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Dental Physiology and Biochemistry — MCQ ও উত্তর

ডেন্টাল ভর্তি · । এই অধ্যায়ে মোট 132টি প্রশ্ন আছে। নিচে প্রথম 10টি প্রশ্ন ও সঠিক উত্তর দেওয়া হলো।

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  1. Which physiological mechanism ensures that an action potential does not propagate backward along the axon?

    1. Relative refractory period
    2. Absolute refractory period
    3. Saltatory conduction
    4. Spatial summation

    সঠিক উত্তর: Absolute refractory period

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option B The correct answer is Absolute refractory period.- During an action potential, the axon membrane undergoes rapid depolarization and repolarization. - The absolute refractory period is the time immediately following an action potential during which the voltage-gated sodium channels are inactivated and cannot be reopened. - This ensures that no new action potential can be initiated in that segment of the membrane, thereby preventing the action potential from traveling backward.- This mechanism enforces a unidirectional flow of the action potential along the axon, moving from the axon hillock toward the axon terminals.- The relative refractory period follows the absolute refractory period and allows a new action potential only if a stronger-than-normal stimulus is present, but it does not fully prevent backward propagation.- Saltatory conduction refers to the jumping of action potentials between nodes of Ranvier to increase conduction speed, but it does not determine directionality.- Spatial summation involves the additive effect of multiple synaptic inputs on a neuron’s ability to reach threshold and does not affect action potential propagation direction.Thus, the key physiological mechanism preventing backward propagation of the action potential along an axon is the absolute refractory period.Reference: Principles of Neural Science, 6th Edition, Chapter 4: The Neuron: Electrical Properties, Page 102

  2. The Goldman-Hodgkin-Katz equation is primarily used to calculate:

    1. The resting membrane potential incorporating multiple ion permeabilities
    2. The threshold potential for action potential initiation
    3. The equilibrium potential of a single ion species
    4. The rate of ion transport across the Na+/K+ ATPase pump

    সঠিক উত্তর: The resting membrane potential incorporating multiple ion permeabilities

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option A - The Goldman-Hodgkin-Katz (GHK) equation is a fundamental tool in understanding the electrical properties of cell membranes. - Unlike the Nernst equation, which calculates the equilibrium potential for a single ion species, the GHK equation takes into account the relative permeabilities and concentrations of multiple ions simultaneously.- This makes it particularly useful for determining the resting membrane potential of cells where ions like potassium (K⁺), sodium (Na⁺), and chloride (Cl⁻) all contribute to the electrical charge across the membrane. - The GHK equation mathematically integrates these factors, providing a more accurate and physiologically relevant value for the membrane potential when several ion species permeate the membrane. - Therefore, it reflects the combined effect of various ions rather than focusing on just one ion as individual equilibrium potentials do.In summary, the GHK equation is primarily used to calculate the resting membrane potential incorporating multiple ion permeabilities, making Option 1 the correct choice.

  3. What primarily causes hyperpolarization during an action potential?

    1. Delayed closing of potassium channels after repolarization
    2. Prolonged opening of sodium channels
    3. Early inactivation of calcium channels
    4. Increased permeability to chloride ions

    সঠিক উত্তর: Delayed closing of potassium channels after repolarization

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option A The primary cause of hyperpolarization during an action potential is the delayed closing of potassium channels after repolarization. - During an action potential, the neuron rapidly depolarizes due to the influx of sodium ions through voltage-gated sodium channels. - Following this, the membrane potential returns to the resting level in a phase called repolarization, which is mainly driven by the efflux of potassium ions through voltage-gated potassium channels.- However, these potassium channels do not close immediately after repolarization. - Instead, they remain open for a short period, allowing excessive potassium ions to leave the cell.- This results in the membrane potential becoming more negative than the resting potential, a state known as hyperpolarization or the after-hyperpolarization phase.Key points to note: - Delayed closing of potassium channels extends potassium efflux beyond the resting membrane potential.- This causes the membrane potential to become more negative than the usual resting level.- The hyperpolarized state makes the neuron less excitable briefly, ensuring proper directional signal propagation.The other options are not correct because: - Prolonged opening of sodium channels would maintain depolarization, not hyperpolarization.- Early inactivation of calcium channels primarily affects calcium influx and does not play a major role in hyperpolarization.- Increased permeability to chloride ions could cause inhibitory effects but is not the primary cause of hyperpolarization during the classic action potential stages.

  4. Which of the following statements about the generation of action potentials is accurate?

    1. Action potentials arise spontaneously without prior stimuli
    2. They depend on graded potentials to reach a threshold level
    3. Action potentials decrease in amplitude as they propagate along the axon
    4. They are initiated by hyperpolarization of the neuronal membrane

    সঠিক উত্তর: They depend on graded potentials to reach a threshold level

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option B The correct answer is: They depend on graded potentials to reach a threshold level.- Action potentials are all-or-none electrical signals generated by neurons to transmit information. - They are not spontaneous but instead require a stimulus to bring the membrane potential to a certain threshold. - This initial stimulus is usually in the form of graded potentials, which are small changes in membrane potential caused by synaptic inputs or sensory stimuli. - When the summed graded potentials reach the threshold voltage (usually around -55 mV), voltage-gated sodium channels open, triggering the rapid depolarization phase of the action potential.Option 1 is incorrect because action potentials do not arise spontaneously; they require a graded potential that reaches threshold.Option 3 is incorrect since action potentials maintain a consistent amplitude as they propagate along the axon due to the regenerative opening of ion channels.Option 4 is incorrect because action potentials are initiated by depolarization, not hyperpolarization; hyperpolarization makes the membrane more negative and moves it further from the threshold.In summary, the generation of an action potential critically depends on graded potentials reaching a threshold level, making option 2 the accurate statement.Reference: Principles of Neural Science, 5th Edition, Chapter 4, p. 123-130

  5. Secondary active transport is driven by which of the following mechanisms?

    1. An electrochemical gradient generated by primary active transport
    2. Direct hydrolysis of ATP by the transporter
    3. Facilitated diffusion down a concentration gradient
    4. Passive diffusion through the lipid bilayer

    সঠিক উত্তর: An electrochemical gradient generated by primary active transport

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option A Secondary active transport is a process by which molecules are moved against their concentration gradient using energy indirectly. The key driving force behind secondary active transport is an electrochemical gradient that has been established by primary active transport. In primary active transport, ATP is directly used to pump ions across the membrane, creating a gradient of ions, usually sodium or hydrogen ions. Secondary active transporters then harness the energy stored in this ion gradient to co-transport other substances either into or out of the cell. This means that while the secondary transporter itself does not hydrolyze ATP, it relies on the ion gradient generated by primary active transport to move molecules against their concentration gradient. The other options are incorrect because: Option 2 describes primary active transport, where ATP is hydrolyzed directly by the transporter. Option 3 refers to facilitated diffusion, which is passive movement down a gradient and does not require energy. Option 4 describes passive diffusion, which occurs through the lipid bilayer without protein assistance and does not require energy. Therefore, the correct mechanism driving secondary active transport is an electrochemical gradient generated by primary active transport. Reference: Ganong's Review of Medical Physiology, 26th Edition, Chapter 3: Membrane Transport

  6. In a relaxed skeletal muscle, which protein is responsible for covering the myosin-binding sites on actin filaments?

    1. Troponin
    2. Tropomyosin
    3. Myosin light chain
    4. Nebulin

    সঠিক উত্তর: Tropomyosin

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option B In a relaxed skeletal muscle, the myosin-binding sites on the actin filaments are blocked, preventing muscle contraction. This is primarily due to the protein tropomyosin, which lies along the length of the actin filament and physically covers these binding sites. When the muscle is at rest, tropomyosin blocks the interaction between actin and myosin, ensuring that cross-bridge formation does not occur. Upon stimulation, calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites, allowing myosin heads to attach to actin and initiate contraction.To clarify the roles of the other proteins listed: - Troponin is a regulatory protein that binds calcium and controls tropomyosin’s position on actin. - The myosin light chain is part of the myosin molecule itself, involved in regulating myosin head activity, not in blocking binding sites. - Nebulin is a structural protein that helps stabilize and regulate the length of actin filaments but does not block binding sites.Therefore, the correct answer is tropomyosin, as it is the protein responsible for covering the myosin-binding sites on actin filaments in relaxed skeletal muscle.Reference: Guyton and Hall Textbook of Medical Physiology, 13th Edition, Chapter 12 - Muscle Physiology

  7. Which neurotransmitter is predominantly released at the motor end plate to trigger skeletal muscle contraction?

    1. Gamma-aminobutyric acid (GABA)
    2. Acetylcholine (ACh)
    3. Norepinephrine
    4. Dopamine

    সঠিক উত্তর: Acetylcholine (ACh)

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option B - The neurotransmitter predominantly released at the motor end plate to trigger skeletal muscle contraction is Acetylcholine (ACh). - When an action potential reaches the axon terminal of a motor neuron, vesicles containing ACh fuse with the presynaptic membrane and release ACh into the synaptic cleft.- ACh then binds to nicotinic acetylcholine receptors on the muscle fiber's motor end plate, causing the opening of ion channels. - This results in an influx of sodium ions, leading to depolarization of the muscle membrane and initiation of an action potential in the muscle fiber.- Consequently, this action potential triggers the process of muscle contraction. - In contrast, other neurotransmitters listed, such as GABA (an inhibitory neurotransmitter in the CNS), norepinephrine (primarily involved in the autonomic nervous system), and dopamine (mainly involved in CNS functions related to reward and motor control), do not play a role at the skeletal muscle motor end plate. Therefore, the exclusive neurotransmitter responsible for initiating skeletal muscle contraction at the neuromuscular junction is Acetylcholine (ACh). Reference: Ganong's Review of Medical Physiology, 26th Edition, Chapter 10: Neuromuscular Transmission and Muscle Contraction

  8. In skeletal muscle fibers, from which intracellular structure are calcium ions (Ca2+) primarily released to initiate contraction?

    1. Mitochondria
    2. Sarcoplasmic reticulum
    3. Transverse tubules
    4. Cytosol

    সঠিক উত্তর: Sarcoplasmic reticulum

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option B - In skeletal muscle fibers, the initiation of contraction is tightly regulated by the availability of calcium ions (Ca2+). - These ions play a crucial role in the interaction between actin and myosin, the proteins responsible for muscle contraction. - The source of these calcium ions is key to understanding muscle physiology.- The primary intracellular structure responsible for releasing Ca2+ is the sarcoplasmic reticulum (SR), a specialized form of the endoplasmic reticulum found in muscle cells. - The sarcoplasmic reticulum stores high concentrations of calcium ions, and upon receiving a signal from the nervous system, it rapidly releases Ca2+ into the cytosol of the muscle fiber. - This sudden increase in cytosolic calcium enables the binding of calcium to troponin, which causes a conformational change that moves tropomyosin away from actin’s myosin-binding sites, thereby facilitating contraction.- While mitochondria are important for energy production and transverse tubules (T-tubules) help propagate action potentials deep into the muscle fiber, they do not serve as the main source of calcium. - The cytosol, in contrast, is the fluid where calcium exerts its effect but does not store or release it in significant amounts.Therefore, the sarcoplasmic reticulum is the correct answer as the intracellular calcium reservoir that initiates skeletal muscle contraction.Reference: Guyton and Hall Textbook of Medical Physiology, 13th Edition, Chapter 9: Muscle Physiology, Page 106

  9. What is the primary function of ATP during the muscle contraction cycle?

    1. Initiates the release of calcium ions from the sarcoplasmic reticulum
    2. Provides energy for myosin to detach from actin
    3. Causes the phosphorylation of troponin to expose binding sites
    4. Acts as a secondary messenger to stimulate muscle contraction

    সঠিক উত্তর: Provides energy for myosin to detach from actin

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option B The primary function of ATP during the muscle contraction cycle is to provide energy for myosin to detach from actin. During the contraction process, myosin heads bind to actin filaments forming cross-bridges, which generates the power stroke necessary for muscle shortening. After the power stroke occurs, ATP binds to the myosin head, causing a conformational change that leads to the detachment of myosin from actin. Without ATP, myosin would remain bound to actin, preventing muscle relaxation and leading to a state known as rigor.It is important to note that ATP is also hydrolyzed to ADP and inorganic phosphate, which re-energizes the myosin head preparing it for the next contraction cycle. This continuous cycle of ATP binding, hydrolysis, and myosin-actin interaction is fundamental to muscle function.The other options are incorrect: - ATP does not initiate the release of calcium ions from the sarcoplasmic reticulum; this is regulated by the action potential and the ryanodine receptors. - Troponin phosphorylation does not occur via ATP during contraction; instead, calcium binds to troponin to expose actin binding sites. - ATP does not function as a secondary messenger in muscle contraction signaling; calcium ions serve this role.Key points: • ATP binds to myosin heads to allow detachment from actin • ATP hydrolysis re-cocks the myosin head for the next power stroke • Essential for muscle relaxation and continued contraction cycles Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 10: Muscle Contraction and the Sliding Filament Theory

  10. Which process is responsible for ending muscle contraction and triggering muscle relaxation?

    1. Reuptake of Ca2+ into the sarcoplasmic reticulum
    2. Release of acetylcholine from the motor neuron
    3. Breakdown of ATP by myosin ATPase
    4. Diffusion of sodium ions into the muscle fiber

    সঠিক উত্তর: Reuptake of Ca2+ into the sarcoplasmic reticulum

    ব্যাখ্যা: সমাধান সঠিক উত্তর: Option A - The process responsible for ending muscle contraction and initiating muscle relaxation is the reuptake of Ca2+ into the sarcoplasmic reticulum.- During muscle contraction, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber. - These calcium ions bind to troponin, causing a conformational change that allows the interaction between actin and myosin, leading to contraction.- To end contraction, calcium must be removed from the cytoplasm. - This occurs through the active transport of Ca2+ back into the sarcoplasmic reticulum via calcium pumps.- As calcium levels in the cytoplasm decrease, calcium dissociates from troponin, resulting in the blocking of the actin-myosin binding sites by tropomyosin. - This stops cross-bridge cycling and allows the muscle fiber to relax.In summary:- Release of acetylcholine initiates contraction, not relaxation.- Breakdown of ATP provides energy for contraction and relaxation but does not directly end contraction.- Diffusion of sodium ions is involved in generating action potentials, not the termination of contraction.Hence, the key step to end contraction and trigger relaxation is the active reuptake of calcium ions into the sarcoplasmic reticulum.Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 9: Muscle Contraction

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