Nerve and Muscle Physiology Practice Questions

Q: How does myasthenia gravis cause muscle weakness?

Blocks ACh receptors at the neuromuscular junctions.. ***Blocks ACh receptors at the neuromuscular junctions.*** - Myasthenia gravis is an **autoimmune disorder** where the body produces **antibodies** that target and block the **nicotinic acetylcholine receptors (AChRs)** at the **neuromuscular junction** [1]. - This blocking prevents acetylcholine from binding effectively to the muscle cell, leading to a **reduced number of available receptors** and thus impaired muscle contraction and characteristic **muscle weakness** [2]. *Increases acetylcholine degradation.* - An increase in **acetylcholine degradation** would mean that **acetylcholinesterase** is overly active, breaking down acetylcholine more quickly. - While this would reduce available acetylcholine, it is **not the primary mechanism** in myasthenia gravis, where the issue is primarily with the receptors themselves [2]. *Inhibits acetylcholine release.* - Conditions like **Lambert-Eaton myasthenic syndrome (LEMS)**, not myasthenia gravis, are characterized by antibodies targeting **presynaptic voltage-gated calcium channels**, leading to **reduced acetylcholine release** [3]. - In myasthenia gravis, acetylcholine is released normally from the presynaptic neuron, but it cannot bind to the blocked receptors. *Decreases acetylcholine production.* - A decrease in acetylcholine production would imply a problem in the **synthesis of the neurotransmitter**, which is typically performed in the presynaptic neuron. - Myasthenia gravis does not primarily affect the production of acetylcholine; rather, it affects the **postsynaptic response** to acetylcholine.

Q: What is the neurotransmitter primarily involved in muscle contraction?

Acetylcholine. ***Acetylcholine*** - **Acetylcholine (ACh)** acts at the **neuromuscular junction** to initiate muscle contraction by binding to nicotinic receptors on the muscle fiber membrane. - This binding causes depolarization and triggers the release of **calcium** from the sarcoplasmic reticulum, essential for the interaction of actin and myosin filaments. *Glutamate* - **Glutamate** is the primary **excitatory neurotransmitter** in the central nervous system, mainly involved in synaptic transmission, learning, and memory. - It does not mediate signal transmission at the **neuromuscular junction** for skeletal muscle contraction. *Dopamine* - **Dopamine** is a neurotransmitter involved in reward, motivation, and motor control pathways within the **central nervous system** (basal ganglia). - It does not play a direct role in the peripheral process of **skeletal muscle contraction** at the neuromuscular junction. *Serotonin* - **Serotonin** primarily regulates mood, sleep, appetite, and gastrointestinal function in the **central nervous system**. - It is not involved in directly signaling **skeletal muscle fibers** for contraction at the neuromuscular junction.

Q: Which enzyme degrades acetylcholine at the neuromuscular junction?

Acetylcholinesterase. ***Acetylcholinesterase*** - This enzyme is located in the **synaptic cleft** and rapidly **hydrolyzes acetylcholine** into acetate and choline, terminating its action. - Its efficient degradation of acetylcholine ensures precise control over muscle contraction and relaxation at the **neuromuscular junction**. *Catechol-O-methyltransferase* - This enzyme is primarily involved in the degradation of **catecholamines** like dopamine, norepinephrine, and epinephrine, not acetylcholine. - It plays a significant role in the metabolism of neurotransmitters in the **central nervous system** and peripheral tissues, but not specifically at the neuromuscular junction for acetylcholine. *Glutaminase* - This enzyme is responsible for converting **glutamine to glutamate**, a crucial step in the synthesis of the excitatory neurotransmitter glutamate. - It is not involved in the degradation of acetylcholine at any synapse. *Monoamine oxidase* - This enzyme metabolizes **monoamine neurotransmitters** such as serotonin, dopamine, and norepinephrine, primarily within the synapse. - It does not act on acetylcholine, which is a **cholinergic neurotransmitter**.

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 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.

Question 1

How does myasthenia gravis cause muscle weakness?

Accepted Answer

Blocks ACh receptors at the neuromuscular junctions.

Answer

Increases acetylcholine degradation.

Answer

Inhibits acetylcholine release.

Answer

Decreases acetylcholine production.

Question 2

What is the neurotransmitter primarily involved in muscle contraction?

Accepted Answer

Acetylcholine

Answer

Glutamate

Answer

Dopamine

Answer

Serotonin

Question 3

Which enzyme degrades acetylcholine at the neuromuscular junction?

Accepted Answer

Acetylcholinesterase

Answer

Catechol-O-methyltransferase

Answer

Glutaminase

Answer

Monoamine oxidase

Question 4

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 5

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 6

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

Accepted Answer

Potassium

Answer

Sodium

Answer

Chloride

Answer

Calcium

Question 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?