Nerve and Muscle Physiology Practice Questions

Q: What type of fibers are postganglionic fibers?

Type C fibers. **Explanation:** The classification of nerve fibers is based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, myelination, and conduction velocity. **1. Why Type C fibers is the correct answer:** Postganglionic autonomic fibers (both sympathetic and parasympathetic) are **Type C fibers**. These are the smallest in diameter (0.4–1.2 μm) and are **unmyelinated**. Due to the lack of myelin and small size, they have the slowest conduction velocity (0.5–2.0 m/s). Their primary function is to carry autonomic signals to effector organs (smooth muscle, cardiac muscle, and glands) and transmit slow pain/temperature sensations. **2. Why other options are incorrect:** * **Type B fibers:** These are small, **myelinated** fibers. In the autonomic nervous system, **preganglionic fibers** are Type B. They conduct impulses faster than Type C fibers but slower than Type A. * **Type A fibers:** These are large, myelinated fibers subdivided into Alpha (proprioception/somatic motor), Beta (touch/pressure), Gamma (muscle spindles), and Delta (fast pain/cold). They are not involved in postganglionic autonomic transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Myelination Rule:** Preganglionic = Myelinated (Type B); Postganglionic = Unmyelinated (Type C). * **Sensitivity to Local Anesthetics:** Type C fibers are highly sensitive to local anesthetics (like Lidocaine), which is why pain and autonomic function are blocked before motor function (Type A). * **Sensitivity to Pressure/Hypoxia:** Type A fibers are most sensitive to pressure; Type B fibers are most sensitive to hypoxia. * **Fast vs. Slow Pain:** Type A-delta fibers carry "fast/sharp" pain, while Type C fibers carry "slow/dull/aching" pain.

Q: Which of the following substances is produced by osteoblasts?

Collagen. **Explanation:** Osteoblasts are the bone-forming cells derived from mesenchymal stem cells. Their primary function is to synthesize and secrete the organic matrix of bone, known as **osteoid**. * **Why Collagen is Correct:** Approximately 90-95% of the organic matrix (osteoid) produced by osteoblasts consists of **Type I Collagen**. This collagen provides the structural framework and tensile strength of the bone. Osteoblasts also secrete non-collagenous proteins like osteocalcin and osteopontin. * **Why other options are incorrect:** * **Calcium:** Calcium is a mineral obtained from the diet and transported via the blood. While osteoblasts facilitate the *deposition* of calcium hydroxyapatite into the matrix, they do not "produce" the element itself. * **Pyrophosphate:** This is a potent **inhibitor** of bone mineralization. It is present in the extracellular fluid to prevent hydroxyapatite precipitation in soft tissues. Osteoblasts actually produce **Alkaline Phosphatase (ALP)** to break down pyrophosphate, thereby allowing mineralization to occur in the bone matrix. * **Monosodium urate:** These are crystals formed by the precipitation of uric acid. They are the causative agents of **Gout** and are not a physiological product of bone cells. **High-Yield NEET-PG Pearls:** * **Marker of Osteoblastic Activity:** Serum **Alkaline Phosphatase (ALP)** and **Osteocalcin** are clinical markers used to assess bone formation rates. * **Collagen Type:** Remember "Type **One** for B**one**." (Type II is for cartilage). * **Mineralization:** Osteoblasts secrete membrane-bound **matrix vesicles** containing ALP and calcium-binding proteins, which serve as the initial sites for hydroxyapatite crystal formation.

Q: The latch bridge mechanism in smooth muscles primarily aids in which of the following?

Sustained contraction. **Explanation:** The **Latch Bridge Mechanism** is a unique physiological feature of smooth muscle that allows for **sustained contraction** (tonus) with minimal energy expenditure. **1. Why "Sustained Contraction" is correct:** In smooth muscle, contraction is regulated by the phosphorylation of the myosin light chain (MLC) by Myosin Light Chain Kinase (MLCK). When the enzyme **Myosin Light Chain Phosphatase (MLCP)** dephosphorylates the myosin while it is still attached to actin, the detachment rate of the myosin heads decreases significantly. These "latch bridges" remain attached for a prolonged period, maintaining tension (tone) without requiring additional ATP hydrolysis. This is crucial for organs like blood vessels and the bladder, which must maintain pressure for long durations. **2. Why the other options are incorrect:** * **Option A (Initiation of contraction):** Initiation is dependent on the increase in intracellular Calcium and the activation of MLCK. The latch mechanism occurs at the *end* of the cycle or during prolonged phases, not at the start. * **Option C (Early dephosphorylation):** While dephosphorylation is part of the process, the latch mechanism is defined by the *result* of that dephosphorylation (prolonged attachment), not the speed of the enzyme itself. **High-Yield NEET-PG Pearls:** * **Energy Efficiency:** Smooth muscle can maintain the same tension as skeletal muscle using **1/10th to 1/300th** of the energy. * **Calmodulin:** Smooth muscle lacks troponin; Calcium binds to **Calmodulin** to initiate the bridge cycle. * **Reverse Latch:** The mechanism is reversed when a new ATP molecule eventually binds or when intracellular calcium levels drop significantly, allowing for relaxation.

Q: What is the role of creatine phosphate in muscle?

Provides instant energy. **Explanation:** **1. Why Option B is Correct:** Creatine phosphate (also known as phosphocreatine) acts as a **high-energy phosphate reservoir** in muscle cells. During the first few seconds of intense muscular activity, the demand for ATP exceeds the rate at which oxidative phosphorylation or glycolysis can produce it. The enzyme **Creatine Kinase (CK)** catalyzes the transfer of a phosphate group from creatine phosphate to ADP, regenerating ATP almost instantaneously. This provides the "instant energy" required for short bursts of maximal effort (e.g., a 100m sprint). **2. Why Other Options are Incorrect:** * **Option A:** Gluconeogenesis (synthesis of glucose from non-carbohydrate sources) occurs primarily in the liver and kidneys, not through creatine phosphate. * **Option C:** Excitation-contraction coupling is mediated by calcium release from the sarcoplasmic reticulum and its binding to Troponin C; creatine phosphate does not play a structural or signaling role in this process. * **Option D:** The stretch reflex is a neurological feedback loop involving muscle spindles and alpha-motor neurons; it is not dependent on the biochemical energy stores of the muscle fiber itself. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Lohmann’s Reaction:** The reversible chemical reaction: $ADP + \text{Creatine Phosphate} \rightleftharpoons ATP + \text{Creatine}$. * **Creatine Kinase (CK) Isoenzymes:** CK-MM (Skeletal muscle), CK-MB (Cardiac muscle - marker for MI), and CK-BB (Brain). * **Creatinine:** A waste product formed by the non-enzymatic breakdown of creatine phosphate. Its excretion rate is relatively constant and used as a marker for GFR. * **Energy Sequence:** ATP stores (1-2 sec) $\rightarrow$ Creatine Phosphate (5-8 sec) $\rightarrow$ Anaerobic Glycolysis $\rightarrow$ Aerobic Metabolism.

Q: What is true about neuropraxia?

Physiological block. **Explanation:** **Neuropraxia** is the mildest form of nerve injury according to **Seddon’s classification**. It is characterized by a **physiological block** of nerve conduction without any structural damage to the axon or the connective tissue sheaths (epineurium, perineurium, and endoneurium). 1. **Why the correct answer is right:** In neuropraxia, the nerve remains anatomically intact, but conduction is temporarily halted, usually due to focal demyelination or ischemia caused by sustained pressure (e.g., Saturday Night Palsy). Since the axon is preserved, there is **no Wallerian degeneration** distal to the site of injury. Recovery is typically spontaneous and complete within days to weeks once the pressure is relieved. 2. **Why the incorrect options are wrong:** * **Blockage of axon:** This is misleading. While conduction is blocked, the physical continuity of the axon is not interrupted. * **Incomplete/Complete transection:** These describe higher grades of injury. **Axonotmesis** involves axonal disruption with preservation of the sheath, while **Neurotmesis** (Option D) involves complete transection of both the axon and the connective tissue, requiring surgical intervention. **High-Yield Facts for NEET-PG:** * **Seddon’s Classification:** Neuropraxia (Mildest) → Axonotmesis → Neurotmesis (Most severe). * **Sunderland’s Classification:** Neuropraxia corresponds to **First-degree** injury. * **Clinical Feature:** Motor fibers are more susceptible than sensory fibers; autonomic functions are usually preserved. * **EMG Finding:** No denervation potentials (fibrillations) are seen because the axon is intact.

Q: The inverse stretch reflex is mediated by which of the following structures?

Golgi tendon organ. **Explanation:** The **Inverse Stretch Reflex** (also known as the autogenic inhibition reflex) is a protective mechanism that prevents muscle damage due to excessive tension. **1. Why Golgi Tendon Organ (GTO) is correct:** The GTO is the sensory receptor for this reflex, located at the junction of muscle fibers and tendons. Unlike the muscle spindle, which responds to changes in muscle *length*, the GTO is arranged in **series** with muscle fibers and responds primarily to **muscle tension** (force). When a muscle undergoes heavy contraction, the GTO is stimulated and sends impulses via **Type Ib afferent fibers** to the spinal cord. These fibers synapse on inhibitory interneurons, which then inhibit the alpha motor neurons of the same muscle, causing it to relax. **2. Why other options are incorrect:** * **Muscle Spindle:** This is the receptor for the **Stretch Reflex** (Myotatic reflex). It is arranged in **parallel** with extrafusal fibers and responds to changes in muscle **length**. * **Trail and Tail fibre endings:** These terms refer to the types of motor nerve endings on intrafusal muscle fibers. **Trail endings** are typically found on nuclear chain fibers, while **Plate endings** (often confused with "tail") are on nuclear bag fibers. They are involved in the efferent (motor) gamma system, not the afferent limb of the inverse stretch reflex. **Clinical Pearls for NEET-PG:** * **Receptor:** Golgi Tendon Organ (Tension sensor). * **Afferent Nerve:** Type Ib (Fast conducting). * **Synapse:** Polysynaptic (involves an inhibitory interneuron). * **Function:** Prevents tendon avulsion and muscle tearing during extreme exertion. * **Clasp-knife response:** In upper motor neuron (UMN) lesions, the sudden relaxation of a spastic muscle upon passive stretching is attributed to the activation of the inverse stretch reflex.

Question 1

Which of the following is true about Duchenne's muscular dystrophy?

Accepted Answer

It is an X-linked recessive disorder.

Answer

Skeletal muscle wasting occurs due to abnormalities of the contractile filaments.

Answer

Muscle wasting occurs in the pelvic region only.

Answer

It is a form of titanopathy.

Question 2

Which of the following factors regulate the rate of firing of axons during nerve conduction?

Accepted Answer

Na+ channels opening

Answer

K+ channels

Answer

Refractory period

Answer

Na+ channels closing

Question 3

What type of fibers are postganglionic fibers?

Accepted Answer

Type C fibers

Answer

Type B fibers

Answer

Both

Answer

None

Question 4

Increase in threshold level on applying a subthreshold, slowly rising stimulus is known as?

Accepted Answer

Accommodation

Answer

Adaptation

Answer

Refractoriness

Answer

Electrotonus

Question 5

Duchenne's muscular dystrophy affects which group of muscles commonly?

Accepted Answer

Calf muscles

Answer

Shoulder muscles

Answer

Forearm muscles

Answer

Respiratory muscles

Question 6

Which of the following substances is produced by osteoblasts?

Accepted Answer

Collagen

Answer

Calcium

Answer

Pyrophosphate

Answer

Monosodium urate

Question 7

The latch bridge mechanism in smooth muscles primarily aids in which of the following?

Accepted Answer

Sustained contraction

Answer

Initiation of contraction

Answer

Early dephosphorylation

Answer

None of the above

Question 8

What is the role of creatine phosphate in muscle?

Accepted Answer

Provides instant energy

Answer

Aids in gluconeogenesis

Answer

Is involved in action-contraction coupling

Answer

Aids in the stretch reflex

Question 9

What is true about neuropraxia?

Accepted Answer

Physiological block

Answer

Blockage of axon

Answer

Incomplete transection of nerve

Answer

Complete transection of nerve

Question 10

The inverse stretch reflex is mediated by which of the following structures?

Accepted Answer

Golgi tendon organ

Answer

Trail fibre ending

Answer

Tail fibre ending

Answer

Muscle spindle

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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