Botox and Neuromuscular Junction
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Questions and Answers

What mechanism does botulinum toxin employ to produce its paralyzing effects?

  • Inhibiting the release of dopamine at synapses
  • Blocking the presynaptic release of acetylcholine (correct)
  • Increasing the reuptake of neurotransmitters
  • Enhancing calcium ion release from the sarcoplasmic reticulum

    • Which of the following statements best describes the process of excitation-contraction coupling?

  • The propagation of an action potential down the T-tubules leads to calcium release from the terminal cisterna (correct)
  • The binding of ATP to myosin activates cross-bridge cycling in muscle fibers
  • The release of acetylcholine triggers muscle contraction through calcium ion influx
  • The process by which calcium ions are synthesized within muscle cells

    • What is the primary cause of muscle weakness in myasthenia gravis?

  • Destruction of myelin sheath surrounding neurons
  • Excessive production of acetylcholine at the neuromuscular junction
  • Blocking of neuromuscular junction by antibodies against acetylcholine receptors (correct)
  • Deficiency of calcium ions needed for muscle contraction

    • Which health problem can botulinum toxin be used to treat?

    <p>Cervical dystonia</p> Signup and view all the answers

    What role do calcium ions play in the contraction of muscle cells?

    <p>They promote the binding of myosin to actin filaments</p> Signup and view all the answers

    What is the primary function of the neuromuscular junction?

    <p>To transmit signals from motor neurons to skeletal muscle cells</p> Signup and view all the answers

    Which ion is primarily responsible for triggering muscle contraction during the excitation-contraction coupling process?

    <p>Calcium (Ca)</p> Signup and view all the answers

    How does botulinum toxin affect muscle function?

    <p>It blocks neurotransmitter release at the neuromuscular junction</p> Signup and view all the answers

    In myasthenia gravis, what is the primary issue affecting muscle function?

    <p>Autoimmune response affecting acetylcholine receptors</p> Signup and view all the answers

    During depolarization of a muscle cell, which ion influx occurs primarily?

    <p>Sodium (Na)</p> Signup and view all the answers

    What effect does hyperpolarization have on a muscle cell's ability to contract?

    <p>It makes the membrane potential more negative and less excitable</p> Signup and view all the answers

    What is the typical resting membrane potential range for skeletal muscle cells?

    <p>-70 to -90 mV</p> Signup and view all the answers

    Which of the following correctly describes repolarization in muscle action potential?

    <p>Potassium ions flow out of the cell restoring the negative potential</p> Signup and view all the answers

    What occurs immediately after the depolarization of the L-type Ca2+ channel in the T-tubule membrane?

    <p>Calcium ions are released from the SR into the cytosol.</p> Signup and view all the answers

    Which receptor is responsible for the release of calcium ions from the SR?

    <p>Ryanodine receptor</p> Signup and view all the answers

    In excitation-contraction coupling, what is the role of increased intracellular calcium concentration?

    <p>It activates troponin C.</p> Signup and view all the answers

    What is the main cause of muscle paralysis in individuals with myasthenia gravis?

    <p>Autoantibodies attacking acetylcholine receptors.</p> Signup and view all the answers

    What is the mechanism of action of curare at the neuromuscular junction?

    <p>It competes with acetylcholine for binding to nicotinic receptors.</p> Signup and view all the answers

    Which of the following describes calcium-induced calcium release (CICR)?

    <p>Intracellular calcium activation of the Ca2+-release channel.</p> Signup and view all the answers

    What is the role of acetylcholine esterase following muscle stimulation?

    <p>To break down acetylcholine at the neuromuscular junction.</p> Signup and view all the answers

    Which type of muscle fiber contraction is directly initiated by excitation-contraction coupling?

    <p>Twitch contraction</p> Signup and view all the answers

    What is one of the medical uses of Botox?

    <p>Temporary smoothing of facial wrinkles</p> Signup and view all the answers

    Which condition can be treated with Botox injections?

    <p>Severe underarm sweating</p> Signup and view all the answers

    How does botulinum toxin induce paralysis?

    <p>By blocking presynaptic release of acetylcholine</p> Signup and view all the answers

    What role does the neuromuscular junction play in muscle contraction?

    <p>It facilitates communication between motor neurons and muscle cells.</p> Signup and view all the answers

    What would typically occur when acetylcholine binds at the motor end plate?

    <p>Depolarization of the muscle cell membrane</p> Signup and view all the answers

    What is the duration of an action potential in muscle cells?

    <p>1 to 5 ms</p> Signup and view all the answers

    What is the resting membrane potential range for skeletal muscle cells?

    <p>-70 to -90 mV</p> Signup and view all the answers

    Which stage of action potential follows depolarization?

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

    What role does the motor neuron serve in muscle contraction?

    <p>It stimulates each muscle cell for contraction.</p> Signup and view all the answers

    What is the typical velocity of conduction for muscle action potentials?

    <p>3 to 5 m/sec</p> Signup and view all the answers

    What indicates the polarized condition of a muscle cell at rest?

    <p>Resting membrane potential</p> Signup and view all the answers

    During which action potential stage does the muscle cell become less negative than its resting state?

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

    What structure electrically isolates skeletal muscle cells from each other?

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

    What triggers the opening of voltage-regulated calcium channels at the axon terminal?

    <p>Arrival of action potential</p> Signup and view all the answers

    What is the immediate effect of calcium entering the axon terminal?

    <p>Fusion of synaptic vesicles</p> Signup and view all the answers

    What is the primary action that acetylcholine performs when it binds to receptor sites on the motor end plate?

    <p>Opens channels for sodium and potassium ion exchange</p> Signup and view all the answers

    What process results from the conformational changes in the L-type Ca2+ channels during depolarization?

    <p>Entry of calcium ions through channel pores</p> Signup and view all the answers

    What role does the action potential play in the release of calcium ions from the terminal cisternae?

    <p>It causes the release of calcium ions from terminal cisternae</p> Signup and view all the answers

    What type of channels are primarily activated in the T tubules during depolarization?

    <p>L-type Ca2+ channels</p> Signup and view all the answers

    Which drug class inhibits the L-type Ca2+ channels known as DHP receptors?

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

    What is primarily responsible for the local depolarization of the motor end plate?

    <p>Sodium ion influx and potassium efflux</p> Signup and view all the answers

    What triggers the release of calcium ions from the sarcoplasmic reticulum (SR) during muscle contraction?

    <p>Depolarization of the L-type Ca2+ channel in the T-tubule membrane</p> Signup and view all the answers

    How does curare affect muscle function at the neuromuscular junction?

    <p>It prevents acetylcholine from depolarizing the motor endplate.</p> Signup and view all the answers

    What is the effect of acetylcholine esterase after muscle stimulation?

    <p>Breaks down acetylcholine to terminate the signal.</p> Signup and view all the answers

    What disease is characterized by antibodies attacking acetylcholine receptors?

    <p>Myasthenia gravis</p> Signup and view all the answers

    What mechanism is involved in calcium-induced calcium release (CICR)?

    <p>Calcium ions facilitate the release of additional calcium from the SR.</p> Signup and view all the answers

    What role do the ryanodine receptors play in skeletal muscle contraction?

    <p>They facilitate the release of calcium ions from the SR.</p> Signup and view all the answers

    What is a characteristic feature of myasthenia gravis?

    <p>Autoimmune attack on acetylcholine receptors</p> Signup and view all the answers

    What is the consequence of curare's effect on nicotinic receptors?

    <p>It leads to muscle paralysis due to inhibition of ACh binding.</p> Signup and view all the answers

    Study Notes

    Botox

    • Botox is a drug made from a toxin produced by the bacterium Clostridium botulinum.
    • Botox is used in small doses to treat health problems like:
      • Temporary smoothing of facial wrinkles.
      • Severe underarm sweating.
      • Cervical dystonia.
      • Blepharospasm.
      • Strabismus.
      • Chronic migraine.
      • Overactive bladder.
    • Botox injections work by weakening or paralyzing certain muscles or by blocking certain nerves.
    • Botox injections are used to block the release of acetylcholine at the neuromuscular junction, causing muscle paralysis.

    Neuromuscular Junction

    • Each skeletal muscle cell is individually stimulated by a motor neuron.
    • The neuromuscular junction is where a motor neuron meets a muscle cell membrane.
    • The arrival of an action potential at the axon terminal triggers the release of acetylcholine.
    • Acetylcholine binds to receptors on the muscle cell membrane, causing depolarization.
    • Depolarization of the muscle cell membrane triggers an action potential that spreads along the sarcolemma and down to T tubules.
    • The action potential in the T tubules causes the release of calcium ions from the terminal cisternae into the cytosol.
    • The increased calcium concentration in the cytosol triggers muscle contraction.

    Calcium Channels and Muscle Contraction

    • The action potential causes a conformational change in the Ca2+-release channel, which is located in the SR membrane.
    • The Ca2+-release channel is also known as the ryanodine receptor.
    • After depolarization of the L-type Ca2+ channel on the T-tubule membrane, calcium stored in the SR rapidly leaves through the Ca2+-release channel.
    • The increase in intracellular calcium concentration activates troponin C, initiating the formation of cross-bridges between myofilaments.
    • The entire process, from depolarization of the T-tubule membrane to the initiation of the cross-bridge cycle of contraction, is called excitation-contraction coupling (ECC).
    • There is direct mechanical coupling between the L-type Ca2+ channel in the T-tubule membrane and the Ca2+-release channel of the SR.
    • Calcium can also directly activate the Ca2+-release channel in the SR, a process known as Ca2+-induced Ca2+ release (CICR).

    Acetylcholine Breakdown

    • Acetylcholine diffuses away from its receptor site after a brief period.
    • Acetylcholine is broken down by the enzyme acetylcholinesterase.

    Myasthenia Gravis

    • Myasthenia gravis is an autoimmune disease affecting one in every 20,000 individuals.
    • Antibodies attack acetylcholine receptors at the neuromuscular junction, leading to muscle weakness and fatigue.

    Curare

    • Curare is a highly toxic substance used by certain indigenous tribes in South America to poison hunting arrows.
    • Curare binds to nicotinic receptors at the neuromuscular junction, preventing acetylcholine binding and depolarization of the motor endplate.
    • This leads to muscle paralysis.
    • The effects of curare can be reversed by high concentrations of acetylcholine.

    Muscle Action Potential

    • The resting membrane potential of a muscle cell is -70 to -90 mV.
    • The duration of a muscle action potential is 1 to 5 milliseconds.
    • The velocity of conduction of a muscle action potential is 3 to 5 m/sec.

    Stages of Action Potential

    • Resting: The cell membrane is polarized, with a negative charge inside the cell and a positive charge outside.
    • Depolarization: The cell membrane becomes permeable to sodium ions, leading to a rapid influx of sodium ions and a reversal of the membrane potential.
    • Repolarization: The cell membrane becomes permeable to potassium ions, leading to a rapid efflux of potassium ions and a return of the membrane potential to its resting state.
    • Hyperpolarization: The cell membrane becomes more permeable to potassium ions, leading to a brief period of hyperpolarization beyond the resting membrane potential.
    • Resting: The cell membrane returns to its resting state, with the sodium-potassium pump actively restoring the resting membrane potential.

    Role of Motor Neuron

    • A motor neuron is a single nerve cell that extends from the brain or spinal cord to a muscle or gland.
    • Skeletal muscle cells are electrically isolated from each other.
    • Each skeletal muscle cell must be stimulated by a motor neuron to contract.

    Muscle Action Potential

    • Resting membrane potential: -70 to -90 mV
    • Duration of action potential: 1 to 5 ms
    • Velocity of conduction: 3 to 5 m/sec

    Stages of Action Potential

    • Resting: The membrane is polarized, with a negative charge inside the cell and a positive charge outside.
    • Depolarization: Sodium ions rush into the cell, making the inside more positive.
    • Repolarization: Potassium ions move out of the cell, restoring the negative charge inside.
    • Hyperpolarization: The membrane potential becomes more negative than the resting potential.
    • Resting: The membrane returns to its resting potential.

    Role of Motor Neuron

    • Each skeletal muscle cell is individually stimulated by a motor neuron.
    • Motor neurons extend from the brain or spinal cord to muscle or glands.
    • Skeletal muscle cells are electrically isolated from each other by endomysium.
    • In order for skeletal muscle cells to contract, each cell must be stimulated by a motor neuron.

    Neuromuscular Junction

    • The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber.
    • The junction is characterized by a synaptic cleft, which is a space between the motor neuron's axon terminal and the muscle fiber's motor end plate.
    • Resting membrane potential: -70 to -90 mV

    Action Potential at Axon Terminal

    • The arrival of an action potential at the axon terminal opens voltage-regulated calcium channels, allowing calcium ions to enter the axon terminal.

    Fusion of Synaptic Vesicles

    • Calcium ions cause synaptic vesicles to fuse with the membrane of the axon terminal.

    Release of Acetylcholine

    • Acetylcholine (ACh) is released into the synaptic cleft by exocytosis.

    Acetylcholine Binding

    • ACh binds to receptor sites on the motor end plate, opening chemically regulated ion channels.
    • This binding causes an influx of sodium ions and a small efflux of potassium ions, leading to local depolarization of the motor end plate.

    The Nicotinic Acetylcholine Receptor

    • The nicotinic acetylcholine receptor is a type of ligand-gated ion channel found at the neuromuscular junction.
    • It is responsible for transmitting signals from motor neurons to muscle fibers.

    Action Potential Propagation

    • The depolarization of the motor end plate triggers an action potential that propagates along the sarcolemma and down to T tubules.

    Calcium Release from Terminal Cisternae

    • The action potential causes the release of calcium ions from the terminal cisternae into the cytosol.

    How Calcium Triggers Contraction

    • The action potential causes depolarization of the T tubules, activating L-type Ca2+ channels.
    • Conformational changes in the L-type Ca2+channels induce a conformational change in the Ca2+-release channel, which is located in the SR membrane.
    • Calcium stored in the SR rapidly leaves through the Ca2+-release channel.

    Excitation-Contraction Coupling (ECC)

    • ECC refers to the process from the depolarization of the T-tubule membrane to the initiation of the cross-bridge cycle of contraction.
    • ECC involves direct mechanical coupling between the L-type Ca2+channel in the T-tubule membrane and the Ca2+-release channel of the SR.

    Breakdown of Acetylcholine

    • After a brief period, acetylcholine diffuses away from its receptor site and the ion channel closes.
    • Acetylcholine is broken down by the enzyme acetylcholinesterase.

    Myasthenia Gravis

    • Myasthenia Gravis is an autoimmune disease that affects the neuromuscular junction.
    • It occurs in 1 in every 20,000 persons.
    • Antibodies attack the acetylcholine receptors, disrupting the transmission of nerve impulses to muscle fibers.

    Curare

    • Curare is a toxin used by some South American tribes.
    • Curare binds directly to nicotinic receptors on the postsynaptic membrane of the neuromuscular junction, preventing the binding of ACh and depolarization of the motor end plate.
    • This leads to muscle paralysis.

    Botox

    • Botox is a drug made from a toxin produced by the bacterium Clostridium botulinum.
    • It blocks presynaptic release of acetylcholine at the neuromuscular junction, inducing partial paralysis and atrophy.
    • Botox injections are used to treat a variety of conditions, including facial wrinkles, excessive sweating, cervical dystonia, and migraine headaches.

    Summary

    • Each skeletal muscle cell is individually stimulated by a motor neuron.
    • The neuromuscular junction mediates communication between a motor neuron and a muscle fiber.
    • Action potentials trigger the release of acetylcholine, leading to depolarization of the motor end plate and initiation of contraction.

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