Nerve and Muscle Physiology Practice Questions

Q: Increase in cytosolic calcium from intracellular storage during smooth muscle contraction is due to?

Inositol trisphosphate (IP3) and diacylglycerol (DAG). **Explanation:** The correct answer is **D. Inositol trisphosphate (IP3) and diacylglycerol (DAG)**. In smooth muscle, contraction is initiated by an increase in cytosolic calcium ($Ca^{2+}$). While some calcium enters via voltage-gated channels, a significant portion is released from the **Sarcoplasmic Reticulum (SR)**—the primary intracellular storage site. This process is mediated by the **Gq-protein coupled receptor (GPCR) pathway**: 1. A ligand (e.g., Norepinephrine, Acetylcholine) binds to a Gq-coupled receptor. 2. This activates **Phospholipase C (PLC)**, which cleaves membrane-bound PIP2 into **IP3** and **DAG**. 3. **IP3** binds to specific IP3-gated calcium release channels on the SR membrane, triggering the efflux of $Ca^{2+}$ into the cytosol. 4. **DAG** activates Protein Kinase C (PKC), which further modulates contractile proteins. **Why the other options are incorrect:** * **A. cAMP:** Generally promotes **smooth muscle relaxation** (e.g., via $\beta_2$ receptors) by inhibiting Myosin Light Chain Kinase (MLCK) and stimulating calcium sequestration back into the SR. * **B. cGMP:** Acts as a second messenger for Nitric Oxide (NO) and Atrial Natriuretic Peptide (ANP). It causes **vasodilation/relaxation** by activating Protein Kinase G (PKG), which dephosphorylates Myosin Light Chains. * **C. cCMP:** This is a minor signaling molecule and does not play a primary role in the standard excitation-contraction coupling of smooth muscle. **High-Yield NEET-PG Pearls:** * **Calmodulin:** Unlike skeletal muscle (which uses Troponin C), smooth muscle $Ca^{2+}$ binds to **Calmodulin** to activate MLCK. * **L-type Calcium Channels:** These are the targets of Calcium Channel Blockers (CCBs) like Nifedipine. * **Pharmacomechanical Coupling:** The ability of IP3 to cause contraction without a change in membrane potential is a unique feature of smooth muscle.

Q: What is the term for a decrease in reflex response after repetitive stimulation?

Fatigue. **Explanation:** The correct answer is **Fatigue**. In the context of reflex activity, repetitive stimulation of the afferent nerve leads to a gradual decrease and eventual disappearance of the reflex response. This occurs primarily due to the **exhaustion of neurotransmitter stores** at the synaptic level within the reflex arc. Unlike nerve fibers, which are relatively indefatigable, the synapse is the most vulnerable site for fatigue in the neural pathway. **Analysis of Options:** * **A. Summation:** This refers to the cumulative effect of multiple stimuli. It can be *temporal* (repeated stimuli from one neuron) or *spatial* (simultaneous stimuli from multiple neurons) that combine to reach the threshold for an action potential. It increases rather than decreases the response. * **C. Irradiation:** This occurs when the strength of a stimulus is increased, causing the impulse to spread to more neurons in the spinal cord, involving more muscle groups (e.g., a strong painful stimulus causing withdrawal of the entire limb instead of just a finger). * **D. Occlusion:** This is a phenomenon where the response to simultaneous stimulation of two afferent nerves is *less* than the sum of their individual responses because they share a common pool of postsynaptic neurons. While it involves a "decrease" in expected output, it is not caused by repetitive stimulation. **High-Yield Facts for NEET-PG:** * **Site of Fatigue:** In a nerve-muscle preparation, the **synapse** (neuromuscular junction) fatigues first, followed by the muscle, while the nerve fiber is the last to fatigue. * **Synaptic Delay:** The time taken for the neurotransmitter to release and act on the postsynaptic membrane (approx. 0.5 ms) is the reason for the delay in reflex action. * **One-way conduction:** Reflexes follow the **Bell-Magendie Law**, stating that impulses pass only from the axon to the dendrite/soma across a synapse.

Q: Which ion has the lowest electrochemical driving force in a typical neuron with a resting membrane potential of -65 millivolts?

Chloride. ### Explanation The **electrochemical driving force** is the net force acting on an ion, determined by the difference between the actual membrane potential ($V_m$) and the ion's equilibrium potential ($E_{ion}$). The formula is: **Driving Force = $V_m - E_{ion}$** **1. Why Chloride (A) is Correct:** In a typical neuron, the resting membrane potential ($V_m$) is approximately **-65 to -70 mV**. The equilibrium potential for Chloride ($E_{Cl}$) is also approximately **-65 to -70 mV**. Because $V_m$ is almost equal to $E_{Cl}$, the net driving force is near zero. Consequently, there is very little net movement of chloride ions across the resting membrane. **2. Why the Other Options are Incorrect:** * **Potassium (B):** $E_K$ is typically around **-90 mV**. The driving force is $|-65 - (-90)| = 25\text{ mV}$. While small, it is significantly higher than that of Chloride. * **Sodium (C):** $E_{Na}$ is approximately **+60 mV**. The driving force is massive: $|-65 - (+60)| = 125\text{ mV}$. This creates a strong inward pressure for $Na^+$ to enter the cell. * **Calcium (D):** $E_{Ca}$ is very positive (approx. **+120 mV**). Combined with a very low intracellular concentration, the driving force is the highest among common ions ($>180\text{ mV}$). **3. High-Yield Facts for NEET-PG:** * **Nernst Equation:** Used to calculate the equilibrium potential for a single ion. * **Goldman-Hodgkin-Katz (GHK) Equation:** Used to calculate the RMP by considering the permeability of all ions. * **RMP Determinant:** The RMP is closest to the equilibrium potential of the ion with the **highest permeability** (which is Potassium at rest). * **Chloride Paradox:** In some neurons, $Cl^-$ is passively distributed, making its $E_{Cl}$ exactly equal to RMP, resulting in zero driving force.

Q: Name the structure that is NOT involved in excitation-contraction coupling in striated muscle?

Microtubules. **Explanation:** **Excitation-Contraction (E-C) Coupling** is the physiological process by which an electrical stimulus (action potential) is converted into a mechanical response (muscle contraction). **Why Microtubules are the correct answer:** Microtubules are components of the cytoskeleton involved in structural integrity, intracellular transport, and cell division. While they provide a framework for the cell, they play **no direct role** in the rapid transmission of electrical impulses or the release of calcium required for E-C coupling. **Analysis of other options:** * **Motor End Plate:** This is the specialized region of the sarcolemma at the neuromuscular junction. It contains nicotinic acetylcholine receptors; its depolarization (End Plate Potential) is the initiating step that triggers the action potential. * **Sarcolemma:** The muscle cell membrane propagates the action potential along its surface and deep into the fiber via **T-tubules**. This ensures that the electrical signal reaches the interior of the muscle fiber. * **Sarcoplasmic Reticulum (SR):** The SR acts as the primary intracellular calcium reservoir. The signal from the T-tubules triggers the release of $Ca^{2+}$ via Ryanodine receptors (RyR), which is the essential "coupling" step that allows actin-myosin interaction. **High-Yield Facts for NEET-PG:** * **The Triad:** In skeletal muscle, a triad consists of one T-tubule and two terminal cisternae of the SR. It is located at the **A-I junction**. * **Voltage Sensor:** The Dihydropyridine (DHP) receptor in the T-tubule acts as the voltage sensor. * **Calcium Release Channel:** The Ryanodine receptor (RyR1 in skeletal muscle) is the channel on the SR. * **Malignant Hyperthermia:** Caused by a mutation in the RyR1 receptor, leading to excessive calcium release upon exposure to volatile anesthetics.

Q: In the Central Nervous System (CNS), myelination is carried out by which type of glial cell?

Oligodendrocytes. **Explanation:** The correct answer is **Oligodendrocytes**. Myelination is the process of forming a lipid-rich insulating layer around axons to increase the speed of nerve impulse conduction (saltatory conduction). In the **Central Nervous System (CNS)**, which includes the brain and spinal cord, this function is performed by oligodendrocytes. A key characteristic of oligodendrocytes is that a single cell can extend multiple processes to myelinate segments of several different axons simultaneously. **Analysis of Incorrect Options:** * **Schwann cells:** These are responsible for myelination in the **Peripheral Nervous System (PNS)**. Unlike oligodendrocytes, one Schwann cell myelinates only a single segment of one axon. * **Astrocytes:** These are the most abundant glial cells in the CNS. They provide structural support, maintain the blood-brain barrier (BBB), regulate the external chemical environment, and form scar tissue (gliosis) after injury. * **Microglia:** These are the resident macrophages of the CNS. They act as the primary immune defense and are derived from the yolk sac (mesodermal origin), unlike other glial cells which are neuroectodermal. **High-Yield Clinical Pearls for NEET-PG:** * **Multiple Sclerosis (MS):** An autoimmune demyelinating disease specifically affecting the **CNS** (oligodendrocytes). * **Guillain-Barré Syndrome (GBS):** An acute inflammatory demyelinating polyneuropathy affecting the **PNS** (Schwann cells). * **Origin:** All glial cells (Astrocytes, Oligodendrocytes, Ependymal cells) originate from the **Neural Tube**, except for Microglia, which originate from **Monocytes/Mesoderm**.

Q: What is the function of synaptobrevin?

Presynaptic vesicle fusion. **Explanation:** The process of neurotransmitter release at the chemical synapse involves a group of proteins known as **SNARE proteins** (Soluble NSF Attachment Protein Receptors). These proteins are essential for the docking and fusion of synaptic vesicles with the presynaptic membrane. **Why Option A is Correct:** **Synaptobrevin** (also called VAMP - Vesicle-Associated Membrane Protein) is a **v-SNARE** (vesicular SNARE). It is located on the membrane of the neurotransmitter vesicle. During synaptic transmission, synaptobrevin interacts with **t-SNAREs** (target SNAREs) on the presynaptic membrane—specifically **Syntaxin** and **SNAP-25**. This interaction forms a "SNARE complex" that pulls the vesicle close to the presynaptic membrane, leading to membrane fusion and exocytosis of the neurotransmitter into the synaptic cleft. **Why Other Options are Incorrect:** * **Option B:** Vesicle fusion occurs at the **presynaptic** terminal to release neurotransmitters. The postsynaptic membrane contains receptors, not fusion machinery for neurotransmitter release. * **Options C & D:** While synaptobrevin is essential for transmission, its primary physiological function is the mechanical act of **fusion**. It does not act as a regulatory inhibitor or an amplifier of the signal itself. **High-Yield Clinical Pearls for NEET-PG:** * **Tetanus Toxin & Botulinum Toxin:** These are proteases that cleave SNARE proteins. * **Tetanus toxin** specifically cleaves **Synaptobrevin** in inhibitory interneurons (Renshaw cells), leading to spastic paralysis. * **Botulinum toxins (B, D, F, G)** also cleave **Synaptobrevin**, while types A and E cleave SNAP-25, leading to flaccid paralysis. * **Synaptotagmin:** This is the calcium sensor on the vesicle that triggers the final fusion step when $Ca^{2+}$ enters the terminal.

Q: The cross-bridges of the sarcomere in skeletal muscle are components of what?

Myosin. ### Explanation **Correct Option: A (Myosin)** In skeletal muscle, the **thick filaments** are composed of the protein **Myosin**. Each myosin molecule consists of a tail (rod) and two globular heads. These heads, along with the "arm" or hinge region, extend outward toward the thin filaments to form **cross-bridges**. These cross-bridges contain two critical binding sites: 1. **Actin-binding site:** To initiate contraction. 2. **ATP-binding site (Myosin ATPase):** To hydrolyze ATP and provide energy for the "power stroke." **Why Incorrect Options are Wrong:** * **B. Actin:** This is the primary component of the **thin filament**. While actin has a binding site for myosin, it does not form the cross-bridge structure itself; it serves as the "ladder" the cross-bridge climbs. * **C. Troponin:** A regulatory protein complex (TnT, TnI, TnC) located on the thin filament. Its role is to bind Calcium (TnC) and shift tropomyosin to uncover the active sites on actin. * **D. Tropomyosin:** A filamentous protein that wraps around actin. In a relaxed state, it physically blocks the myosin-binding sites on actin, preventing cross-bridge formation. **High-Yield NEET-PG Pearls:** * **The Power Stroke:** Occurs when **ADP and Pi (inorganic phosphate) are released** from the myosin head, causing it to tilt toward the M-line. * **Detachment:** Binding of a **new ATP molecule** is required for the myosin head to detach from actin. * **Rigor Mortis:** Occurs due to the lack of ATP; without ATP, the cross-bridges cannot detach, leaving the muscle in a rigid, contracted state. * **H-Zone:** This region of the sarcomere contains **only thick filaments (myosin)** and lacks cross-bridges in its central "bare zone."

Question 1

Increase in cytosolic calcium from intracellular storage during smooth muscle contraction is due to?

Accepted Answer

Inositol trisphosphate (IP3) and diacylglycerol (DAG)

Answer

Cyclic adenosine monophosphate (cAMP)

Answer

Cyclic guanosine monophosphate (cGMP)

Answer

Cyclic cytidine monophosphate (cCMP)

Question 2

What is the term for a decrease in reflex response after repetitive stimulation?

Accepted Answer

Fatigue

Answer

Summation

Answer

Irradiation

Answer

Occlusion

Question 3

Which ion has the lowest electrochemical driving force in a typical neuron with a resting membrane potential of -65 millivolts?

Accepted Answer

Chloride

Answer

Potassium

Answer

Sodium

Answer

Calcium

Question 4

In a nerve, the magnitude of the action potential overshoot is normally a function of which of the following?

Accepted Answer

Extracellular sodium concentration

Answer

Magnitude of the stimulus

Answer

Intracellular potassium concentration

Answer

Resting membrane potential

Question 5

Name the structure that is NOT involved in excitation-contraction coupling in striated muscle?

Accepted Answer

Microtubules

Answer

Sarcolemma

Answer

Sarcoplasmic Reticulum

Answer

Motor end plate

Question 6

In the Central Nervous System (CNS), myelination is carried out by which type of glial cell?

Accepted Answer

Oligodendrocytes

Answer

Schwann cells

Answer

Astrocytes

Answer

Microglia

Question 7

Staircase phenomenon (Treppe) is due to?

Accepted Answer

Increased availability of intracellular calcium

Answer

Synthesis of stable troponin C molecules

Answer

Summation

Answer

Tetanus

Question 8

The plateau phase of the myocardial action potential is primarily due to which ion movement?

Accepted Answer

Influx of Ca++

Answer

Efflux of Na+

Answer

Influx of K+

Answer

Closure of voltage-gated K+ channels

Question 9

What is the function of synaptobrevin?

Accepted Answer

Presynaptic vesicle fusion

Answer

Postsynaptic vesicle fusion

Answer

Inhibits synaptic transmission

Answer

Amplify synaptic transmission

Question 10

The cross-bridges of the sarcomere in skeletal muscle are components of what?

Accepted Answer

Myosin

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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