Introduction to Muscle Anatomy
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Questions and Answers

What is the primary function of muscle tissue?

  • To store energy
  • To provide electrical signals
  • To facilitate biochemical reactions
  • To convert biochemical reactions into mechanical work (correct)

    • Which type of muscle is responsible for the contraction of internal organs?

  • Skeletal Muscle
  • Striated Muscle
  • Cardiac Muscle
  • Smooth Muscle (correct)

    • Which layer of connective tissue surrounds an entire skeletal muscle?

  • Perimysium
  • Epimysium (correct)
  • Endomysium
  • Fascia

    • What primarily occupies the space within a muscle fiber?

    <p>Myofibrils</p> Signup and view all the answers

    Which type of muscle cannot initiate contraction on its own?

    <p>Skeletal Muscle</p> Signup and view all the answers

    What comprises the bundles of contractile and elastic proteins in a muscle fiber?

    <p>Myofibrils</p> Signup and view all the answers

    What role do mitochondria play within muscle fibers?

    <p>ATP synthesis</p> Signup and view all the answers

    What is generated by muscles to help maintain body temperature?

    <p>Heat</p> Signup and view all the answers

    What are the main contractile proteins in muscle tissue?

    <p>Actin and myosin</p> Signup and view all the answers

    Which structure within the sarcomere consists of only thick filaments?

    <p>H Zone</p> Signup and view all the answers

    What role do troponin and tropomyosin play in muscle contraction?

    <p>They regulate muscle contraction.</p> Signup and view all the answers

    How is a sarcomere defined structurally?

    <p>As the section of myofibril between two Z discs</p> Signup and view all the answers

    What is the significance of the M Line in a sarcomere?

    <p>It serves as the attachment site for thick filaments.</p> Signup and view all the answers

    What is the primary function of nebulin in the muscle sarcomere?

    <p>To align thin filaments</p> Signup and view all the answers

    Which of the following statements about muscle contraction is true?

    <p>The excitation-contraction coupling begins with an action potential.</p> Signup and view all the answers

    What does the I Band consist of in a sarcomere?

    <p>Only thin filaments</p> Signup and view all the answers

    What role does acetylcholinesterase play in muscle contraction?

    <p>It removes ACh from the synaptic cleft to stop the signal.</p> Signup and view all the answers

    Which ion's influx leads to local depolarization during muscle activation?

    <p>Na+</p> Signup and view all the answers

    What happens after the DHP receptor changes conformation?

    <p>It opens SR Ca2+ channels releasing Ca2+ into the cytosol.</p> Signup and view all the answers

    In what position does tropomyosin need to be for myosin to bind to actin?

    <p>On position</p> Signup and view all the answers

    During the power stroke of the crossbridge cycle, what happens to the myosin head?

    <p>It pivots toward the M line and pulls the filament.</p> Signup and view all the answers

    What initiates the binding of myosin to actin during the crossbridge cycle?

    <p>Binding of Ca2+ to troponin.</p> Signup and view all the answers

    Which statement correctly describes the crossbridge attachment during muscle contraction?

    <p>About 50% of crossbridges are attached at any given time.</p> Signup and view all the answers

    What is the primary energy source used by myosin during the crossbridge cycle?

    <p>ATP</p> Signup and view all the answers

    What characterizes complete (fused) tetanus?

    <p>Rapid stimulation rate preventing relaxation</p> Signup and view all the answers

    What is a defining feature of a motor unit?

    <p>It includes a somatic motor neuron and its associated muscle fibers</p> Signup and view all the answers

    How does an isometric contraction create force without changing muscle length?

    <p>Elastic elements within the muscle stretch to absorb the force</p> Signup and view all the answers

    Where is smooth muscle typically found?

    <p>In the walls of hollow organs and blood vessels</p> Signup and view all the answers

    What distinguishes single-unit smooth muscle from multi-unit smooth muscle?

    <p>Single-unit muscle fibers are not electrically coupled</p> Signup and view all the answers

    How does contraction in smooth muscle differ from contraction in skeletal muscle?

    <p>Smooth muscle can maintain contractions longer without fatigue</p> Signup and view all the answers

    What is the size comparison of smooth muscle fibers to skeletal muscle fibers?

    <p>Smooth muscle fibers are smaller and similar to myofibrils</p> Signup and view all the answers

    Which of these statements is true regarding isotonic contractions?

    <p>They involve changes in muscle length while moving a constant load</p> Signup and view all the answers

    What is the role of calcium ions ($Ca^{2+}$) in smooth muscle contraction?

    <p>Calcium ions activate myosin through phosphorylation.</p> Signup and view all the answers

    How do calcium ions enter smooth muscle cells to initiate contraction?

    <p>Through stretch-activated channels and voltage-gated channels.</p> Signup and view all the answers

    What is the function of myosin light chain kinase (MLCK) in smooth muscle cells?

    <p>MLCK activates myosin by phosphorylating its light chain.</p> Signup and view all the answers

    What distinguishes the contraction process of smooth muscle from that of cardiac muscle?

    <p>Smooth muscle utilizes phosphorylation to regulate contraction.</p> Signup and view all the answers

    What structures anchor actin in smooth muscle cells?

    <p>Dense bodies</p> Signup and view all the answers

    What happens to myosin when it is phosphorylated by MLCK?

    <p>It can now interact with actin to allow crossbridge cycling.</p> Signup and view all the answers

    What is a notable feature of smooth muscle cell organization compared to skeletal muscle?

    <p>Smooth muscle does not have a banding pattern due to lack of sarcomeres.</p> Signup and view all the answers

    Which vesicles in smooth muscle are specialized for cell signaling?

    <p>Caveolae</p> Signup and view all the answers

    Study Notes

    Introduction to Muscle

    • Muscle is specialized tissue that converts biochemical reactions into mechanical work.
    • Two primary functions are contraction and expansion.
    • Muscles generate heat and contribute to body temperature regulation.
    • There are three types of muscle in the human body: skeletal, smooth, and cardiac.
    • Skeletal muscle is attached to bones, contracts in response to somatic motor neurons, and cannot initiate its own contraction.
    • Smooth muscle is found in internal organs and tubes and influences the movement of materials through the body.
    • Cardiac muscle is found only in the heart and is responsible for pumping blood.
    • This unit primarily focuses on skeletal muscle.

    Gross Structure of Skeletal Muscle

    • Skeletal muscle is responsible for positioning and movement of the skeleton and makes up ~40% of body weight.
    • Skeletal muscle is covered by the epimysium, a connective tissue sheath.
    • Inside are bundles of muscle tissue called fascicles, covered by the perimysium, another connective tissue sheath.
    • Nerves and blood vessels are also found within skeletal muscle.
    • Muscle fibers (muscle cells) are found within each fascicle, covered by the endomysium, the innermost connective tissue sheath.
    • Myofibrils are the functional units of skeletal muscle and are found within muscle fibers.
    • Myofibrils contain many glycogen granules for energy storage and mitochondria for ATP synthesis.

    Structure of a Muscle Fibre

    • Muscle fibers are long, cylindrical cells with several hundred nuclei on the surface.
    • The cell membrane of a muscle fiber is called the sarcolemma.
    • Myofibrils occupy most of the space in a muscle fiber.
    • Myofibrils are bundles of contractile elastic proteins.

    Organization of the Myofibril

    • Myofibrils consist of bundles of contractile elastic proteins.
    • These proteins include contractile proteins (actin & myosin), regulatory proteins (troponin & tropomyosin), and accessory proteins (nebulin & titin).
    • Myosin is a motor protein consisting of two coiled protein molecules with a head and tail region.
    • Actin is composed of G-actin subunits that polymerize to form chains (F-actin).
    • Two F-actin chains twist together to form the thin filament, which associates with regulatory proteins (troponin & tropomyosin) that regulate muscle contraction.

    The Sarcomere

    • Myofibrils have stripes called striations, giving skeletal muscle its striated appearance.
    • The sarcomere is the repeating pattern of striations and is made up of organized thick and thin filaments.
    • The Z-line is the site of attachment for thin filaments, with one sarcomere containing two Z discs and the filaments between them.
    • The I band is a region containing only thin filaments.
    • The A band is a region containing both thick and thin filaments.
    • The H zone is a region containing only thick filaments.
    • The M line is the site of attachment for thick filaments and is the center of the sarcomere.

    Skeletal Muscle Contraction

    • Skeletal muscles contract in response to signals from the nervous system.
    • Excitation-contraction coupling is the process that leads to muscle contraction and involves electrical and mechanical events.
    • The process starts with an action potential in the muscle membrane, initiated by a neurotransmitter called acetylcholine (ACh).
    • ACh binds to nicotinic cholinergic receptors on the motor end plate, which are Na+/K+ channels.
    • The binding of ACh opens the channels, allowing Na+ and K+ to move across the membrane.
    • This influx of Na+ exceeds the efflux of K+, resulting in a local depolarization called an end plate potential (EPP).
    • The EPP travels down the T-tubule system, which contains dihydropyridine receptors (DHP receptors), or L-type calcium channels.
    • Depolarization changes the conformation of the DHP receptors, which are physically linked to ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR).
    • This conformational change in the DHP receptors opens the RyR Ca2+ channels on the SR.
    • Ca2+ leaves the SR and increases cytosolic Ca2+ concentrations.
    • Ca2+ then binds to troponin, which shifts tropomyosin into the “on” position, exposing actin binding sites for the myosin head.
    • Myosin can now bind to actin and go through the cross bridge cycle.

    Crossbridge Cycle

    • The crossbridge cycle is the process by which myosin converts chemical energy (ATP) into movement.
    • Myosin binds to actin, releases inorganic phosphate, pivots toward the center of the sarcomere (power stroke), and detaches with the binding of a new ATP molecule.
    • This process repeats as long as Ca2+ is bound to troponin.
    • During contraction, the crossbridges do not all move simultaneously.
    • At any given time, 50% of the crossbridges are attached and producing contraction.
    • Incomplete (unfused) tetanus is the result of slow stimulation rates that allow for slight relaxation between stimuli.
    • Complete (fused) tetanus is the result of fast stimulation rates that don't allow for relaxation.

    Motor Unit

    • The motor unit is the fundamental unit of contraction in skeletal muscle.
    • A muscle is made up of many motor units.
    • Each motor unit consists of a group of muscle fibers and the somatic motor neuron that controls them.
    • All muscle fibers within a motor unit are of the same skeletal muscle fiber type.
    • An action potential in the somatic motor neuron causes contraction of all muscle fibers in the motor unit.
    • Each motor unit contracts in an all-or-none fashion.

    Body Movement

    • Two main types of muscle contraction are isotonic and isometric.
    • Isotonic contractions create force and move a load, with a constant load and a change in muscle length.
    • Isometric contractions generate force without movement, maintain constant muscle length, and usually involve a load greater than the force that can be applied.

    Smooth Muscle

    • Smooth muscle is found in the walls of hollow organs and tubes but not attached to bones.
    • Some important smooth muscles include the bladder sphincter, intestine, and walls of blood vessels.

    Arrangement of Smooth Muscle Cells

    • Smooth muscle cells can be arranged in two ways: single unit and multi-unit.
    • Single-unit smooth muscle cells are not individually stimulated and are found in the walls of internal organs like blood vessels.
    • Multi-unit smooth muscle cells individually innervated and found in the iris of the eye and parts of the reproductive organs.

    Differences Between Smooth & Skeletal Muscle

    • Smooth muscle contraction changes muscle shape, not just length, unlike skeletal muscle.
    • Smooth muscle develops tension more slowly than skeletal muscle.
    • Smooth muscle can sustain contraction for longer without fatiguing.
    • Smooth muscle fibers are much smaller than skeletal muscle fibers.
    • Actin and myosin are not arranged into sarcomeres in smooth muscle.
    • Actin and myosin are arranged in long bundles diagonally around the periphery of the cell.
    • Actin is anchored at cell membrane structures called dense bodies, not Z lines like in skeletal muscle.
    • Smooth muscle cells lack T-tubules and have less sarcoplasmic reticulum.
    • Smooth muscle cells have special vesicles called caveolae for cell signaling.

    Smooth Muscle Contraction

    • Smooth muscle contraction is regulated by phosphorylation of myosin, unlike skeletal muscle, which is regulated by troponin/tropomyosin interaction with actin.
    • An increase in cytosolic Ca2+ triggers smooth muscle contraction.
    • Ca2+ enters from the extracellular fluid via voltage-gated channels, stretch-activated channels, or chemically-gated channels.
    • Ca2+ entry from the ECF triggers the release of SR Ca2+.
    • Ca2+ then binds to calmodulin (CaM) in the cytosol.
    • The Ca2+/CaM complex activates myosin light chain kinase (MLCK), which phosphorylates the myosin light chain in the head, activating myosin’s ATPase activity.
    • This phosphorylation of myosin allows it to interact with actin and initiate crossbridge cycling for smooth muscle contraction.

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    Description

    This quiz covers the fundamentals of muscle tissue, focusing specifically on skeletal muscle. It explores its structure, types, functions, and role in body movement and temperature regulation. Test your understanding of muscle physiology and anatomy.

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