Nerve and Muscle Physiology Practice Questions

Q: What type of stimulus does a muscle spindle detect?

Length. **Explanation:** The **muscle spindle** is a specialized sensory receptor located within the belly of skeletal muscles. Its primary physiological function is to detect changes in **muscle length** and the rate of change in length. 1. **Why "Length" is correct:** Muscle spindles are arranged in **parallel** with extrafusal muscle fibers. When a muscle is stretched, the intrafusal fibers of the spindle are also stretched, triggering sensory signals via Type Ia and Type II afferent fibers. This information is crucial for the **stretch reflex** (myotatic reflex), which maintains muscle tone and prevents over-stretching. 2. **Why other options are incorrect:** * **Tension (Option A):** This is detected by the **Golgi Tendon Organ (GTO)**. GTOs are arranged in **series** with muscle fibers and respond to force or contraction strength to prevent tendon avulsion. * **Proprioception (Option B):** While muscle spindles *contribute* to proprioception (the sense of body position), "Proprioception" is a broad sensory category, not a specific stimulus. "Length" is the specific physical parameter detected. * **Stretch (Option D):** While "stretch" is the action that triggers the spindle, in physiological terms, the spindle is defined as a **linear length detector**. In many exams, "Length" is considered the more precise physiological answer over "Stretch." **High-Yield Clinical Pearls for NEET-PG:** * **Innervation:** Sensory (Ia - dynamic/velocity; II - static/length) and Motor (**Gamma motor neurons** - maintain spindle sensitivity during contraction). * **Alpha-Gamma Co-activation:** Ensures the muscle spindle remains sensitive even when the muscle shortens. * **Inverse Myotatic Reflex:** Mediated by the GTO to cause muscle relaxation when tension is too high.

Q: Relaxation of skeletal muscle occurs by?

Removal of sarcoplasmic calcium. ### Explanation **Correct Answer: D. Removal of sarcoplasmic calcium** The fundamental mechanism of skeletal muscle relaxation is the reduction of cytosolic calcium levels. In the resting state, the **Troponin-Tropomyosin complex** physically blocks the myosin-binding sites on actin. For relaxation to occur, calcium must be actively pumped back into the **Sarcoplasmic Reticulum (SR)** via the **SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase)** pump. As sarcoplasmic calcium concentrations drop, calcium dissociates from **Troponin C**, allowing the tropomyosin to return to its inhibitory position, thereby preventing cross-bridge cycling. **Analysis of Incorrect Options:** * **A & B:** While the removal of Acetylcholine (by Acetylcholinesterase) and the subsequent closure of nicotinic receptors stop the generation of new action potentials, they do not directly cause relaxation. Relaxation only occurs once the calcium already released into the sarcoplasm is sequestered. * **C:** Binding of calcium with Troponin C is the trigger for **contraction**, not relaxation. This binding causes a conformational change that moves tropomyosin away from the active sites on actin. **High-Yield NEET-PG Pearls:** * **SERCA Pump:** This is an ATP-dependent primary active transporter. Therefore, muscle relaxation is an **active process**. * **Calsequestrin:** A protein within the SR that binds to calcium, allowing the SR to store high concentrations of $Ca^{2+}$ without an osmotic penalty. * **Rigor Mortis:** Occurs because ATP is required for the SERCA pump to remove calcium and for the myosin head to detach from actin. Without ATP, the muscle remains locked in a contracted state. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR)**, leading to excessive calcium release and sustained contraction.

Q: All of the following are true about excitation-contraction coupling except?

Calcium binds to troponin to initiate muscle contraction.. **Explanation** Excitation-Contraction (E-C) coupling is the physiological process where an electrical stimulus (action potential) is converted into a mechanical response (muscle contraction). **Why Option D is the "Except" (Correct Answer):** While it is a common fact that calcium binds to troponin, the question asks for the specific mechanism that **initiates** the cross-bridge cycle. In skeletal muscle, calcium binds specifically to **Troponin C**. This binding causes a conformational change in **Troponin I**, which subsequently moves **Tropomyosin** away from the active sites on actin. Therefore, the initiation of contraction is technically the **unmasking of the actin-binding site**, allowing myosin heads to attach. In the context of NEET-PG, precision regarding the specific troponin subunit (Troponin C) and the role of tropomyosin is often tested. **Analysis of Other Options:** * **Option A:** Correct statement. The process begins when an action potential reaches the motor nerve terminal, triggering the release of **Acetylcholine (ACh)** into the synaptic cleft. * **Option B:** Correct statement. Relaxation is an active process. Calcium is pumped back into the sarcoplasmic reticulum (SR) via the **SERCA pump** (Sarcoplasmic/Endoplasmic Reticulum Ca2+ ATPase). * **Option C:** Correct statement. Depolarization of the T-tubules activates **DHP receptors**, which then open **Ryanodine receptors (RyR)** on the SR, releasing calcium into the sarcoplasm. **High-Yield Clinical Pearls for NEET-PG:** * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR1)**, leading to excessive calcium release upon exposure to succinylcholine or halothane. * **Calsequestrin:** A protein inside the SR that binds calcium, allowing it to be stored at high concentrations. * **Dantrolene:** The drug of choice for Malignant Hyperthermia as it inhibits RyR1.

Q: Which event will NOT occur during the depolarization phase?

K+ channels open. ### Explanation The correct answer is **C. K+ channels open**. #### 1. Underlying Medical Concept Depolarization is the phase of an action potential where the membrane potential becomes more positive (less negative). In a typical nerve or muscle cell, this is driven by the **influx of cations** (positive ions) into the cell. * **K+ channels** are primarily responsible for the **repolarization** phase. When K+ channels open, K+ ions exit the cell (efflux) along their concentration gradient, making the interior of the cell more negative again. Therefore, K+ channel opening is an event that follows depolarization, rather than occurring during it. #### 2. Analysis of Incorrect Options * **A. Na+ channels open:** This is the hallmark of the depolarization phase in neurons and skeletal muscle. Rapid opening of voltage-gated Na+ channels leads to a massive Na+ influx, causing the rapid upstroke of the action potential. * **B. Ca+ channels open:** In specific tissues, such as cardiac pacemaker cells (SA node) and smooth muscle, depolarization is mediated by the opening of voltage-gated Ca²+ channels. Even in ventricular myocytes, Ca²+ channels open during the late phase of depolarization to contribute to the plateau phase. #### 3. High-Yield Clinical Pearls for NEET-PG * **Threshold Potential:** The specific membrane potential (usually -55mV in nerves) at which voltage-gated Na+ channels open to trigger depolarization. * **Tetrodotoxin (TTX):** A potent neurotoxin (from Pufferfish) that blocks voltage-gated Na+ channels, preventing depolarization. * **Hyperkalemia:** Increases resting membrane potential (making it less negative), which initially makes cells more excitable but eventually leads to inactivation of Na+ channels, causing paralysis or arrhythmias. * **Absolute Refractory Period:** Occurs during depolarization and early repolarization when Na+ channels are either already open or in an inactivated state.

Q: Which one of the following muscle proteins plays an important role in the contraction of both smooth and striated muscle?

Actin. **Explanation:** The fundamental mechanism of muscle contraction in both striated (skeletal and cardiac) and smooth muscle is the **Sliding Filament Theory**. This process relies on the interaction between two primary contractile proteins: **Actin** (thin filament) and **Myosin** (thick filament). **Why Actin is the Correct Answer:** Actin is a highly conserved structural protein that forms the backbone of the thin filaments. Regardless of the muscle type, contraction cannot occur without the cross-bridge cycling between actin and myosin. While the regulatory mechanisms (how contraction is triggered) differ between muscle types, the physical "sliding" of actin over myosin is the universal final common pathway for tension generation. **Analysis of Incorrect Options:** * **Calmodulin (A):** This is the primary calcium-binding regulatory protein in **smooth muscle** only. It activates Myosin Light Chain Kinase (MLCK). It is not involved in striated muscle contraction, where calcium binds to Troponin C. * **Troponin (B):** This regulatory complex (consisting of Troponin I, T, and C) is unique to **striated muscle**. Smooth muscle lacks troponin entirely. * **Tropomyosin (C):** While tropomyosin is present in both, its role in smooth muscle is less clearly defined and not essential for the initiation of contraction in the same way it is for striated muscle (where it blocks the myosin-binding site on actin). **NEET-PG High-Yield Pearls:** * **Striated Muscle:** Regulation is **actin-linked** (via the Troponin-Tropomyosin complex). * **Smooth Muscle:** Regulation is **myosin-linked** (via phosphorylation of the Myosin Light Chain). * **Commonality:** Both require ATP and an increase in intracellular Calcium, though the source of calcium in smooth muscle is primarily extracellular/sarcolemmal, whereas in skeletal muscle, it is the Sarcoplasmic Reticulum.

Q: What are postganglionic sympathetic fibres?

C fibres. ### Explanation The classification of nerve fibres is a high-yield topic in neurophysiology, primarily based on the **Erlanger-Gasser classification**, which categorizes fibres according to their diameter, myelination, and conduction velocity. **Why C fibres are the correct answer:** Postganglionic sympathetic fibres are unique because they are **unmyelinated**. According to the Erlanger-Gasser classification, **Type C fibres** are the only unmyelinated fibres. They have the smallest diameter (0.4–1.2 μm) and the slowest conduction velocity (0.5–2.0 m/s). These fibres carry postganglionic autonomic signals and slow pain (prolonged, burning sensation). **Analysis of Incorrect Options:** * **Option A (A-alpha):** These are the largest, most heavily myelinated fibres. They function as somatic motor fibres (to extrafusal muscle fibres) and carry proprioception (from muscle spindles and Golgi tendon organs). * **Option C (B fibres):** These are small, lightly myelinated fibres. Crucially, they represent **preganglionic** autonomic fibres (both sympathetic and parasympathetic). * **Option D (A-gamma):** These are myelinated fibres that supply the intrafusal muscle fibres of the muscle spindle, regulating muscle tone. **NEET-PG High-Yield Pearls:** 1. **The "B" vs. "C" Rule:** Always remember: **B** comes before **C** in the alphabet, just as **Preganglionic (B)** comes before **Postganglionic (C)** in the autonomic pathway. 2. **Susceptibility:** * **Local Anesthetics:** Block **C fibres** first (smallest diameter). * **Pressure:** Affects **A fibres** first (largest diameter). * **Hypoxia:** Affects **B fibres** first. 3. **Fast vs. Slow Pain:** Fast pain is carried by **A-delta** fibres, while slow, chronic pain is carried by **C fibres**.

Question 1

What type of stimulus does a muscle spindle detect?

Accepted Answer

Length

Answer

Tension

Answer

Proprioception

Answer

Stretch

Question 2

Relaxation of skeletal muscle occurs by?

Accepted Answer

Removal of sarcoplasmic calcium

Answer

Removal of Acetylcholine from the synaptic cleft

Answer

Closure of the nicotinic cholinergic receptor

Answer

Binding of calcium with troponin

Question 3

Synaptic potentials can be recorded by which of the following methods?

Accepted Answer

Microelectrode

Answer

Patch clamp technique

Answer

Voltage clamp technique

Answer

Electroencephalography (EEG)

Question 4

Which statement is TRUE regarding nerve conduction?

Accepted Answer

It follows the all-or-none phenomenon.

Answer

Conduction is dependent on the amplitude.

Answer

A propagated action potential is generated in the axon hillock.

Answer

Conduction is faster in myelinated fibers than in unmyelinated fibers.

Question 5

In muscle cells, resting membrane potential is equal to the isoelectric potential of which ion?

Accepted Answer

K+

Answer

Na+

Answer

Cl-

Answer

Mg++

Question 6

All of the following are true about excitation-contraction coupling except?

Accepted Answer

Calcium binds to troponin to initiate muscle contraction.

Answer

Acetylcholine is released at the nerve terminal.

Answer

Calcium is pumped back into the sarcoplasmic reticulum during relaxation.

Answer

Calcium is released from the sarcoplasmic reticulum during contraction.

Question 7

Which event will NOT occur during the depolarization phase?

Accepted Answer

K+ channels open

Answer

Na+ channels open

Answer

Ca+ channels open

Answer

None of the above

Question 8

Which one of the following muscle proteins plays an important role in the contraction of both smooth and striated muscle?

Accepted Answer

Actin

Answer

Calmodulin

Answer

Troponin

Answer

Tropomyosin

Question 9

The length of a muscle at which tension is maximum is called:

Accepted Answer

Optimal length

Answer

Active length

Answer

Resting length

Answer

Maximal length

Question 10

What are postganglionic sympathetic fibres?

Accepted Answer

C fibres

Answer

A alpha fibres

Answer

B fibres

Answer

A gamma fibres

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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