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Questions and Answers
What is the primary role of tropomyosin during muscle contraction?
What is the primary role of tropomyosin during muscle contraction?
Which component of troponin is responsible for binding calcium ions?
Which component of troponin is responsible for binding calcium ions?
What happens during the cross-bridge movement phase of muscle contraction?
What happens during the cross-bridge movement phase of muscle contraction?
What initiates the process of muscle contraction at the molecular level?
What initiates the process of muscle contraction at the molecular level?
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Which phase of muscle contraction involves the formation of the actin-myosin cross-bridge?
Which phase of muscle contraction involves the formation of the actin-myosin cross-bridge?
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How does rigor mortis occur post-mortem?
How does rigor mortis occur post-mortem?
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Which of the following best describes the role of ATP during muscle contraction?
Which of the following best describes the role of ATP during muscle contraction?
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What ion plays a critical role in muscle contraction by enabling cross-bridge formation?
What ion plays a critical role in muscle contraction by enabling cross-bridge formation?
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During muscle relaxation, what happens to the calcium ions?
During muscle relaxation, what happens to the calcium ions?
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What is the impact of a muscle action potential on the sarcolemma?
What is the impact of a muscle action potential on the sarcolemma?
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Which neurotransmitter is NOT involved in the autonomic nervous system's regulation of smooth muscle contraction?
Which neurotransmitter is NOT involved in the autonomic nervous system's regulation of smooth muscle contraction?
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What is the role of the pacemaker potential in cardiac muscle?
What is the role of the pacemaker potential in cardiac muscle?
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Which local factor does NOT affect smooth muscle contraction?
Which local factor does NOT affect smooth muscle contraction?
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What structure connects cardiac muscle cells to form an electrical syncytium?
What structure connects cardiac muscle cells to form an electrical syncytium?
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Which of the following does NOT describe a characteristic of cardiac muscle?
Which of the following does NOT describe a characteristic of cardiac muscle?
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Which term describes the heart's response to hormonal influences on contraction strength?
Which term describes the heart's response to hormonal influences on contraction strength?
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What mechanism is primarily responsible for the release of Ca2+ during cardiac muscle contraction?
What mechanism is primarily responsible for the release of Ca2+ during cardiac muscle contraction?
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What is the main effect of the prolonged refractory period in cardiac muscle?
What is the main effect of the prolonged refractory period in cardiac muscle?
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What structure forms the neuromuscular junction?
What structure forms the neuromuscular junction?
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What is the action that occurs immediately after the influx of Ca2+ at the nerve ending?
What is the action that occurs immediately after the influx of Ca2+ at the nerve ending?
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Which neurotransmitter is exclusively used at the neuromuscular junction?
Which neurotransmitter is exclusively used at the neuromuscular junction?
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In the case of a heavier load during isometric contraction, which of the following occurs?
In the case of a heavier load during isometric contraction, which of the following occurs?
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What is the primary outcome of a single action potential in muscle fibers?
What is the primary outcome of a single action potential in muscle fibers?
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Which of the following describes the phenomenon of temporal summation?
Which of the following describes the phenomenon of temporal summation?
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What happens if a muscle's tension does not exceed the load during isometric contraction?
What happens if a muscle's tension does not exceed the load during isometric contraction?
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What does the all-or-none rule state regarding muscle contraction?
What does the all-or-none rule state regarding muscle contraction?
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What is a motor unit composed of?
What is a motor unit composed of?
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Which type of muscle fiber is classified for its fast contraction and high glycolytic capacity?
Which type of muscle fiber is classified for its fast contraction and high glycolytic capacity?
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What effect does strength training have on muscle fibers?
What effect does strength training have on muscle fibers?
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What is the primary energy source for muscle contraction in skeletal muscle?
What is the primary energy source for muscle contraction in skeletal muscle?
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What happens to muscle fibers during disuse atrophy?
What happens to muscle fibers during disuse atrophy?
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What is the characteristic of slow-oxidative muscle fibers?
What is the characteristic of slow-oxidative muscle fibers?
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What role does skeletal muscle tone play?
What role does skeletal muscle tone play?
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Which statement about muscle fiber types is accurate?
Which statement about muscle fiber types is accurate?
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What is a primary cause of sore muscles?
What is a primary cause of sore muscles?
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Which of the following is an AChE inhibitor used in the treatment of myasthenia gravis?
Which of the following is an AChE inhibitor used in the treatment of myasthenia gravis?
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Which symptom is most commonly associated with myasthenia gravis?
Which symptom is most commonly associated with myasthenia gravis?
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What complication can occur as a result of muscle weakness in myasthenia gravis?
What complication can occur as a result of muscle weakness in myasthenia gravis?
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What physical tasks may become difficult for someone with myasthenia gravis?
What physical tasks may become difficult for someone with myasthenia gravis?
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What is a characteristic feature of Duchenne muscular dystrophy?
What is a characteristic feature of Duchenne muscular dystrophy?
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What effect does neuromuscular junction pathophysiology have on muscle function?
What effect does neuromuscular junction pathophysiology have on muscle function?
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How does muscle weakness from myasthenia gravis primarily affect the body?
How does muscle weakness from myasthenia gravis primarily affect the body?
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Study Notes
Tropomyosin
- Arranged in a thread-like manner along the actin filament's spiral furrows.
- Stabilizes filament structure by binding to troponin T1 and T2.
- Controls access to the cross-bridge binding site on actin:
- During muscle relaxation, it covers the cross-bridge binding site.
- During muscle contraction, binding of Ca2+ to troponin causes tropomyosin to move into the spiral furrows, exposing the cross-bridge binding site for myosin.
Troponin
- Attached to tropomyosin at regular intervals.
- Composed of three subunits:
- Troponin C: Contains the Ca2+-binding site.
- Troponin I: Inhibits the interaction between myosin and actin.
- Troponin T: Binds to tropomyosin.
Cross-Bridge Cycle
- A series of steps that describe the interaction between myosin and actin during muscle contraction.
- Closely linked to ATP consumption at the myosin head.
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Formation of Cross-Bridge:
- Binding of Ca2+ to troponin C moves tropomyosin, exposing the cross-bridge binding site on actin.
- The myosin head binds to the exposed binding site on actin, forming the actin-myosin cross-bridge.
- ATP is not consumed during the binding step.
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Cross-Bridge Movement:
- ATP is hydrolyzed at the myosin head's enzymatic site (ATPase activity).
- The released energy drives the flexing movement of the myosin head (hinge movement), causing a power stroke.
- This movement slides the myosin filament along the actin filament, resulting in shortening of the sarcomere.
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Uncoupling of Cross-Bridge:
- Binding of a new ATP molecule to the myosin head disrupts the cross-bridge between myosin and actin.
Rigor Mortis
- A state of muscle rigidity occurring after death due to ATP depletion.
- The absence of ATP prevents the uncoupling of the cross-bridge, leading to sustained muscle contraction.
- Begins 3-4 hours after death, reaches completion around 12 hours, and dissipates after 48-60 hours.
Excitation-Contraction Coupling
- The sequence of events that link the electrical excitation of the sarcolemma (action potential) to the formation of the cross-bridge in the myofilaments.
- The muscle fiber's action potential (lasting 1-2 ms) disappears before the onset of contraction, which takes much longer.
- The process occurs as follows:
- Neuromuscular stimulation → Action potential (membrane excitation) → Transmission of excitation via T-tubules → Ca2+ release from SR → Action of Ca2+ on myofibrils → Contraction of contractile proteins.
Role of Ca2+ in Muscle Contraction
- The key regulator of muscle contraction and relaxation.
- Action potential propagation (neuron) → End-plate potential → Depolarization of sarcolemma → Opening of L-type Ca2+-channels (DHP receptor) in the sarcolemma → Ca2+ influx into the cytosol → Ca2+ release from SR (Ryanodine receptor, Ca2+-induced Ca2+ release) → Binding of Ca2+ to troponin C → Formation of cross-bridges between myosin and actin → Cross-bridge cycle → Muscle contraction → Outflux of Ca2+ (Na+-Ca2+ exchanger, Ca2+-pump) → Restoration of intracellular Ca2+ concentration → Uncoupling of cross-bridges → Relaxation of muscle.
Energy for Muscle Contraction
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Role of ATP:
- Binding to the myosin head disrupts the cross-bridge.
- Provides energy for the power stroke of the myosin head during the cross-bridge cycle.
- Powers the operation of the Ca2+-pump.
Neuromuscular Junction (NMJ)
- The junction between a motor nerve ending and a muscle fiber.
- Contains a synaptic cleft (the space separating the nerve and muscle).
Transmission of Excitation at the NMJ
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Excitation of Motor Neuron:
- Increased Ca2+ influx at the nerve ending triggers the release of acetylcholine-containing synaptic vesicles.
- Acetylcholine binds to receptors on the motor end plate, leading to increased Na+ and K+ influx.
- This depolarization, known as the end-plate potential (EPP), triggers an action potential in the muscle fiber, ultimately causing contraction.
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Role of Acetylcholinesterase:
- Breaks down acetylcholine into choline and acetic acid, allowing for the recycling of acetylcholine and termination of signal.
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Differences from interneuronal synapses:
- Only acetylcholine is used as a neurotransmitter.
- A single EPP can exceed the threshold for an action potential in the muscle. In contrast, multiple EPSPs are necessary to reach threshold in interneuronal synapses.
- IPSPs do not occur at the NMJ.
Types of Muscle Contraction
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Twitch: A single contraction in a muscle fiber caused by a single action potential.
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Twitch Curve: A graphical representation of a twitch, showing:
- Latent period (time delay between stimulation and the onset of contraction).
- Contraction period (muscle shortening).
- Relaxation period (muscle returning to its resting length).
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Twitch Curve: A graphical representation of a twitch, showing:
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Summation: The increase in muscle tension due to a series of action potentials.
- Temporal Summation: Multiple action potentials occur before the muscle has fully relaxed from the previous contraction, leading to a greater force of contraction.
- Spatial Summation: The recruitment of additional motor units, resulting in a stronger contraction.
- Isotonic Contraction: Muscle tension remains constant while muscle length changes.
- Isometric Contraction: Muscle length remains constant while muscle tension changes.
- All-or-None Rule: A muscle fiber will contract with maximal force if stimulated, or not at all, regardless of the strength of the stimulus.
Control of Muscular Strength
- Motor Unit: A motor neuron and all the muscle fibers it innervates.
- Size of Motor Unit: The number of muscle fibers innervated by a single motor neuron. Smaller motor units provide finer control, while larger motor units generate more force.
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Muscle strength is controlled by:
- Spatial Summation: Increasing the number of activated motor units.
- Frequency of Stimulation: Increasing the frequency of action potentials.
Types of Skeletal Muscle Fiber
- Classified based on both speed of contraction and metabolic pathways for ATP production:
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Slow-Oxidative Fibers (Type I):
- Contract slowly.
- Rely heavily on oxidative metabolism for ATP production.
- Red muscle fibers.
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Fast-Oxidative Fibers (Type IIa):
- Contract relatively fast.
- Primarily rely on oxidative metabolism, but can also use glycolysis.
- Intermediate in color.
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Fast-Glycolytic Fibers (Type IIb):
- Contract very fast.
- Heavily rely on glycolytic metabolism for ATP production.
- White muscle fibers.
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Slow-Oxidative Fibers (Type I):
Exercise Effects
-
Strength Training:
- Muscle hypertrophy (increase in muscle fiber size).
- Increased contractile protein content.
- Increased anaerobic glycolytic enzyme content.
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Endurance Training:
- No change in muscle fiber size.
- Increased capillary density.
- Increased mitochondrial enzyme content.
- Exercise does not change the type of muscle fiber present.
Skeletal Muscle Tone
- A constant, low level of muscle tension present even at rest.
- Due to nerve activity from the spinal cord.
Sore Muscle
- Caused by connective tissue injury and histamine release.
Pathophysiology of the Neuromuscular Junction
-
Acetylcholinesterase Inhibitors:
- Nerve gases and organophosphate pesticides (e.g., malathion, parathion) inhibit acetylcholinesterase, leading to excessive acetylcholine accumulation and muscle spasms.
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Blocking Acetylcholine-Receptor Interaction:
- Curare (D-tubocurarine) blocks acetylcholine from binding to its receptor, causing muscle paralysis.
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Myasthenia Gravis:
- An autoimmune disease where the body produces antibodies against acetylcholine receptors.
- Results in muscle weakness and fatigue.
- Treatment involves acetylcholinesterase inhibitors (e.g., neostigmine, physostigmine).
Muscular Dystrophy
- A group of genetic disorders characterized by progressive muscle weakness and degeneration.
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Duchenne Muscular Dystrophy:
- Caused by defects in the dystrophin gene, which codes for a protein that helps stabilize muscle fibers.
- Primarily affects boys.
- Leads to progressive muscle weakness, primarily in the legs and hips, eventually affecting breathing muscles.
Smooth Muscle
- Located in the walls of internal organs and blood vessels.
- Contracts more slowly and over a longer duration than skeletal muscle.
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Regulation of Smooth Muscle Contraction:
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Autonomic Nervous System:
- Diffuse junctions with multiple neurotransmitter release points called varicosities.
- Neurotransmitters include acetylcholine and norepinephrine.
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Hormones:
- Angiotensin, vasopressin, endothelin, and bradykinin.
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Local Factors:
- Paracrine agents (e.g., NO, eicosanoids).
- Acidity, oxygen concentration, osmolarity, and ion composition of the extracellular fluid.
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Stretch:
- Stretch-activated (mechanosensitive) Ca2+ channels can trigger contraction.
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Autonomic Nervous System:
Cardiac Muscle
- Found only in the heart.
- Exhibits features of both skeletal and smooth muscle.
-
Morphological Characteristics:
- Striated muscle fibers for strong contractions.
- Gap junctions for electrical coupling between cells.
- Intercalated disks: Specialized junctions containing gap junctions and desmosomes for structural and electrical connection.
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Electrical Characteristics:
- Pacemaker Potential: Spontaneous electrical activity originates in the sinoatrial (SA) node, leading to regular heartbeats.
- Conduction System: Includes the SA node, atrioventricular (AV) node, bundles of His, and Purkinje fibers, ensuring coordinated electrical activation of the heart.
- Electrical Syncytium: Gap junctions allow rapid propagation of action potentials between cells, creating a functional unit.
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Functional Characteristics:
- Blood Circulation: Primary function of the heart.
- Contraction Mechanism: Similar to skeletal muscle, involving the interaction of actin and myosin, with abundant sarcoplasmic reticulum (SR) and T-tubules.
- Ca2+ Dependence: Highly dependent on extracellular Ca2+ influx for contraction.
- Hormone Responsiveness: Affected by hormones like epinephrine and norepinephrine.
- Spontaneous Excitation: Possesses inherent rhythmicity in the SA node.
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Autonomic Nervous System Modulation:
- Sympathetic nerves: Increase heart rate and contractility.
- Parasympathetic nerves: Decrease heart rate.
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Myocardial Action Potential:
- SA Node: Spontaneous depolarization and action potential generation.
- AV Node: Slow conduction allowing coordinated atrioventricular contractions.
- Ventricular Muscle: Prolonged action potential compared to skeletal muscle.
Cardiac Muscle Contraction
- Ca2+ Release-Induced Ca2+ Release: Ca2+ influx into the cell triggers further Ca2+ release from the SR, amplifying the intracellular Ca2+ signal.
- Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA): A Ca2+ pump located in the SR that actively removes Ca2+ from the cytosol, leading to muscle relaxation.
- Na+/Ca2+ Exchanger: A protein in the sarcolemma that utilizes the electrochemical gradient of Na+ to remove Ca2+ from the cell.
Cardiac Control by Autonomic Nervous System
- Chronotropy: Regulation of heart rate.
- Dromotropy: Modulation of conduction speed.
- Inotropy: Control of contractile force.
- Lusitropy: Affects the rate of ventricular relaxation.
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Description
Test your knowledge on tropomyosin, troponin, and the cross-bridge cycle involved in muscle contraction. This quiz covers the structures and functions of key proteins and their role in the process of muscle movement. Understand how Ca2+ influences these interactions.
