Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 331: 'Lock jaw' indicates

A. Fracture of condyle
B. Ankylosis
C. Spasm of masseter muscle (Correct Answer)
D. Inflammatory trismus

Explanation: ***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.

Question 332: Sweat glands are supplied by only:

A. Sensory nerves
B. Motor nerves
C. Sympathetic nerves (Correct Answer)
D. Parasympathetic nerves

Explanation: ***Sympathetic nerves*** - Sweat glands (both eccrine and apocrine) are primarily innervated by the **sympathetic nervous system**. - While sympathetic fibers typically release **norepinephrine**, the sympathetic innervation of sweat glands is unique as it releases **acetylcholine**, acting on muscarinic receptors. *Sensory nerves* - **Sensory nerves** transmit information *from* the periphery *to* the central nervous system, detecting stimuli like touch, temperature, and pain. - They do not directly control the function or secretion of sweat glands. *Motor nerves* - **Motor nerves** innervate muscles, causing contraction, and some glands, but are not the primary innervation for sweat glands. - Their main role is in mediating voluntary and involuntary muscle movements. *Parasympathetic nerves* - The **parasympathetic nervous system** is primarily involved in "rest and digest" functions, such as stimulating digestion and reducing heart rate. - It plays a very limited role, if any, in the direct innervation and control of sweat gland secretion.

Question 333: Hypokalemia causes:

A. Hyperpolarisation (Correct Answer)
B. Increased amplitude of action potential
C. Tetany
D. Resting membrane potential becomes less negative

Explanation: ***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).

Question 334: Sensory nerve action potential in nerve conduction velocity (NCV) studies is used to calculate:

A. Conduction velocity
B. Sensory conduction (Correct Answer)
C. Motor conduction
D. Muscular contraction

Explanation: ***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.

Question 335: What primarily causes vasoconstriction in skin?

A. Warm climate
B. Wheal and flare
C. Sympathetic (Correct Answer)
D. Parasympathetic

Explanation: ***Sympathetic*** - The **sympathetic nervous system** primarily controls vasoconstriction in the skin via **adrenergic receptors** on vascular smooth muscle. - Activation of these nerves releases **norepinephrine**, leading to the contraction of smooth muscle and thus narrowing of blood vessels. *Warm climate* - A warm climate typically causes **vasodilation** in the skin, allowing for increased blood flow to the surface to facilitate heat loss. - This is a physiological response to prevent **overheating**, not to vasoconstrict. *Wheal and flare* - A **wheal and flare reaction** is a localized immune response, often to allergens, characterized by histamine release causing **vasodilation** (flare) and edema (wheal). - This reaction involves localized vasodilation, not systemic or primary vasoconstriction. *Parasympathetic* - The **parasympathetic nervous system** generally has minimal direct control over cutaneous vascular tone, especially regarding vasoconstriction. - Its primary role in the skin is related to **glandular secretions**, such as sweat production.

Question 336: Sensory fiber with least conduction velocity:

A. beta fiber
B. C- fiber (Correct Answer)
C. A-delta fiber
D. Alpha fiber

Explanation: ***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 337: If a person has normal musculature but has difficulty swallowing, which nerves should be tested for function?

A. Hypoglossal and phrenic
B. Hypoglossal and splanchnic
C. Glossopharyngeal and vagus (Correct Answer)
D. Splanchnic and vagus

Explanation: ***Glossopharyngeal and vagus*** - The **glossopharyngeal nerve (CN IX)** is responsible for the **afferent limb of the gag reflex** and sensation from the posterior tongue and pharynx, crucial for initiating swallowing. - The **vagus nerve (CN X)** innervates most muscles of the **pharynx and larynx**, controlling swallowing and protecting the airway. *Hypoglossal and phrenic* - The **hypoglossal nerve (CN XII)** controls **tongue movements**, which are important for bolus formation and propulsion but not directly for pharyngeal contraction. - The **phrenic nerve** primarily innervates the **diaphragm** and is essential for respiration, not swallowing. *Hypoglossal and splanchnic* - As mentioned, the **hypoglossal nerve** controls **tongue movement**. - **Splanchnic nerves** are part of the autonomic nervous system, primarily involved in **visceral innervation** of abdominal and pelvic organs, not the direct motor control of swallowing muscles. *Splanchnic and vagus* - **Splanchnic nerves** are involved in **abdominal and pelvic visceral function**, not directly in the pharyngeal phase of swallowing. - While the **vagus nerve** is critical for swallowing, the combination with splanchnic nerves is incorrect for targeted testing of dysphagia.

Question 338: Calcium does not bind to

A. Troponin
B. Calmodulin
C. Tropomyosin (Correct Answer)
D. None of the options

Explanation: ***Tropomyosin*** - **Tropomyosin** is a protein that winds around **actin filaments** and, in relaxed muscle, blocks the **myosin-binding sites** on actin, preventing contraction. - Calcium does **not directly bind** to tropomyosin; rather, its binding to **troponin** causes a conformational change that moves tropomyosin away from the binding sites. - **This is the correct answer** because tropomyosin lacks calcium-binding sites. *Incorrect: Troponin* - **Troponin** is a complex of three proteins (**troponin I, T, and C**) that is crucial for muscle contraction. - **Troponin C** is the specific subunit that **binds calcium ions**, initiating the cascade of events leading to muscle contraction. - This option is incorrect because troponin DOES bind calcium. *Incorrect: Calmodulin* - **Calmodulin** is a ubiquitous **calcium-binding messenger protein** expressed in all eukaryotic cells. - It mediates many crucial cellular processes by interacting with and regulating various protein targets (e.g., kinases, phosphatases) when it **binds to calcium ions**. - This option is incorrect because calmodulin DOES bind calcium. *Incorrect: None of the options* - This option would suggest that all the listed proteins bind calcium. - Since **tropomyosin does NOT bind calcium**, this option is incorrect.

Question 339: Demyelination is the major feature of Multiple Sclerosis. Which of the following cells forms myelin in the central nervous system?

A. Astrocytes
B. Ependymal cells
C. Microglia
D. Oligodendrocytes (Correct Answer)

Explanation: ***Oligodendrocytes*** - **Oligodendrocytes** are glial cells exclusively found in the **central nervous system (CNS)** that produce and maintain the **myelin sheath**. - The myelin sheath, formed by these cells, insulates axons and allows for rapid, efficient **saltatory conduction** of action potentials. *Astrocytes* - **Astrocytes** are star-shaped glial cells that provide structural and metabolic support for neurons, regulate the **blood-brain barrier**, and maintain the extracellular environment. - They do not form myelin; instead, they play roles in **neurotransmission** and response to injury. *Ependymal cells* - **Ependymal cells** line the ventricles of the brain and the central canal of the spinal cord, forming an interface between the neural tissue and **cerebrospinal fluid (CSF)**. - They are involved in **CSF production** and circulation, having no role in myelination. *Microglia* - **Microglia** are the resident immune cells of the CNS, functioning as **macrophages** to remove cellular debris, pathogens, and damaged neurons. - They are crucial for immune surveillance and inflammatory responses but do not produce myelin.

Question 340: Physiological unlocking is caused by -

A. Sartorius
B. Rectus femoris
C. Semimembranosus
D. Popliteus (Correct Answer)

Explanation: ***Popliteus*** - The **popliteus muscle** is responsible for the **physiological unlocking** mechanism of the knee joint at the beginning of flexion from a fully extended position. - It **internally rotates the tibia** on the femur (or externally rotates the femur on the tibia) to disengage the femoral condyles from their locked position, allowing flexion to initiate. *Sartorius* - The **sartorius muscle** is a strong flexor, abductor, and external rotator of the hip, and a flexor and internal rotator of the knee joint. - It does not primarily contribute to the unlocking mechanism of the knee. *Rectus femoris* - The **rectus femoris** is one of the quadriceps muscles and is a powerful extensor of the knee. - It plays no role in initiating knee flexion by unlocking the joint. *Semimembranosus* - The **semimembranosus** is part of the hamstring group, primarily involved in knee flexion and hip extension. - While it contributes to knee flexion, it does not perform the specific rotational movement required for unlocking the knee.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

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Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

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Muscle Metabolism and Fatigue

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Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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