Nerve and Muscle Physiology Practice Questions

Q: What is the contractile unit of a muscle?

Sarcomere. ### Explanation **Correct Answer: A. Sarcomere** The **sarcomere** is defined as the functional and structural unit of contraction in skeletal muscle. It is the segment of a myofibril located between two successive **Z-lines**. During muscle contraction (based on the Sliding Filament Theory), the distance between the Z-lines decreases as actin (thin) filaments slide over myosin (thick) filaments, effectively shortening the sarcomere. Because this is the smallest component of the muscle capable of independent contraction, it is designated the "contractile unit." **Analysis of Incorrect Options:** * **B. Sarcolemma:** This is the cell membrane of a muscle fiber. While it conducts action potentials via voltage-gated channels, it does not perform the mechanical act of contraction. * **C. Myofibril:** These are long, cylindrical bundles of myofilaments found within the muscle cell. A myofibril is composed of many sarcomeres arranged in series; thus, it contains the contractile units but is not the unit itself. * **D. Sarcotubular System:** This consists of the T-tubules and the Sarcoplasmic Reticulum (SR). Its primary role is **Excitation-Contraction Coupling** (releasing $Ca^{2+}$), not the mechanical contraction. **High-Yield NEET-PG Pearls:** * **Sarcomere Composition:** Contains one full **A-band** (anisotropic) and two **half I-bands** (isotropic). * **Contraction Dynamics:** During contraction, the **H-zone** and **I-band** shorten/disappear, but the **A-band width remains constant**. * **Titin:** The largest protein in the human body; it acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the sarcomere. * **L-Tubules:** The longitudinal part of the sarcotubular system (SR) specifically stores calcium via the protein **calsequestrin**.

Q: White fibers are predominantly present in which of the following muscle types?

Extraocular muscle. **Explanation:** Skeletal muscle fibers are classified into two main types based on their metabolic and contractile properties: **Type I (Red/Slow-twitch)** and **Type II (White/Fast-twitch)**. **Why Extraocular Muscles are correct:** Extraocular muscles require extremely rapid, precise, and short-duration movements (saccades) to track objects. Therefore, they are predominantly composed of **Type II (White) fibers**. These fibers have high myosin ATPase activity, allowing for fast contraction, but they rely on anaerobic glycolysis and contain less myoglobin and mitochondria, making them prone to fatigue. **Analysis of Incorrect Options:** * **A, C, and D (Calf, Back, and Hip muscles):** These are primarily **postural muscles** (e.g., Soleus in the calf, Erector spinae in the back). Postural muscles must maintain sustained contractions for long periods against gravity. Consequently, they are rich in **Type I (Red) fibers**, which contain high amounts of myoglobin and mitochondria, utilizing aerobic metabolism to resist fatigue. **High-Yield NEET-PG Pearls:** * **Type I (Red):** "One Slow Red Ox" — Type **I**, **Slow**-twitch, **Red** (high myoglobin), **Ox**idative phosphorylation. Found in the **Soleus** (classic exam example). * **Type II (White):** Fast-twitch, glycolytic, low myoglobin. Found in **Extraocular muscles** and **Gastrocnemius** (for sprinting). * **Intermediate Fibers (Type IIa):** These are fast-oxidative glycolytic fibers that share characteristics of both types. * **Muscle Plasticity:** While genetics determine the baseline ratio, endurance training can increase the oxidative capacity of fibers, whereas weightlifting increases the size of Type II fibers.

Q: Which type of nerve fibers are least susceptible to pressure?

Type C fibers. **Explanation:** The susceptibility of nerve fibers to various insults depends on their diameter and myelination. According to the **Gasser-Erlanger classification**, nerve fibers are categorized into Types A, B, and C. **Why Type C is correct:** Type C fibers are the **least susceptible to pressure**. They are the smallest in diameter and are unmyelinated. Because they lack a bulky myelin sheath and have a smaller surface area, they are mechanically more resilient to compression compared to the larger, myelinated fibers. **Why the other options are incorrect:** * **Type A fibers:** These are the **most susceptible to pressure**. They are large-diameter, heavily myelinated fibers (e.g., alpha motor neurons, sensory fibers for touch/pressure). Their large size and complex myelin structure make them highly vulnerable to mechanical deformation and ischemia caused by pressure. * **Type B fibers:** These are intermediate-sized, preganglionic autonomic fibers. While more resistant than Type A, they are more susceptible to pressure than Type C. * **Type D fibers:** This is not a standard category in the Gasser-Erlanger classification of nerve fibers. **High-Yield Clinical Pearls for NEET-PG:** To remember the susceptibility patterns, use the mnemonic **"P-A-I-N"**: 1. **P**ressure: Most susceptible = **Type A**; Least = **Type C**. 2. **H**ypoxia (Ischemia): Most susceptible = **Type B**; Least = **Type C**. 3. **L**ocal Anesthetics: Most susceptible = **Type C**; Least = **Type A**. (Small, unmyelinated fibers are blocked first). * **Order of block by Local Anesthetics:** Type C > Type B > Type A. * **Order of loss in Pressure:** Type A > Type B > Type C (This explains why "Saturday Night Palsy" affects motor function and touch before pain sensation).

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

-50 mV. **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).

Q: Which sensory fibers are least susceptible to hypoxia?

Type IV. **Explanation:** The susceptibility of nerve fibers to various insults depends on their physiological characteristics. The sensitivity of nerve fibers to hypoxia, pressure, and local anesthetics follows a specific order, often tested in NEET-PG. **Why Type IV is correct:** Type IV fibers (Erlanger-Gasser **Type C fibers**) are the least susceptible to hypoxia. These are small-diameter, unmyelinated fibers that conduct slow pain and temperature sensations. Because they have a lower metabolic rate and lack the complex machinery of thick myelin sheaths, they can withstand oxygen deprivation longer than larger, myelinated fibers. **Why the other options are incorrect:** * **Type I (A-alpha):** These are the largest, most heavily myelinated fibers (proprioception, somatic motor). They have the highest metabolic demand and are the **most susceptible to pressure**, but they are highly sensitive to hypoxia compared to Type IV. * **Type II (A-beta):** These are medium-sized myelinated fibers (touch, pressure). Like Type I, they are more sensitive to hypoxia than the unmyelinated Type IV fibers. * **Type III (A-delta):** These are small, thinly myelinated fibers (fast pain, cold). While more resistant than Type I, they are still more sensitive to hypoxia than Type IV. **High-Yield Clinical Pearls for NEET-PG:** To remember the sensitivity of nerve fibers, use the following hierarchy (from Most Sensitive to Least Sensitive): 1. **Hypoxia:** B > A > C (Type IV is C, thus least sensitive). 2. **Pressure:** A > B > C (Large fibers are compressed first; C is least sensitive). 3. **Local Anesthetics:** C > B > A (Small, unmyelinated fibers are blocked first). *Note: Type IV fibers are the only unmyelinated sensory fibers; Types I, II, and III are all subtypes of myelinated Type A fibers.*

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

Slower net exit of K+. ### 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 1

What is the contractile unit of a muscle?

Accepted Answer

Sarcomere

Answer

Sarcolemma

Answer

Myofibril

Answer

Sarcotubular System

Question 2

Saltatory conduction refers to which process?

Accepted Answer

Propagation of action potential between the nodes of Ranvier.

Answer

Propagation of action potential along the myelin sheath.

Answer

Propagation of action potential along the axon proper.

Answer

Propagation of action potential through dendrites.

Question 3

White fibers are predominantly present in which of the following muscle types?

Accepted Answer

Extraocular muscle

Answer

Calf muscle

Answer

Back muscle

Answer

Hip muscle

Question 4

Optic nerve fibres, once cut, do not regenerate because they are not covered by which of the following structures?

Accepted Answer

Neurilemma

Answer

Myelin sheath

Answer

Both myelin sheath and neurilemma

Answer

Neither myelin sheath nor neurilemma

Question 5

Which type of nerve fibers are least susceptible to pressure?

Accepted Answer

Type C fibers

Answer

Type A fibers

Answer

Type B fibers

Answer

Type D fibers

Question 6

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

Accepted Answer

It contracts when stretched in the absence of any extrinsic innervation

Answer

Does not have actin and myosin

Answer

Cannot do sustained contraction

Answer

Does not require calcium for contraction

Question 7

True about nerve impulse is:

Accepted Answer

Travels in one direction at a synapse

Answer

Travels in one direction along an axon

Answer

If the current is increased too slowly, the nerve responds faster

Answer

Travels with the speed of electric current

Question 8

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

Accepted Answer

-50 mV

Answer

-75 mV

Answer

-90 mV

Answer

-35 mV

Question 9

Which sensory fibers are least susceptible to hypoxia?

Accepted Answer

Type IV

Answer

Type I

Answer

Type II

Answer

Type III

Question 10

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

Accepted Answer

Slower net exit of K+

Answer

Delayed influx of Na+

Answer

Na+ influx above zero potential

Answer

Slow closure of K+ channels

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?