Excitation of Skeletal Muscle: Neuromuscular Transmission
128 Questions
0 Views

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to lesson

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What role does acetylcholinesterase play in muscle fiber excitation?

  • It enhances the action of acetylcholine.
  • It stimulates the release of more acetylcholine.
  • It increases the duration of acetylcholine presence.
  • It breaks down acetylcholine in the synaptic space. (correct)

    • Why is there a limited duration for acetylcholine action in the synaptic space?

  • It is predominantly destroyed by acetylcholinesterase. (correct)
  • It requires re-synthesis before action can occur.
  • It is quickly absorbed by muscle fibers.
  • It binds permanently to receptors.

    • What effect does the rapid removal of acetylcholine have on muscle fibers?

  • It increases the muscle's sensitivity to stimulation.
  • It prevents over-excitation and facilitates recovery. (correct)
  • It prolongs muscle contraction.
  • It leads to constant relaxation of the muscle.

    • What is a consequence of fatigue at the neuromuscular junction?

    <p>It inhibits re-excitation of the muscle fiber.</p> Signup and view all the answers

    What happens to acetylcholine after it diffuses out of the synaptic space?

    <p>It becomes unavailable for muscle fiber excitation.</p> Signup and view all the answers

    What could potentially cause a weakened end plate potential?

    <p>Decreased release of acetylcholine by toxins.</p> Signup and view all the answers

    How long does acetylcholine typically remain active in the synaptic space?

    <p>A few milliseconds.</p> Signup and view all the answers

    What physiological process is similar to fatigue of the neuromuscular junction?

    <p>Fatigue in the central nervous system due to over-excited synapses.</p> Signup and view all the answers

    What is the main role of calcium ions at the neuromuscular junction?

    <p>To stimulate the fusion of acetylcholine vesicles with the neural membrane</p> Signup and view all the answers

    Which structure is primarily responsible for insulating the motor end plate?

    <p>Schwann cells</p> Signup and view all the answers

    What is the width of the synaptic space at the neuromuscular junction?

    <p>20 to 30 nanometers</p> Signup and view all the answers

    What is the primary function of the acetylcholine receptors located in the muscle fiber membrane?

    <p>To open ion channels in response to acetylcholine binding</p> Signup and view all the answers

    What type of process is involved in the release of acetylcholine from vesicles?

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

    What are the smaller folds at the bottom of the synaptic gutter called?

    <p>Subneural clefts</p> Signup and view all the answers

    Which of the following accurately describes the neuromuscular junction structure?

    <p>It includes a complex of branching nerve terminals that penetrate the muscle fiber membrane.</p> Signup and view all the answers

    What initiates the opening of voltage-gated calcium channels at the motor end plate?

    <p>Depolarization of the nerve terminal membrane due to an action potential</p> Signup and view all the answers

    What role do mitochondria play in the axon terminal?

    <p>They provide adenosine triphosphate (ATP) for neurotransmitter synthesis.</p> Signup and view all the answers

    What is the function of acetylcholine at the motor end plate?

    <p>It excites the muscle fiber, leading to contraction.</p> Signup and view all the answers

    Where are acetylcholine-gated ion channels primarily located?

    <p>Near the dense bar areas of the muscle fiber membrane.</p> Signup and view all the answers

    What initiates the opening of the acetylcholine-gated ion channel?

    <p>The conformational change when two acetylcholine molecules bind to alpha subunits.</p> Signup and view all the answers

    What is concentrated in the synaptic vesicles at the axon terminal?

    <p>Acetylcholine for neurotransmission.</p> Signup and view all the answers

    Which structure is involved in the facilitation of synaptic transmission at the motor end plate?

    <p>Dense bar that organizes neurotransmitter release.</p> Signup and view all the answers

    What happens after acetylcholine is emptied into the synaptic space?

    <p>A conformational change allows channel opening.</p> Signup and view all the answers

    What is located immediately below the dense bar areas in the muscle fiber membrane?

    <p>Subneural clefts where synaptic communication occurs.</p> Signup and view all the answers

    What role do calcium ions play when an action potential arrives at the nerve terminal?

    <p>They increase the rate of fusion of vesicles with the membrane.</p> Signup and view all the answers

    Which statement accurately describes the function of acetylcholinesterase?

    <p>It inactivates acetylcholine to terminate its action.</p> Signup and view all the answers

    What occurs as a result of D-tubocurarine application at the neuromuscular junction?

    <p>It blocks acetylcholine receptors.</p> Signup and view all the answers

    What happens to acetylcholine after it is released into the synaptic space?

    <p>It is broken down by acetylcholinesterase.</p> Signup and view all the answers

    What type of drugs can prevent impulses from reaching the muscle at the neuromuscular junction?

    <p>Curariform drugs.</p> Signup and view all the answers

    What is the primary effect of diisopropyl fluorophosphate on the nervous system?

    <p>It inactivates acetylcholinesterase for a prolonged period.</p> Signup and view all the answers

    How many acetylcholine molecules are stored in a single vesicle?

    <p>About 10,000 molecules.</p> Signup and view all the answers

    What consequence can occur from excessive stimulation of the muscle by acetylcholine?

    <p>Muscle spasms or laryngeal spasm.</p> Signup and view all the answers

    What role does acetylcholinesterase play in the neuromuscular junction?

    <p>It splits acetylcholine into acetate ion and choline.</p> Signup and view all the answers

    What is the consequence of the antibodies in myasthenia gravis?

    <p>They block or destroy acetylcholine receptors at the postsynaptic junction.</p> Signup and view all the answers

    How does myasthenia gravis primarily affect muscle function?

    <p>By reducing the strength of end plate potentials to initiate action potentials.</p> Signup and view all the answers

    What are ‘coated pits’ in the context of the neuromuscular junction?

    <p>They are membrane invaginations involved in vesicle formation.</p> Signup and view all the answers

    What typically triggers the rapid reformation of vesicles at the nerve terminal?

    <p>The contraction of proteins like clathrin in the nerve ending.</p> Signup and view all the answers

    In myasthenia gravis, how many vesicles are typically available for transmission at the neuromuscular junction?

    <p>Around 125 vesicles.</p> Signup and view all the answers

    What is the timeframe for the sequence of events following an action potential in the neuromuscular junction?

    <p>5 to 10 milliseconds.</p> Signup and view all the answers

    What happens if the disease intensity of myasthenia gravis is severe enough?

    <p>The patient may die from paralysis.</p> Signup and view all the answers

    What role do dihydropyridine receptors play in the excitation-contraction coupling process?

    <p>They activate calcium release channels in the sarcoplasmic reticulum.</p> Signup and view all the answers

    How do T tubules interact with the extracellular fluid?

    <p>They open to the exterior, allowing communication with extracellular fluid.</p> Signup and view all the answers

    What initiates the release of calcium ions from the sarcoplasmic reticulum?

    <p>Changes in voltage sensed by dihydropyridine receptors.</p> Signup and view all the answers

    What structural feature facilitates the branching of T tubules among myofibrils?

    <p>The interlacing of T tubules among themselves.</p> Signup and view all the answers

    What happens to the calcium release channels after they are activated?

    <p>They remain open for a few milliseconds to release calcium.</p> Signup and view all the answers

    What is the primary effect of calcium ions released into the sarcoplasm?

    <p>They cause muscle fibers to contract.</p> Signup and view all the answers

    Why is the calcium pump important after muscle contraction occurs?

    <p>It removes calcium ions from the myofibrillar fluid.</p> Signup and view all the answers

    In which location do dihydropyridine receptors initiate the physiological changes leading to muscle contraction?

    <p>At the junction of T tubules and sarcoplasmic reticulum.</p> Signup and view all the answers

    What mechanism is primarily responsible for the rapid removal of acetylcholine from the synaptic space?

    <p>Degradation by acetylcholinesterase</p> Signup and view all the answers

    Which of the following effects is associated with the fatigue of the neuromuscular junction?

    <p>Reduced availability of acetylcholine</p> Signup and view all the answers

    What can occur when acetylcholine diffuses out of the synaptic space?

    <p>Reduction in the strength of muscle contraction</p> Signup and view all the answers

    Which statement best describes the time frame of acetylcholine action in the synaptic space?

    <p>Remains effective for a few milliseconds to excite muscle fibers</p> Signup and view all the answers

    What is one potential outcome of acetylcholine being rapidly removed from the synaptic space?

    <p>Prevention of excessive muscle contraction</p> Signup and view all the answers

    What is the main function of synaptic vesicles at the neuromuscular junction?

    <p>Store and release acetylcholine into the synaptic space</p> Signup and view all the answers

    Which of the following structures is primarily identified at the neuromuscular junction?

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

    How does the presence of acetylcholinesterase in the synaptic space primarily affect neuromuscular transmission?

    <p>It degrades acetylcholine to terminate its action</p> Signup and view all the answers

    What is the approximate number of acetylcholine vesicles released when a nerve impulse reaches the neuromuscular junction?

    <p>125</p> Signup and view all the answers

    Which of the following best describes the synaptic space at the neuromuscular junction?

    <p>It contains acetylcholinesterase which rapidly degrades acetylcholine</p> Signup and view all the answers

    What is the role of myelinated nerve fibers in the excitation of skeletal muscle?

    <p>They increase the speed of action potential transmission</p> Signup and view all the answers

    What type of motor control do large motoneurons in the anterior horns of the spinal cord provide?

    <p>Gross motor control for large muscle groups</p> Signup and view all the answers

    What causes a conformational change that opens acetylcholine-gated ion channels?

    <p>Binding of two acetylcholine molecules to the alpha subunits</p> Signup and view all the answers

    Where are mitochondria primarily located in relation to the synaptic space?

    <p>Near the mouths of the subneural clefts</p> Signup and view all the answers

    What is the role of the dense bar in the structure of the neuromuscular junction?

    <p>To serve as a site for acetylcholine release</p> Signup and view all the answers

    What is primarily concentrated in the synaptic vesicles at the axon terminal?

    <p>Acetylcholine for muscle excitation</p> Signup and view all the answers

    What specific structural characteristic helps in the facilitation of synaptic transmission?

    <p>Subneural clefts that increase receptor surface area</p> Signup and view all the answers

    What triggers the release of acetylcholine from the vesicles at the neuromuscular junction?

    <p>Elevation of intracellular calcium levels</p> Signup and view all the answers

    Which type of channel remains constricted until acetylcholine binds to it?

    <p>Acetylcholine-gated ion channels</p> Signup and view all the answers

    Which component supports the release sites for neurotransmitters at the neuromuscular junction?

    <p>Dense bars that organize membrane proteins</p> Signup and view all the answers

    What immediate effect does acetylcholine have upon binding to its receptors?

    <p>Opening of ion channels for sodium influx</p> Signup and view all the answers

    What is the primary result of botulinum toxin's effect at the neuromuscular junction?

    <p>Decreased quantity of acetylcholine release</p> Signup and view all the answers

    What physiological effect occurs when stimulation of the nerve fiber exceeds 100 impulses per second for several minutes?

    <p>Diminished number of acetylcholine vesicles</p> Signup and view all the answers

    What prevents chloride ions from passing through the acetylcholine-gated channel at the neuromuscular junction?

    <p>Negative charges at the channel mouth</p> Signup and view all the answers

    What happens to acetylcholine once it activates the acetylcholine receptors?

    <p>It gets destroyed by acetylcholinesterase</p> Signup and view all the answers

    What characterizes the safety factor at the neuromuscular junction under normal conditions?

    <p>It typically results in more impulses than needed for excitation</p> Signup and view all the answers

    What is a consequence of the destruction of acetylcholine by acetylcholinesterase in the synaptic space?

    <p>Decreased efficiency of signal transmission</p> Signup and view all the answers

    What typically limits the effective stimulation of the neuromuscular junction during extended activity?

    <p>Loss of acetylcholine due to continuous release</p> Signup and view all the answers

    What typically occurs to the motor end plate during fatigue of the neuromuscular junction?

    <p>Reduced release of acetylcholine vesicles</p> Signup and view all the answers

    Why is the safety factor at the neuromuscular junction considered high?

    <p>It ensures a high margin of safety for muscle activation</p> Signup and view all the answers

    How does acetylcholine behave in the synaptic space once it has been released?

    <p>It continuously activates receptors until degraded</p> Signup and view all the answers

    What is the primary consequence of the sudden influx of sodium ions into the muscle fiber during neuromuscular transmission?

    <p>Creation of end plate potential</p> Signup and view all the answers

    What is the effect of opening acetylcholine-gated channels in the muscle fiber membrane?

    <p>Elevated positive charge inside the fiber</p> Signup and view all the answers

    How does the molecular structure of the acetylcholine receptor contribute to its function?

    <p>It forms a tubular channel for sodium ions</p> Signup and view all the answers

    What is the total molecular weight of the acetylcholine receptor complex?

    <p>275,000</p> Signup and view all the answers

    What initiates the action potential in a muscle fiber following the creation of an end plate potential?

    <p>A local positive potential change</p> Signup and view all the answers

    Which protein subunits comprise the acetylcholine receptor complex?

    <p>Two alpha, one beta, one delta, and one gamma</p> Signup and view all the answers

    What potential change occurs in the muscle fiber when sodium ions enter during neuromuscular transmission?

    <p>Positive potential change of 50 to 75 millivolts</p> Signup and view all the answers

    What event follows the creation of the end plate potential in the muscle fiber membrane?

    <p>Propagation of an action potential along the membrane</p> Signup and view all the answers

    What is not a characteristic of the acetylcholine receptor in muscle fibers?

    <p>It selectively allows potassium ions to flow</p> Signup and view all the answers

    What is a significant outcome of the action potential that spreads along the muscle membrane?

    <p>It leads to muscle fiber contraction</p> Signup and view all the answers

    What primarily prevents negatively charged ions from passing through the acetylcholine-gated channel?

    <p>The strong negative charges in the channel's mouth</p> Signup and view all the answers

    Why do sodium ions predominantly flow through acetylcholine-gated channels in muscle tissues?

    <p>They are attracted by the negative potential inside the muscle membrane</p> Signup and view all the answers

    What characteristic of the acetylcholine-gated channel contributes to ion selectivity?

    <p>The selective permeability based on ion size</p> Signup and view all the answers

    What is the typical resting membrane potential of a muscle fiber?

    <p>-80 to -90 millivolts</p> Signup and view all the answers

    What factors lead to a greater influx of sodium ions relative to potassium ions in muscle fibers?

    <p>Negative potential inside the muscle membrane and lower sodium concentration outside</p> Signup and view all the answers

    Which ions are primarily involved in the acetylcholine-gated channel's activity?

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

    Why does the acetylcholine-gated channel not allow chloride ions to pass through?

    <p>The negative charges at the channel's mouth repel negatively charged chloride ions</p> Signup and view all the answers

    What occurs when calcium ion concentration inside the nerve terminal increases?

    <p>It causes an immediate release of acetylcholine into the synaptic space.</p> Signup and view all the answers

    What is the primary action of diisopropyl fluorophosphate on acetylcholinesterase?

    <p>It inactivates acetylcholinesterase for an extended period.</p> Signup and view all the answers

    What is a potential consequence of excessive acetylcholine accumulation at the neuromuscular junction?

    <p>Muscle spasms and possible respiratory failure.</p> Signup and view all the answers

    What is the mechanism of action for curariform drugs at the neuromuscular junction?

    <p>They block acetylcholine receptors on muscle fibers.</p> Signup and view all the answers

    What primary function do the small vesicles at the nerve terminal serve?

    <p>They transport acetylcholine to the synaptic cleft for exocytosis.</p> Signup and view all the answers

    What is the primary impact of opening acetylcholine-gated channels in muscle fibers?

    <p>It permits an influx of sodium ions and positive charges.</p> Signup and view all the answers

    Which proteins are primarily involved in the structure of the receptor complex for acetylcholine?

    <p>Two alpha, one beta, one delta, and one gamma proteins.</p> Signup and view all the answers

    What electrical change occurs inside the muscle fiber when sodium ions rush in due to opened acetylcholine-gated channels?

    <p>A positive potential increase of 50 to 75 millivolts.</p> Signup and view all the answers

    What is created as a result of the end plate potential within the muscle fiber?

    <p>An action potential that propagates along the muscle membrane.</p> Signup and view all the answers

    What is the primary difference between acetylcholine and drugs like methacholine or nicotine?

    <p>These drugs persist in action due to slower degradation.</p> Signup and view all the answers

    What structural configuration do the proteins in the acetylcholine receptor create?

    <p>A circular formation that allows selective ion passage.</p> Signup and view all the answers

    What triggers a new action potential in muscle fibers after contraction?

    <p>Localized depolarization areas initiating due to ion leakage.</p> Signup and view all the answers

    How does the end plate potential contribute to muscle contraction?

    <p>It generates a local change that can trigger an action potential.</p> Signup and view all the answers

    What substances inactivate acetylcholinesterase, enhancing neuromuscular transmission?

    <p>Neostigmine, physostigmine, and diisopropyl fluorophosphate.</p> Signup and view all the answers

    Which structure is primarily responsible for the formation of small vesicles in motoneurons?

    <p>Golgi apparatus.</p> Signup and view all the answers

    What type of potential is specifically referred to as the result of sodium influx through acetylcholine-gated channels?

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

    What is the primary consequence of the positive potential change created by sodium ions in the muscle fiber?

    <p>Initiation of an action potential leading to contraction.</p> Signup and view all the answers

    What is the impact of drugs that stimulate the muscle fiber by mimicking acetylcholine?

    <p>They initiate constant contractions and spasms.</p> Signup and view all the answers

    Under what conditions is measurable fatigue at the neuromuscular junction likely to occur?

    <p>Only at the most exhausting levels of muscle activity.</p> Signup and view all the answers

    What initiates depolarization of the muscle fiber membrane at the motor end plate?

    <p>Binding of acetylcholine to receptors.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of drugs that stimulate the neuromuscular junction?

    <p>They are destroyed by cholinesterase.</p> Signup and view all the answers

    What role does the neuromuscular junction play in chemical transmission of signals?

    <p>It acts as a connection point where muscle fibers receive excitatory signals.</p> Signup and view all the answers

    Which process primarily contributes to the rapid elimination of acetylcholine from the synaptic space?

    <p>Degradation by acetylcholinesterase</p> Signup and view all the answers

    What is the primary location of acetylcholinesterase in relation to the synaptic structure?

    <p>Associated with the spongy connective tissue in the synaptic space</p> Signup and view all the answers

    What effect does acetylcholine diffusion out of the synaptic space have on muscle fibers?

    <p>Decreases the availability of the neurotransmitter</p> Signup and view all the answers

    Which factor is likely responsible for the cessation of muscle contraction following an action potential?

    <p>Insufficient acetylcholine concentration in the synaptic space</p> Signup and view all the answers

    What does the phenomenon of fatigue at the neuromuscular junction primarily indicate?

    <p>Decreased efficiency of neurotransmitter release</p> Signup and view all the answers

    How does the action of acetylcholinesterase affect muscle fiber recovery time?

    <p>It accelerates recovery by reducing acetylcholine levels quickly</p> Signup and view all the answers

    What is the underlying reason for a weakened end plate potential due to botulinum toxin?

    <p>Decreased release of acetylcholine</p> Signup and view all the answers

    What role does the brief presence of acetylcholine in the synaptic space play during muscle excitation?

    <p>It allows for immediate and short-lived excitation</p> Signup and view all the answers

    In terms of neurotransmitter action, what is a similarity between the neuromuscular junction and synapses in the central nervous system?

    <p>Both can experience fatigue when overstimulated</p> Signup and view all the answers

    What is the typical timeframe within which acetylcholine acts before being cleared from the synaptic space?

    <p>A fraction of a second</p> Signup and view all the answers

    Study Notes

    Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling

    • Skeletal muscle fibers are innervated by large myelinated nerve fibers originating from motoneurons in the spinal cord.
    • Each nerve fiber branches, typically stimulating 3-hundreds of muscle fibers
    • Each nerve ending forms a neuromuscular junction with the muscle fiber near its midpoint.
    • Two percent of muscle fibers have more than one junction.
    • The neuromuscular junction is also called the motor end plate.

    Physiology of the Neuromuscular Junction

    • Nerve terminals invaginate into the muscle fiber surface, but remain outside the plasma membrane.
    • The motor end plate is insulated by Schwann cells.
    • The invaginated muscle membrane is called the synaptic gutter or synaptic trough.
    • The space between the terminal and fiber membrane is the synaptic space/cleft (20-30 nanometers).
    • Subneural clefts in the gutter increase surface area for transmitter action.
    • Mitochondria in the axon terminal supply ATP for acetylcholine synthesis.
    • Acetylcholine is rapidly absorbed into synaptic vesicles.
    • Acetylcholinesterase in the synaptic space destroys acetylcholine.

    Secretion of Acetylcholine

    • Nerve impulses trigger the release of about 125 acetylcholine vesicles into the synaptic space.
    • Voltage-gated calcium channels open upon action potential arrival.
    • Calcium ions diffuse from the space, attracting acetylcholine vesicles to the neural membrane.
    • Vesicles fuse with the membrane and release acetylcholine via exocytosis.
    • Calcium ion entry is the key stimulus for acetylcholine release.

    Effect of Acetylcholine on Postsynaptic Membrane

    • Acetylcholine receptors are gated ion channels.
    • These receptors are primarily near the mouths of the subneural clefts.
    • Acetylcholine binding opens the channels, allowing sodium, potassium, and calcium ions to flow.
    • Sodium ions flow inwards due to the negative interior potential.
    • This creates a local positive potential change—end-plate potential (EPP).
    • EPPs initiate action potentials that spread along the muscle membrane, causing contraction.

    Destruction of Acetylcholine

    • Acetylcholinesterase rapidly breaks down acetylcholine in the synaptic cleft.
    • This prevents continued muscle re-excitation after the initial action potential.

    End-Plate Potential and Muscle Fiber Excitation

    • Sodium influx creates electrical potential changes near the end plate—end-plate potential (EPP).
    • Sufficient EPPs trigger action potential initiation in the muscle cell membrane.
    • Curare blocks acetylcholine's effect (weakened EPP).
    • Botulinum toxin reduces acetylcholine release. (weakened EPP).

    Safety Factor and Fatigue

    • The neuromuscular junction has a high safety factor (EPP is stronger than required for muscle excitation).
    • Prolonged high-frequency stimulation causes the junctional fatigue (reduced acetylcholine release).

    Myasthenia Gravis

    • Autoimmune disease where antibodies attack acetylcholine receptors.
    • Results in reduced transmission between the nerve and muscle, causing muscle weakness.

    Muscle Action Potential

    • Skeletal muscle action potential duration: 1-5 milliseconds
    • Skeletal muscle resting membrane potential: -80 to -90 mV.
    • Conduction velocity: ~3-5 m/s (Significantly slower than nerve fibers).

    Spread of Action Potential

    • Action potentials along the surface membrane don't penetrate deep into the fiber.
    • Transverse tubules (T tubules) conduct the action potential deep into the fiber, where they initiate calcium release from the sarcoplasmic reticulum (SR).

    Transverse Tubule-Sarcoplasmic Reticulum System

    • T tubules are extensions of the sarcolemma (muscle cell membrane).
    • They conduct action potentials deeply into the muscle fiber.
    • The SR is a specialized smooth endoplasmic reticulum surrounding myofibrils.
    • SR contains terminal cisternae adjacent to T tubules and longitudinal tubules.
    • Action potentials in T-tubules cause calcium release (from the SR terminal cisternae) into the cytosol.

    Calcium Ions and Muscle Contraction

    • Calcium ions initiate muscle contraction.
    • Calcium pumps in the SR remove calcium ions from the cytosol.
    • This enables muscle relaxation to occur.

    Studying That Suits You

    Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

    Quiz Team

    Related Documents

    Chapter 7: Excitation of Skeletal Muscle (PDF)

    Description

    Explore the fascinating processes of neuromuscular transmission and excitation-contraction coupling in skeletal muscle fibers. Understand how nerve fibers interact with muscle fibers at the motor end plate and the structural adaptations that facilitate this communication. Test your knowledge on key physiological concepts related to muscle excitation.

    More Like This

    Use Quizgecko on...
    Browser
    Open
    Browser
    Browser