Physiology Chapter on Neuromuscular Transmission
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Questions and Answers

What is the primary function of skeletal muscles related to posture?

  • Static force production for maintaining posture (correct)
  • Heat production
  • Kinetic force production for locomotion
  • Venous drainage

    • How does myosin interact with ATP during muscle contraction?

  • It hydrolyzes ATP and uses the energy for contraction (correct)
  • It solely binds to ATP without hydrolyzing it
  • It stores ATP for later use
  • It employs ATP for binding to actin only

    • Which component is involved in the flexing mechanism of myosin cross-bridges?

  • The ATP binding site on actin
  • The amine terminal of actin
  • The head structure of tropomyosin
  • The hinges of the myosin protein (correct)

    • What role do tropomyosin molecules play in muscle contraction?

    <p>They cover active sites on actin during resting conditions</p> Signup and view all the answers

    Which statement describes skeletal muscle tissue?

    <p>It is responsible for voluntary movements in the body</p> Signup and view all the answers

    What is the primary function of Troponin-I in muscle contraction?

    <p>Binding to actin to inhibit actomyosin ATPase activity</p> Signup and view all the answers

    What percentage of the human body is comprised of skeletal muscles?

    <p>40%</p> Signup and view all the answers

    Where are T tubules located in mammalian skeletal muscle fibers?

    <p>At the junction of the A and I bands</p> Signup and view all the answers

    What is the characteristic feature of thin filament proteins?

    <p>Each thin filament consists of 300-400 actin molecules</p> Signup and view all the answers

    What role does calcium play in muscle contraction?

    <p>It binds to Troponin-C to initiate contraction</p> Signup and view all the answers

    Which function does skeletal muscle NOT perform?

    <p>Hormonal secretion</p> Signup and view all the answers

    What is the function of the ryanodine receptor in the sarcoplasmic reticulum?

    <p>To release calcium ions into the cytoplasm</p> Signup and view all the answers

    What structures make up the triad in skeletal muscle fibers?

    <p>One T tubule and two terminal cisternae of the sarcoplasmic reticulum</p> Signup and view all the answers

    How does an action potential influence muscle fiber contraction?

    <p>It initiates the release of acetylcholine at the neuromuscular junction</p> Signup and view all the answers

    What is the role of the motor end plate in muscle contraction?

    <p>It contains receptors for neurotransmitters like acetylcholine</p> Signup and view all the answers

    Which of the following components is NOT found in the structure of troponin?

    <p>Troponin-x</p> Signup and view all the answers

    What is the role of curare in neuromuscular transmission?

    <p>It competes with acetylcholine for receptor sites.</p> Signup and view all the answers

    What is the primary effect of anticholinesterase drugs in the treatment of myasthenia gravis?

    <p>They prevent the breakdown of acetylcholine.</p> Signup and view all the answers

    What characterizes the progression of myasthenia gravis symptoms?

    <p>Weakness that worsens with rapid activity.</p> Signup and view all the answers

    During the pathophysiology of myasthenia gravis, what role do autoantibodies play?

    <p>They bind to and decrease the number of acetylcholine receptors.</p> Signup and view all the answers

    What is a common autoimmune condition associated with myasthenia gravis?

    <p>Rheumatoid arthritis.</p> Signup and view all the answers

    What chest imaging is recommended for newly diagnosed myasthenia gravis patients?

    <p>CT scan to check for thymoma.</p> Signup and view all the answers

    What percentage of myasthenia gravis patients is likely to have thymic abnormalities?

    <p>More than 75%.</p> Signup and view all the answers

    What is the typical age distribution for women diagnosed with myasthenia gravis?

    <p>20s.</p> Signup and view all the answers

    What is the role of acetylcholinesterase in the synaptic cleft?

    <p>To degrade ACh and prevent prolonged stimulation</p> Signup and view all the answers

    During neuromuscular transmission, what triggers the release of ACh from synaptic vesicles?

    <p>Influx of Ca+2 ions into the nerve terminal</p> Signup and view all the answers

    What results from an increase in Na+ conductance during neuromuscular transmission?

    <p>End-plate potential (EPP)</p> Signup and view all the answers

    Why does neuromuscular transmission exhibit unidirectionality?

    <p>The muscle membrane does not release neurotransmitters</p> Signup and view all the answers

    What occurs during the depolarization of the muscle cell membrane?

    <p>End-plate potential generation</p> Signup and view all the answers

    What physiological property makes neuromuscular transmission prone to fatigue?

    <p>Depletion of ACh vesicles</p> Signup and view all the answers

    Which of the following describes the synaptic cleft?

    <p>Extracellular space containing basal lamina and acetylcholinesterase</p> Signup and view all the answers

    What is contained within each synaptic vesicle released during neuromuscular transmission?

    <p>2000 to 10,000 ACh molecules</p> Signup and view all the answers

    Study Notes

    Neuromuscular Transmission

    • Neuromuscular transmission is the process of transmitting nerve impulses from alpha motor neurons (αMNs) to skeletal muscle fibers.
    • It is a crucial process for voluntary muscle movement.

    Physiology of Skeletal Muscles

    • Skeletal muscles are attached to bones and are responsible for voluntary movement.
    • Human bodies contain over 400 skeletal muscles.
    • Skeletal muscle functions include:
      • Static force production for maintaining posture and stabilizing joints.
      • Kinetic force production for locomotion and breathing.
      • Heat production.
      • Help venous drainage.

    Muscle Proteins

    • Myosin protein:

      • Contains an actin-binding site, ATP binding site, and a catalytic site for ATP hydrolysis (myosin ATPase).
      • The head of the myosin protein extends to the side to form an arm.
      • Arms and heads together are called cross-bridges.
      • Cross-bridges are flexible at two points called hinges.

    • Thin filament proteins:

      • Each thin filament contains 300-400 actin molecules.
      • Actin molecules have specific myosin cross-bridge binding sites (active sites), an ATP binding site, and a catalytic site for ATP hydrolysis (actomyosin ATPase).
      • Tropomyosin molecules cover the active sites on actin under resting conditions, preventing contraction.
      • Troponin is a complex of three globular protein molecules:
        • Troponin-I binds actin to inhibit actomyosin ATPase activity.
        • Troponin-T binds to tropomyosin.
        • Troponin-C binds Ca2+.

    Tubular System

    • Transverse (T) Tubules:

      • Invaginations of the sarcolemma.
      • Located at the junction of A and I bands in mammalian skeletal muscle.
      • Propagate action potentials from the sarcolemma into the T tubule network.
      • Contain extracellular fluid (ECF).

    • Sarcoplasmic Reticulum (SR):

      • ER of the muscle fiber that surrounds each myofibril.
      • Contains high levels of calcium ions (Ca2+) used to initiate muscle contraction.
      • Expanded ends form terminal cisternae (TC), which make contact with T tubules on either side of the sarcomere at the A and I band junction (triad).
      • Foot processes span the space between T tubule and SR membranes.
      • The SR membrane contains the ryanodine receptor, which is a Ca2+ release channel.
      • The T-tubule membrane contains the dihydropyridine (DHP) receptor, which opens the ryanodine Ca2+ release channel on the SR.

    Neuromuscular Junction

    • The neuromuscular junction (NMJ) is the site where a motor neuron communicates with a muscle fiber.
    • Action potentials in the motor neuron cause the release of acetylcholine (ACh) at the NMJ, triggering muscle contraction.
    • Physiologic anatomy of the NMJ:
      • Alpha motor neurons (αMNs) branch to innervate multiple muscle fibers.
      • Each muscle fiber receives only one axon terminal containing ACh vesicles.
      • The nerve ending fits into a depression in the muscle membrane called the motor end plate (MEP).
      • MEP is rich in Ach receptors and contains junctional folds.
      • The synaptic cleft is the extracellular space between the nerve terminal and muscle membrane, containing basal lamina and the enzyme acetylcholinesterase (AChE).

    Neuromuscular Transmission

    • The sequence of events during neuromuscular transmission includes:
      • Action potential reaches the nerve ending, opening voltage-gated Ca2+ channels.
      • Ca2+ influx into the nerve terminal binds to calmodulin and activates CaM kinases.
      • ACh vesicles fuse with the membrane and release their contents by exocytosis.
      • ACh diffuses across the synaptic cleft and binds to receptors on the MEP, opening ion channels.
      • Na+ influx and K+ efflux depolarize the muscle membrane, creating an end-plate potential (EPP).
      • EPP triggers action potentials on either side of the MEP, which propagate along the muscle fiber and initiate contraction.
      • Acetylcholinesterase (AChE) rapidly degrades ACh in the synaptic cleft to prevent continuous stimulation of the muscle fiber.

    Properties of Neuromuscular Transmission

    • Unidirectional: transmission occurs from neuron to muscle fiber.
    • Delay: a short delay (0.5 msec) between nerve stimulation and muscle contraction.
    • Fatiguability: prolonged stimulation leads to fatigue as ACh vesicles become depleted.
    • Ionic influences: ion concentrations affect neuromuscular transmission.
    • Drug effects: drugs can stimulate or block neuromuscular transmission.

    Myasthenia Gravis (MG)

    • A serious autoimmune disease characterized by muscle weakness and fatigue.
    • Caused by antibodies against ACh receptors, leading to decreased receptor numbers at the NMJ.
    • Symptoms worsen with repeated stimulation due to decreased ACh release.
    • Can be ocular, bulbar, or generalized.
    • Patients may have a family history of autoimmune disorders, and thymic abnormalities are frequently observed.

    Treatment of Myasthenia Gravis

    • Chest CT scan is indicated to rule out thymoma.
    • Anticholinesterase drugs such as physostigmine and neostigmine can ameliorate symptoms by increasing ACh levels.
    • Thymectomy is considered, especially when thymoma is suspected.

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    Neuromuscular Transmission & Myasthenia Gravis PDF

    Description

    This quiz covers the essential concepts of neuromuscular transmission and the physiology of skeletal muscles. It explores the mechanisms behind voluntary muscle movement, muscle structure, and the roles of various muscle proteins. Test your understanding of these fundamental topics in human physiology.

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