Muscle Physiology

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What are the three types of muscle tissue is the muscular system?

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What are the three types of muscle tissue is the muscular system?

Skeletal, Cardiac, and Smooth

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What is skeletal muscle?

  • Attached to bones of skeleton

  • Voluntary (consciously controlled)

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What is cardiac muscle?

  • Makes up most of the wall of the heart

  • Involuntary (non-consciously controlled)

  • Responsible for pumping action of the heart

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What is smooth muscle?

  • Found in walls of internal organs, such as those of digestive tract.

  • Involuntary (non-consciously controlled)

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Skeletal muscles

  • Over 600 skeletal muscles in the body

  • Attached to bones, and skin of face

  • Under conscious control (voluntary)

  • Are organs of the muscular system

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What is skeletal muscle composed of?

  • Skeletal muscle tissue

  • Nervous tissue

  • Blood

  • Connective tissue

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What is Fascia?

Thin covering of connective tissue around a muscle.

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What are tendons?

Cord-like mass of connective tissues that connect muscles to a bone.

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What is Aponeurosis?

Sheet-like mass of connective tissue that connects a muscle to bone, skin, or another muscle.

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What does the Epimysium surround?

Surrounds whole muscles; lies beneath fascia.

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What does the Perimysium surround?

Surrounds fascicles within a muscle.

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What does the Endomysium surround?

Surrounds muscle fibers (cells) within a fascicle.

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What is a compartment?

Space containing groups of muscles, blood vessels, and nerves enclosed by fascia.

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What is Compartment Syndrome?

  • It’s fluid accumulation with a compartment which results in the increase of pressure in the compartments.

  • It leads to deficiency of oxygen and nurtients.

  • Also causes severe pain.

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  • Fascia of each muscle is part of network of fasciae

  • Portion surrounding muscles is deep fascia, which connects to subcutaneous fascia under skin

  • Network also connects to subserous fascia of serous membranes

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Skeletal muscle fiber (cell) are…


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What is sarcolemma?

Cell membrane of muscle fiber

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What is the sarcoplasm?

Cytoplasm of muscle fiber

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Myofribils consist what?

Thin actin filaments and thick myosin filaments.

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What is sarcomeres?

Units that connect end-to-end, to make up myofibrils. (Functional unit of the muscular system)

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What is the Sarcoplasmic reticulum (SR) and what does it store?

It is the endoplasmic reticulum of muscles and it stores calcium.

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What is the transverse (“T”) tubule responsible for?

It relays electrical impulses to the SR.

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What does the Triad consist of?

1 T tubule and 2 SR cisternae

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What does myofibrils consist of that connect from end-to-end?


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How are Striation patterns made?

They’re made by the arrangement of myofilaments in the myofibrils.

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The A sarcomere consists of what structures?

I band- Light band; composed of thin actin filaments

A band- Dark band; composed of thick myosin filaments with portions overlapped with thin actin filaments.

H zone- Center of A band; composed of thick myosin filaments.

Z line (Z disc)- Sarcomere boundary; in center of I band which anchors filaments in place.

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

<p>I band- Light band; composed of thin actin filaments</p><p>A band- Dark band; composed of thick myosin filaments with portions overlapped with thin actin filaments.</p><p>H zone- Center of A band; composed of thick myosin filaments.</p><p>Z line (Z disc)- Sarcomere boundary; in center of I band which anchors filaments in place.</p><p>M line- Center of sarcomere and A band; anchors thick filaments</p>
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Thick filaments

  • Composed of myosin proteins

  • Heads form cross-bridges with thin filaments

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Thin filaments

  • Composed of actin protein

  • Associated with troponin and tropomyosin, which prevent cross-bridge formation when muscle is not contracting.

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Heavy chains and light chains form what?

Heavy chains intertwine to form myosin tail

Light chains form myosin globurlar head

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Actin filaments made up of what?

G actin (globular) subunits

  • G actin subunits bears active sites for myosin head attachment during contraction.

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G actin subunits link together to form what?

Long fibrous F actin (filamentous)

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____ F actin strands twist together to form a thin filament.


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What does tropomyosin and troponin do?

They regulate proteins bound to actin.

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Contraction of a skeletal muscle fiber

  • Requires interaction from several chemical and cellular components.

  • Results from a movement within the myofibrils, in which the actin and myosin filaments slide past each other, shortening the sarcomeres

  • Muscle fiber shortens and pulls on attachment points.

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Neuronmuscular Junction (NMJ)

  • A type of Synapse

  • Also called a myoneural junction

  • Site where an axon of motor neuron and skeletal muscle fiber interact

  • Skeletal muscle fibers contract only when stimulated by a motor neuron

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Parts of a Neuromuscular Junction consist of..

  • Motor neurons

  • Motor end plates

  • Synaptic Cleft

  • Synaptic vesicles

  • Neurotransmitters

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Motor neuron

Neuron that controls skeletal muscle fibers

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Motor end plate

Specialized folded portion of skeletal muscle fiber, where fiber binds to neurotransmitter.

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Synaptic Cleft

Space between neuron and muscle fiber, across which neurotransmitter travels.

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Synaptic vesicles

Membrane-bound sacs containing neurotransmitters

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Chemical released by motor neuron to deliver message to muscle fiber

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Steps for contraction at neuromuscular junction

  • Action potential arrives at the end of a motor neuron

  • Action potential sends an electrical signal which opens a voltage channel

  • Calcium rushes into end of motor neuron and stimulates release of ACh from synaptic vesicles

  • ACh diffuses arcross synaptic cleft and bind to receptors

  • AcH receptors open a channel where Na ions enter and K ion exits.

  • A change occurs in the membrane potential called the end plate potential

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Generations of an Action Potential Across the Sarcolemma

  • Resting sarcolemma is polarized, meaning a voltage exists across membrane

  • Action potential is caused by changes in electrical charges

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Three steps of action potential across the sarcolemma.

  • End plate potential- generated at the neuromuscular junction

  • Depolarization- generating and propagating an action potential

  • Repolarization- restoring the sarcolemma to its initial polarized state (negative inside, positive outside)

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Refractory period

Muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete.

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What is excitation-contractions coupling?

Connnection between muscle fiber stimulation and muscle contraction.

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During muscle relaxation:

  • Calcium ions are stored in sarcoplasmic reticulum

  • Troponin-tropomyosin complexes cover binding sites on actin filaments

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Upon muscle stimulation:

  • Muscle impulses travel down the T-tubules causes SR to release

  • Calcium ions binds to troponin to change its shape

  • Each tropomyosin is held in place by a troponin molecule; change in shape of troponin alters the position of tropomyosin

  • Binding sites on actin are now exposed

  • Myosin heads bind to actin, forming cross-bridges

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Cross-Bridge Cycling in order

  1. Myosin head attaches to actin binding site, forming cross-bridge

  2. Myosin cross-bridge pulls thin filaments towards center of sarcomere

  3. ADP and Phosphate are released from myosin

  4. New ATP binds to myosin

  5. Linkage between actin and myosin cross-bridge break

  6. ATP splits

  7. Myosin cross-bridge goes back to original position, ready to bind to another binding site on actin.

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  • Acetylcholinesterase rapidly decomposes ACh remaining the synapse

  • Muscle impulse stops when ACh is decomposed

  • Stimules to sarcolemma and muscle fiber membrane ceases.

  • Calcium pumps calcium ions back into sarcoplasmic reticulum

  • Troponin-tropomyosin complex again covers binding sites on actin.

  • Myosin and actin binding are now prevented.

  • Muscle fiber relaxes.

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Sliding Filament Model of Muscle Contraction:

  • When sarcomeres shorten, thick and thin filaments slide past each other

  • H zones and I bands narrow (some texts state the H zones will disappear and I bands shorten)

  • Z lines move closer together (pulled toward M line)

  • A bands move closer to each other

  • Thin and thick filaments do not change length

  • Overlap between filaments increases

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Myasthenia Gravis (MG)

  • An autoimmune disorder, which antibodies attack acetylcholine receptors on skeletal muscle fibers (motor end plates) in neuromuscular junctions.

  • Person may have only one-third normal number of ACh receptors

  • May lead to widespread muscle weakness and muscle fatigue

  • Treatments for it include drugs that inhibit acetylcholinesterase and immunosuppressant drugs

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Muscular Dystrophy

  • Deficiency or abnormality/mutation in scarce muscle protein called dystrophin.

  • Dystrophin binds to internal side of muscle cell membrane, and holds them together during contraction.

  • In absence of normal dystrophin, cells lose normal structure and die.

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Clostridium Botulinum

  • Anaerobic bacteria that produces a toxin that caused food poisoning.

  • Grows in food that is not properly preserved

  • Causes digestive, muscular, and respiratory symptoms.

  • Treated with an antitoxin

  • Used in small doses as Botox to smooth wrinkles, treat mirgraines. Blocks the release of Ach causing paralysis.

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ATP reserves

  • First source of energy for muscle contraction

  • Muscle cells store only a small amount-must be able to regenerate ATP

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Creatine Phosphate

  • Initial source of energy to regenerate ATP from ADP and P.

  • Stores energy in phosphate bond

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Cellular respiration

  • Must be used to fuel longer periods of muscle contraction

  • Breaks down glucose to produce ATP

  • Glucose stored as glycogen in muscle cells

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Types of phases of cellular respiration:

  • Anaerobic Phase

  • Aerobic Phase

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Anaerobic (does NOT require O2) Phase

  • Glycolysis

  • Occurs in cytoplasm/cytosol

  • Produces little ATP- 2 ATP

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Aerobic Phase

  • Citric acid cycle and electron transport chain system

  • Occurs in the mitochondria

  • Produces the most ATP (max is 30 in a muscle cell)

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What does myoglobin do?

It stores extra oxygen in muscles (increases the amount of oxygen available in the muscle cells to support aerobic respiration.

  • Important because blood flow may decrease when contracting muscle fibers compress blood vessels.

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Oxygen Debt

The amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations.

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Anaerobic (Lactic Acid) Threshold

  • Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O2

  • Lactic acid is produced which goes to the liver to be converted to glucose. (Requires ATP)

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Muscle Fatigue

Inability to contract muscles (For example, during prolonged exercise)

  • Common causes of muscle fatigue

    • Decreased blood flow

    • Ion imbalances across the sarcolemma

    • Loss of desire to continue exercise (psychological)

    • Accumulation of lactic acid (controversial)

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Muscle Cramp

  • Not fully understood

  • Sustained, involuntary muscle contraction

  • May be caused by changes in electrolytes concentration in extracellular fluids in the area

    • Sodium

    • Chlorine

    • Magnesium

    • Potassium

    • Calcium

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Heat production

  • Heat is a byproduct of cellular respiration in active cells. In other words, active cells generate heat.

    • Muscle contraction will release heat

  • Muscle cells are a major source of body heat. Muscle tissue is a large portion of body mass.

  • More than half the energy release in cellular respiration becomes heat; less than half is transferred to ATP.

  • Blood transports heat throughout body core.

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Threshold Stimulus

Minimum strength of stimulation of a muscle fiber required to cause contractions

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What is a twitch?

  • A contractile response of a single muscle fiber to a single impulse

  • Twitches vary in strengths (contributing factors):

    • Development of fatigue

    • Length to which fiber is stretched before stimulation

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Period of times associated with a twitch

  • Latent period- Delay between stimulation and start of contraction (approximately 2 milliseconds)

  • Period of contraction- Fiber pulls at attachments

  • Period of relaxation- Pulling forces decrease

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What does a myogram do?

It records events of a muscle twitch.

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Length-Tension Relationship

  • The length of muscle fiber before stimulation determines the amount of force it can develop.

  • Optimum starting length is resting length of the fiber which allows the greatest force to develop.

  • Stretched muscle fiber develop less force, since some myosin heads cannot reach binding sites on actin.

  • Shortened muscle fiber also develop less force, since compressed sarcomeres cannot shorten further.

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Whole muscle contraction

  • The actions we perform usually require the contribution of multiple muscle fibers simultaneously.

  • We can electrically stimulate the whole muscle which contracts the muscle.

  • When contracted muscles will pull on levers (bones).

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  • Process by which the force of individual muscle fiber twitch combine, when frequency of stimulation increases.

  • Produces sustained contractions and can lead to partial or complete tetanic contractions

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What are the types of Tetanus?

Partial Tetany and Complete Tetany

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What is Partial Tetany and Complete Tetany?

Partial Tetany

  • Occurs at higher frequencies of stimulation

  • Time spent in relaxation between twitches become very brief

Complete Tetany

  • Occurss at very high frequencies of stimulation

  • Does not occur in body, only in laboratories

  • Forceful sustained contraction has no relaxation between twitches.

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What is a motor unit?

A motor neuron plus all of the muscle fibers it controls.

(Most muscle fibers have only one motor end plate)

  • A whole muscle consists of many motor units

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Precise movements of motor units

Produced with fewer muscle fibers in a motor unit.

  • Example: the motor unit of the muscle that moves the eye including fewer than 10 muscle fibers per motor unit and can produce very slight movements

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Coarse movements of motor units

Produced with large numbers of fibers in a motor unit

  • Motor units of the large muscle in the back may include a hundred or more muscle fibers. Movements are larger scale than the eye.

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Sustained contractions

  • Smaller motor units (smaller diameter axons)- recruited first

  • Large motor units (larger diameter axons)- recruited later and with greater force

  • Summation and recruitment can produce sustained contractions of increasing strength

  • Whole muscle contractions are smooth movements. Individuals twitches do not normally occur. Partial or tetanic contractions of muscle fibers are common.

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What is muscle tone (tonus)?

Continuous state of partial contractions in resting muscles.

  • Contributes to maintaining posture

  • Taught neck, head, trunk, and lower limbs enable a person to sit up, stand, hold up head etc.

  • Lose consciousness and the body collapses

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What happens in isotonic contractions?

Muscle contracts and changes length. (Means equal force)

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Types of Isotonic contractions

  • Concentric- shortening contraction; occurs when lifting an object

  • Eccentric- lengthening contraction; occurs when force not sufficient to lift object (lowering a hand weight)

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Isometric contractions

  • Muscle contracts but does NOT change length

  • Means “equal length”

  • Tension develops, but parts attached to muscle do not move (no movement-holding object in place)

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Muscles vary in the following ways:

  • Contraction speed: Slow or Fast

  • Method of ATP production: Oxidatively or Glycolytically

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Three types of muscle fibers found in human:

  • Slow Twitch fibers (Type I)

  • Fast-twitch fatigue-resistant fibers (Type IIa)

  • Fast-twitch glycolytic fibers (Type IIb)

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Slow-twitch fibers (Type I) (Will be on test)

  • Always oxidative

  • Resistant to fatigue (due to high ATP production)

  • Red fibers (High myoglobin)

  • Abundant myoglobin (stores oxygen in muscles)

  • Good blood supply

  • Many mitochondria

  • Slow ATPase activity; slow to contract

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Fast-twitch fatigue-resistant fibers (Type IIa) (Might be on test so still know it)

  • Intermediate twitch fibers

  • Intermediate oxidative capacity

  • Intermediate amount of myoglobin

  • White fibers (have less myoglobin)

  • Resistant to fatigue

  • Rapid ATPase activity

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Fast-twitch glycolytic fibers (Type IIb) (Will be on test)

  • Anaerobic respiration (glycolysis)

  • White fibers (less myoglobin)

  • Poorer blood supply than slow-twitch fibers

  • Fewer mitochondria than slow twitch

  • More SR than slow-twitch (SR to store and absorb calcium ions)

  • Susceptible to fatigue

  • Fast ATPase activity; contract rapidly

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What is hypertrophy and atrophy?

  • Hypertrophy- enlargement of skeletal muscle that is exercised

  • Atrophy- decrease in size (actin and myosin filaments) and strength of skeletal muscle that is unused. Can happen if neurons no longer supply a signal.

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What does Aerobic and Forceful exercise stimulate?

  • Aerobic exercise simulates slow-twitch fibers; in response, fibers increase their capillaries and mitochondria

  • Forceful exercise simulates mainly fast-twitch fibers; in response, fibers produce new actin and myosin filaments, and the muscle enlarges.

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Smooth muscle

Compared to skeletal muscle fibers, smooth muscle fibers are:

  • Shorter

  • Single, centrally located nucleus

  • Elongated with tapering ends (tissue looks like spindles)

  • Myofilaments randomly organized (so they have actin, myosin, and myofibrils)

  • Lack striations

  • Lack transverse tubules

  • Sarcoplasmic reticulum (SR) not well developed

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Smooth muscle SIMILARITIES to skeletal muscle

  • Mechanisms reflect reactions to actin and myosin

  • Triggered by membrane impulses and release of calcium ions

  • Both use energy from ATP molecules (have mitochondria)

  • Neurotransmitters: acetylcholine

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Smooth muscle DIFFERENCES to skeletal muscle

  • Does not have troponin; instead has calmodulin-binds to calcium activating contraction

  • Calcium needed diffuses into the cell from extracellular fluid not SR


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What are the two types of smooth muscle?

  • Multi-unit smooth muscle

  • Visceral smooth muscle

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Multi-unit smooth muscle

  • Cells are less organized

  • Function as separate units

  • Fibers function independently

  • Iris of eye, walls of blood vessels

  • Stimulated by neurons, hormones

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What role does hormones play in multi-unit smooth muscles?

It can stimulate or inhibit contractions or may alter the degree of response to neurotransmitters.

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Visceral smooth muscles

  • Single-unit smooth muscle; cells respond as a unit

  • Sheets of spindle-shaped muscle fibers

  • Fibers held together by gap junctions

  • Exhibit rhythmicity

  • Conduct peristalsis

  • Walls of most hollow organs (intestines, stomach, etc.)

  • More common type of smooth muscle

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Smooth muscle contractions RESEMBLE skeletal muscle contractions in the following ways:

  • Interaction between actin and myosin

  • Both use calcium and ATP

  • Both are triggered by membrane impulses

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Smooth muscle contractions DIFFER from skeletal muscle contraction in the following ways:

  • Smooth muscle lacks troponin; used calmodulin instead

  • Two neurotransmitters affect smooth muscle: Acetylcholine (Ach) and norepinephrine (NE)

  • Hormones can stimulate or inhibit smooth muscle

  • Stretching can trigger smooth muscle contractions

  • Smooth muscle slower to contraction and relax

  • Smooth muscle more resistant to fatigue

  • Smooth muscles can change length without changing tautness.

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Cardiac Muscle

  • Located only in the heart

  • Striated muscle cells

  • Muscle fiber joined together by intercalated discs

  • Fibers branch, contains a single nucleus; has many mitochondria

  • Network of fibers contracts as a unit (syncytium)

  • Self-exciting and rhythmic

  • Does not require stimulation from neurons

  • Longer refractory period (muscle fiber cannot be stimulated until repolarization is complete) than skeletal muscle.

  • No sustained or tetanic contractions

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