Nerve and Muscle Physiology Practice Questions

Q: Miss X lifts weights. When she contracts her biceps muscle, all of the following parts of the muscle shorten EXCEPT the:

A bands. This question tests your understanding of the **Sliding Filament Theory** of muscle contraction. ### 1. Why the Correct Answer (A Band) is Right According to the Sliding Filament Theory, muscle contraction occurs when thin (actin) filaments slide over thick (myosin) filaments. * The **A band** represents the entire length of the **thick (myosin) filaments**. * Since the filaments themselves do not change in length—they merely slide past one another—the A band remains **constant** during both contraction and relaxation. ### 2. Why the Other Options are Incorrect * **Sarcomeres (Option A):** The sarcomere is the functional unit of contraction, defined as the distance between two Z lines. As filaments slide, the sarcomere length decreases. * **I bands (Option B):** The I band consists only of thin (actin) filaments. During contraction, actin filaments are pulled toward the center of the sarcomere (M line), increasing the zone of overlap and causing the I band to **shorten**. * **Inter-Z line distance (Option D):** This is synonymous with the sarcomere length. As the muscle contracts, Z lines are pulled closer together, thus the distance **shortens**. ### 3. High-Yield NEET-PG Pearls * **What shortens:** Sarcomere, I band, H zone, and Inter-Z line distance. * **What remains constant:** A band, length of thick filaments, and length of thin filaments. * **The H zone** (the central part of the A band containing only myosin) disappears during maximal contraction. * **Mnemonic:** "**HI**" disappears (**H** zone and **I** band shorten), but the "**A**" stays the same (**A** band). * **Energy Requirement:** ATP is required for both contraction (cross-bridge cycling) and relaxation (calcium reuptake into the sarcoplasmic reticulum).

Q: What is the resting membrane potential (RMP) of smooth muscle?

-50 mV. **Explanation:** The resting membrane potential (RMP) of **smooth muscle** is typically less negative than that of skeletal muscle or neurons, generally ranging between **-50 mV and -60 mV**. The correct option is **-50 mV**. **Why -50 mV is correct:** Smooth muscle cells have a higher permeability to sodium ($Na^+$) and calcium ($Ca^{2+}$) ions at rest compared to skeletal muscles. Additionally, the $Na^+$-$K^+$ pump in smooth muscle is less active in maintaining a high gradient. This results in a "less negative" or more depolarized RMP, which allows these muscles to be more easily excited by hormonal or mechanical stimuli. **Analysis of Incorrect Options:** * **-75 mV:** This is closer to the RMP of **cardiac atrial cells** or certain large nerve fibers. * **-90 mV (Option C):** This is the characteristic RMP for **skeletal muscle fibers** and **ventricular cardiomyocytes**. It is highly negative due to high resting permeability to potassium ($K^+$). * **-35 mV (Option D):** This is too depolarized for a stable RMP; however, it may be reached during the peak of "slow wave" potentials before an action potential is triggered. **NEET-PG High-Yield Pearls:** 1. **Instability:** Unlike skeletal muscle, the RMP of smooth muscle is often unstable (e.g., **Slow Waves** or Pacemaker potentials in the gut). 2. **Action Potential:** In most smooth muscles, the upstroke of the action potential is caused by the influx of **Calcium ($Ca^{2+}$)** rather than Sodium. 3. **L-type Ca channels:** These are the primary channels involved in smooth muscle contraction and are the targets for Calcium Channel Blockers (CCBs). 4. **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle cells are connected by **gap junctions**, allowing them to contract as a single unit (syncytium).

Q: The phase of after depolarization in cardiac action potential is characterized by which of the following?

Slower net exit of K+. ### Explanation In the cardiac action potential (specifically the ventricular muscle fiber), the **Phase 3 (Rapid Repolarization)** is primarily driven by the efflux of $K^+$ ions through voltage-gated potassium channels. **Why Option D is Correct:** As repolarization nears completion, the membrane potential approaches the resting level. During the terminal portion of this phase, there is a **slower net exit of $K^+$**. This occurs because the driving force for potassium decreases as the membrane potential gets closer to the equilibrium potential for $K^+$. This deceleration in $K^+$ efflux leads to a "tailing off" effect known as **after-depolarization** (or the terminal phase of repolarization), before the membrane stabilizes at Phase 4. **Analysis of Incorrect Options:** * **Option A & B:** Sodium ($Na^+$) influx is the hallmark of **Phase 0 (Depolarization)**. By the time the cell reaches the after-depolarization stage, $Na^+$ channels are inactivated (refractory period). $Na^+$ influx above zero potential specifically refers to the "overshoot" during Phase 0. * **Option C:** While $K^+$ channels eventually close to return to the resting state, the specific phenomenon of after-depolarization is defined by the **rate of ion movement (flux)** rather than the mechanical closure speed of the channels themselves. ### High-Yield Clinical Pearls for NEET-PG: * **Phase 2 (Plateau Phase):** Unique to cardiac muscle; caused by a balance between $Ca^{2+}$ influx (L-type channels) and $K^+$ efflux. * **Refractory Period:** The absolute refractory period (ARP) in cardiac muscle lasts until mid-Phase 3, preventing tetany and allowing the heart to act as a pump. * **Early After-Depolarizations (EADs):** Occur during Phase 2 or 3; if they reach threshold, they can trigger "Torsades de Pointes," especially in Long QT Syndrome.

Q: What is a miniature end plate potential (MEPP)?

Recorded in resting muscle. **Explanation:** A **Miniature End Plate Potential (MEPP)** is a small, spontaneous depolarization of the muscle post-synaptic membrane. **1. Why the correct answer is right:** MEPPs are **recorded in resting muscle** because they occur without any nerve stimulation. They are caused by the random, spontaneous release of a **single quantum** (one vesicle) of acetylcholine (ACh) from the presynaptic terminal into the neuromuscular junction. Since these occur randomly at rest, they do not trigger an action potential. **2. Why the incorrect options are wrong:** * **Option A:** Anticholinesterases (like Neostigmine) inhibit the enzyme that breaks down ACh. This actually **increases** the amplitude and duration of MEPPs, rather than inhibiting them. * **Option C:** MEPPs are **sub-threshold** potentials. A single MEPP is too small to reach the threshold required to open voltage-gated sodium channels; therefore, it cannot cause muscle excitation or contraction. Excitation requires an End Plate Potential (EPP), which is the summation of multiple MEPPs. * **Option D:** The typical amplitude of a MEPP is approximately **0.5 to 1.0 mV**. An amplitude of 5 to 10 mV is too high for a single quantum. **High-Yield Clinical Pearls for NEET-PG:** * **Quantal Release:** The MEPP is the smallest unit of electrical activity at the NMJ, representing the "quantum" nature of neurotransmitter release. * **Calcium Independence:** While normal EPPs are strictly calcium-dependent, spontaneous MEPPs can occur even in low-calcium environments. * **Clinical Correlation:** In **Lambert-Eaton Myasthenic Syndrome**, the *frequency* of MEPPs may be normal, but the EPP is reduced due to antibodies against presynaptic Ca²⁺ channels. In **Myasthenia Gravis**, the *amplitude* of MEPPs is reduced due to the destruction of post-synaptic ACh receptors.

Question 1

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

Question 2

Miss X lifts weights. When she contracts her biceps muscle, all of the following parts of the muscle shorten EXCEPT the:

Accepted Answer

A bands

Answer

Sarcomeres

Answer

I bands

Answer

Inter-Z line distance

Question 3

True about nerve impulse is:

Accepted Answer

Travels in one direction at a synapse

Answer

Travels in one direction along an axon

Answer

If the current is increased too slowly, the nerve responds faster

Answer

Travels with the speed of electric current

Question 4

What is the resting membrane potential (RMP) of smooth muscle?

Accepted Answer

-50 mV

Answer

-75 mV

Answer

-90 mV

Answer

-35 mV

Question 5

The phase of after depolarization in cardiac action potential is characterized by which of the following?

Accepted Answer

Slower net exit of K+

Answer

Delayed influx of Na+

Answer

Na+ influx above zero potential

Answer

Slow closure of K+ channels

Question 6

What is a miniature end plate potential (MEPP)?

Accepted Answer

Recorded in resting muscle

Answer

Inhibited by anticholinesterases

Answer

Responsible for excitation of the muscle

Answer

Of 5 to 10 mV amplitude

Question 7

A travelling nerve impulse does not depolarize the area immediately behind it, because:

Accepted Answer

It is refractory

Answer

It is hyperpolarized

Answer

It is not self-propagating

Answer

The condition is always orthodromic

Question 8

Which one of the following is absent in smooth muscles?

Accepted Answer

Troponin

Answer

Actin

Answer

Myosin

Answer

Myosin adenine triphosphatase

Question 9

Which of the following statements is FALSE regarding the stretch reflex?

Accepted Answer

Relaxation of the muscle is the response.

Answer

Stretch of the muscle initiates the reflex.

Answer

The muscle spindle is the sense organ.

Answer

Impulses are conducted by fast sensory fibers.

Question 10

If a supramaximal stimulus is applied to an excitable tissue like a nerve or a muscle and it elicits a response, then the tissue is said to be in which state?

Accepted Answer

Relative refractory period

Answer

Absolute refractory period

Answer

Latent period

Answer

After-depolarization

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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