Muscle Structure and Contraction
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Questions and Answers

What is the functional unit of skeletal muscle?

  • Muscle Fibre
  • Fascicle
  • Sarcomere (correct)
  • Myofibril

    • Which of the following connective tissue surrounds each individual muscle fibre?

  • Fascicle
  • Endomysium (correct)
  • Epimysium
  • Perimysium

    • What happens to the A band during muscle contraction?

  • It becomes longer
  • It disappears
  • It becomes shorter
  • It stays the same width (correct)

    • What is the main structural component of thin filaments?

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

    What is the function of the myosin head?

    <p>Both A and B</p> Signup and view all the answers

    How does the actin filament structure appear?

    <p>A helix</p> Signup and view all the answers

    What is the structure of a muscle fibre?

    <p>A bundle of myofibrils</p> Signup and view all the answers

    What is the function of perimysium connective tissue?

    <p>Surrounds each fascicle</p> Signup and view all the answers

    What is the result of tropomyosin covering the myosin binding site?

    <p>Muscle relaxation occurs</p> Signup and view all the answers

    What is a motor unit composed of?

    <p>A single motor neuron and multiple muscle fibres</p> Signup and view all the answers

    What is the role of calmodulin in smooth muscle contraction?

    <p>To activate myosin light chain kinase (MLCK)</p> Signup and view all the answers

    What is the characteristic of smooth muscle that allows it to generate action potentials without external stimuli?

    <p>Pacemaker cells in hollow organ walls</p> Signup and view all the answers

    What is the result of phosphorylation of myosin heads in smooth muscle contraction?

    <p>Myosin heads bind to actin-binding sites</p> Signup and view all the answers

    What are the specialized regions of the axon that release neurotransmitters in smooth muscle contraction?

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

    What is the result of calcium being released into the sarcoplasm?

    <p>Calcium binds to the troponin, moving the troponin and tropomyosin away from the myosin binding site</p> Signup and view all the answers

    What breaks the bond between the myosin head and the actin filament?

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

    What is the result of the myosin head returning to its starting position?

    <p>The myosin filaments slide along the actin</p> Signup and view all the answers

    What is necessary for muscle relaxation to occur?

    <p>The absence of motor neuron firing</p> Signup and view all the answers

    What type of ATPase is responsible for returning calcium to the stores or expelling it from the cell?

    <p>Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA)</p> Signup and view all the answers

    What is the result of the ATPase in the Sarcoplasmic Reticulum removing calcium from the cytosol?

    <p>Troponin releases calcium</p> Signup and view all the answers

    What is the term for the process by which the myosin filaments slide along the actin filaments?

    <p>Sliding filament theory</p> Signup and view all the answers

    What is the primary role of calcium ions in the contraction mechanism?

    <p>To bind to TnC of Troponin Complex, causing a conformational change that moves tropomyosin away from myosin's binding sites</p> Signup and view all the answers

    What is the result of the overlap between the myosin and actin filaments increasing?

    <p>The I band decreases in size</p> Signup and view all the answers

    What is the result of the bending of myosin heads in the power stroke?

    <p>The actin filament is pulled towards the centre of the sarcomere</p> Signup and view all the answers

    What is the function of the voltage-sensitive proteins inside the T-tubules?

    <p>To change conformation and allow depolarization to spread</p> Signup and view all the answers

    What is the primary function of the nicotinic ACh receptors in the neuromuscular junction?

    <p>To bind to acetylcholine and allow influx of sodium ions</p> Signup and view all the answers

    What is the result of the conformational change in troponin complex due to calcium ion binding?

    <p>Tropomyosin moves away from myosin's binding sites on actin</p> Signup and view all the answers

    What is the role of the action potential in the skeletal muscle in excitation-contraction coupling?

    <p>To allow depolarization to spread over the sarcolemma and through the T-tubules</p> Signup and view all the answers

    What is the primary function of the adaptive immune system?

    <p>Highly specific defence with memory</p> Signup and view all the answers

    What is the site of haematopoiesis?

    <p>Bone marrow</p> Signup and view all the answers

    Which type of stem cell can generate only one specific cell type?

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

    What is the relationship between the innate and adaptive immune systems?

    <p>The adaptive system is dependent on the innate system for activation</p> Signup and view all the answers

    What is the characteristic of the innate immune system?

    <p>Rapid response with no memory of previous exposures</p> Signup and view all the answers

    Which type of stem cell is capable of producing nearly all cell types in the three germ layers?

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

    What is the result of haematopoiesis?

    <p>Development of stem cells into protective cells</p> Signup and view all the answers

    What is the characteristic of oligopotent stem cells?

    <p>Able to produce cells within a closely related family</p> Signup and view all the answers

    Which type of phagocytic cell is responsible for phagocytosing bone?

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

    What is the primary function of CD4 proteins?

    <p>Recognizing antigens presented by antigen-presenting cells</p> Signup and view all the answers

    What is the difference between MHC Class I and MHC Class II?

    <p>MHC I is present on all nucleated cells, while MHC II is only present on antigen-presenting cells</p> Signup and view all the answers

    What is the role of Th cells after activation by APCs?

    <p>They activate B cells and stimulate antibody production</p> Signup and view all the answers

    What is the site of maturation of B cells?

    <p>Lymph nodes</p> Signup and view all the answers

    What is the function of M1 monocytes?

    <p>Fighting against infections and foreign invaders</p> Signup and view all the answers

    What attracts phagocytes to the site of infection or injury?

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

    What is the function of CD8 cells?

    <p>Recognizing antigens on the surface of infected cells</p> Signup and view all the answers

    What is the primary function of the lymphoid cells found in the epithelium of the gut?

    <p>To produce antibodies against pathogens</p> Signup and view all the answers

    Which part of the Waldeyer's ring is responsible for taking up antigens expressed by pathogens?

    <p>Microfold (M) cells</p> Signup and view all the answers

    What is the primary role of the spleen in the immune system?

    <p>To filter blood and recycle old red blood cells</p> Signup and view all the answers

    What is the characteristic of acute inflammation that distinguishes it from chronic inflammation?

    <p>It occurs in response to injury or infection</p> Signup and view all the answers

    What is the function of the lymph node below the M cells in the Waldeyer's ring?

    <p>To release mature activated B cells (plasma cells) to the epithelium</p> Signup and view all the answers

    What is the term for the immune response that occurs in the gut?

    <p>Gut-associated lymphoid tissue (GALT)</p> Signup and view all the answers

    What is the role of the M cells in the Waldeyer's ring?

    <p>To allow immune cells to enter and interact with the epithelium</p> Signup and view all the answers

    What is the result of the interaction between the M cells and immune cells in the Waldeyer's ring?

    <p>The activation of immune cells to combat infection</p> Signup and view all the answers

    What is the primary function of myeloid cells in the immune system?

    <p>To mature into innate immune cells in the bone marrow</p> Signup and view all the answers

    What is the primary function of M1 monocytes?

    <p>Fighting against infections and foreign invaders</p> Signup and view all the answers

    What is the primary function of haematopoiesis in the immune system?

    <p>To develop protective cells from stem cells</p> Signup and view all the answers

    Which type of phagocytic cell is responsible for fighting parasitic infections?

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

    What is the result of the complement system activation?

    <p>Inflammation, opsonization, and direct killing of target cells</p> Signup and view all the answers

    Which type of stem cell can generate all cell types needed for a new organism?

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

    Which of the following phagocytic cells is responsible for phagocytosing bone?

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

    What is the primary function of CD4 proteins?

    <p>Recognizing antigens presented by antigen-presenting cells (APCs)</p> Signup and view all the answers

    Where is red bone marrow typically found in adults?

    <p>At the end of bones (epiphysis)</p> Signup and view all the answers

    What is the relationship between the innate and adaptive immune systems?

    <p>The innate immune system is the first line of defense, while the adaptive immune system is the second line of defense</p> Signup and view all the answers

    What is the primary function of eosinophils in the immune response?

    <p>To modulate allergic responses</p> Signup and view all the answers

    What is the difference between MHC Class I and MHC Class II?

    <p>MHC I is on all nucleated cells, while MHC II is on antigen-presenting cells (APCs)</p> Signup and view all the answers

    Which type of stem cell is capable of producing cells within a closely related family?

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

    What is the characteristic of yellow bone marrow?

    <p>It has a high fat content</p> Signup and view all the answers

    What is the site of haematopoiesis in the body?

    <p>Bone marrow</p> Signup and view all the answers

    What is the role of Th cells after activation by APCs?

    <p>Activating B cells</p> Signup and view all the answers

    Which type of stem cell is limited to generating a few cell types within the same family?

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

    What is the site of maturation of B cells?

    <p>Lymph nodes</p> Signup and view all the answers

    What is the difference between monocytes and macrophages?

    <p>Monocytes are found in blood circulation, while macrophages are found in tissues</p> Signup and view all the answers

    What attracts phagocytes to the site of infection or injury?

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

    What is the result of haematopoiesis in the immune system?

    <p>Development of protective cells from stem cells</p> Signup and view all the answers

    What is the primary function of the complement system in the immune response?

    <p>To mark pathogens for destruction by phagocytic cells</p> Signup and view all the answers

    Which type of stem cell can generate only one specific cell type?

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

    What is the primary function of CD8 cells?

    <p>Recognizing antigens on the surface of infected cells</p> Signup and view all the answers

    Study Notes

    Muscle Organisation

    • Skeletal muscles and cardiac muscles are striated, while smooth muscles are non-striated.

    Structure of Skeletal Muscle

    • The functional unit of skeletal muscle is the sarcomere, which is replicated down the length of the muscle to form myofibrils.
    • Myofibrils are bundled together to form muscle fibers (a cell).
    • Muscle fibers are bundled and wrapped in connective tissue to form fascicles.
    • Fascicles are in turn bundled to form muscle.

    Connective Tissue

    • Endomysium: surrounds each individual muscle fiber.
    • Perimysium: surrounds each fascicle.
    • Epimysium: surrounds the entire muscle.

    Sarcomere Structure

    • A band: stays the same width during contraction.
    • H zone and I band: become shorter during contraction.
    • Thick filaments: formed by myosin, consisting of two identical subunits.
    • Thin filaments: formed by actin, which interacts with myosin cross-bridge.
    • Tropomyosin and troponin are associated with actin.

    Myosin Structure

    • Myosin head: has an actin binding site and ATPase site.
    • Actin (Thin Filament) Structure: forms a helix, with tropomyosin molecules blocking myosin binding sites.

    Contraction Mechanism

    • Power stroke: bending of myosin heads pulls the actin filament towards the center of the sarcomere.
    • Role of calcium: increasing calcium ions bind to troponin complex, causing a conformational change that moves tropomyosin away from myosin's binding site on actin.

    Neuromuscular Junction

    • Neuromuscular Junction (NMJ): a specialized synapse connecting motor neuron to skeletal muscle fiber.
    • Excitation-contraction coupling: action potential at axon terminal opens voltage-gated calcium channels, releasing acetylcholine into the synaptic cleft, which activates nicotinic ACh receptors, triggering muscle contraction.

    Sliding Filament Theory

    • Myosin heads form a cross-bridge with actin filament.
    • Attached heads pull on the actin filament through relaxation.
    • ATP attaches to the myosin head, breaking the bond with the actin filament.
    • ATP hydrolyses and "cocks" the myosin head back to the starting position, causing the myosin filaments to slide along the actin, resulting in muscle contraction.

    Muscle Relaxation

    • Relaxation requires breaking the cross-bridge cycle.
    • In the absence of motor neuron firing, calcium channels close.
    • ATPases in the sarcoplasmic reticulum quickly return calcium to stores or expel it from the cell.
    • SERCA (Sarco/Endoplasmic Reticulum Calcium ATPases) causes troponin (TnC) to release calcium, reversing the conformational change in tropomyosin.

    Motor Unit

    • A motor unit consists of a single motor neuron and muscle fibers.

    Smooth Muscle Contraction

    • Depolarization opens L-type voltage-gated calcium channels, allowing calcium influx.
    • Intracellular calcium binds to calmodulin, activating myosin light chain kinase (MLCK).
    • MLCK phosphorylates myosin heads through ATP hydrolysis.
    • Phosphorylated myosin heads can attach to actin-binding sites, causing muscle fiber contraction.

    Control of Smooth Muscle Contraction

    • Smooth muscle contraction is involuntary.
    • Varicosities (swellings of axons) filled with neurotransmitters contact the sarcolemma.
    • Smooth muscle can be spontaneously active, generating action potentials without external stimuli, often due to pacemaker cells in hollow organ walls.

    Innate Immune System vs Adaptive Immune System

    • Innate Immune System: present from birth, non-specific defense, rapid response, no memory of previous exposures
    • Adaptive Immune System: develops over time, highly specific defense, slower response, memory, involves B cells, T cells, and antibodies

    Haematopoiesis

    • Definition: development of stem cells and precursor cells into protective cells
    • Origin: bone marrow, travel through circulation, and mature throughout the body
    • 5 types of stem cells:
      • Totipotent: can generate all cell types needed for a new organism
      • Pluripotent: capable of producing nearly all cell types, in three germ layers
      • Multipotent: able to produce cells within a closely related family
      • Oligopotent: limited to generating a few cell types within the same family
      • Unipotent: can produce only one specific cell type

    Waldeyer's Ring

    • Located in the upper airway
    • Consists of 4 tonsillar structures, containing lymphoid tissue and lymphocytes (T and B cells)

    Microfold (M) Cells

    • Role: take up antigens expressed by pathogens
    • Structure: 'U' shaped, with a pocket toward the basal end, allowing T cells, B cells, and Antigen-Presenting Cells (APCs) to enter
    • Function: facilitate interaction with immune cells

    Tonsils

    • Part of the mucosa-associated lymphoid tissue (MALT)
    • Contain lymphocytes (T and B cells)

    Peyer's Patches

    • Lymphoid cells found in the epithelium of the gut
    • Part of the gut-associated lymphoid tissue (GALT)
    • Function: supply the gut with B cells

    Spleen Functions

    • Blood filter
    • Recycles old red blood cells
    • Stores platelets and other white blood cells
    • Facilitates opsonization (coating bacteria with antibodies) for bacterial clearance

    Acute Inflammation vs Chronic Inflammation

    • Acute Inflammation: occurs in response to injury or infection, immune cells are brought to the site → clear infection or damaged tissue, resolves once threat is eliminated, typically lasts up to 2 weeks
    • Chronic Inflammation: (not mentioned)

    Monocyte Functions

    • M1 Monocytes: fight against infections and foreign invaders
    • M2 Monocytes: healing

    Phagocytic Cells

    • 4 types: Monocytes, Macrophages, Neutrophils, Osteoclasts
    • Function: consume invading pathogens
    • Attracted by: chemoattractant

    MHC (Major Histocompatibility Complex)

    • Crucial component for T cell maturation and function
    • MHC Class I (MHC I): on all nucleated/infected cells
    • MHC Class II (MHC II): only on antigen-presenting cells (APCs)

    CD4 Proteins

    • Recognize antigens presented by APCs
    • CD4 + MHC 2

    CD8 Cells

    • Recognize antigens on the surface of infected cells
    • CD8 + MHC I

    B Cell Maturation

    • Occurs in lymph nodes

    T and B Cell Activation

    • T cells await activation in the lymphatic system (lymph vessels or lymphatic vessels)
    • After T cell activation by APCs: Th cells (CD4 + MHC 2) are activated, and they, in turn, activate B cells, which mature into plasma cells, and enter the circulation.

    Types of Bone Marrow

    • Red bone marrow: produces red cells and most white blood cells, typically found at the end of bones (epiphysis) in adults
    • Yellow bone marrow: produces some white blood cells, typically found in the canal of long bones in adults, and has a high fat content

    Eosinophil Cells

    • Involved in fighting parasitic infections and modulating allergic responses

    Complement System

    • Activates inflammation
    • Opsonization (labelling) of pathogens and cells for clearance/destruction
    • Direct killing of target cells/microbes by lysis

    Myeloid Cells

    • Many innate immune cells mature in the bone marrow or shortly after entering circulation
    • Maturing cells become myeloid cells

    Monocytes and Macrophages

    • Monocytes: present in blood circulation (m1 + m2)
    • Macrophages: mainly found in tissues, being tissue residents
    • M1 Monocytes: fight against infections and foreign invaders
    • M2 Monocytes: involved in healing

    Phagocytic Cells

    • 4 types: monocytes, macrophages, neutrophils, and osteoclasts
    • Phagocytic cells consume invading pathogens
    • Chemoattractant: attracts phagocytes towards site of infection/injury

    MHC 1 and MHC 2

    • MHC 1: on all nucleated/infected cells
    • MHC 2: only on antigen-presenting cells (APCs)
    • MHC is crucial for T cell maturation and function

    CD4 Proteins and CD8 Cells

    • CD4: recognises antigens presented by APCs
    • CD4 + MHC 2
    • CD8: recognises antigens on the surface of infected cells
    • CD8 + MHC I

    B Cell Maturation

    • Maturation of B cells occurs in lymph nodes

    T Cell Activation

    • After T cell activation by APCs, Th cells (CD4 + MHC 2) are activated, and they, in turn, activate B cells
    • B cells mature into plasma cells, which enter the circulation

    Innate Immune System vs Adaptive Immune System

    • Innate Immune System: present from birth, non-specific defence, rapid response, no memory of previous exposures, includes physical barriers, chemicals, and basic cellular components
    • Adaptive Immune System: develops over time, highly specific defence, slower response, memory, involves B cells, T cells, and antibodies, communicates with the innate system for a more specific response

    Haematopoiesis

    • Haematopoiesis: stem cells and precursor cells develop into protective cells
    • Originating in the bone marrow, travel through circulation, and mature throughout the body

    Types of Stem Cells

    • Totipotent: can generate all cell types needed for a new organism
    • Pluripotent: capable of producing nearly all cell types in the three germ layers
    • Multipotent: able to produce cells within a closely related family
    • Oligopotent: limited to generating a few cell types within the same family
    • Unipotent: can produce only one specific cell type

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    Description

    Learn about the structure and morphology of muscles, including skeletal muscle organisation and contraction. Understand the functional unit of sarcomere and myofibrils.

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