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: 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: A 30-year-old female presents with muscle weakness and fatigue. Her serum calcium levels are elevated. Which of the following is a likely physiological consequence of hypercalcemia?

Decreased neuromuscular excitability. ***Decreased neuromuscular excitability*** - Elevated **calcium levels** stabilize the neuronal cell membrane, making it **less permeable to sodium ions**. - This reduces the likelihood of depolarization and action potential generation, leading to **muscle weakness** and **fatigue**. *Increased risk of hypocalcemia* - **Hypercalcemia** refers to elevated calcium levels, so an increased risk of hypocalcemia (low calcium levels) is contradictory. - The body's regulatory mechanisms would try to decrease calcium in response to hypercalcemia, not cause hypocalcemia. *Decreased neuronal conduction velocity* - While hypercalcemia can affect neuronal function, it primarily **decreases excitability** rather than directly slowing conduction velocity along the axon. - The main impact is on the threshold for firing an action potential. *Increased neuromuscular excitability and tetany* - **Increased neuromuscular excitability** and **tetany** are characteristic symptoms of **hypocalcemia** (low calcium), not hypercalcemia. - In hypocalcemia, the neuronal membrane becomes more permeable to sodium, leading to spontaneous depolarization and muscle spasms.

Q: A patient presents with muscle cramps and tetany. Laboratory results show hypocalcemia. Which of the following physiological mechanisms is most likely responsible for these symptoms?

Increased permeability to sodium. ***Increased permeability to sodium*** - **Hypocalcemia** reduces the threshold for excitation, making nerve and muscle cells **hyperexcitable**. - This occurs because fewer calcium ions stabilize the cell membrane, leading to an easier influx of **sodium ions** and subsequent depolarization. *Decreased neuronal excitability* - This is incorrect as **hypocalcemia** actually leads to **increased neuronal excitability**, causing symptoms like tetany and cramps. - Reduced extracellular calcium increases membrane permeability to sodium, making it easier for neurons to fire action potentials. *Increased binding of calcium to albumin* - While increased binding of calcium to albumin (e.g., in alkalosis) can lead to **hypocalcemia**, it describes a cause of hypocalcemia, not the physiological mechanism directly responsible for the symptoms of muscle cramps and tetany. - The direct mechanism causing symptoms relates to the *effect* of reduced free calcium on nerve and muscle cell membranes. *Decreased release of parathyroid hormone* - **Decreased PTH release** is a common *cause* of hypocalcemia (e.g., hypoparathyroidism), but it is not the direct physiological mechanism behind the muscle cramps and tetany. - PTH regulates calcium levels but does not directly mediate the effect of low calcium on nerve and muscle cells.

Q: What is the role of myelin in the nervous system?

Increase conduction velocity. ***Increase conduction velocity*** - **Myelin** acts as an electrical insulator around the axon, preventing the leakage of ions across the membrane. - This **insulation** allows action potentials to "jump" between the **Nodes of Ranvier** (a process called **saltatory conduction**), significantly increasing the speed of nerve signal transmission. - This is the **primary and most important role** of myelin, increasing conduction velocity up to 100-fold. *Facilitate neurotransmitter release* - **Neurotransmitter release** occurs at the **axon terminal** in response to the arrival of an action potential and influx of calcium ions, a process not directly mediated by myelin. - **Myelin** primarily functions to speed up signal propagation along the axon, not to influence the final output at the synapse. *Form synapses* - **Synapses** are specialized junctions between neurons where information is transmitted, typically formed by the **axon terminal** of one neuron and the **dendrites** or cell body of another. - **Myelin** is located along the axon, not at the synaptic cleft, and does not directly participate in synapse formation. *Contribute to axonal integrity* - While myelin and myelinating cells (oligodendrocytes and Schwann cells) do provide **metabolic support** and **trophic factors** to axons, this is a **secondary function**. - The **primary role** of myelin is to increase conduction velocity through saltatory conduction, making it the best answer to this question.

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

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

Accepted Answer

Potassium

Answer

Sodium

Answer

Chloride

Answer

Calcium

Question 3

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 4

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 5

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 6

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 7

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 8

A 30-year-old female presents with muscle weakness and fatigue. Her serum calcium levels are elevated. Which of the following is a likely physiological consequence of hypercalcemia?

Accepted Answer

Decreased neuromuscular excitability

Answer

Decreased neuronal conduction velocity

Answer

Increased risk of hypocalcemia

Answer

Increased neuromuscular excitability and tetany

Question 9

A patient presents with muscle cramps and tetany. Laboratory results show hypocalcemia. Which of the following physiological mechanisms is most likely responsible for these symptoms?

Accepted Answer

Increased permeability to sodium

Answer

Increased binding of calcium to albumin

Answer

Decreased release of parathyroid hormone

Answer

Decreased neuronal excitability

Question 10

What is the role of myelin in the nervous system?

Accepted Answer

Increase conduction velocity

Answer

Form synapses

Answer

Facilitate neurotransmitter release

Answer

Contribute to axonal integrity

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?