Nerve and Muscle Physiology Practice Questions

Q: After the application of one stimulus, the time period during which a second, stronger stimulus can evoke an impulse is called?

Relative refractory period. ### Explanation **Correct Answer: B. Relative Refractory Period** The **Relative Refractory Period (RRP)** is the interval following the Absolute Refractory Period during which a second action potential can be generated, but only if the stimulus is **stronger than the initial threshold stimulus**. * **Mechanism:** During this phase, voltage-gated $Na^+$ channels have begun to recover from their inactivated state (transitioning back to the closed/resting state), and $K^+$ channels are wide open (causing hyperpolarization). Because the membrane is further from the threshold and some $Na^+$ channels are still inactive, a suprathreshold stimulus is required to trigger a response. **Why other options are incorrect:** * **A. Absolute Refractory Period (ARP):** This is the period from the start of the upstroke until roughly one-third of repolarization. During this time, **no stimulus**, regardless of strength, can excite the nerve because $Na^+$ channels are either already open or in an inactivated state. * **C. Latent Period:** This is the time delay between the application of a stimulus and the first measurable response (e.g., the time between nerve stimulation and muscle contraction). * **D. Local Response:** This refers to a non-propagated, graded potential (sub-threshold) that does not obey the "All-or-None" law. **High-Yield NEET-PG Pearls:** * **ARP** determines the **maximum frequency** of impulse discharge in a neuron. * The ARP corresponds to the period from the firing level until repolarization is about **one-third complete**. * In **cardiac muscle**, the refractory period is significantly longer than in skeletal muscle, which prevents **tetanization** of the heart. * **Accommodation:** If a nerve is subjected to a slowly increasing constant current, the threshold rises; this is due to the inactivation of $Na^+$ channels during the slow depolarization.

Q: After an injury to an axon, the subsequent degeneration of all axonal fibres distal to the injury, with all fibres proximal to the injury remaining unaffected, is known as which type of degeneration?

Wallerian degeneration.. **Explanation:** **Wallerian Degeneration (Orthograde Degeneration)** refers to the pathological changes that occur in the **distal segment** of an axon following a traumatic injury or transection. Since the distal portion is separated from the neuronal cell body (the metabolic center), it loses its supply of essential proteins and nutrients. This leads to the disintegration of the axon and the breakdown of the myelin sheath, which are subsequently cleared by macrophages and Schwann cells. **Analysis of Incorrect Options:** * **Retrograde Degeneration:** This refers to pathological changes occurring **proximal** to the site of injury (towards the cell body). It involves the breakdown of the axon up to the first node of Ranvier and characteristic changes in the cell body known as *chromatolysis* (swelling of the cell body and displacement of the nucleus). * **Transneuronal Degeneration:** This occurs when the death of one neuron leads to the degeneration of another neuron with which it synapses. It can be anterograde (affecting the postsynaptic neuron) or retrograde (affecting the presynaptic neuron). **High-Yield NEET-PG Pearls:** * **Chromatolysis:** The hallmark of retrograde degeneration in the cell body; involves the disappearance of Nissl granules (RER). * **Regeneration:** In the Peripheral Nervous System (PNS), Schwann cells form **Bungner bands**, which act as physical guides for regenerating axonal sprouts. * **Rate of Growth:** Regenerating nerve fibers typically grow at a rate of **1–4 mm/day**. * **Blood-Brain Barrier:** Wallerian degeneration is slower in the CNS compared to the PNS because oligodendrocytes do not provide the same growth-promoting environment as Schwann cells.

Q: Which of the following statements regarding skeletal and cardiac muscle is false?

Both have graded contractions.. ### Explanation The correct answer is **C**, as the statement "Both have graded contractions" is false. **1. Why Option C is False (The Correct Answer):** Skeletal muscle fibers follow the **"All-or-None Law"** at the cellular level. However, the muscle as a whole can produce graded contractions through motor unit recruitment and frequency summation. In contrast, **cardiac muscle** functions as a **functional syncytium** due to gap junctions in intercalated discs. When one cell is excited, the entire myocardium contracts as a single unit. Therefore, cardiac muscle cannot recruit additional motor units; it follows the all-or-none law for the entire organ, making "graded contractions" (in the sense of recruitment) impossible for the heart. **2. Analysis of Incorrect Options:** * **Option A:** Both muscles require calcium to bind to **Troponin C**, which moves tropomyosin and allows actin-myosin cross-bridge formation. * **Option B:** In skeletal muscle, the Dihydropyridine (DHP) receptor acts as a voltage sensor physically coupled to the Ryanodine receptor (RyR1). It triggers calcium release from the Sarcoplasmic Reticulum (SR) without needing extracellular calcium influx. Thus, it is independent of extracellular $Ca^{2+}$. * **Option D:** Cardiac muscle has a prolonged **Plateau Phase (Phase 2)** caused by the opening of L-type $Ca^{2+}$ channels, which prevents tetany and allows for adequate ventricular filling. **High-Yield NEET-PG Pearls:** * **Trigger for $Ca^{2+}$ release:** In skeletal muscle, it is **Voltage-gated** (electromechanical coupling). In cardiac muscle, it is **Calcium-Induced Calcium Release (CICR)** via RyR2. * **Refractory Period:** Cardiac muscle has a very long absolute refractory period (250ms), making it **impossible to tetanize**, unlike skeletal muscle. * **Source of $Ca^{2+}$:** Skeletal muscle relies 100% on the SR; Cardiac muscle relies on both the SR and extracellular fluid (ECF).

Q: Which of the following decreases in length during the contraction of a skeletal muscle fiber?

I band of the sarcomere. ### Explanation The mechanism of skeletal muscle contraction is best explained by the **Sliding Filament Theory**. According to this theory, muscle contraction occurs when thin (actin) filaments slide over thick (myosin) filaments, pulling the Z-lines closer together. #### Why the Correct Answer is Right: * **I band (Isotropic band):** This region contains only thin filaments. During contraction, as the thin filaments slide toward the center of the sarcomere (H-zone), the distance between the thick filaments of adjacent sarcomeres decreases. Consequently, the **I band shortens** and may even disappear during maximal contraction. * **H zone:** Although not an option here, it is important to note that the H zone (containing only thick filaments) also decreases in length or disappears. #### Why the Other Options are Wrong: * **A band (Anisotropic band):** This represents the entire length of the thick filaments. Since the filaments themselves do not shrink or fold but merely slide, the **A band remains constant** in length. * **Thick and Thin Filaments:** The individual protein filaments (myosin and actin) **do not change their physical length**. Their appearance of "shortening" the sarcomere is due to an increase in their degree of overlap, not a change in their actual dimensions. #### High-Yield Clinical Pearls for NEET-PG: * **Sarcomere:** Defined as the distance between two **Z-lines**; it is the functional unit of contraction and shortens during muscle activity. * **Mnemonic:** During contraction, **"HI"** (H-zone and I-band) **shortens**, while the **A-band** stays the **Same**. * **Titans:** The protein **Titin** acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the muscle. * **Calcium Binding:** Contraction is initiated when $Ca^{2+}$ binds to **Troponin C**, leading to the displacement of tropomyosin and exposing the myosin-binding sites on actin.

Q: The 'all or none' law refers to which physiological phenomenon?

Action potential. The **'All-or-None' Law** is a fundamental principle in neurophysiology stating that if a stimulus is strong enough to reach the **threshold potential** (typically -55mV), an action potential of maximal and constant amplitude is triggered. If the stimulus is sub-threshold, no action potential occurs at all. ### Why Action Potential is Correct: The action potential is an active, regenerative electrical event. Once the threshold is crossed, voltage-gated sodium channels open in a positive feedback loop (Hodgkin cycle). The resulting depolarization does not vary in size based on the strength of the stimulus; instead, a stronger stimulus increases the **frequency** of firing, not the **amplitude** of the individual spike. ### Why Other Options are Incorrect: * **Resting Membrane Potential (RMP):** This is a static state (typically -70mV in neurons) maintained by ion leak channels and the Na+/K+ ATPase pump. It is a baseline condition, not a triggered response that follows an all-or-none rule. * **Membrane Potential:** This is a general term for the voltage difference across a membrane. It includes **graded potentials** (like EPSPs or IPSPs), which do *not* follow the all-or-none law; their magnitude is proportional to the stimulus intensity and they decay over distance. ### NEET-PG High-Yield Pearls: * **Applicability:** The All-or-None law applies to **individual nerve fibers** and **individual muscle fibers**. It does **not** apply to a whole nerve trunk or a whole skeletal muscle, which show "graded" responses due to the recruitment of multiple motor units. * **Exception:** Cardiac muscle as a whole (syncytium) follows the All-or-None law because of gap junctions. * **Key Distinction:** Action potentials are **non-decremental** (do not lose strength over distance), whereas graded potentials are **decremental**.

Q: Magnitude of action potential is determined by:

Na+. **Explanation:** The magnitude (amplitude) of an action potential is primarily determined by the **extracellular concentration of Sodium (Na+)**. 1. **Why Na+ is correct:** According to the **Hodgkin-Huxley model**, the rising phase of an action potential is caused by a rapid influx of Na+ ions through voltage-gated channels. The peak of the action potential tends to approach the **Equilibrium Potential of Sodium (ENa)**, which is approximately +60 mV. If the extracellular Na+ concentration decreases, the concentration gradient weakens, leading to a lower peak and reduced magnitude of the action potential. 2. **Why other options are incorrect:** * **K+ (Potassium):** While K+ is crucial for establishing the **Resting Membrane Potential (RMP)** and mediating repolarization, it does not determine the peak magnitude of the depolarization phase. * **Ca++ (Calcium):** Extracellular calcium levels primarily affect the **threshold** for firing an action potential. Low Ca++ (hypocalcemia) makes the nerve more excitable (tetany) by lowering the threshold, but it doesn't dictate the total magnitude. * **Mg++ (Magnesium):** Magnesium acts as a physiological calcium channel blocker and stabilizes membranes, but it is not the primary ion responsible for the action potential spike. **High-Yield NEET-PG Pearls:** * **Amplitude vs. Frequency:** Nerve signals use **Frequency Coding**. The magnitude (amplitude) of an action potential is constant for a given nerve fiber (All-or-None Law); only the frequency of firing changes with stimulus intensity. * **Hyponatremia:** Severe hyponatremia can decrease action potential amplitude, potentially leading to neurological symptoms. * **Tetrodotoxin (Pufferfish):** Blocks voltage-gated Na+ channels, completely abolishing the action potential magnitude.

Question 1

In muscle cells, resting membrane potential is equal to the isoelectric potential of which ion?

Accepted Answer

K+

Answer

Na+

Answer

Cl-

Answer

Mg++

Question 2

After the application of one stimulus, the time period during which a second, stronger stimulus can evoke an impulse is called?

Accepted Answer

Relative refractory period

Answer

Absolute refractory period

Answer

Latent period

Answer

Local response

Question 3

A 17-year-old soccer player suffered a fracture to the left tibia. After his lower leg has been in a cast for 8 weeks, he found that the left gastrocnemius muscle is significantly smaller in circumference than it was before the fracture. What is the most likely explanation?

Accepted Answer

Temporary reduction in actin and myosin protein synthesis.

Answer

Decrease in the number of individual muscle fibers in the muscle.

Answer

Decrease in blood flow to the muscle caused by constriction from the cast.

Answer

Progressive denervation.

Question 4

The length of a muscle at which tension is maximum is called:

Accepted Answer

Optimal length

Answer

Active length

Answer

Resting length

Answer

Maximal length

Question 5

After an injury to an axon, the subsequent degeneration of all axonal fibres distal to the injury, with all fibres proximal to the injury remaining unaffected, is known as which type of degeneration?

Accepted Answer

Wallerian degeneration.

Answer

Retrograde degeneration.

Answer

Transneuronal degeneration.

Answer

None of the above.

Question 6

Which of the following statements regarding skeletal and cardiac muscle is false?

Accepted Answer

Both have graded contractions.

Answer

Both require calcium for excitation-contraction coupling.

Answer

The excitation-contraction coupling of skeletal muscle is independent of extracellular calcium.

Answer

A plateau phase is seen in the action potential of cardiac muscle.

Question 7

Which of the following decreases in length during the contraction of a skeletal muscle fiber?

Accepted Answer

I band of the sarcomere

Answer

A band of the sarcomere

Answer

Thick filaments

Answer

Thin filaments

Question 8

An action potential is produced by which of the following ionic movements?

Accepted Answer

Sodium influx

Answer

Sodium efflux

Answer

Potassium influx

Answer

Potassium efflux

Question 9

The 'all or none' law refers to which physiological phenomenon?

Accepted Answer

Action potential

Answer

Resting potential

Answer

Membrane potential

Answer

None of the above

Question 10

Magnitude of action potential is determined by:

Accepted Answer

Na+

Answer

Mg++

Answer

K++

Answer

Ca++

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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