Nerve and Muscle Physiology Practice Questions

Q: Which of the following mechanisms actively maintains the ionic concentration gradients essential for the resting membrane potential in neurons?

Sodium-potassium pump. ***Sodium-potassium pump*** - The **sodium-potassium pump (Na⁺/K⁺-ATPase)** actively transports **3 sodium ions out** and **2 potassium ions into** the cell, requiring ATP. - This establishes and maintains the **concentration gradients** (high K⁺ inside, high Na⁺ outside) that are essential for the resting membrane potential. - While **potassium leak channels** create the primary electrical potential (~-70 mV), the pump maintains the gradients that allow these channels to function and contributes an additional **-5 to -10 mV** through its electrogenic activity. *Calcium channels* - **Calcium channels** mediate **calcium influx** during action potentials, triggering neurotransmitter release and other cellular processes. - They do not establish or maintain the ionic gradients responsible for the **resting membrane potential**. *Chloride channels* - **Chloride channels** help stabilize membrane potential and mediate inhibitory signals but do not actively maintain the primary **Na⁺ and K⁺ gradients** underlying resting potential. - They play a secondary role compared to the active transport mechanisms. *Voltage-gated sodium channels* - **Voltage-gated sodium channels** are closed at rest and open during **action potential depolarization** to allow rapid Na⁺ influx. - They propagate action potentials but do not maintain the resting concentration gradients.

Q: A patient experiences sudden muscle weakness after consuming a large amount of potassium-rich food. Which physiological mechanism is likely responsible for the muscle weakness?

Depolarization of the nerve membrane. ***Depolarization of the nerve membrane*** - A large intake of potassium-rich food can lead to **hyperkalemia**, causing the **resting membrane potential** of nerve and muscle cells to become less negative (depolarized). - While initial depolarization can fire action potentials, sustained depolarization inactivates **voltage-gated sodium channels**, preventing further firing and leading to **muscle weakness** or paralysis. *Decreased potassium efflux* - In situations of hyperkalemia, there might be a *relative* decrease in potassium efflux compared to the elevated extracellular potassium, but the primary mechanism leading to weakness is due to the sustained depolarization of the membrane. - Reduced potassium efflux wouldn't directly cause muscle weakness; rather, it would contribute to the inability to repolarize effectively. *Increased sodium influx* - While depolarization is linked to sodium channel function, the underlying issue in hyperkalemia leading to weakness is the sustained depolarization that inactivates these channels, not an increased sodium influx itself. - An uncontrolled increase in sodium influx would typically lead to increased excitability, not weakness, unless it's part of a cycle that inactivates voltage-gated channels. *Hyperpolarization of the nerve membrane* - **Hyperpolarization** would make the nerve membrane more negative and *less* excitable, eventually leading to muscle weakness by making it harder to reach the threshold for an action potential. - However, **hyperkalemia** causes **depolarization** (less negative resting membrane potential), making this option incorrect for the given scenario.

Q: Which ion primarily determines the resting membrane potential of excitable cells?

Potassium. ***Potassium*** - The resting membrane potential is primarily determined by the **efflux of potassium ions** through **leak channels**, making the inside of the cell more negative. - The high **intracellular concentration of potassium** and the greater permeability of the membrane to potassium at rest are key factors. *Sodium* - While sodium channels are critical for the **depolarization phase** of an action potential, the cell membrane is much less permeable to **sodium** at rest. - The **sodium-potassium pump** actively transports sodium out of the cell, contributing indirectly to the resting potential. *Chloride* - **Chloride ions** can influence the resting membrane potential, particularly in certain cells, but their contribution is generally less significant than potassium. - In many neurons, chloride flow helps to **stabilize or hyperpolarize** the membrane, but it's not the primary determinant of the resting state. *Calcium* - **Calcium ions** are crucial for various cellular processes, including neurotransmitter release and muscle contraction, but they play a minimal direct role in establishing the resting membrane potential. - The cell maintains a very **low intracellular calcium concentration**, and its channels are largely closed at rest.

Q: Which of the following best describes the role of the sarcoplasmic reticulum in muscle cells?

Stores calcium ions. ***Stores calcium ions*** - The **sarcoplasmic reticulum (SR)** is a specialized endoplasmic reticulum in muscle cells that primarily functions as a **storage site for Ca2+ ions**. - During muscle contraction, the SR releases stored **calcium ions** into the sarcoplasm, which then bind to troponin, initiating the contractile process. *Generates ATP* - The primary organelle responsible for **ATP generation** in muscle cells is the **mitochondria**, through cellular respiration. - While muscle contraction requires ATP, the **sarcoplasmic reticulum** itself does not produce it. *Contains actin and myosin* - **Actin and myosin** are the primary contractile proteins found within the **myofibrils** of muscle cells, not within the sarcoplasmic reticulum. - The sarcoplasmic reticulum surrounds the myofibrils but does not contain these filaments internally. *Produces muscle proteins* - **Muscle proteins** like actin and myosin are synthesized on **ribosomes** located in the cytoplasm or on the rough endoplasmic reticulum. - The **sarcoplasmic reticulum** is mainly involved in calcium handling, not protein synthesis.

Q: A patient with hypocalcemia presents with tetany. Which physiological process is most likely responsible for the muscle spasms?

Increased neuronal excitability. ***Increased neuronal excitability*** - **Hypocalcemia** reduces the extracellular calcium concentration, which destabilizes the neuronal membrane by reducing calcium's normal stabilizing effect on **voltage-gated sodium channels**. - This lowers the threshold for depolarization, making neurons **more excitable** and prone to spontaneous action potentials, resulting in the involuntary muscle contractions characteristic of **tetany**. - The mechanism: Ca²⁺ normally binds to negatively charged membrane sites, raising the voltage threshold needed for sodium channel activation. Low calcium removes this stabilization. *Decreased neuronal excitability* - This would lead to a reduction in nerve impulses and muscle activity, which is the opposite of what is seen in tetany. - Conditions causing decreased neuronal excitability often result in muscle weakness or paralysis, not spasms. *Increased muscle contractility* - While tetany involves muscle contraction, the primary issue in hypocalcemia is enhanced *neuronal* firing that *initiates* the contractions, not an inherent increase in the muscle's ability to contract independently of neuronal input. - Calcium's direct role in muscle contraction (binding to troponin) is largely intracellular and not the primary driver of tetany caused by *extracellular* hypocalcemia. *Decreased muscle contractility* - This would result in weaker muscle contractions or paralysis, which is contrary to the clinical presentation of tetany. - Decreased muscle contractility is not associated with hypocalcemia-induced spasms.

Q: A patient with severe muscle weakness is found to have reduced acetylcholine release at the neuromuscular junction. Which physiological process is most likely impaired?

Exocytosis of acetylcholine. ***Exocytosis of acetylcholine*** - **Reduced acetylcholine release** directly indicates a problem with the process by which neurotransmitters are expelled from the presynaptic terminal, which is **exocytosis**. - Impaired exocytosis would lead to an insufficient amount of acetylcholine reaching the postsynaptic membrane, causing **severe muscle weakness**. - This is the most direct physiological explanation when the primary finding is **reduced release** at the neuromuscular junction. *Synthesis of acetylcholine* - While impaired synthesis would eventually lead to reduced release, the immediate problem described is **reduced release**, implying acetylcholine is present but not being liberated effectively. - Problems with synthesis often manifest as a more chronic, rather than acute, decline in neurotransmitter levels. - Synthesis occurs in the cytoplasm before vesicular packaging, so this is upstream from the release mechanism. *Reuptake of choline* - Impaired reuptake of choline would affect the *recycling* of choline for subsequent acetylcholine synthesis, but it does not directly explain an immediate reduction in the *release* of already synthesized acetylcholine. - This would primarily affect the long-term availability of neurotransmitter precursors, not the exocytotic process itself. - Choline reuptake occurs at the presynaptic terminal after ACh breakdown in the synaptic cleft. *Storage of acetylcholine in vesicles* - If storage were impaired, acetylcholine might be degraded within the presynaptic terminal rather than being packaged into vesicles for release. - This could lead to reduced release, but the question specifically points to the **release mechanism** as the primary issue. - When release is specifically impaired, the exocytotic machinery (calcium-dependent fusion and SNARE proteins) is most directly implicated.

Q: Which neurotransmitter primarily mediates the effects of the parasympathetic nervous system?

Acetylcholine. ***Acetylcholine*** - **Acetylcholine** (ACh) is the primary neurotransmitter released by postganglionic neurons in the **parasympathetic nervous system**, mediating its "rest and digest" effects. - ACh acts on **muscarinic receptors** in target organs, leading to responses such as decreased heart rate, increased digestion, and pupillary constriction. *Dopamine* - **Dopamine** is primarily involved in reward, motivation, and motor control within the central nervous system, and it is not a primary mediator of the parasympathetic system. - While dopamine receptors exist in the peripheral nervous system, its role in mediating direct parasympathetic effects is limited compared to ACh. *Norepinephrine* - **Norepinephrine** is the primary neurotransmitter of the **sympathetic nervous system**, responsible for "fight or flight" responses. - It acts on **adrenergic receptors** at target organs, producing effects opposite to those of the parasympathetic system (e.g., increased heart rate, vasoconstriction). *GABA* - **GABA** (gamma-aminobutyric acid) is the main **inhibitory neurotransmitter** in the central nervous system, promoting relaxation and reducing neuronal excitability. - It has a negligible or indirect role in mediating the direct effector functions of the peripheral autonomic nervous system, particularly the parasympathetic branch.

Q: Which ion is most crucial for the propagation of action potentials in neurons?

Sodium. ***Sodium*** - The rapid influx of **sodium ions** through voltage-gated sodium channels is responsible for the **depolarization phase** of the action potential. - This depolarization is the primary event that triggers and propagates the nerve impulse along the axon. *Calcium* - While calcium is crucial for **neurotransmitter release** at the synaptic terminal, it is not the primary ion responsible for the initial depolarization and propagation of the action potential in the axon itself. - Influx of **calcium ions** plays a more significant role in action potentials in certain cell types like cardiac muscle cells. *Magnesium* - Magnesium acts as a cofactor for many enzymes and plays a role in nerve conduction indirectly, but it is not directly involved in the rapid ion flux that generates an action potential. - High magnesium levels can actually **reduce neuronal excitability** by blocking NMDA receptors. *Potassium* - **Potassium ions** are primarily responsible for the **repolarization phase** of the action potential, where they exit the cell to restore the negative resting membrane potential. - Their efflux also contributes to the **hyperpolarization** that follows an action potential.

Question 1

Which of the following mechanisms actively maintains the ionic concentration gradients essential for the resting membrane potential in neurons?

Accepted Answer

Sodium-potassium pump

Answer

Calcium channels

Answer

Chloride channels

Answer

Voltage-gated sodium channels

Question 2

A patient experiences sudden muscle weakness after consuming a large amount of potassium-rich food. Which physiological mechanism is likely responsible for the muscle weakness?

Accepted Answer

Depolarization of the nerve membrane

Answer

Decreased potassium efflux

Answer

Increased sodium influx

Answer

Hyperpolarization of the nerve membrane

Question 3

Which ion primarily determines the resting membrane potential of excitable cells?

Accepted Answer

Potassium

Answer

Sodium

Answer

Chloride

Answer

Calcium

Question 4

A patient presents with symptoms of muscle weakness and fatigue. Serum potassium levels are significantly elevated. How does hyperkalemia affect the resting membrane potential and action potential generation in neurons?

Accepted Answer

Depolarizes the resting membrane potential, making action potentials harder to generate

Answer

Hyperpolarizes the resting membrane potential, making action potentials harder to generate

Answer

No change in resting membrane potential, no change in action potential generation

Answer

Hyperpolarizes the resting membrane potential, making action potentials easier to generate

Question 5

A neurology patient exhibits muscle weakness and fatigue. Electromyography shows a decreased amplitude of muscle action potentials. Which ion channel malfunction is most likely responsible for this finding?

Accepted Answer

Sodium channels in the sarcolemma

Answer

Calcium channels in the sarcoplasmic reticulum

Answer

Potassium channels in the sarcolemma

Answer

Chloride channels in the sarcolemma

Question 6

Which of the following best describes the role of the sarcoplasmic reticulum in muscle cells?

Accepted Answer

Stores calcium ions

Answer

Generates ATP

Answer

Contains actin and myosin

Answer

Produces muscle proteins

Question 7

A patient with hypocalcemia presents with tetany. Which physiological process is most likely responsible for the muscle spasms?

Accepted Answer

Increased neuronal excitability

Answer

Decreased neuronal excitability

Answer

Increased muscle contractility

Answer

Decreased muscle contractility

Question 8

A patient with severe muscle weakness is found to have reduced acetylcholine release at the neuromuscular junction. Which physiological process is most likely impaired?

Accepted Answer

Exocytosis of acetylcholine

Answer

Synthesis of acetylcholine

Answer

Reuptake of choline

Answer

Storage of acetylcholine in vesicles

Question 9

Which neurotransmitter primarily mediates the effects of the parasympathetic nervous system?

Accepted Answer

Acetylcholine

Answer

Dopamine

Answer

Norepinephrine

Answer

GABA

Question 10

Which ion is most crucial for the propagation of action potentials in neurons?

Accepted Answer

Sodium

Answer

Calcium

Answer

Magnesium

Answer

Potassium

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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