Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 91: What is a characteristic of smooth muscle cells in the intestine?

A. Does not have actin and myosin
B. Cannot do sustained contraction
C. It contracts when stretched in the absence of any extrinsic innervation (Correct Answer)
D. Does not require calcium for contraction

Explanation: **Explanation:** The correct answer is **C: It contracts when stretched in the absence of any extrinsic innervation.** Smooth muscle in the intestine is classified as **Unitary (Visceral) Smooth Muscle**. A hallmark of this tissue is its **myogenic activity**. These cells possess stretch-activated calcium channels; when the muscle fiber is stretched (e.g., by a food bolus), these channels open, leading to depolarization and contraction. This response occurs independently of the extrinsic nervous system, though it can be modulated by it. **Analysis of Incorrect Options:** * **Option A:** All muscle types (skeletal, cardiac, and smooth) require **actin and myosin** for contraction. While smooth muscle lacks organized sarcomeres (making it non-striated), it contains dense bodies that anchor these filaments. * **Option B:** Smooth muscle is specialized for **sustained contractions** (tonus) with very little energy expenditure. This is achieved through the **"Latch-bridge mechanism,"** where dephosphorylated myosin remains attached to actin for prolonged periods. * **Option D:** **Calcium is essential** for smooth muscle contraction. However, unlike skeletal muscle (which uses Troponin), smooth muscle calcium binds to **Calmodulin**, which then activates Myosin Light Chain Kinase (MLCK). **High-Yield NEET-PG Pearls:** * **Gap Junctions:** Unitary smooth muscle cells are electrically coupled via gap junctions, allowing them to contract as a single syncytium. * **Pacemaker Activity:** The **Interstitial Cells of Cajal (ICC)** generate the "Slow Waves" (Basal Electrical Rhythm) in the GI tract. * **Caveolae:** Smooth muscle lacks a well-developed T-tubule system; instead, it has rudimentary invaginations called caveolae. * **Caldesmon & Calponin:** These are regulatory proteins in smooth muscle that inhibit the ATPase activity of myosin.

Question 92: Miss X lifts weights. When she contracts her biceps muscle, all of the following parts of the muscle shorten EXCEPT the:

A. Sarcomeres
B. I bands
C. A bands (Correct Answer)
D. Inter-Z line distance

Explanation: This question tests your understanding of the **Sliding Filament Theory** of muscle contraction. ### 1. Why the Correct Answer (A Band) is Right According to the Sliding Filament Theory, muscle contraction occurs when thin (actin) filaments slide over thick (myosin) filaments. * The **A band** represents the entire length of the **thick (myosin) filaments**. * Since the filaments themselves do not change in length—they merely slide past one another—the A band remains **constant** during both contraction and relaxation. ### 2. Why the Other Options are Incorrect * **Sarcomeres (Option A):** The sarcomere is the functional unit of contraction, defined as the distance between two Z lines. As filaments slide, the sarcomere length decreases. * **I bands (Option B):** The I band consists only of thin (actin) filaments. During contraction, actin filaments are pulled toward the center of the sarcomere (M line), increasing the zone of overlap and causing the I band to **shorten**. * **Inter-Z line distance (Option D):** This is synonymous with the sarcomere length. As the muscle contracts, Z lines are pulled closer together, thus the distance **shortens**. ### 3. High-Yield NEET-PG Pearls * **What shortens:** Sarcomere, I band, H zone, and Inter-Z line distance. * **What remains constant:** A band, length of thick filaments, and length of thin filaments. * **The H zone** (the central part of the A band containing only myosin) disappears during maximal contraction. * **Mnemonic:** "**HI**" disappears (**H** zone and **I** band shorten), but the "**A**" stays the same (**A** band). * **Energy Requirement:** ATP is required for both contraction (cross-bridge cycling) and relaxation (calcium reuptake into the sarcoplasmic reticulum).

Question 93: True about nerve impulse is:

A. Travels in one direction along an axon
B. If the current is increased too slowly, the nerve responds faster
C. Travels in one direction at a synapse (Correct Answer)
D. Travels with the speed of electric current

Explanation: ### Explanation **1. Why Option C is Correct:** The transmission of a nerve impulse at a chemical synapse is **unidirectional** (one-way). This is due to the structural polarity of the synapse: neurotransmitters are stored in vesicles only in the **presynaptic terminal**, and the specific receptors for those neurotransmitters are located on the **postsynaptic membrane**. This ensures that the signal always travels from the axon terminal of one neuron to the dendrite or cell body of the next. **2. Why Other Options are Incorrect:** * **Option A:** While impulses usually travel one way in the body (*orthodromic*), an isolated nerve fiber can conduct impulses in **both directions** (*antidromic*) if stimulated in the middle. The "one-way" rule is a property of the synapse, not the axon itself. * **Option B:** If a stimulus is applied too slowly, the nerve fails to fire due to a phenomenon called **Accommodation**. This happens because slow depolarization allows time for $K^+$ channels to open and $Na^+$ channels to inactivate, raising the threshold for excitation. * **Option D:** Nerve impulses are electrochemical signals, not pure electricity. They travel at speeds ranging from **0.5 to 120 m/s**, which is significantly slower than the speed of light/electric current (approx. 300,000 km/s). **3. High-Yield Facts for NEET-PG:** * **Bell-Magendie Law:** States that in the spinal cord, sensory impulses enter via dorsal roots and motor impulses exit via ventral roots (unidirectional flow). * **Synaptic Delay:** The time required for neurotransmitter release and binding (usually **0.5 msec**). This is the slowest part of nerve conduction. * **Erlanger-Gasser Classification:** Type **A-alpha** fibers are the fastest (proprioception/somatic motor), while **Type C** fibers are the slowest (pain/temperature, unmyelinated). * **Saltatory Conduction:** Occurs in myelinated fibers where the impulse "jumps" between **Nodes of Ranvier**, increasing velocity and conserving energy.

Question 94: What is the resting membrane potential (RMP) of smooth muscle?

A. -50 mV (Correct Answer)
B. -75 mV
C. -90 mV
D. -35 mV

Explanation: **Explanation:** The resting membrane potential (RMP) of **smooth muscle** is typically less negative than that of skeletal muscle or neurons, generally ranging between **-50 mV and -60 mV**. The correct option is **-50 mV**. **Why -50 mV is correct:** Smooth muscle cells have a higher permeability to sodium ($Na^+$) and calcium ($Ca^{2+}$) ions at rest compared to skeletal muscles. Additionally, the $Na^+$-$K^+$ pump in smooth muscle is less active in maintaining a high gradient. This results in a "less negative" or more depolarized RMP, which allows these muscles to be more easily excited by hormonal or mechanical stimuli. **Analysis of Incorrect Options:** * **-75 mV:** This is closer to the RMP of **cardiac atrial cells** or certain large nerve fibers. * **-90 mV (Option C):** This is the characteristic RMP for **skeletal muscle fibers** and **ventricular cardiomyocytes**. It is highly negative due to high resting permeability to potassium ($K^+$). * **-35 mV (Option D):** This is too depolarized for a stable RMP; however, it may be reached during the peak of "slow wave" potentials before an action potential is triggered. **NEET-PG High-Yield Pearls:** 1. **Instability:** Unlike skeletal muscle, the RMP of smooth muscle is often unstable (e.g., **Slow Waves** or Pacemaker potentials in the gut). 2. **Action Potential:** In most smooth muscles, the upstroke of the action potential is caused by the influx of **Calcium ($Ca^{2+}$)** rather than Sodium. 3. **L-type Ca channels:** These are the primary channels involved in smooth muscle contraction and are the targets for Calcium Channel Blockers (CCBs). 4. **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle cells are connected by **gap junctions**, allowing them to contract as a single unit (syncytium).

Question 95: The phase of after depolarization in cardiac action potential is characterized by which of the following?

A. Delayed influx of Na+
B. Na+ influx above zero potential
C. Slow closure of K+ channels
D. Slower net exit of K+ (Correct Answer)

Explanation: ### Explanation In the cardiac action potential (specifically the ventricular muscle fiber), the **Phase 3 (Rapid Repolarization)** is primarily driven by the efflux of $K^+$ ions through voltage-gated potassium channels. **Why Option D is Correct:** As repolarization nears completion, the membrane potential approaches the resting level. During the terminal portion of this phase, there is a **slower net exit of $K^+$**. This occurs because the driving force for potassium decreases as the membrane potential gets closer to the equilibrium potential for $K^+$. This deceleration in $K^+$ efflux leads to a "tailing off" effect known as **after-depolarization** (or the terminal phase of repolarization), before the membrane stabilizes at Phase 4. **Analysis of Incorrect Options:** * **Option A & B:** Sodium ($Na^+$) influx is the hallmark of **Phase 0 (Depolarization)**. By the time the cell reaches the after-depolarization stage, $Na^+$ channels are inactivated (refractory period). $Na^+$ influx above zero potential specifically refers to the "overshoot" during Phase 0. * **Option C:** While $K^+$ channels eventually close to return to the resting state, the specific phenomenon of after-depolarization is defined by the **rate of ion movement (flux)** rather than the mechanical closure speed of the channels themselves. ### High-Yield Clinical Pearls for NEET-PG: * **Phase 2 (Plateau Phase):** Unique to cardiac muscle; caused by a balance between $Ca^{2+}$ influx (L-type channels) and $K^+$ efflux. * **Refractory Period:** The absolute refractory period (ARP) in cardiac muscle lasts until mid-Phase 3, preventing tetany and allowing the heart to act as a pump. * **Early After-Depolarizations (EADs):** Occur during Phase 2 or 3; if they reach threshold, they can trigger "Torsades de Pointes," especially in Long QT Syndrome.

Question 96: What is a miniature end plate potential (MEPP)?

A. Inhibited by anticholinesterases
B. Recorded in resting muscle (Correct Answer)
C. Responsible for excitation of the muscle
D. Of 5 to 10 mV amplitude

Explanation: **Explanation:** A **Miniature End Plate Potential (MEPP)** is a small, spontaneous depolarization of the muscle post-synaptic membrane. **1. Why the correct answer is right:** MEPPs are **recorded in resting muscle** because they occur without any nerve stimulation. They are caused by the random, spontaneous release of a **single quantum** (one vesicle) of acetylcholine (ACh) from the presynaptic terminal into the neuromuscular junction. Since these occur randomly at rest, they do not trigger an action potential. **2. Why the incorrect options are wrong:** * **Option A:** Anticholinesterases (like Neostigmine) inhibit the enzyme that breaks down ACh. This actually **increases** the amplitude and duration of MEPPs, rather than inhibiting them. * **Option C:** MEPPs are **sub-threshold** potentials. A single MEPP is too small to reach the threshold required to open voltage-gated sodium channels; therefore, it cannot cause muscle excitation or contraction. Excitation requires an End Plate Potential (EPP), which is the summation of multiple MEPPs. * **Option D:** The typical amplitude of a MEPP is approximately **0.5 to 1.0 mV**. An amplitude of 5 to 10 mV is too high for a single quantum. **High-Yield Clinical Pearls for NEET-PG:** * **Quantal Release:** The MEPP is the smallest unit of electrical activity at the NMJ, representing the "quantum" nature of neurotransmitter release. * **Calcium Independence:** While normal EPPs are strictly calcium-dependent, spontaneous MEPPs can occur even in low-calcium environments. * **Clinical Correlation:** In **Lambert-Eaton Myasthenic Syndrome**, the *frequency* of MEPPs may be normal, but the EPP is reduced due to antibodies against presynaptic Ca²⁺ channels. In **Myasthenia Gravis**, the *amplitude* of MEPPs is reduced due to the destruction of post-synaptic ACh receptors.

Question 97: A travelling nerve impulse does not depolarize the area immediately behind it, because:

A. It is hyperpolarized
B. It is refractory (Correct Answer)
C. It is not self-propagating
D. The condition is always orthodromic

Explanation: ### Explanation **Why the correct answer is right:** The unidirectional propagation of a nerve impulse is primarily due to the **Refractory Period**. When an action potential occurs, the voltage-gated Na⁺ channels in that segment of the membrane undergo a conformational change to an **inactivated state**. During this time (the Absolute Refractory Period), no amount of stimulus can reopen these channels. Consequently, even though the local current flows in both directions, the area immediately behind the impulse cannot be re-excited, ensuring the impulse moves only forward toward the axon terminal. **Analysis of Incorrect Options:** * **A. It is hyperpolarized:** While the "after-hyperpolarization" phase occurs due to delayed closure of K⁺ channels, it is not the primary reason for the lack of depolarization. The fundamental barrier is the inactivation of Na⁺ channels (refractoriness). * **C. It is not self-propagating:** This is factually incorrect. Nerve impulses are, by definition, self-propagating through local circuit flow (Hermann’s theory). * **D. The condition is always orthodromic:** This is a descriptive term, not a mechanism. **Orthodromic** means the impulse travels in the natural direction (soma to synapse). While impulses are usually orthodromic, this is a *result* of the refractory period, not the underlying physiological cause. **High-Yield Clinical Pearls for NEET-PG:** * **Absolute Refractory Period (ARP):** Corresponds to the period from the threshold to the early part of repolarization. It sets the **upper limit** for the frequency of action potentials. * **Relative Refractory Period (RRP):** Corresponds to the later part of repolarization; a supra-threshold stimulus can trigger a response here. * **Molecular Basis:** ARP is due to the **inactivation gate (h-gate)** of the Na⁺ channel closing. * **Antidromic Conduction:** If an axon is stimulated in the middle experimentally, the impulse travels in both directions, but in a living body, synapses act as "one-way valves" to maintain orthodromic flow.

Question 98: Which one of the following is absent in smooth muscles?

A. Actin
B. Myosin
C. Troponin (Correct Answer)
D. Myosin adenine triphosphatase

Explanation: ### Explanation The correct answer is **Troponin**. **1. Why Troponin is the correct answer:** In striated muscles (skeletal and cardiac), the troponin complex (Troponin I, T, and C) plays a critical role in calcium-mediated contraction by uncovering the myosin-binding sites on actin. However, **smooth muscle lacks troponin**. Instead, it utilizes a calcium-binding protein called **Calmodulin**. When calcium enters the smooth muscle cell, it binds to calmodulin, forming a complex that activates **Myosin Light Chain Kinase (MLCK)**. This enzyme phosphorylates the myosin head, allowing it to bind to actin and initiate contraction. **2. Why the other options are incorrect:** * **Actin (A) and Myosin (B):** These are the fundamental contractile proteins present in all muscle types (skeletal, cardiac, and smooth). While smooth muscle does not have the organized sarcomere structure seen in striated muscle, it still relies on the sliding filament mechanism between actin (thin filaments) and myosin (thick filaments). * **Myosin ATPase (D):** This enzyme is located on the myosin head and is essential for hydrolyzing ATP to provide the energy required for the cross-bridge cycle. While the ATPase activity in smooth muscle is significantly slower than in skeletal muscle (leading to slower contraction), it is definitely present. **3. High-Yield Facts for NEET-PG:** * **Caldesmon and Calponin:** These are unique proteins in smooth muscle that inhibit the actin-myosin interaction, functionally replacing the inhibitory role of Troponin I. * **Dense Bodies:** Smooth muscles lack Z-discs; instead, actin filaments are attached to **dense bodies**, which are the functional equivalents of Z-discs. * **Latch-bridge Mechanism:** This unique feature allows smooth muscle to maintain prolonged tension with very little ATP consumption. * **Caveolae:** These are small invaginations of the sarcolemma in smooth muscle that act like the T-tubule system of skeletal muscle to facilitate calcium entry.

Question 99: Which of the following statements is FALSE regarding the stretch reflex?

A. Stretch of the muscle initiates the reflex.
B. Relaxation of the muscle is the response. (Correct Answer)
C. The muscle spindle is the sense organ.
D. Impulses are conducted by fast sensory fibers.

Explanation: The **Stretch Reflex** (Myotatic Reflex) is a monosynaptic reflex that serves as a fundamental mechanism for maintaining muscle tone and posture. ### **Explanation of the Correct Answer (B)** The statement "Relaxation of the muscle is the response" is **FALSE**. When a muscle is stretched, the reflex arc triggers a **contraction** of the same (agonist) muscle to resist the stretch and maintain a constant muscle length. Relaxation occurs in the *antagonist* muscle via reciprocal inhibition, but the primary response of the stretch reflex itself is contraction. ### **Analysis of Other Options** * **A. Stretch of the muscle initiates the reflex:** This is **TRUE**. The stimulus is a sudden or sustained longitudinal stretch of the muscle fibers. * **C. The muscle spindle is the sense organ:** This is **TRUE**. Muscle spindles are specialized encapsulated structures located in parallel with extrafusal fibers that act as stretch receptors. * **D. Impulses are conducted by fast sensory fibers:** This is **TRUE**. The primary afferents from the muscle spindle are **Type Ia nerve fibers**, which are the largest and fastest-conducting fibers in the body (70–120 m/s). ### **High-Yield Clinical Pearls for NEET-PG** * **Monosynaptic Nature:** The stretch reflex is the only monosynaptic reflex in the human body (one synapse between the Ia afferent and the alpha motor neuron). * **Dynamic vs. Static:** Nuclear bag fibers (Ia afferents) mediate the dynamic response (tendon jerks), while nuclear chain fibers (Group II afferents) mediate the static response (muscle tone). * **Gamma Motor Neurons:** These innervate the contractile ends of the muscle spindle, maintaining its sensitivity even when the muscle is contracted (Alpha-Gamma co-activation). * **Clinical Correlation:** Exaggerated stretch reflexes (hyperreflexia) are a hallmark of **Upper Motor Neuron (UMN)** lesions, while diminished reflexes (hyporeflexia) indicate **Lower Motor Neuron (LMN)** lesions.

Question 100: If a supramaximal stimulus is applied to an excitable tissue like a nerve or a muscle and it elicits a response, then the tissue is said to be in which state?

A. Absolute refractory period
B. Relative refractory period (Correct Answer)
C. Latent period
D. After-depolarization

Explanation: ### Explanation **1. Why the Correct Answer is Right (Relative Refractory Period):** The **Relative Refractory Period (RRP)** is the interval following an action potential during which a second response can be elicited, but only if the stimulus is **stronger than normal (supramaximal)**. * **Mechanism:** During RRP, some voltage-gated Na+ channels have recovered from their inactivated state to their closed (resting) state, making them available for activation. However, K+ conductance is still high (hyperpolarization), which opposes depolarization. Therefore, a supramaximal stimulus is required to overcome this "threshold shift" and trigger a new action potential. **2. Why the Incorrect Options are Wrong:** * **A. Absolute Refractory Period (ARP):** During this phase, the tissue is completely non-excitable. No matter how strong the stimulus (even supramaximal), a second action potential cannot be generated because Na+ channels are either already open or in an inactivated state. * **C. Latent Period:** This is the time delay between the application of a stimulus and the first detectable response. It is a timing phase, not a state of excitability defined by stimulus intensity. * **D. After-depolarization:** Also known as the "negative after-potential," this is a phase where the membrane potential is slightly more positive than the resting level before returning to baseline. While excitability is actually increased here, the question specifically defines the state by the requirement of a supramaximal stimulus, which characterizes the RRP. **3. High-Yield Clinical Pearls for NEET-PG:** * **ARP vs. RRP:** ARP sets the **upper limit** for the maximum frequency of nerve impulses. * **Accommodation:** If a nerve is subjected to a slowly increasing constant current, the threshold for activation rises. This is called accommodation. * **Hypocalcemia:** Low extracellular calcium lowers the threshold for activation, making nerves more excitable (leading to tetany), effectively shortening the refractory periods. * **Cardiac Muscle:** The ARP in cardiac muscle is exceptionally long (250ms), which prevents tetanization of the heart—a vital physiological safeguard.

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

Practice Questions

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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