Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 421: Myosin and actin filaments are kept in place by?

A. Tropomyosin
B. Troponin
C. Actinin
D. Titin (Correct Answer)

Explanation: ***Titin*** - **Titin** is a large, elastic protein that extends from the **Z-disc to the M-line** in a sarcomere. - It plays a crucial role in maintaining the structural integrity and **passive elasticity** of muscle cells by keeping myosin and actin filaments properly aligned. *Tropomyosin* - **Tropomyosin** is a regulatory protein that wraps around the actin filament, **blocking the myosin-binding sites** in relaxed muscle. - Its primary function is to regulate muscle contraction, not to structurally anchor the filaments in place. *Troponin* - **Troponin** is a complex of three proteins (**troponin I, T, and C**) that is associated with tropomyosin on the actin filament. - It binds **calcium ions**, leading to a conformational change that moves tropomyosin away from the myosin-binding sites, allowing contraction to occur. *Actinin* - **Alpha-actinin** is a protein found in the **Z-disc** that helps to anchor the plus ends of actin filaments to the Z-disc. - While it's involved in actin filament organization, it does not directly keep both myosin and actin filaments in their precise functional arrangement along the length of the sarcomere in the same way titin does.

Question 422: Protein connecting Z-lines to M-lines is:

A. Kinin
B. Desmin
C. Titin (Correct Answer)
D. Actin

Explanation: ***Titin*** - **Titin** is a giant protein that extends from the **Z-disc to the M-line** in the sarcomere, acting as a molecular spring. - It maintains the **structural integrity** of the sarcomere and provides passive elasticity to muscles. *Kinin* - **Kinin** is a protein involved in **inflammation and blood pressure regulation**, not a structural component of muscle sarcomeres. - Examples include **bradykinin**, which mediates pain and vasodilation. *Desmin* - **Desmin** is an **intermediate filament protein** that forms a scaffold around the Z-discs, linking myofibrils together. - While it connects myofibrils, it does not directly span between the Z-line and M-line within a single sarcomere. *Actin* - **Actin** is a primary component of **thin filaments** in the sarcomere and participates in muscle contraction, but it does not connect the Z-line to the M-line. - Thin filaments are anchored at the **Z-disc** but only extend partway into the A-band.

Question 423: All of the following physiological processes occur during growth at the epiphyseal plate except:

A. Proliferation and hypertrophy
B. Calcification and ossification
C. Angiogenesis and remodeling
D. Replacement of red bone marrow with yellow marrow (Correct Answer)

Explanation: ***Replacement of red bone marrow with yellow marrow*** - The replacement of **red bone marrow** with **yellow marrow** is a process that occurs in the **diaphysis (shaft)** of long bones with aging, not directly within the **epiphyseal plate** during growth. - While it's a normal physiological change in bone, it's distinct from the primary mechanisms of **longitudinal bone growth** occurring at the growth plate. *Proliferation and hypertrophy* - **Chondrocytes** in the **proliferative zone** of the epiphyseal plate divide rapidly, increasing in number. - In the **hypertrophic zone**, these chondrocytes enlarge significantly, contributing to the lengthening of the bone. *Calcification and ossification* - The hypertrophied chondrocytes in the **calcification zone** undergo apoptosis, and their extracellular matrix becomes calcified. - In the **ossification zone**, osteoblasts invade the calcified cartilage and lay down new bone matrix, replacing the cartilage with bone. *Angiogenesis and remodeling* - **Angiogenesis** (formation of new blood vessels) is essential for delivering osteoblasts and nutrients to the epiphyseal plate and removing chondrocytes. - **Bone remodeling**, involving both bone formation and resorption, occurs as part of the ossification process to shape the new bone and maintain its structural integrity.

Question 424: Stroking the sole of the foot from the heel to the toe results in flexion of the toe and is known as:

A. Babinski reflex.
B. Plantar response. (Correct Answer)
C. Landau reflex.
D. Labyrinthine reflex.

Explanation: ***Plantar response*** - This describes the **normal physiological response** to stroking the sole of the foot, resulting in **flexion of the toes**. - It is evaluated as part of a **neurological examination** to assess the integrity of the corticospinal tracts. *Babinski reflex* - The **Babinski reflex** specifically refers to the **abnormal dorsiflexion** of the great toe and fanning of the other toes, which indicates an upper motor neuron lesion in adults. - While it is a type of plantar reflex, the description in the question (flexion of the toe) refers to the normal plantar response, not the Babinski sign. *Landau reflex* - The **Landau reflex** is a postural reflex observed in infants, where they extend their head, trunk, and legs when held prone in the air. - It is an important developmental milestone and is not related to foot stimulation or toe flexion. *Labyrinthine reflex* - The **labyrinthine reflex** (or labyrinthine righting reactions) refers to reflexes originating from the inner ear's vestibular system that help maintain the proper orientation of the head in space. - It plays a crucial role in **balance and posture** and is not elicited by stroking the foot.

Question 425: Renshaw cell inhibition is?

A. Feedback facilitation
B. Feed forward inhibition
C. Feed forward facilitation
D. Feedback inhibition (Correct Answer)

Explanation: ***Feedback inhibition*** - **Renshaw cells** are inhibitory interneurons in the spinal cord that receive excitatory input from an alpha motor neuron collateral and, in turn, inhibit the same alpha motor neuron, creating a **negative feedback loop**. - This mechanism helps to stabilize motor neuron firing rates and prevent over-excitation, refining motor control. *Feedback facilitation* - This would imply a mechanism where the output of a system enhances its own input, which is not the function of Renshaw cells. - While feedback loops exist in the nervous system, **Renshaw cells** explicitly mediate an inhibitory feedback. *Feed forward inhibition* - **Feedforward inhibition** involves a presynaptic neuron exciting an inhibitory interneuron, which then inhibits a postsynaptic neuron without direct input from the postsynaptic neuron. - This is a different type of circuit, often used for sharpening responses or anticipating activity, and does not describe the Renshaw cell mechanism. *Feed forward facilitation* - **Feedforward facilitation** occurs when a neuron's activity is enhanced by an upstream signal, anticipating a future need, rather than as a consequence of its own firing. - This mechanism is not what **Renshaw cells** do; their action is a direct response to the output of the alpha motor neuron.

Question 426: Which of the following statements correctly describes Group B sympathetic postganglionic nerve fibers?

A. Myelinated fibers that are parasympathetic postganglionic.
B. Unmyelinated fibers primarily conveying pain.
C. Myelinated fibers associated with the autonomic nervous system. (Correct Answer)
D. Myelinated fibers that are sympathetic postganglionic.

Explanation: ***Myelinated fibers associated with the autonomic nervous system.*** * **Group B fibers** are **myelinated autonomic nerve fibers** that serve as **preganglionic neurons** in both sympathetic and parasympathetic divisions of the autonomic nervous system. * These fibers have a diameter of approximately **1-3 μm** and conduction velocity of **3-15 m/s**. * The key characteristic is that they are **preganglionic autonomic fibers**, not postganglionic. * This is the most accurate description among the given options. *Myelinated fibers that are sympathetic postganglionic.* * This is **incorrect** because **sympathetic postganglionic fibers** are predominantly **unmyelinated Group C fibers**, not myelinated Group B fibers. * Group B fibers represent **preganglionic** autonomic neurons, not postganglionic. * The question stem itself contains an error in referring to "Group B sympathetic postganglionic" as this combination does not exist in standard classification. *Unmyelinated fibers primarily conveying pain.* * This description refers to **Group C fibers**, which are **unmyelinated** and primarily transmit **slow pain**, temperature, and postganglionic autonomic signals. * Group C fibers have conduction velocities of **0.5-2 m/s**, much slower than Group B fibers. *Myelinated fibers that are parasympathetic postganglionic.* * This is **incorrect** because **parasympathetic postganglionic fibers** are also **unmyelinated Group C fibers**. * Like sympathetic postganglionic neurons, parasympathetic postganglionic fibers lack significant myelination. * Group B fibers are **preganglionic**, serving both sympathetic and parasympathetic divisions.

Question 427: What is the role of tropomyosin in muscle contraction?

A. Helps in the fusion of actin and myosin
B. Slides over myosin
C. Causes Ca2+ release
D. Covers the binding sites on actin to regulate the interaction with myosin (Correct Answer)

Explanation: **Covers the binding sites on actin to regulate the interaction with myosin.** - **Tropomyosin** is a regulatory protein that wraps around the actin helix, physically blocking the **myosin-binding sites** on actin in a relaxed muscle. - Its role is to prevent the formation of **cross-bridges** between actin and myosin until calcium is present, thereby regulating muscle contraction. *Helps in the fusion of actin and myosin* - Tropomyosin does not facilitate the fusion of actin and myosin; rather, it **regulates their interaction** by blocking or unblocking binding sites. - The actual binding (cross-bridge formation) occurs when **myosin heads** attach to actin binding sites, not a fusion process. *Slides over myosin* - Tropomyosin does not slide over myosin; instead, it **slides along the actin filament** in response to calcium binding to troponin. - The sliding filament model describes how **actin and myosin filaments slide past each other**, but tropomyosin's movement is confined to the actin filament. *Causes Ca2+ release* - **Tropomyosin does not cause Ca2+ release**; its position is influenced by calcium. - **Ca2+ is released from the sarcoplasmic reticulum** and binds to troponin, which then causes tropomyosin to shift.

Question 428: In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?

A. Sarcomere
B. Sarcolemma
C. Myofibril (Correct Answer)
D. Myofilament

Explanation: ***Myofibril*** - A **myofibril** is a cylindrical organelle that runs longitudinally inside a muscle fiber and contains **contractile proteins**. - Myofibrils are composed of repeating units called **sarcomeres**, which are the fundamental units of muscle contraction. *Sarcomere* - A **sarcomere** is the basic contractile unit of a muscle fiber, extending from one Z-disc to the next. - While it is a key component for muscle contraction, it is a **segment within a myofibril**, not the threadlike component that extends the entire length of the fiber. *Sarcolemma* - The **sarcolemma** is the cell membrane of a muscle fiber, responsible for transmitting nerve impulses to the muscle cell. - It encloses the muscle fiber but is not an internal, threadlike contractile component. *Myofilament* - **Myofilaments** are the individual protein filaments (actin and myosin) that make up a sarcomere within a myofibril. - They are the **smallest contractile elements**, but they are not the threadlike structure that extends along the entire muscle fiber.

Question 429: Which of the following statements about nerve impulses is true?

A. Travels slower than the speed of electrical signals in wires.
B. Nerve impulses respond to increased stimulus strength.
C. Travels in one direction at synapses. (Correct Answer)
D. Nerve impulses can propagate bidirectionally along an axon.

Explanation: ***Travels in one direction at synapses.*** - At **synapses**, neurotransmitters are released from the **presynaptic neuron** and bind to receptors on the **postsynaptic neuron**, ensuring unidirectional signal transmission. - This **unidirectional flow** is crucial for organized communication within the nervous system. *Travels slower than the speed of electrical signals in wires.* - While impressive, **nerve impulse conduction** involves electrochemical processes that are significantly slower than the almost instantaneous flow of electrons in wires. - The maximum speed of nerve conduction in humans is about **120 m/s**, whereas electrical signals in wires approach the speed of light. *Nerve impulses respond to increased stimulus strength.* - Nerve impulses (action potentials) operate on an **"all-or-none" principle**; once the **threshold potential** is reached, the amplitude of the action potential is maximal and does not increase with a stronger stimulus. - The nervous system encodes stimulus strength by *increasing the frequency* of action potentials, not their amplitude. *Nerve impulses can propagate bidirectionally along an axon.* - This is **FALSE**. Nerve impulses propagate **unidirectionally** along the axon due to the **refractory period**. - After an action potential passes through a segment, that segment enters an **absolute refractory period** during which it cannot fire again, preventing backward propagation. - This ensures orderly signal transmission from the axon hillock toward the axon terminals.

Question 430: Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

A. 50 Hz
B. 75 Hz
C. 25 Hz (Correct Answer)
D. 100 Hz

Explanation: ***25 Hz*** - **Tetanizing frequency** is the minimum stimulation frequency required to produce tetanus (sustained muscle contraction without complete relaxation between stimuli). - For **incomplete tetanus** to occur, the next stimulus must arrive during the relaxation phase, before the muscle fully relaxes. - The critical time window is the **latent period + contraction period** = 10 ms + 40 ms = 50 ms. However, to ensure summation occurs reliably during relaxation, stimuli typically arrive at a slightly higher frequency. - **Practical tetanizing frequency** = approximately 1/(40 ms) = **25 Hz**, which ensures stimuli arrive during the latter part of contraction or early relaxation phase, producing sustained tension. - This frequency allows sufficient overlap for tetanic fusion while accounting for the physiological requirements of the muscle twitch cycle. *50 Hz* - This frequency (one stimulus every 20 ms) would produce a **complete tetanus** with no visible relaxation between stimuli. - This is higher than the minimum tetanizing frequency required for this muscle with its 100 ms total twitch duration. - While this would produce tetanus, it exceeds the minimum frequency needed. *75 Hz* - This very high frequency (one stimulus every 13.3 ms) would produce a **smooth, complete tetanus**. - This is approximately 3 times the minimum tetanizing frequency and represents excessive stimulation. - Such high frequencies are well beyond what is needed to prevent relaxation in this muscle. *100 Hz* - This extremely high frequency (one stimulus every 10 ms, equivalent to the latent period alone) would produce **maximal tetanic fusion**. - This is 4 times the minimum tetanizing frequency needed for this muscle. - While physiologically possible, this represents supramaximal stimulation frequency for tetanus production in this scenario.

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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