Neuromuscular Junction and Muscle Contraction
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Questions and Answers

Acetylcholine is released into the synaptic cleft by ______

exocytosis

Acetylcholine binds to ligand-gated ______ channels on the motor end plate

sodium

Acetylcholinesterase removes acetylcholine from the ______

synaptic cleft

Troponin anchors to the ______ to prevent tropomyosin from uncovering active sites.

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

In relaxed muscle, tropomyosin covers the active sites on the ______ actin subunits

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

Choline is symported with ______ in the presynaptic terminal

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

Myosin heads bind to active sites on ______ molecules to form cross bridges.

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

Excitation-contraction coupling occurs at the ______ which links electrical and mechanical components.

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

During muscle relaxation, calcium diffuses away from the ______ molecules.

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

Isometric contractions occur when the muscle does not ______ but the tension increases.

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

The ______ is the area where stimulation of muscle cells by a nerve happens.

<p>neuromuscular junction</p> Signup and view all the answers

The axonal endings contain small membranous sacs called ______ that hold acetylcholine.

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

The muscle plasma membrane at the neuromuscular junction is known as the ______ plate.

<p>motor end</p> Signup and view all the answers

There is a space between axonal ends and muscle fibers known as the ______ cleft.

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

The first step of muscle contraction at the neuromuscular junction involves an ______ potential arriving at the presynaptic terminal.

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

In response to the action potential, ______ gated calcium channels in the presynaptic membrane open.

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

The muscular system integrates its physiology with the pathophysiology of common ______ and conditions.

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

A penetrative mind and retentive memory are sought through the prayer for ______ before class.

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

Muscle atrophy is age related reduction in muscle mass and regulation of muscle ______.

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

Muscle fatigue is a temporary state of reduced work ______.

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

Cramps are caused by dehydration and ion ______.

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

Tendinitis is inflammation of a tendon or its attachment point due to overuse of a skeletal ______.

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

Myasthenia gravis is an autoimmune disease characterized by the production of autoantibodies that bind to ______ receptors.

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

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene on the ______ chromosome.

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

Muscle soreness occurs after vigorous exercise and is caused by ______ chemical influx into the muscle fibers.

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

The head of myosin molecule binds to an active site on ______.

<p>G actin</p> Signup and view all the answers

Electromyography is a diagnostic test used to evaluate the electrical activity of ______.

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

ATPase is found on ______.

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

A part of the sarcolemma that invaginates into the interior of skeletal muscle fibers is the ______.

<p>T tubule</p> Signup and view all the answers

The type of muscle contraction when a weight lifter is unable to move an object is mostly ______.

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

Calcium ions combine with ______ during muscle contraction.

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

The sequence of muscle contraction starts with calcium ions combining with ______.

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

During muscle contraction, tropomyosin pulls away from ______.

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

The bending and straightening of actin molecules during contraction occur because they have a ______ portion.

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

Which type of muscle is classified as voluntary?

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

What is the basic structural and functional unit of a skeletal muscle?

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

Which option is not considered a function of the muscular system?

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

Which of the following correctly defines extensibility?

<p>Ability of a muscle to be stretched beyond its normal resting length and still contract</p> Signup and view all the answers

Which structure of the muscle fiber is crucial for regulating intracellular calcium levels?

<p>Sarcoplasmic Reticulum</p> Signup and view all the answers

Which of the following definitions best describes the concept of excitability in muscle tissue?

<p>The ability to conduct an electrical impulse</p> Signup and view all the answers

What happens to a muscle's sarcomeres during contraction?

<p>They slide past one another, causing overall shortening of the muscle</p> Signup and view all the answers

Which of the following types of muscle is involuntary?

<p>Cardiac muscle</p> Signup and view all the answers

Which type of muscle is classified as voluntary?

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

What is the primary structural and functional unit of a skeletal muscle?

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

Which of the following is not a recognized function of the muscular system?

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

How is extensibility defined in the context of muscles?

<p>Ability to be stretched beyond normal resting length and still contract</p> Signup and view all the answers

Which structure in the muscle fiber is responsible for regulating intracellular calcium levels?

<p>Sarcoplasmic Reticulum</p> Signup and view all the answers

Which function of the muscular system involves the ability of skeletal muscle to support an upright position?

<p>Maintenance of Posture</p> Signup and view all the answers

What is one function of skeletal muscles in relation to body temperature?

<p>Producing body heat</p> Signup and view all the answers

How does muscle contraction contribute to respiration?

<p>By contracting the diaphragm and thoracic muscles</p> Signup and view all the answers

Which of the following is not generally accepted as a function of the muscular system?

<p>Facilitating digestion</p> Signup and view all the answers

What is produced as a by-product of skeletal muscle contraction?

<p>Body heat</p> Signup and view all the answers

In what way do skeletal muscles contribute to communication?

<p>By controlling facial expressions</p> Signup and view all the answers

What role do muscles play in communication?

<p>They contract to create facial expressions</p> Signup and view all the answers

Which type of muscle contraction causes the heart to beat?

<p>Cardiac muscle contraction</p> Signup and view all the answers

What functional property of muscle tissue refers to the ability to forcibly shorten?

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

How does smooth muscle contribute to the function of organs and vessels?

<p>By contracting to facilitate movement</p> Signup and view all the answers

What is the role of excitability in muscle tissue?

<p>It enables response to stimuli</p> Signup and view all the answers

What does the term extensibility refer to in muscle tissue?

<p>The ability to be stretched beyond its normal resting length</p> Signup and view all the answers

Which protein is responsible for holding actin filaments together in muscle fibers?

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

What is the primary role of the sarcoplasmic reticulum in a muscle cell?

<p>Storing calcium ions</p> Signup and view all the answers

Which component of the muscle fiber runs from one Z line to another?

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

What structure surrounds each fascicle in a skeletal muscle?

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

What is the main factor that determines the contractile strength of a muscle?

<p>Number of cross-bridges formed</p> Signup and view all the answers

What is the role of troponin in muscle contraction?

<p>To regulate the binding of myosin to actin</p> Signup and view all the answers

Which type of muscle tissue is characterized by voluntary control?

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

What is the fibrous covering of a skeletal muscle called?

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

What do myofibrils in a muscle cell primarily consist of?

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

What occurs after acetylcholine unbinds from the ligand-gated sodium channels?

<p>The ligand-gated sodium channels close.</p> Signup and view all the answers

Which statement best describes the role of acetylcholinesterase in muscle contraction?

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

How is choline reintroduced into the presynaptic terminal?

<p>It is symported with sodium.</p> Signup and view all the answers

What is the order of events that occurs after sodium channels open in the muscle fiber?

<p>Depolarization occurs, generating an action potential.</p> Signup and view all the answers

Which component of the actin myofilament prevents active sites from being exposed in the relaxed muscle state?

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

What is the role of the presynaptic terminal in muscle contraction at the neuromuscular junction?

<p>To contain synaptic vesicles that store neurotransmitters</p> Signup and view all the answers

Which component of the neuromuscular junction directly interacts with acetylcholine after its release?

<p>Motor end plate</p> Signup and view all the answers

What is the first step in the process of muscle contraction at the neuromuscular junction?

<p>Arrival of an action potential at the presynaptic terminal</p> Signup and view all the answers

What separates axonal endings from muscle fibers at the neuromuscular junction?

<p>Synaptic cleft</p> Signup and view all the answers

Which of the following best describes the function of acetylcholine in muscle contraction?

<p>Stimulates muscle fibers to contract</p> Signup and view all the answers

In the context of the muscular system, what is the main role of calcium ions during contraction?

<p>To bind to troponin, triggering a conformational change</p> Signup and view all the answers

What is the overarching purpose of the muscular system's integration with pathophysiology?

<p>To understand the effects of diseases on muscle function</p> Signup and view all the answers

What is the primary role of troponin in skeletal muscle contraction?

<p>It binds calcium and prevents tropomyosin from uncovering active sites.</p> Signup and view all the answers

Which of the following statements best describes the process of muscle relaxation?

<p>Calcium diffuses away from troponin, allowing tropomyosin to cover the active sites.</p> Signup and view all the answers

What are axonal endings primarily responsible for at the neuromuscular junction?

<p>Storing and releasing acetylcholine</p> Signup and view all the answers

During excitation-contraction coupling, what must occur for the muscle to contract?

<p>Action potential must propagate along the sarcolemma.</p> Signup and view all the answers

What characterizes isometric muscle contractions?

<p>The muscle does not shorten and tension increases.</p> Signup and view all the answers

What function do the heads of myosin serve in muscle contraction?

<p>They bind to G actin active sites to form cross bridges.</p> Signup and view all the answers

At what age does the loss of muscle fibers begin?

<p>25 years old</p> Signup and view all the answers

Which mechanism is NOT involved in muscle fatigue?

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

What is a common cause of muscle cramps?

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

Which condition is characterized by chronic widespread pain in skeletal muscles?

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

Myasthenia gravis primarily affects which type of receptors?

<p>Acetylcholine receptors</p> Signup and view all the answers

In Duchenne muscular dystrophy, muscle weakness first begins in which area?

<p>Hip muscles</p> Signup and view all the answers

Which of the following is a common diagnostic test for Duchenne muscular dystrophy?

<p>Serum creatinine</p> Signup and view all the answers

What physiological condition is muscle soreness associated with?

<p>Inflammatory response</p> Signup and view all the answers

Which statement about the function of tropomyosin is accurate?

<p>Tropomyosin has a binding site for calcium</p> Signup and view all the answers

What structure of the muscle fibers facilitates the transmission of action potentials into the interior of the cell?

<p>T tubule</p> Signup and view all the answers

When a weight lifter is unable to move a heavy weight, what type of muscle contraction is primarily involved?

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

During muscle contraction, which of the following correctly lists the sequence of events involving calcium ions?

<p>2, 3, 7</p> Signup and view all the answers

What feature of actin allows for the bending and straightening during muscle contraction?

<p>Hingelike portion</p> Signup and view all the answers

Which molecule is primarily responsible for the ATPase activity linked to muscle contraction?

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

Which process is initiated when calcium ions combine with troponin?

<p>Tropomyosin exposes active sites on actin</p> Signup and view all the answers

What role does the sarcoplasmic reticulum play in muscle contraction?

<p>Storage and release of calcium ions</p> Signup and view all the answers

Study Notes

Neuromuscular Junction

  • The neuromuscular junction is formed by an axonal ending and a motor end plate

  • The axonal ending contains small membrane sacs known as synaptic vesicles which contain acetylcholine

  • The motor end plate is an area on the muscle plasma membrane where it forms the junction with the axonal ending

  • The motor end plate also contains acetylcholine receptors

  • Axonal ends and muscle fibers are separated by a synaptic cleft

Muscle Contraction at NMJ

  • Muscle contraction happens in these steps:
  • Action potential arrives at the presynaptic terminal, causing voltage gated calcium channels in the presynaptic membrane to open, allowing calcium to flow in.
  • Calcium influx initiates release of acetylcholine from synaptic vesicles.
  • Acetylcholine is released into the synaptic cleft by exocytosis
  • Acetylcholine binds to ligand-gated sodium channels in the motor end plate causing them to open.
  • Sodium enters the muscle fiber, causing depolarization. If depolarization passes threshold, an action potential is generated.
  • Acetylcholine then unbinds from the ligand-gated channels, causing them to close.
  • Acetylcholinesterase removes acetylcholine from the synaptic cleft, preventing further stimulation.
  • Choline is symported with sodium in the presynaptic terminal
  • Acetylcholine is reformed within the presynaptic terminal

Actin and Myosin Myofilament Structure

  • Actin Myofilament Component:

  • Globular (G) actin: Globular subunits that form a strand caller fibrous (F) actin.

  • Tropomyosin: In relaxed muscle, covers the active sites on G actin subunits.

  • Troponin: Anchors troponin to actin, preventing tropomyosin from uncovering G actin active sites in a relaxed muscle, binds calcium.

  • Myosin Myofilament Component:

  • Myosin Molecules:

  • Two myosin heavy chains

  • Two myosin heads:

  • Bind to active sites on actin molecules to form cross bridges

  • Attached to the rod portion by a hinge region

  • Heads are ATPase enzymes

Excitation-Contraction Coupling

  • Excitation-contraction coupling occurs at the triad, which is a link between the electrical component of muscle contraction and the mechanical component.
  • For a skeletal muscle to contract:
  • Muscle must be stimulated by a nerve ending
  • Action potential must propagate along its sarcolemma
  • There must be a rise in intracellular calcium levels

Cross-Bridge Movement

  • Steps involved in Cross-Bridge Cycle:
  • The myosin heads bind to active sites on actin, forming cross bridges.
  • The myosin heads swivel, pulling the thin filaments toward the center of the sarcomere.
  • ATP binds to myosin heads causing them to detach from the active sites on actin.
  • The myosin heads split ATP into ADP and inorganic phosphate, providing energy to re-energize the myosin heads.

Muscle Relaxation

  • Muscle relaxation occurs in these steps
  • Acetylcholine is no longer released at the NMJ.
  • Cessation of action potential along sarcolemma stops calcium release.
  • Calcium diffuses away from the troponin molecules.
  • Tropomyosin covers the G actin active sites.
  • No formation of cross-bridges.

Types of Muscle Contractions

  • Isometric Contractions: Muscle does not shorten, tension increases.

  • Isotonic Contractions: Muscle shortens, tone remains constant throughout contraction.

  • Concentric Contractions: Muscle shortens, tone increases.

  • Eccentric Contractions: Muscle lengthens, tone increases.

Disorders of the Muscular System

  • Aging:

  • Muscle atrophy: Physiologic, age-related reduction in muscle mass.

  • Loss of muscle fibers begins as early as 25 years old.

  • Surface area of the NMJ decreases along with the number of motor neurons

  • Muscle Fatigue:

  • Physiologic condition, temporary state of reduced work capacity.

  • Mechanisms:

  • Acidosis and ATP depletion

  • Oxidative stress

  • Local Inflammatory response

  • Muscle Soreness:

  • Occurs after vigorous exercise.

  • Causes: Inflammatory chemical influx into muscle fibers

  • Cramps:

  • Painful spastic contractions of skeletal muscle.

  • Causes: Dehydration, Ion imbalance.

  • Tendonitis: Inflammation of a tendon or its attachment point due to overuse.

  • Fibromyalgia: Non-life-threatening chronic widespread pain in skeletal muscles with no known cure, often referred to as chronic muscle pain syndrome.

  • Myasthenia Gravis:

  • Autoimmune disease

  • Production of antibodies against acetylcholine receptors

  • Diagnostics:

  • Electromyography

  • Identification of autoantibodies

  • Duchenne Muscular Dystrophy:

  • Caused by mutations in the dystrophin gene on the X chromosome.

  • Progressive muscle weakness and muscle contractures.

  • Slow motor development with progressive weakness and muscle wasting.

  • Muscular weakness begins in hip muscles, leading to a waddling gait.

  • Patient uses hands and arms to rise from floor.

  • Diagnostics:

  • Serum creatinine

  • Muscle biopsies

  • Immunohistochemical studies

Formative Assessment Answers

  1. e. Secretability
  2. b. The head of a myosin molecule binds to an active site on G actin.
  3. a. T tubule
  4. b. Isometric contraction
  5. c. 2, 3, 7

Functions of the Muscular System

  • The muscular system is responsible for movement of the body.
  • Muscles maintain posture by keeping the body sitting and standing erect.
  • Muscles are crucial for respiration, as contractions of the thoracic and diaphragmatic muscles aid in breathing.
  • Muscle contractions produce heat as a byproduct, contributing to body temperature regulation.
  • The muscular system plays a role in communication by enabling various movements for speech, facial expressions, and gestures.
  • Smooth muscle contraction within internal organs and vessels leads to their constriction.
  • The contraction of cardiac muscle is responsible for the heartbeat.

Functional Properties of Muscle Tissue

  • Contractility: The ability of a muscle to shorten forcefully. Muscle shortening is forceful, while lengthening is passive.
  • Excitability: The ability of a muscle cell to be stimulated by a nerve to contract.
  • Extensibility: The ability of a muscle to be stretched beyond its normal resting length and still be able to contract.
  • Elasticity: The ability of a muscle to return to its original resting length after it has been stretched.

Neuromuscular Junction

  • Formed by axonal endings and motor end plate
  • Axonal endings have synaptic vesicles containing acetylcholine
  • Motor end plate is the muscle plasma membrane at the junction
  • Motor end plate contains acetylcholine receptors
  • Axonal ends and muscle fibers are separated by a space called the synaptic cleft

Muscle Contraction Steps at NMJ

  • Action potential arrives at the presynaptic terminal
  • Voltage gated calcium channels open
  • Acetylcholine is released into the synaptic cleft by exocytosis
  • Acetylcholine binds to ligand-gated sodium channels on the motor end plate
  • Ligand gated sodium channels open and sodium enters the muscle fiber
  • Acetylcholine unbinds from the ligand gated channels
  • Acetylcholinesterase removes acetylcholine from the synaptic cleft
  • Choline is symported with sodium in the presynaptic terminal
  • Acetylcholine is reformed within the presynaptic terminal

Actin and Myosin Myofilament Structure

  • Actin myofilament component: globular (G) actin, tropomyosin, troponin
  • Globular (G) actin forms a strand called fibrous (F) actin
  • Tropomyosin covers the active sites on the G actin subunits in a relaxed muscle
  • Troponin anchors to actin and prevents tropomyosin from uncovering G actin active sites in a relaxed muscle
  • Troponin binds calcium
  • Myosin myofilament component: myosin molecules
  • Myosin molecules have two myosin heavy chains and two myosin heads
  • The myosin heads bind to active sites on actin molecules to form cross bridges
  • Myosin heads are attached to the rod portion by a hinge region
  • Myosin heads are ATPase enzymes

Excitation-Contraction Coupling

  • Occurs at the triad, which links the electrical component of muscle contraction to the mechanical component
  • A skeletal muscle must be stimulated by a nerve ending to contract
  • Stimulation propagates an action potential along the sarcolemma
  • Intracellular calcium levels rise to trigger contraction

Cross-Bridge Movement

  • Calcium binds to troponin and causes tropomyosin to move away from the active sites on actin
  • Myosin heads bind to the exposed active sites forming cross bridges
  • The myosin head pivots and pulls the thin filament toward the center of the sarcomere using ATP
  • This process is repeated as long as calcium is present
  • Muscle relaxation occurs when acetylcholine is no longer released at the neuromuscular junction

Types of Muscle Contractions

  • Isometric contractions: muscle does not shorten, tension increases
  • Isotonic contractions: muscle shortens, tone remains constant
  • Isotonic contractions include: concentric and eccentric contractions

Disorders of the Muscular System:

  • Aging: muscle atrophy, loss of muscle fibers, reduction in NMJ surface area, decrease in number of motor neurons
  • Muscle fatigue: temporary state of reduced work capacity due to acidosis, ATP depletion, oxidative stress, local inflammatory response
  • Muscle soreness: occurs after vigorous exercise due to inflammatory chemical influx into muscle fibers
  • Cramps: painful, spastic contractions of skeletal muscle caused by dehydration or ion imbalance
  • Tendinitis: inflammation of a tendon or its attachment point due to overuse of a skeletal muscle
  • Fibromyalgia: non-life-threatening chronic widespread pain in skeletal muscles, chronic muscle pain syndrome
  • Myasthenia gravis: autoimmune disease, production of autoantibodies that bind to acetylcholine receptors
  • Duchenne muscular dystrophy: mutations in the dystrophin gene on the x chromosome, progressive muscle weakness, slow motor development with progressive weakness and muscle wasting, muscular weakness begins in hip muscles

Diagnostic Tests for Muscular System Disorders

  • Electromyography: used to diagnose myasthenia gravis
  • Serum creatinine: used to diagnose Duchenne muscular dystrophy
  • Muscle biopsies: used to diagnose Duchenne muscular dystrophy
  • Immunohistochemical studies: used to diagnose Duchenne muscular dystrophy

Formative Assessment Answers

  • 1. e. Secretability
  • 2. b. The head of myosin molecule binds to an active site on G actin
  • 3. a. T tubule
  • 4. b. Isometric contraction
  • 5. c. 2,3,7

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Description

Explore the fascinating mechanisms of the neuromuscular junction and how muscle contraction is triggered. This quiz covers the role of acetylcholine, the synaptic cleft, and the sequence of events leading to muscle fiber depolarization. Test your understanding of these critical physiological processes.

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