Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 311: Inverse stretch reflex is mediated :

A. Unmyelinated C fibres
B. Dorsal Column
C. Muscle spindle
D. Golgi tendon organ (Correct Answer)

Explanation: ***Golgi tendon*** - The **Golgi tendon organ (GTO)** is a **proprioceptor** located at the junction of muscle fibers and tendons, sensitive to changes in muscle tension. - When muscle tension becomes excessive, the GTO is activated, inhibiting the alpha motor neurons innervating that muscle, leading to muscle relaxation, which is the **inverse stretch reflex**. *Unmyelinated C fibres* - These fibers are primarily involved in transmitting **slow, dull pain** and **temperature sensations**, but not proprioceptive reflexes. - Their conduction velocity is much slower than that required for rapid protective reflexes. *Dorsal Column* - The dorsal column-medial lemniscus pathway is responsible for transmitting **fine touch, vibration, and proprioception** to the brain, but it is an ascending sensory pathway and does not directly mediate spinal reflexes. - This pathway is involved in conscious perception, not the direct arc of a reflex. *Muscle spindle* - The **muscle spindle** is responsible for the **stretch reflex** (myotatic reflex), which causes muscle contraction in response to stretch. - It detects changes in **muscle length and rate of change of length**, which is distinct from the inverse stretch reflex mediated by the GTO.

Question 312: Delivery of stimulus above threshold intensity leads to a constant amplitude of AP and is known as:

A. Electrotonic potential
B. All or none law (Correct Answer)
C. Absolute refractory period
D. Relative refractory period

Explanation: ***All or none law*** - The **all-or-none law** states that if a stimulus reaches or exceeds the **threshold intensity**, a neuron will fire an action potential of a constant, maximal amplitude. - If the stimulus is below the threshold, no action potential will fire, meaning there is no partial or submaximal action potential. *Electrotonic potential* - **Electrotonic potentials** are subthreshold, local changes in membrane potential that decay with distance and time. - They are **graded**, meaning their amplitude is proportional to the stimulus intensity, unlike the fixed amplitude of an action potential. *Absolute refractory period* - The **absolute refractory period** is the time during an action potential when the membrane is completely unresponsive to further stimulation, no matter how strong. - This period is due to the **inactivation of voltage-gated sodium channels**, preventing another action potential from being generated. *Relative refractory period* - The **relative refractory period** is the time following the absolute refractory period when a **larger-than-normal stimulus** is required to elicit another action potential. - This occurs because some potassium channels are still open, and the membrane is hyperpolarized, making it harder to reach the threshold.

Question 313: Resting membrane potential of nerve fibre is close to isoelectric potential of:

A. Sodium ions
B. Potassium ions (Correct Answer)
C. Chloride ions
D. Magnesium ions

Explanation: ***Potassium ions*** - The **resting membrane potential** is primarily determined by the **equilibrium potential of potassium ions** because the membrane is far more permeable to potassium than to other ions at rest. - Due to the high **permeability to K+**, a significant outward flow of potassium ions occurs, making the inside of the cell negative relative to the outside, approaching the **Nernst potential for K+**. *Sodium ions* - The membrane has very low permeability to **sodium ions** at rest, so **Na+ influx** only slightly affects the resting potential. - The **Nernst potential for Na+** is positive, which is opposite to the negative resting membrane potential. *Chloride ions* - While chloride ions contribute to the **resting membrane potential**, their contribution is typically less significant than potassium due to varying membrane permeability in different neurons. - In many cells, chloride ions follow the electrical gradient set by other ions and do not actively determine the resting potential. *Magnesium ions* - **Magnesium ions** play crucial roles as cofactors for enzymes and in neurotransmission but have minimal direct influence on establishing the **resting membrane potential**. - The membrane is largely **impermeable to Mg2+** at rest, and their concentration gradients do not establish the baseline voltage.

Question 314: Which of the following receptors mediate stretch reflex?

A. Golgi tendon organ
B. Muscle spindle (Correct Answer)
C. Meissner's corpuscles
D. Merkel's disc

Explanation: ***Muscle spindle*** - Muscle spindles are **stretch-sensitive receptors** located within the muscle belly that detect changes in muscle length and the rate of change in length. - When a muscle is stretched, the muscle spindles are activated, sending signals via **afferent neurons** to the spinal cord, which then initiates a reflex contraction of the same muscle to counteract the stretch—this is the basis of the stretch reflex. *Golgi tendon organ* - **Golgi tendon organs** are located in the tendons and respond to changes in **muscle tension**, not muscle length. Its primary role is to prevent excessive muscle contraction. - When activated by high tension, Golgi tendon organs inhibit the muscle, leading to relaxation (inverse stretch reflex), which is opposite to the stretch reflex. *Meissner's corpuscles* - **Meissner's corpuscles** are **mechanoreceptors** located in the superficial layers of the skin, primarily responsible for detecting **light touch** and **vibrations**. - They are not involved in the regulation of muscle length or tension and therefore do not mediate the stretch reflex. *Merkel's disc* - **Merkel's discs** are **mechanoreceptors** found in the basal layer of the epidermis, specialized for detecting **sustained pressure** and **texture**. - These receptors contribute to fine tactile discrimination but are unrelated to the proprioceptive mechanisms of the stretch reflex.

Question 315: Which of the following is true regarding Na+ (sodium) ions?

A. Does not help other ions in transport
B. Responsible for depolarization (Correct Answer)
C. Responsible for the resting membrane potential
D. Sodium ion is responsible for Donnan effect

Explanation: ***Responsible for depolarization*** - The rapid influx of **Na+ ions** into the cell through voltage-gated sodium channels is the primary event that causes **depolarization** during an action potential. - This influx makes the inside of the cell more positive, shifting the membrane potential from negative toward positive values. *Sodium ion is responsible for Donnan effect* - The **Donnan effect** describes the unequal distribution of permeable ions across a semi-permeable membrane due to the presence of impermeant charged molecules (e.g., proteins). - **Na+ ions are small, permeable ions** - they do not create the Donnan effect. The effect is caused by large, non-diffusible charged molecules like proteins, not by sodium ions. *Does not help other ions in transport* - The **sodium-potassium pump (Na+/K+-ATPase)** actively transports Na+ out of the cell and K+ into the cell, maintaining their concentration gradients. - These Na+ gradients are crucial for **secondary active transport**, where the energy from Na+ moving down its electrochemical gradient is used to move other ions (e.g., in Na+-glucose cotransport) or molecules against their gradients. *Responsible for the resting membrane potential* - The **resting membrane potential** is primarily established by the differential permeability of the membrane to K+ ions and the activity of the Na+/K+-ATPase. - While Na+ leaking into the cell contributes slightly, the dominant factor is the efflux of **K+ ions** through leak channels, as the membrane is much more permeable to K+ than to Na+ at rest.

Question 316: A 10-year-old boy cuts his finger with a pocketknife and immediately applies pressure to the damaged area with his other hand to partially alleviate the pain. Inhibition of pain signals by tactile stimulation of the skin is mediated by which type of afferent neurons from mechanoreceptors?

A. Aδ
B. Type C
C. Aβ (Correct Answer)
D. Aα

Explanation: ***Aβ*** - **Aβ (A-beta) fibers** are large, myelinated afferent neurons that transmit discriminative touch and proprioception. - According to the **gate control theory of pain**, activation of these Aβ fibers by tactile stimulation can inhibit the transmission of pain signals (carried by Aδ and C fibers) in the spinal cord, explaining why rubbing an injured area can reduce pain. *Aδ* - **Aδ (A-delta) fibers** are thinly myelinated afferent neurons that transmit sharp, localized, and fast pain, as well as cold and touch. - While they are involved in pain transmission, they do not primarily mediate the inhibition of pain signals through tactile stimulation, but rather the initial painful sensation. *Type C* - **Type C fibers** are unmyelinated afferent neurons that transmit slow, dull, aching, and burning pain, as well as warmth and some touch. - These fibers are primarily responsible for the prolonged, chronic pain sensation and are inhibited by Aβ fiber activity, not the mediators of the pain inhibition themselves. *Aα* - **Aα (A-alpha) fibers** are the largest and fastest myelinated afferent neurons, primarily responsible for proprioception from muscle spindles (sensory information about muscle length and stretch), and motor innervation to extrafusal muscle fibers. - They are not directly involved in the tactile inhibition of pain signals.

Question 317: Type I muscle fibers are

A. Glycolytic
B. Anaerobic
C. Large
D. Red (Correct Answer)

Explanation: ***Red*** - Type I muscle fibers are also known as **slow-twitch oxidative fibers**, characterized by their **high myoglobin content** which gives them a red appearance. - They are rich in **mitochondria** and have a large supply of capillaries, making them highly efficient at **aerobic metabolism**. *Glycolytic* - **Glycolytic fibers** (Type IIb, or fast-twitch glycolytic) primarily rely on **anaerobic glycolysis** for energy. - These fibers are generally **white** due to lower myoglobin content and fewer mitochondria. *Anaerobic* - **Anaerobic metabolism** is characteristic of **Type II (fast-twitch)** muscle fibers, which are suited for short bursts of high-intensity activity. - Type I fibers primarily use **aerobic metabolism** and are fatigue-resistant, suited for sustained activity. *Large* - **Type II (fast-twitch)** muscle fibers tend to be **larger in diameter** than Type I fibers, allowing them to generate greater force. - Type I fibers are generally **smaller** and produce less tension but are more resistant to fatigue.

Question 318: Pressure sensation is carried by

A. Aγ fibres
B. Aα fibres
C. Aβ fibres (Correct Answer)
D. Aδ fibres

Explanation: ***Aβ fibres*** - **Aβ fibres** are large, myelinated nerve fibres responsible for transmitting touch, vibration, and **pressure sensations**. - Their significant myelination allows for rapid conduction of these **discriminative sensory inputs**. *Aγ fibres* - **Aγ fibres** are motor efferent fibres that innervate the **intrafusal muscle fibres** of muscle spindles, regulating their sensitivity. - They are not involved in the direct transmission of **sensory information** like pressure. *Aα fibres* - **Aα fibres** are the largest and fastest conducting nerve fibres, primarily responsible for **proprioception** (muscle stretch) and **motor efferents** to skeletal muscles (extrafusal fibres). - While they carry some sensory information, they are not the primary carriers of **pressure sensation**. *Aδ fibres* - **Aδ fibres** are lightly myelinated and transmit sensations of **fast pain**, cold temperature, and crude touch. - They conduct more slowly than Aβ fibres and are not primarily responsible for **pressure sensation**.

Question 319: Group 2 sensory fibres are attached to:

A. Annulospiral ending
B. Pacinian corpuscle
C. Golgi tendon
D. Flower spray ending (Correct Answer)

Explanation: ***Flower spray ending*** - **Group II sensory fibers** (also known as Aβ fibers) innervate the **flower spray endings** (secondary endings) of **muscle spindles**. - These endings detect changes in muscle length and contribute to proprioception, responding primarily to static muscle stretch. *Annulospiral ending* - The **annulospiral ending** (primary ending) is innervated by **Group Ia afferent fibers**, not Group II. - These fibers respond to both the rate of change of muscle length (dynamic response) and static muscle length. *Pacinian corpuscle* - **Pacinian corpuscles** are rapidly adapting mechanoreceptors found in subcutaneous tissue and deep fascia, detecting **vibration and deep pressure**. - While they are innervated by Aβ fibers (equivalent to Group II classification), they are **cutaneous mechanoreceptors**, not associated with muscle spindles like the flower spray endings. *Golgi tendon* - The **Golgi tendon organ** is a proprioceptor that detects **muscle tension** and is innervated by **Group Ib afferent fibers**, not Group II. - Its primary role is to monitor tension and provide protective feedback against excessive muscle force.

Question 320: Oligodendrocytes are important in:

A. Chemotaxis
B. Myelin formation (Correct Answer)
C. Blood brain barrier
D. Phagocytosis

Explanation: ***Myelin formation*** - **Oligodendrocytes** are responsible for producing and maintaining the **myelin sheath** around axons in the **central nervous system (CNS)**. - The myelin sheath acts as an electrical insulator, allowing for the **rapid and efficient transmission of nerve impulses**. *Chemotaxis* - **Chemotaxis** is the movement of a cell or organism in response to a chemical stimulus. - This process is primarily associated with **immune cells** like leukocytes, not oligodendrocytes. *Blood brain barrier* - The **blood-brain barrier (BBB)** is primarily formed by **tight junctions** between **endothelial cells** of cerebral capillaries. - **Astrocytes** also play a crucial role in maintaining the integrity and function of the BBB, but not oligodendrocytes. *Phagocytosis* - **Phagocytosis** is the process by which cells engulf large particles, such as bacteria or dead cells. - This function is primarily carried out by **microglia**, the resident immune cells of the CNS, which clear cellular debris and pathogens.

Practice by Chapter

Resting Membrane Potential

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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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