Nerve and Muscle Physiology Practice Questions

Q: Which of the following is associated with the 'All-or-none' law?

Action potential. ### Explanation **1. Why Action Potential is Correct:** The **All-or-None Law** states that if a stimulus is strong enough to reach the **threshold potential** (usually -55mV), an action potential of constant magnitude and shape will be generated. If the stimulus is sub-threshold, no action potential occurs at all. Once triggered, the amplitude and velocity of the action potential remain independent of the strength of the stimulus. Therefore, the cell either "fires" completely or not at all. **2. Why Other Options are Incorrect:** * **Resting Membrane Potential (RMP):** This is the static electrical potential across a cell membrane when the cell is not excited (typically -70mV to -90mV). It is maintained by ion pumps and leak channels and does not follow the all-or-none principle; it can fluctuate slightly based on metabolic conditions. * **Membrane Potential:** This is a general term for the voltage difference across a membrane at any given time. It includes graded potentials (like EPSPs or IPSPs), which are **proportional** to the stimulus intensity and do not follow the all-or-none law. **3. High-Yield Clinical Pearls for NEET-PG:** * **Graded Potentials:** Unlike action potentials, local potentials (e.g., end-plate potentials, receptor potentials) are **not** all-or-none; they are proportional to stimulus strength and can be summated. * **Refractory Period:** The absolute refractory period ensures that action potentials are discrete events, preventing them from merging. * **Exceptions:** While a single nerve fiber follows the all-or-none law, a **whole nerve trunk** (composed of many fibers) does not, as it shows "graded" responses due to the recruitment of different fibers with varying thresholds.

Q: Action potential is transmitted in myofibrils via which structure?

T-Tubules. **Explanation:** The transmission of an action potential from the sarcolemma (cell membrane) into the interior of a muscle fiber is mediated by the **T-tubules (Transverse tubules)**. **1. Why T-Tubules are correct:** T-tubules are deep invaginations of the sarcolemma that run perpendicular to the myofibrils. When an action potential spreads across the muscle surface, it travels down the T-tubules to reach the deep-seated sarcoplasmic reticulum. This triggers the **Dihydropyridine (DHP) receptors** in the T-tubule membrane, which are mechanically coupled to **Ryanodine receptors (RyR)** on the sarcoplasmic reticulum, leading to calcium release and muscle contraction. This process is known as **Excitation-Contraction Coupling**. **2. Why other options are incorrect:** * **Terminal cisterns:** These are enlarged areas of the sarcoplasmic reticulum that *store* calcium. While they release calcium upon stimulation, they do not transmit the action potential itself. * **Longitudinal tubules:** These are the central portions of the sarcoplasmic reticulum primarily involved in calcium reuptake via SERCA pumps, not the initial transmission of the electrical impulse. * **Sarcomere:** This is the basic structural and functional unit of a myofibril (between two Z-lines). It is the site of contraction, not the conduit for the action potential. **High-Yield Clinical Pearls for NEET-PG:** * **The Triad:** In skeletal muscle, a triad consists of one T-tubule and two flanking terminal cisternae, typically located at the **A-I junction**. (In cardiac muscle, it is a *dyad* located at the Z-line). * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive calcium release when exposed to volatile anesthetics (e.g., Halothane). * **L-type Calcium Channels:** The DHP receptors in T-tubules act as voltage sensors in skeletal muscle but function as actual calcium channels in cardiac muscle.

Q: All of the following are true regarding the Golgi tendon organ, EXCEPT:

Detects changes in muscle length. ### Explanation The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons. Its primary function is to monitor **muscle tension** rather than length. **Why Option B is the Correct Answer (The Exception):** The GTO is arranged **in series** with muscle fibers. When a muscle contracts, it pulls on the tendon, activating the GTO. It is highly sensitive to changes in **tension/force**. In contrast, the **Muscle Spindle** is arranged in parallel and is the receptor responsible for detecting changes in **muscle length** and the rate of change in length. **Analysis of Other Options:** * **Option A & C:** The GTO mediates the **Inverse Stretch Reflex** (Autogenic Inhibition). When excessive tension is detected, the GTO sends impulses via **Ib afferent fibers** to the spinal cord. These synapse with inhibitory interneurons that **inhibit the alpha motor neuron** of the same muscle, causing it to relax. This serves as a protective mechanism to prevent tendon avulsion or muscle tearing. * **Option D:** The GTO is indeed an **encapsulated sensory receptor** consisting of a network of collagen fibers enclosed in a capsule. **High-Yield Facts for NEET-PG:** * **Muscle Spindle:** Detects **Length**; Afferents: **Ia** (primary) and **II** (secondary); Reflex: **Stretch Reflex** (monosynaptic). * **Golgi Tendon Organ:** Detects **Tension**; Afferents: **Ib**; Reflex: **Inverse Stretch Reflex** (polysynaptic). * **Clasp-Knife Phenomenon:** This clinical sign (seen in UMN lesions) is attributed to the activation of the inverse stretch reflex mediated by GTOs when a spastic muscle is forcefully stretched.

Q: Following traumatic peripheral nerve transection, re-growth usually occurs at which of the following rates?

1 mm per day. **Explanation:** The correct answer is **1 mm per day (Option B)**. Following a peripheral nerve injury (transection), the distal segment undergoes **Wallerian degeneration**, while the proximal segment undergoes **regeneration**. Once the axonal sprouts cross the lesion site and enter the distal endoneurial tubes, they grow toward the target organ. This regenerative process is driven by slow axonal transport and typically occurs at a rate of **1 mm/day** (or approximately 1 inch per month). **Analysis of Incorrect Options:** * **Option A (0.1 mm/day):** This rate is too slow. While initial "die-back" and the latent period before sprouting may delay visible progress, the actual growth rate is significantly higher. * **Option C (5 mm/day):** This is an overestimation. While some experimental conditions or very young patients might show slightly faster rates, the standard physiological average used in clinical practice is 1 mm/day. * **Option D (1 cm/day):** This is physiologically impossible for human nerve regeneration. Such a rapid rate would lead to recovery in days for injuries that typically take months. **High-Yield Facts for NEET-PG:** * **Hoffmann-Tinel Sign:** A clinical test where distal percussion over the regenerating nerve elicits a tingling sensation. The distal-most point of tingling indicates the extent of axonal regrowth, allowing clinicians to track the 1 mm/day progress. * **Factors affecting regeneration:** Regeneration is faster in **crush injuries** (Sunderland Grade 2) than in complete transections because the endoneurial sheath remains intact to guide the axons. * **Proximal vs. Distal:** Regeneration is generally faster in proximal segments compared to distal segments. * **CNS vs. PNS:** Regeneration occurs in the PNS (aided by Schwann cells) but is virtually absent in the CNS due to inhibitory factors like Nogo-A and the absence of a basement membrane.

Question 1

Which of the following is associated with the 'All-or-none' law?

Accepted Answer

Action potential

Answer

Resting potential

Answer

Membrane potential

Answer

None of the above

Question 2

Action potential is transmitted in myofibrils via which structure?

Accepted Answer

T-Tubules

Answer

Terminal cisterns

Answer

Longitudinal tubules

Answer

Sarcomere

Question 3

What happens to muscle during rigor mortis?

Accepted Answer

Stiffens and shortens

Answer

Stiffens

Answer

Shortens

Answer

Stiffens and lengthens

Question 4

What is saltatory conduction?

Accepted Answer

The jumping of depolarization from node to node in myelinated axons.

Answer

Conduction from a presynaptic to a postsynaptic neuron.

Answer

Conduction within the muscles of the T-tubule system.

Answer

Conduction of an impulse in both directions along an axon.

Question 5

What is the first change that occurs in a cut nerve?

Accepted Answer

Degeneration of the myelin sheath

Answer

Schwann cell proliferation

Answer

Chromatolysis

Answer

Degeneration of the neurilemma

Question 6

Skeletal muscle contraction ends when:

Accepted Answer

Calcium levels decrease outside the sarcoplasmic reticulum

Answer

Ions move out of the cytoplasm

Answer

Acetylcholine is absorbed from the neuromuscular junction

Answer

Receptors close and are drawn inward

Question 7

All of the following are true regarding the Golgi tendon organ, EXCEPT:

Accepted Answer

Detects changes in muscle length

Answer

Mediates the inverse stretch reflex

Answer

Inhibits the alpha motor neuron

Answer

Is an encapsulated sensory receptor

Question 8

After-hyperpolarization during nerve conduction is due to:

Accepted Answer

Slow entry of K+

Answer

Slow entry of Na+

Answer

Pumping of Na+ outside

Answer

Pumping of K+ outside

Question 9

Following traumatic peripheral nerve transection, re-growth usually occurs at which of the following rates?

Accepted Answer

1 mm per day

Answer

0.1 mm per day

Answer

5 mm per day

Answer

1 cm per day

Question 10

Which nerve fiber is most commonly affected by pressure?

Accepted Answer

AP

Answer

Aa

Answer

Ay

Answer

C

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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