Nerve and Muscle Physiology Practice Questions

Q: What are the key molecules involved in the differences between skeletal and smooth muscle contraction and relaxation?

Troponin and Myosin. The fundamental difference between skeletal and smooth muscle contraction lies in the **regulatory proteins** and the **state of the myosin head**. ### 1. Why "Troponin and Myosin" is Correct * **Skeletal Muscle (Troponin-regulated):** Contraction is "actin-linked." Calcium binds to **Troponin C**, causing a conformational change in tropomyosin that uncovers the active sites on actin. * **Smooth Muscle (Myosin-regulated):** Contraction is "myosin-linked." Smooth muscle lacks troponin. Instead, Calcium binds to **Calmodulin**, which activates **Myosin Light Chain Kinase (MLCK)**. This enzyme phosphorylates the myosin head, enabling it to bind to actin. * **Relaxation:** In skeletal muscle, relaxation occurs via simple calcium sequestration. In smooth muscle, it requires **Myosin Light Chain Phosphatase** to dephosphorylate the myosin head. ### 2. Analysis of Incorrect Options * **Option A & D:** While MLCK is vital for smooth muscle, these options are incomplete. They focus only on the activation enzyme or ions, ignoring the structural protein differences (Troponin vs. Myosin state) that define the two systems. * **Option B:** Troponin is exclusive to skeletal and cardiac muscle; it does not explain the mechanism in smooth muscle. ### 3. High-Yield NEET-PG Pearls * **Calmodulin** in smooth muscle is functionally analogous to **Troponin C** in skeletal muscle. * **Latch-bridge mechanism:** Unique to smooth muscle; allows for prolonged tone with minimal ATP consumption. * **Caveolae:** Smooth muscle lacks a well-developed T-tubule system; instead, it uses these membrane invaginations. * **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle uses **gap junctions** for syncytial contraction (e.g., GI tract, uterus).

Q: In a neuron, where does graded electrogenesis occur?

Soma - dendritic zone. **Explanation:** **1. Why the Correct Answer is Right:** Graded electrogenesis refers to the production of **graded potentials** (local changes in membrane potential) that vary in amplitude depending on the strength of the stimulus. In a neuron, the **soma-dendritic zone** is the primary receptive area. It contains a high density of ligand-gated and modality-gated ion channels. When neurotransmitters bind to receptors here, they produce Excitatory Post-Synaptic Potentials (EPSPs) or Inhibitory Post-Synaptic Potentials (IPSPs). These potentials are non-propagated, decremental (fade over distance), and can be summed—the hallmarks of graded electrogenesis. **2. Why the Other Options are Incorrect:** * **Initial Segment:** This is the "Trigger Zone." It has the highest density of voltage-gated $Na^+$ channels and the lowest threshold for firing. Its role is to convert graded potentials into all-or-none **Action Potentials**, not to generate graded potentials. * **Axon:** The axon is specialized for the **conduction** of action potentials via saltatory or continuous conduction. It follows the "All-or-None" law, which is the opposite of graded electrogenesis. * **Nerve Ending:** This zone is specialized for **secretion** (neurotransmitter release) via voltage-gated $Ca^{2+}$ channels. While some local potentials can occur, it is not the primary site for electrogenesis. **3. High-Yield Facts for NEET-PG:** * **Summation:** Graded potentials can be **Temporal** (high frequency from one source) or **Spatial** (multiple sources). Action potentials cannot summate due to the refractory period. * **Threshold:** The initial segment (Axon Hillock) is the most excitable part of the neuron because it requires the least amount of depolarization to reach the threshold. * **Comparison:** Unlike action potentials, graded potentials have **no refractory period**.

Q: In a sarcomere, which band or zone is NOT characterized by the presence of actin filaments?

H band. **Explanation:** The sarcomere is the functional unit of skeletal muscle, extending from one Z-line to the next. Its appearance is defined by the arrangement of thick (myosin) and thin (actin) filaments. **Why the H band is correct:** The **H band** (from German *heller*, meaning brighter) is the central region of the A band. It contains **only thick (myosin) filaments**. During muscle contraction, actin filaments slide over myosin toward the center, narrowing the H band, but at rest, it is characterized specifically by the absence of actin. **Analysis of incorrect options:** * **I band:** This is the isotropic band which contains **only thin (actin) filaments**. It spans across two adjacent sarcomeres, bisected by the Z disc. * **M band:** Located in the dead center of the H zone, the M line (M band) contains proteins (like myomesin) that anchor the thick filaments. While it is within the H zone, the question asks for the "band or zone" defined by the absence of actin; the H zone is the primary anatomical region meeting this criteria. * **Z band (Z disc):** This is the boundary of the sarcomere where **actin filaments are anchored** via the protein alpha-actinin. **High-Yield NEET-PG Pearls:** 1. **"A" is for All:** The **A band** (Anisotropic) contains the entire length of the thick filament, including the area where actin and myosin overlap. Its length **remains constant** during contraction. 2. **Contraction Dynamics:** During contraction, the **H zone and I band shorten**, while the Z lines move closer together. 3. **Titans of the Sarcomere:** **Titin** is the largest protein in the body; it acts as a spring, connecting the Z disc to the M line, providing passive elasticity.

Q: Which of the following is NOT true about the tendon organ?

Contracting. The **Golgi Tendon Organ (GTO)** is a high-threshold mechanoreceptor located in the tendons of skeletal muscles. It functions as a tension sensor, monitoring the force of muscle contraction. ### Why "Contracting" is the Correct Answer The GTO is stimulated by **tension**, not by the act of contraction itself (though contraction creates tension). When tension becomes excessive, the GTO triggers the **Inverse Stretch Reflex** (Autogenic Inhibition). This reflex causes the muscle to **relax** rather than contract. Therefore, "Contracting" is the opposite of its physiological effect. ### Analysis of Incorrect Options * **A. Inhibitory:** The GTO sends impulses via **Ib afferent fibers** to the spinal cord, where they synapse with inhibitory interneurons. These interneurons inhibit the alpha motor neurons of the agonist muscle, leading to relaxation. * **B. Negative Feedback:** It operates on a negative feedback loop to regulate muscle tension. If tension increases, the GTO reduces motor output to prevent injury, maintaining tension within a physiological range. * **C. Protective:** Its primary role is to prevent tendon avulsion or muscle tearing during extreme exertion by forcing the muscle to "give way" (the **Clasp-knife response**). ### NEET-PG High-Yield Pearls * **Afferent Fiber:** Ib (Fast-conducting). * **Reflex:** Inverse Stretch Reflex (Disynaptic). * **GTO vs. Muscle Spindle:** The Muscle Spindle (Ia/II fibers) responds to **length/stretch** and causes contraction; the GTO responds to **tension** and causes relaxation. * **Clinical Sign:** The **Clasp-knife phenomenon** seen in upper motor neuron lesions is mediated by the GTO.

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).

Question 1

What are the key molecules involved in the differences between skeletal and smooth muscle contraction and relaxation?

Accepted Answer

Troponin and Myosin

Answer

Troponin and Myosin light chain kinase

Answer

Troponin

Answer

Myosin light chain kinase and Calcium ions

Question 2

In a neuron, where does graded electrogenesis occur?

Accepted Answer

Soma - dendritic zone

Answer

Initial segment

Answer

Axon

Answer

Nerve ending

Question 3

In a sarcomere, which band or zone is NOT characterized by the presence of actin filaments?

Accepted Answer

H band

Answer

I band

Answer

M band

Answer

Z band

Question 4

Which of the following is NOT true about the tendon organ?

Accepted Answer

Contracting

Answer

Inhibitory

Answer

Negative feedback

Answer

Protective

Question 5

What is the contractile unit of a muscle?

Accepted Answer

Sarcomere

Answer

Sarcolemma

Answer

Myofibril

Answer

Sarcotubular System

Question 6

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 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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