Nerve and Muscle Physiology Practice Questions

Q: The effect seen due to decreased serum calcium concentration is

Excitability of muscle. ***Excitability of the muscle*** - A decrease in serum calcium concentration (**hypocalcemia**) reduces the threshold potential for sodium channels, making nerve and muscle cells **more excitable**. - This increased excitability can lead to symptoms like **tetany**, muscle spasms, and even convulsions. *Depression of Nervous system* - This is typically seen with **hypercalcemia** (increased serum calcium), where elevated calcium levels stabilize nerve membranes, making them less excitable. - **Hypocalcemia**, conversely, leads to neuronal hyperexcitability, not depression. *Increase the renal absorption* - Renal calcium reabsorption is primarily regulated by **parathyroid hormone (PTH)**. Low serum calcium stimulates PTH release, which *increases* renal calcium reabsorption to restore calcium levels. - This is a *physiological response* to hypocalcemia, not an *effect* of hypocalcemia on neural or muscular function. *Relaxation of muscle* - Muscle relaxation requires ATP and the re-sequestration of calcium into the sarcoplasmic reticulum, and is not a direct consequence of low extracellular calcium. - Instead, **hypocalcemia** causes increased muscle **contraction** and spasms due to enhanced neuromuscular excitability.

Q: Nernst equilibrium potential for Cl– is

–70mV. ***–70mV*** - The **Nernst equilibrium potential** for an ion represents the membrane potential at which there is no net movement of that ion across the membrane, as the **electrical gradient** exactly balances the **chemical (concentration) gradient**. - For Cl⁻, the extracellular concentration is **higher than intracellular**, creating a **chemical gradient that drives Cl⁻ inward**. - At **–70mV** (negative inside), the **electrical gradient repels Cl⁻ outward** (negative ions repelled by negative charge), balancing the inward chemical drive and achieving equilibrium. - This is the typical value in **most neurons**. *–90mV* - A potential of **–90mV** is typically the **equilibrium potential for potassium (K⁺)** in neurons, reflecting K⁺'s high intracellular concentration. - In **skeletal muscle**, ECl can approach –90mV, but in neurons it is typically less negative (around –70mV). *+80mV* - At a **positive membrane potential**, the electrical gradient would **attract Cl⁻ into the cell** (negative ion attracted to positive charge). - This would **reinforce** the inward chemical gradient, causing net Cl⁻ influx, not equilibrium. - This value does not represent the equilibrium potential for any major physiological ion. *+60mV* - At **+60mV**, Cl⁻ would experience both electrical attraction inward and chemical drive inward, causing **strong net influx**, not equilibrium. - This value approximates the **equilibrium potential for sodium (Na⁺)**, which has high extracellular concentration and reaches equilibrium at positive potentials.

Q: Nuclear bag fibre detects

Sense dynamic length of muscle. ***Sense dynamic length of muscle*** - **Nuclear bag fibres** are a type of intrafusal muscle fibre found in **muscle spindles** that are sensitive to the **rate of change of muscle length (dynamic stretch)**. - They transmit this information via **primary (annulospiral) afferent fibres** (Ia afferents) to the central nervous system, contributing to reflexes and proprioception. *Involved in reciprocal innervations* - **Reciprocal inhibition** involves the relaxation of an antagonist muscle when the agonist contracts, a function coordinated by interneurons in the spinal cord, not directly detected by nuclear bag fibers. - While muscle spindles contribute to the overall reflex arc, reciprocal innervation is a broader reflex phenomenon, not a primary detection function of nuclear bag fibres themselves. *Alpha motor neuron stimulation* - **Alpha motor neurons** innervate **extrafusal muscle fibres** causing muscle contraction, and they are not directly detected by nuclear bag fibres. - **Gamma motor neurons** innervate the intrafusal fibres (including nuclear bag fibres) to maintain their sensitivity during muscle contraction, but this is an efferent control, not a detection function. *Senses muscle tension* - **Muscle tension** is primarily detected by **Golgi tendon organs (GTOs)**, which are located in the tendons and respond to changes in tension during muscle contraction. - While nuclear bag fibres detect muscle stretch, their primary role is related to length and rate of change of length, not directly tension.

Q: The most significant immediate result of lowered serum calcium is

Hyperirritability of nerves and muscles. ***Hyperirritability of nerves and muscles*** - Lowered serum calcium (hypocalcemia) decreases the threshold potential of excitable cells, leading to **increased neuronal and muscular excitability**. - This can manifest as **tetany**, muscle cramps, paresthesias, and in severe cases, seizures. *Decalcification of bones* - **Chronic hypocalcemia** can lead to secondary hyperparathyroidism, which may eventually cause bone decalcification. - This is a **long-term effect**, not an immediate significant result of acutely lowered serum calcium. *Decalcification of teeth* - Tooth decalcification is primarily associated with **fluoride deficiency**, poor oral hygiene, or acidic erosion, not directly with acute systemic hypocalcemia. - The calcium in teeth is **highly stable** and less readily mobilized than bone calcium in response to acute serum calcium changes. *Weakened heart action* - While severe **hypocalcemia can impair myocardial contractility** and lead to a weakened heart action, it is often preceded or accompanied by significant neuromuscular symptoms. - **Hyperkalemia** is more classically associated with immediate life-threatening cardiac dysfunction, while hypocalcemia primarily affects nerve and muscle excitability first.

Q: Which of the following neurotransmitters has only inhibitory action?

GABA. ***GABA*** - **Gamma-aminobutyric acid (GABA)** is the primary **inhibitory neurotransmitter** in the central nervous system. - It works by reducing neuronal excitability, often by allowing **chloride ions** into the cell, leading to hyperpolarization. *Glutamine* - **Glutamine** is an **amino acid** that is a precursor to the excitatory neurotransmitter **glutamate** and the inhibitory neurotransmitter GABA, but it is not a neurotransmitter itself. - It plays a crucial role in the **glutamate-GABA cycle**, but its direct action is not neurotransmission. *All of the options* - This option is incorrect because only GABA among the choices has a solely inhibitory action; **glutamine** is a precursor, and **aspartate** is primarily excitatory. - Not all substances listed are neurotransmitters, nor do they all have purely inhibitory actions. *Aspartate* - **Aspartate** is an **excitatory neurotransmitter** and plays a role in synaptic plasticity and learning. - It primarily acts on **NMDA receptors**, similar to glutamate, to depolarize neurons.

Q: The increase in threshold of a receptor when a series of stimuli of subthreshold intensity are applied in succession is called __________.

Accommodation. ***Accommodation*** - This describes the phenomenon where the **threshold of excitability increases** when a nerve or receptor is subjected to **repeated subthreshold stimuli** or a slowly rising stimulus. - As subthreshold stimuli are applied in succession, voltage-gated **sodium channels progressively inactivate** without triggering an action potential. - This inactivation causes the threshold to **increase**, making the receptor or nerve fiber **less excitable** and requiring a stronger stimulus to fire. - This is the **correct answer** as it precisely matches the definition in the question stem. *Adaptation* - This refers to the **decrease in receptor responsiveness** when exposed to a **constant or prolonged stimulus**. - Receptors reduce their firing rate over time even though the stimulus continues (e.g., wearing clothes, background noise). - Adaptation is about **decreased response magnitude**, not an increase in threshold due to subthreshold stimuli. - This is a different phenomenon from what the question describes. *Resistance* - In physiology, resistance typically refers to the **opposition to flow** (e.g., vascular resistance, airway resistance) or the ability to **withstand effects** of drugs or pathogens. - It does not describe changes in **neuronal or receptor threshold** due to stimulation patterns. *Initiation* - This term refers to the **beginning or triggering** of a process, such as the initiation of an action potential or biochemical cascade. - It does not describe the **change in excitability threshold** that occurs with repeated subthreshold stimulation.

Q: 'Lock jaw' indicates

Spasm of masseter muscle. ***Spasm of masseter muscle*** - **\"Lockjaw\"** (or trismus) is a classic symptom referring to the inability to open the mouth fully, primarily caused by **spasm of the masseter muscle**. - This symptom is most notably associated with **tetanus**, where the powerful toxins cause sustained muscle contractions. *Fracture of condyle* - A condylar fracture can cause pain and difficulty in opening the mouth, but it typically presents with **malocclusion** and **deviation of the jaw** towards the fractured side. - While it restricts mouth opening, the primary mechanism is mechanical obstruction and pain, not generalized muscle spasm. *Ankylosis* - **Ankylosis** is the fusion or stiffening of a joint, in this case, the **temporomandibular joint (TMJ)**. - It would lead to chronic and often complete loss of mouth opening, but it's a structural problem rather than an acute muscular spasm. *Inflammatory trismus* - **Inflammatory trismus** can result from infections or inflammation in the oral and maxillofacial regions (e.g., pericoronitis, abscesses). - This type of trismus is caused by inflammation and pain response, leading to reflexive muscle guarding, which is distinct from the sustained, involuntary spasm seen in conditions like tetanus.

Q: Hypokalemia causes:

Hyperpolarisation. ***Hyperpolarisation*** - Hypokalemia leads to an **increased gradient of potassium ions** across the cell membrane, causing more K+ to leave the cell. - This efflux of positive charge makes the **resting membrane potential more negative**, thus hyperpolarizing the cell and making it less excitable. *Increased amplitude of action potential* - Hypokalemia primarily affects the **resting membrane potential** and excitability, not the amplitude of the action potential itself once it's triggered. - The amplitude of an action potential is mainly determined by the **influx of sodium ions** through voltage-gated channels. *Tetany* - **Tetany** is typically associated with **hypocalcemia**, which increases neuronal excitability. - Hypokalemia generally leads to **decreased neuronal and muscle excitability**, causing weakness and paralysis rather than tetany. *Resting membrane potential becomes less negative* - If the resting membrane potential becomes less negative, it's referred to as **depolarization**, which is associated with **hyperkalemia** (reduced potassium gradient, less K+ efflux). - In hypokalemia, the increased potassium gradient causes the membrane to become **more negative** (hyperpolarized).

Q: Sensory nerve action potential in nerve conduction velocity (NCV) studies is used to calculate:

Sensory conduction. ***Sensory conduction*** - The **sensory nerve action potential (SNAP)** directly measures the electrical activity of **sensory nerve fibers**. - This allows for the calculation of **sensory nerve conduction velocity** and amplitude, assessing the function of these specific nerve pathways. *Conduction velocity* - This is a general term and does not specify whether referring to **sensory** or **motor nerve conduction velocity**. - While SNAPs contribute to conduction velocity calculations, this option is too broad and not as precise as "Sensory conduction." *Motor conduction* - **Motor nerve conduction** is measured using **compound muscle action potentials (CMAPs)**, which reflect the activity of motor nerve fibers and the muscle itself. - SNAPs specifically assess sensory nerves and are not used to calculate motor conduction parameters. *Muscular contraction* - **Muscular contraction** is the physiological response to nerve stimulation, but it is not directly calculated using the **sensory nerve action potential**. - Muscle contraction is more directly assessed through **electromyography (EMG)** and motor nerve conduction studies.

Q: Sensory fiber with least conduction velocity:

C- fiber. ***C- fiber*** - **C-fibers** are **unmyelinated** and have a very small diameter, leading to the **slowest conduction velocity** among all nerve fiber types. - They transmit **slow, dull pain**, temperature, and crude touch sensations. - Conduction velocity: **0.5-2 m/s** *Beta fiber* - **Beta fibers** (Aβ fibers) are **myelinated sensory fibers**, have a medium-to-large diameter, and conduct impulses at a faster rate than C-fibers. - They transmit touch, pressure, vibration, and proprioception. - Conduction velocity: **30-70 m/s** *A-delta fiber* - **A-delta fibers** (Aδ fibers) are **thinly myelinated sensory fibers** that conduct faster than C-fibers but slower than A-beta fibers. - They transmit **sharp, acute pain**, cold temperature, and pressure sensations. - Conduction velocity: **12-30 m/s** *Alpha fiber* - **Alpha fibers** (Aα fibers) are the **largest diameter**, most heavily **myelinated** nerve fibers, resulting in the **fastest conduction velocity**. - They include alpha motor neurons for **skeletal muscle contraction** and Ia/Ib sensory afferents for proprioception. - Conduction velocity: **70-120 m/s**

Question 1

The effect seen due to decreased serum calcium concentration is

Accepted Answer

Excitability of muscle

Answer

Depression of nervous system

Answer

Increased renal absorption

Answer

Relaxation of muscle

Question 2

Nernst equilibrium potential for Cl– is

Accepted Answer

–70mV

Answer

–90mV

Answer

+80mV

Answer

+60mV

Question 3

Nuclear bag fibre detects

Accepted Answer

Sense dynamic length of muscle

Answer

Involved in reciprocal innervations

Answer

Alpha motor neuron stimulation

Answer

Senses muscle tension

Question 4

The most significant immediate result of lowered serum calcium is

Accepted Answer

Hyperirritability of nerves and muscles

Answer

Decalcification of bones

Answer

Decalcification of teeth

Answer

Weakened heart action

Question 5

Which of the following neurotransmitters has only inhibitory action?

Accepted Answer

GABA

Answer

Glutamine

Answer

All of the options

Answer

Aspartate

Question 6

The increase in threshold of a receptor when a series of stimuli of subthreshold intensity are applied in succession is called __________.

Accepted Answer

Accommodation

Answer

Resistance

Answer

Initiation

Answer

Adaptation

Question 7

'Lock jaw' indicates

Accepted Answer

Spasm of masseter muscle

Answer

Fracture of condyle

Answer

Ankylosis

Answer

Inflammatory trismus

Question 8

Hypokalemia causes:

Accepted Answer

Hyperpolarisation

Answer

Increased amplitude of action potential

Answer

Tetany

Answer

Resting membrane potential becomes less negative

Question 9

Sensory nerve action potential in nerve conduction velocity (NCV) studies is used to calculate:

Accepted Answer

Sensory conduction

Answer

Conduction velocity

Answer

Motor conduction

Answer

Muscular contraction

Question 10

Sensory fiber with least conduction velocity:

Accepted Answer

C- fiber

Answer

beta fiber

Answer

A-delta fiber

Answer

Alpha fiber

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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