Nerve and Muscle Physiology Practice Questions

Q: Where does the initiation of an impulse start?

Axon hillock and initial segment. The initiation of a nerve impulse (action potential) occurs at the **Axon Hillock and Initial Segment** (often collectively called the **Trigger Zone**). ### Why Option B is Correct The initiation of an action potential depends on the density of **voltage-gated sodium (Na+) channels**. The initial segment of the axon has a significantly higher concentration of these channels compared to the rest of the neuron. Consequently, this region has the **lowest threshold** for excitation, meaning it requires the least amount of depolarization to trigger an all-or-none action potential. While the axon hillock funnels the graded potentials, the actual spike usually begins in the adjacent initial segment. ### Why Other Options are Incorrect * **Axon (A):** While the action potential *propagates* along the axon, it does not *initiate* there. The distal axon serves as a conduction cable. * **Cell Body / Soma (C):** The soma contains a relatively low density of voltage-gated Na+ channels. It primarily functions to integrate incoming signals (EPSPs and IPSPs) rather than generating the spike. * **Dendritic Tree (D):** Dendrites are the primary sites for receiving inputs. They generate **graded potentials** (local changes in membrane potential) which decay over distance and do not typically initiate an action potential. ### High-Yield NEET-PG Pearls * **Threshold Value:** The threshold at the initial segment is approximately **-35 to -40 mV**, whereas the rest of the soma requires depolarization to about -10 mV. * **Orthodromic vs. Antidromic:** Normal conduction from the soma to the axon terminal is "orthodromic." Experimental stimulation of an axon that travels backward to the soma is "antidromic." * **Myelination:** In myelinated neurons, the action potential "jumps" between **Nodes of Ranvier** (Saltatory conduction), where Na+ channel density is also very high.

Q: In an isotonic contraction of the muscle, what occurs?

The muscle moves a load through a distance.. ### Explanation **1. Why Option A is Correct:** In an **isotonic contraction** (Greek: *iso* = same, *tonos* = tension), the muscle tension remains constant once it exceeds the weight of the load. Because the tension is sufficient to overcome the resistance, the muscle fibers shorten, resulting in the **movement of a load through a distance**. This represents external work being done ($Work = Force \times Distance$). **2. Analysis of Incorrect Options:** * **Option B:** This describes an **isometric contraction**. In isometric contractions, the load is too heavy for the muscle to move; the muscle develops tension, but its length remains constant, and no external work is performed. * **Option C:** In isotonic contraction, the **latent period is longer** than in isometric contraction. This is because, in an isotonic contraction, the muscle must first build enough tension to equal the load (the isometric phase) before shortening can begin. * **Option D:** During an isotonic contraction, the muscle **shortens** (concentric) or lengthens under controlled tension (eccentric). However, the standard definition in this context refers to shortening to move a load. The length never increases during the active contractile phase of a typical isotonic twitch. **3. NEET-PG High-Yield Pearls:** * **Isotonic vs. Isometric:** In isometric contraction, the **Elastic Component (SEC)** is stretched while the **Contractile Component (CC)** shortens, keeping the overall length the same. In isotonic, the CC shortens further after the SEC is stretched. * **Work Done:** Isotonic contraction performs external work; Isometric contraction performs zero external work (all energy is dissipated as heat). * **Velocity:** The velocity of muscle shortening is maximal when the load is zero and decreases as the load increases (**Force-Velocity Relationship**).

Q: Which fibres are most sensitive to local anesthesia?

C fibres. ### Explanation The sensitivity of nerve fibers to local anesthetics (LA) is determined by the **size of the fiber** and the **presence of myelin**. Local anesthetics work by blocking voltage-gated sodium channels. **Why C Fibers are the most sensitive:** According to the traditional physiological classification, **C fibers** are the most sensitive to local anesthesia because they are the **smallest in diameter** and **unmyelinated**. Because they lack a myelin sheath, the drug can easily access the axonal membrane along its entire length. In contrast, myelinated fibers require the anesthetic to reach a high enough concentration to block at least three successive Nodes of Ranvier to stop conduction. **Analysis of Options:** * **Option A (A fibers):** These are large, myelinated fibers. Due to their large diameter and thick myelin sheath, they are the **least sensitive** (most resistant) to local anesthesia. Within this group, A-alpha (motor) are the hardest to block, while A-delta (pain/temperature) are blocked earlier. * **Option B (B fibers):** These are small, preganglionic autonomic myelinated fibers. While they are very sensitive (often blocked even before C fibers in clinical practice due to their anatomical location), in standard physiological testing based on fiber diameter alone, C fibers are considered the most sensitive. **High-Yield Clinical Pearls for NEET-PG:** 1. **Order of Blockade (Clinical):** Autonomic (B) > Pain/Temperature (C & A-delta) > Touch/Pressure (A-beta) > Motor (A-alpha). 2. **Order of Recovery:** Exactly the reverse of the blockade. 3. **Sensitivity to Hypoxia:** B fibers > A fibers > C fibers (C fibers are most resistant to pressure and hypoxia). 4. **Sensitivity to Pressure:** A fibers > B fibers > C fibers. 5. **Rule of Thumb:** Smaller diameter and lack of myelin generally increase sensitivity to local anesthetics.

Q: What is the resting membrane potential of a muscle fiber?

-90mV. The Resting Membrane Potential (RMP) is the electrical potential difference across the cell membrane when the cell is at rest. In skeletal muscle fibers, the RMP is typically **-90 mV**. ### Why -90 mV is Correct The RMP is primarily determined by the equilibrium potential of Potassium ($K^+$). According to the Nernst equation, the equilibrium potential for $K^+$ is approximately -94 mV. Because the resting muscle membrane is highly permeable to $K^+$ (via leak channels) and relatively impermeable to $Na^+$, the RMP sits very close to the $K^+$ equilibrium potential. The slight difference (-90 mV vs -94 mV) is due to a minor influx of $Na^+$ and the electrogenic contribution of the $Na^+-K^+$ ATPase pump. ### Analysis of Incorrect Options * **-70 mV (Option B):** This is the typical RMP for **large neurons**. Neurons have a slightly higher RMP than muscle fibers, making them more excitable. * **-60 mV (Option A):** This is closer to the RMP of **smooth muscle cells** (ranging from -50 to -60 mV) or the threshold potential for firing an action potential in many excitable tissues. * **-80 mV (Option C):** While some cardiac cells or specific fibers may show this value, -90 mV is the standard physiological value cited for skeletal muscle and ventricular myocytes. ### High-Yield NEET-PG Pearls * **Skeletal Muscle vs. Nerve:** Muscle fibers have a more negative RMP (-90 mV) compared to nerves (-70 mV). * **Ionic Basis:** $K^+$ efflux is the most important factor in establishing RMP. * **Maintenance:** The $Na^+-K^+$ ATPase pump maintains the concentration gradients but only contributes about -4 to -5 mV directly to the RMP. * **Clinical Correlation:** Hypokalemia hyperpolarizes the membrane (makes it more negative), making it harder to trigger an action potential, leading to muscle weakness.

Question 1

Where does the initiation of an impulse start?

Accepted Answer

Axon hillock and initial segment

Answer

Axon

Answer

Cell body

Answer

Dendritic tree

Question 2

All are true about local potential except?

Accepted Answer

Summation is not possible

Answer

Does not follow the all-or-none law

Answer

Can be depolarized or hyperpolarized

Answer

Is proportional to stimulus strength

Question 3

Which of the following statements about smooth muscle is false?

Accepted Answer

The musculature of the uterus is made of multi-unit smooth muscle.

Answer

Ca2+ is the excitation-contraction coupler.

Answer

The "latch" mechanism facilitates prolonged holding of contractions of smooth muscle.

Answer

Shows the property of plasticity.

Question 4

In an isotonic contraction of the muscle, what occurs?

Accepted Answer

The muscle moves a load through a distance.

Answer

The muscle is not able to move the load.

Answer

The latent period of contraction is shorter than in isometric contraction.

Answer

The length of the muscle is increased.

Question 5

Which nerve fiber exhibits the slowest velocity of conduction?

Accepted Answer

C fibers

Answer

Alpha (Aa)

Answer

Alpha-delta (Aδ)

Answer

Beta (Aβ)

Question 6

Tapping the tendon of a muscle may elicit what response?

Accepted Answer

Contraction of the muscle

Answer

Relaxation of the muscle

Answer

No response

Answer

Fibrillation of the muscle

Question 7

Which fibres are most sensitive to local anesthesia?

Accepted Answer

C fibres

Answer

A fibres

Answer

B fibres

Answer

None of the above

Question 8

What is the resting membrane potential of a muscle fiber?

Accepted Answer

-90mV

Answer

-60mV

Answer

-70mV

Answer

-80mV

Question 9

Repolarization of a neuron is due to which of the following ionic movements?

Accepted Answer

Potassium influx

Answer

Sodium influx

Answer

Potassium efflux

Answer

All of the above

Question 10

Mutation of the gene coding for ryanodine receptors is implicated in malignant hyperthermia. Which of the following statements best explains the increased heat production in malignant hyperthermia?

Accepted Answer

Increased muscle metabolism due to excess calcium ions

Answer

Thermic effect of food

Answer

Increased sympathetic discharge

Answer

Mitochondrial thermogenesis

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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