Nerve and Muscle Physiology — MCQs
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Which of the following is/are true about nodes of Ranvier? 1. No myelin 2. Rich in sodium channels
A 50-year-old woman who is a known diabetic and hypertensive has developed vesicular eruption on the trunk in T6 dermatome. The dermatologist confirmed it as Herpes Zoster. She is complaining of severe pain and increased sensitivity to touch. Which of the following receptors sense sustained pressure?
Which is NOT a function of muscles?
Annulospiral endings are present on:
Sweat glands receive cholinergic innervation from
Nuclear bag fibre detects
Lurching Gait is due to paralysis of which of the following?
The most significant immediate result of lowered serum calcium is
Which of the following neurotransmitters has only inhibitory action?
The increase in threshold of a receptor when a series of stimuli of subthreshold intensity are applied in succession is called __________.
Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs
Question 321: Which of the following is/are true about nodes of Ranvier? 1. No myelin 2. Rich in sodium channels
- A. None of the options
- B. No myelin
- C. Both (Correct Answer)
- D. Rich in sodium channels
Explanation: ***Both*** - Nodes of Ranvier are **gaps in the myelin sheath** that occur at regular intervals along a myelinated axon, making them areas with **no myelin**. - These nodes are crucially important because they have a **high concentration of voltage-gated sodium channels**, which enables the propagation of action potentials via **saltatory conduction**. - Both statements are **physiologically accurate** descriptions of the nodes of Ranvier, and their unique structure and composition are fundamental to **efficient transmission of nerve impulses**. *None of the options* - This option is incorrect because both statements, "No myelin" and "Rich in sodium channels," are **physiologically accurate** descriptions of the nodes of Ranvier. *No myelin* - While it is true that nodes of Ranvier are **devoid of myelin**, this statement alone does not encompass the full functional importance of these structures. - Their role in nerve impulse conduction also heavily relies on the presence of specific ion channels, which this option does not include. *Rich in sodium channels* - It is accurate that nodes of Ranvier have a **high density of voltage-gated sodium channels**, which is essential for regenerating the action potential. - However, this statement alone omits the key structural feature that they are also **unmyelinated gaps**, which is equally important to their function.
Question 322: A 50-year-old woman who is a known diabetic and hypertensive has developed vesicular eruption on the trunk in T6 dermatome. The dermatologist confirmed it as Herpes Zoster. She is complaining of severe pain and increased sensitivity to touch. Which of the following receptors sense sustained pressure?
- A. Pacinian corpuscle
- B. Ruffini corpuscles (Correct Answer)
- C. Meissner's corpuscles
- D. Merkel cells
Explanation: ***Ruffini corpuscles*** - **Ruffini corpuscles** are slow-adapting mechanoreceptors located deep in the dermis and subcutaneous tissue. - They are responsible for sensing **sustained pressure**, skin stretch, and contribute to the perception of object slippage and grasp. *Pacinian corpuscle* - **Pacinian corpuscles** are rapidly adapting receptors that detect **vibration** and **deep pressure**. - They are not primarily involved in sensing sustained pressure due to their rapid adaptation. *Merkel cells* - **Merkel cells** are slow-adapting mechanoreceptors found in the basal epidermis. - They are crucial for sensing **light touch**, **texture**, and **two-point discrimination**, but not deep or sustained pressure. *Meissner's corpuscles* - **Meissner's corpuscles** are rapidly adapting mechanoreceptors located in the dermal papillae. - They are specialized for detecting **light touch** and **low-frequency vibration**, particularly important for discriminative touch.
Question 323: Which is NOT a function of muscles?
- A. Cause movement
- B. Maintain posture
- C. Produce heat
- D. Absorb nutrients (Correct Answer)
Explanation: ***Absorb nutrients*** - Muscles are primarily involved in **movement**, **posture**, and **heat generation**, not direct nutrient absorption. - **Nutrient absorption** mainly occurs in the **gastrointestinal tract**, with specialized cells designed for this function. *Cause movement* - **Skeletal muscles** contract to pull on bones, generating the forces necessary for **locomotion** and other body movements. - **Smooth and cardiac muscles** are responsible for involuntary movements like **peristalsis** and **heart pumping**. *Maintain posture* - **Skeletal muscles** continuously contract to oppose gravity and maintain the body's **erect position** and stability. - This sustained, low-level contraction is known as **muscle tone**. *Produce heat* - **Muscle contraction** is an inefficient process, with a significant portion of the energy converted into **heat**. - This heat production is crucial for maintaining **body temperature**, especially during exercise or in cold environments.
Question 324: Annulospiral endings are present on:
- A. Nuclear bag fibers
- B. Both (Correct Answer)
- C. Nuclear chain fibers
- D. None of the options
Explanation: ***Both*** - **Annulospiral endings** are the primary sensory afferents (Type Ia) that innervate the central, non-contractile region of both **nuclear bag fibers** and **nuclear chain fibers** within the muscle spindle. - These endings respond to changes in **muscle length** and the **rate of change of muscle length**, playing a crucial role in the stretch reflex. *Nuclear bag fibers* - These are intrafusal muscle fibers that have their nuclei clustered in a central "bag-like" region. They primarily detect the **rate of change of muscle length (dynamic response)**. - While they are innervated by annulospiral endings, so are nuclear chain fibers. *Nuclear chain fibers* - These intrafusal fibers have their nuclei arranged in a single row or "chain." They primarily detect **static changes in muscle length**. - They are also innervated by annulospiral endings, indicating that the annulospiral endings are not exclusive to nuclear bag fibers. *None of the options* - This option is incorrect because annulospiral endings are definitively present on both nuclear bag and nuclear chain fibers.
Question 325: Sweat glands receive cholinergic innervation from
- A. Pre-ganglionic parasympathetic
- B. Post ganglionic parasympathetic
- C. Post ganglionic sympathetic (Correct Answer)
- D. Preganglionic sympathetic
Explanation: ***Post ganglionic sympathetic*** - Sweat glands are one of the few exceptions where the **postganglionic sympathetic neurons** release **acetylcholine** (cholinergic innervation) instead of norepinephrine. - This specialized innervation stimulates increased sweat production for **thermoregulation**. *Pre-ganglionic parasympathetic* - Pre-ganglionic parasympathetic fibers release acetylcholine, but they synapse with post-ganglionic neurons in **ganglia located near or within target organs**, not directly innervating sweat glands. - Their primary role is in **rest-and-digest functions**, not sweat production. *Post ganglionic parasympathetic* - Post-ganglionic parasympathetic fibers typically innervate **smooth muscles and glands** in organs like the gut or bladder, releasing acetylcholine to promote relaxation or secretion. - They do not innervate sweat glands; sudoriferous glands fall under the sympathetic division for their innervation. *Preganglionic sympathetic* - Pre-ganglionic sympathetic neurons originate in the **thoracolumbar spinal cord** and release acetylcholine at the ganglia. - They do not directly innervate sweat glands; they synapse with **post-ganglionic sympathetic neurons** first.
Question 326: Nuclear bag fibre detects
- A. Involved in reciprocal innervations
- B. Alpha motor neuron stimulation
- C. Sense dynamic length of muscle (Correct Answer)
- D. Senses muscle tension
Explanation: ***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.
Question 327: Lurching Gait is due to paralysis of which of the following?
- A. Gluteus medius (Correct Answer)
- B. Adductor magnus
- C. Hamstrings
- D. Quadriceps femoris
Explanation: ***Gluteus medius*** * Paralysis of the **gluteus medius** leads to a **Trendelenburg gait** or **lurching gait**, where the pelvis drops on the unsupported side during walking. * This muscle is crucial for **stabilizing the pelvis** during the single-limb support phase of gait. *Adductor Magnus* * Paralysis of the adductor magnus would primarily affect **thigh adduction** and extension, not directly causing a lurching gait. * Problems with this muscle might impact the ability to bring the legs together or stabilize the leg during certain movements. *Hamstrings* * The hamstrings are responsible for **knee flexion** and **hip extension**. * Paralysis would result in difficulty bending the knee and limited hip extension, potentially leading to a stiff-knee gait, but not typically a lurching gait. *Quadriceps femoris* * The quadriceps femoris is essential for **knee extension** and is critical for activities like standing, walking, and climbing stairs. * Paralysis would cause the knee to buckle, leading to a **knee-hyperflexion gait** or difficulty with weight-bearing on that leg.
Question 328: The most significant immediate result of lowered serum calcium is
- A. Decalcification of bones
- B. Decalcification of teeth
- C. Weakened heart action
- D. Hyperirritability of nerves and muscles (Correct Answer)
Explanation: ***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.
Question 329: Which of the following neurotransmitters has only inhibitory action?
- A. Glutamine
- B. All of the options
- C. GABA (Correct Answer)
- D. Aspartate
Explanation: ***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.
Question 330: The increase in threshold of a receptor when a series of stimuli of subthreshold intensity are applied in succession is called __________.
- A. Accommodation (Correct Answer)
- B. Resistance
- C. Initiation
- D. Adaptation
Explanation: ***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.
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