigue• Red fibers

•Contain most myoglobin• Good blood supply•Back muscles

Fast-twitch glycolytic fibers (type IIa)

• white fibers (less myoglobin)• poorer blood supply• susceptible to fatigue•Hand muscles, eye muscles

Fast-twitch fatigue-resistant fibers (type IIb)

• intermediate fibers• oxidative• intermediate amount of myoglobin• pink to red in color•resistant to fatigue•Limb muscles

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• Spasm•Sudden involuntary contraction of a large

group of muscles

• Tremor• Involuntary contraction of opposing muscle

groups

• Fasciculation• Involuntary, brief twitch of a muscle visible

under the skin•Occurs irregularly and doesn’t move the

affected muscle

Abnormal Contractions

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•Fibrillation•Similar to fasciculation except it is not visible

under the skin

•Tic•Twitch made involuntarily by muscles under

voluntary control

•Eyelids or facial muscles are examples

•Generally tics are of psychological origin

Abnormal Contractions

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Smooth Muscle Fibers

Compared to skeletal muscle fibers• shorter• single, centrally located nucleus• elongated with tapering ends• myofilaments randomly organized• lack striations• lack transverse tubules• sarcoplasmic reticula not well developed

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Types of Smooth Muscle

Visceral Smooth Muscle• single-unit smooth muscle• sheets of muscle fibers• fibers held together by gap junctions• exhibit rhythmicity• exhibit peristalsis• walls of most hollow organs

Multiunit Smooth Muscle• less organized• function as separate units • fibers function separately• irises of eye• walls of blood vessels

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Smooth Muscle Contraction

• Resembles skeletal muscle contraction• interaction between actin and myosin• both use calcium and ATP• both are triggered by membrane impulses

• Different from skeletal muscle contraction• smooth muscle lacks troponin• smooth muscle uses calmodulin • two neurotransmitters affect smooth muscle

• acetlycholine and norepinephrine• hormones affect smooth muscle• stretching can trigger smooth muscle contraction• smooth muscle slower to contract and relax• smooth muscle more resistant to fatigue• smooth muscle can change length without changing

tautness

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• Fibers are quadrangular• Single nucleus• More and larger mitochondria• Contain actin and myosin• Fibers branched and interconnected• 2 separate networks (atria and ventricles)

intercalated disc separate each fiber in a network• impulse stimulates the entire network,

contraction of the entire network

Cardiac Muscle

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• atria contract—blood to the ventricle

• ventricles contract—blood to the arteries and through the body

• auto-rhythmicity—nerve impulses only increase or decrease the rhythmic contractions

• remains contracted 10-15 times longer

• extra refractory period—allows heart to rest and prevents tetanus

Cardiac Muscle

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Characteristics of Muscle Tissue

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Skeletal Muscle Actions

• origin – immovable end• insertion – movable end

• prime mover (agonist) – primarily responsible for movement• synergists – assist prime mover• antagonist – resist prime mover’s action and cause movement in the opposite direction

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Body Movement

Four Basic Components of Lever1. rigid bar – bones2. fulcrum – point on which bar moves; joint3. object - moved against resistance; weight4. force – supplies energy for movement; muscles

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Levers and Movement

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Major Skeletal Muscles

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Major Skeletal Muscles

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Muscles of Facial Expression

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Muscles of Mastication

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Muscles of Facial Expression and Mastication

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Muscles That Move the Head and Vertebral Column

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Muscles That Move the Head and Vertebral Column

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Muscles That Move the Pectoral Girdle

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Muscles That Move the Pectoral Girdle

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Muscles That Move the Arm

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Muscles That Move the Arm

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Muscles That Move the Arm

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Muscles That Move the Forearm

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Muscles That Move the Forearm

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Muscles That Move the Forearm

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Cross Section of the Forearm

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Muscles That Move the Hand

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Muscles That Move the Hand

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Muscles of the Abdominal Wall

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Muscles of the Abdominal Wall

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Muscles of the Pelvic Outlet

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Muscles of Pelvic Outlet

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Muscles That Move the Thigh

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Muscles That Move the Thigh

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Muscles That Move the Thigh

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Muscles That Move the Leg

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Muscles That Move the Leg

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Muscles That Move the Leg

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Muscles That Move the Leg

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Muscles That Move the Foot

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Muscles That Move the Foot

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Muscles That Move the Foot

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Life-Span Changes

• myoglobin, ATP, and creatine phosphate decline

• by age 80, half of muscle mass has atrophied

• adipose cells and connective tissues replace muscle tissue

• exercise helps to maintain muscle mass and function

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Homeostatic Imbalances (Disorders)• May involve

– lack of nutrients, – disease,– injury,– atrophy, – neurological problem– accumulation of toxic products

Clinical Application

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• Fibrosis• Formation of fibrous connective tissue where it

normally does not exist

• Mature skeletal and cardiac muscle cannot undergo mitosis. Damaged fibers are replaced with fibrous connective tissue

• Most often the result of muscle injury or degeneration

Clinical Application

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• Fibromyalgia (algia=painful condition)• Common non-articular rheumatic disorders

• pain, tenderness• stiffness of muscles, tendons, and surrounding soft tissue

• Affects the fibrous connective tissue components of tendons and ligaments

• May be caused or aggravated by physical or mental stress, trauma, exposure to dampness or cold, poor sleep

• Relieved by heat, massage, and rest

Clinical Application

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• Dystrophies (disorders)– Muscle destroying diseases– degeneration of individual muscle fibers

which leads to a progressive atrophy of the skeletal muscle

– Skeletal muscles affected bilaterally– classified by mode of inheritance, age of

onset, and clinical characteristics

Clinical Application

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Clinical Application

Myasthenia Gravis

• autoimmune disorder• receptors for ACh on muscle cells are attacked• weak and easily fatigued muscles result• difficulty swallowing and chewing• ventilator needed if respiratory muscles are affected• treatments include

• drugs that boost ACh• removing thymus gland• immunosuppressant drugs• antibodies

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• Duchenne Muscular dystrophy (DMD)•Most common form

•Genetic

•Gene identified and DNA sequence worked out (could lead to replacement therapy to prevent muscle loss

Clinical Application