Nerve and Muscle Physiology Practice Questions

Q: Which of the following best describes an attribute of visceral smooth muscle that is not shared by skeletal muscle?

Contracts in response to stretch. **Explanation:** The correct answer is **B. Contracts in response to stretch.** **Why it is correct:** Visceral (unitary) smooth muscle, found in the walls of hollow organs like the gut and ureters, exhibits a unique property called **stress-relaxation** and the **myogenic response**. When these muscles are stretched, mechanically gated calcium channels open, leading to depolarization and subsequent contraction. This allows organs to move contents forward (peristalsis) or maintain tone despite distension. In contrast, skeletal muscle is neurogenic; it requires a motor nerve impulse (ACh release) to initiate contraction and does not contract simply by being stretched. **Why the other options are incorrect:** * **A. Contraction is ATP dependent:** This is a shared feature. Both skeletal and smooth muscles require ATP for cross-bridge cycling and for the sequestration of calcium by the SERCA pump. * **C. Does not contain actin filaments:** This is incorrect. Both muscle types contain actin (thin) and myosin (thick) filaments. While smooth muscle lacks organized sarcomeres and troponin, it relies heavily on actin-myosin interaction for force generation. * **D. High rate of cross-bridge cycling:** This is a characteristic of **skeletal muscle**. Smooth muscle is known for its **low** rate of cross-bridge cycling, which allows for the "Latch State"—a mechanism where it maintains prolonged tension with minimal energy (ATP) consumption. **High-Yield NEET-PG Pearls:** * **Latch State:** Unique to smooth muscle; allows for sustained contraction (e.g., vascular tone) without fatigue. * **Calmodulin:** Smooth muscle lacks troponin; calcium binds to Calmodulin, which then activates **Myosin Light Chain Kinase (MLCK)** to initiate contraction. * **Caveolae:** These are the functional equivalents of T-tubules in smooth muscle.

Q: Calcium initiates skeletal muscle contraction by which mechanism?

Binding to troponin. **Explanation:** In skeletal muscle, contraction is regulated by the **Troponin-Tropomyosin complex** located on the actin (thin) filament. **1. Why the correct answer is right:** When an action potential reaches the muscle fiber, it triggers the release of Calcium ($Ca^{2+}$) from the **Sarcoplasmic Reticulum (SR)** into the sarcoplasm. This $Ca^{2+}$ binds specifically to **Troponin C**. This binding induces a conformational change in the entire troponin complex, which physically pulls the **tropomyosin** molecule away from the active sites on the actin filament. This exposes the binding sites, allowing the myosin heads to form cross-bridges with actin, initiating the power stroke. **2. Why the incorrect options are wrong:** * **Option A:** Calcium binds to Troponin, not Tropomyosin. Tropomyosin’s role is to physically mask the binding sites; it moves only after Troponin C is activated. * **Option B:** In skeletal muscle, the primary source of $Ca^{2+}$ is the **Sarcoplasmic Reticulum**, not the sarcolemma (extracellular fluid). This is a key distinction from cardiac muscle, which requires extracellular $Ca^{2+}$ influx. * **Option D:** Calcium *uncovers* the binding sites; it does not cover them. Tropomyosin is the protein that covers the sites in a relaxed state. **High-Yield Clinical Pearls for NEET-PG:** * **Troponin Subunits:** Remember **T** (binds to **T**ropomyosin), **I** (**I**nhibitory; binds to actin), and **C** (binds to **C**alcium). * **Rigor Mortis:** Occurs due to the lack of ATP, which is required for the *detachment* of myosin from actin. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR1)**, leading to excessive $Ca^{2+}$ release from the SR. * **Dihydropyridine Receptor (DHPR):** Acts as a voltage sensor in the T-tubule that triggers the RyR1.

Q: Which muscles are primarily used during the stance and swing phases of normal walking?

Gastrocnemius. **Explanation:** The gait cycle consists of two main phases: the **Stance phase** (60%) and the **Swing phase** (40%). Efficient walking requires coordinated muscle activation to provide propulsion and clearance. **Why Gastrocnemius is the correct answer:** The **Gastrocnemius** (along with the Soleus) is the primary muscle responsible for the **"Push-off"** or "Toe-off" stage at the end of the stance phase. It performs powerful plantarflexion, providing the necessary forward propulsive force to transition the limb into the swing phase. While many muscles contribute to walking, the plantarflexors are the chief contributors to the mechanical work required for forward progression. **Analysis of Incorrect Options:** * **Popliteus:** Known as the "Key to the knee," its primary role is to **unlock the knee** by medially rotating the tibia on the femur (or laterally rotating the femur on the tibia) to initiate flexion from a fully extended position. It is not a primary locomotor muscle. * **Tibialis Anterior:** This muscle is most active during the **initial swing phase** (for foot clearance/dorsiflexion) and at **heel strike** (to eccentrically control foot drop). It does not provide the primary power for the stance-to-swing transition. * **Iliopsoas:** This is a powerful hip flexor. While it helps initiate the swing phase by accelerating the thigh forward, it is not considered the primary driver of the gait cycle compared to the propulsive power of the posterior compartment of the leg. **High-Yield Clinical Pearls for NEET-PG:** * **Trendelenburg Gait:** Caused by weakness of Gluteus Medius/Minimus (hip abductors). * **Foot Drop:** Caused by paralysis of the Tibialis Anterior (Common Peroneal Nerve injury). * **Stance Phase Stages:** Heel strike → Foot flat → Mid-stance → Heel-off → Toe-off. * **Energy Efficiency:** The center of gravity follows a sinusoidal curve during walking to minimize energy expenditure.

Q: The action of acetylcholine at the neuromuscular junction is terminated primarily by?

Enzymatic breakdown by acetylcholinesterase. ### Explanation **1. Why Option D is Correct:** The primary mechanism for terminating the action of Acetylcholine (ACh) at the neuromuscular junction (NMJ) is **rapid enzymatic hydrolysis**. The enzyme **Acetylcholinesterase (AChE)** is concentrated in the synaptic cleft, specifically within the basal lamina. It breaks down ACh into **Choline and Acetate**. This process is incredibly efficient (one of the fastest enzymatic reactions in the body), ensuring that the muscle fiber can be repolarized quickly to respond to subsequent nerve impulses. **2. Why Other Options are Incorrect:** * **Option A (Diffusion):** While a small amount of ACh may diffuse away from the cleft, it is a minor contributor compared to the rapid enzymatic destruction. * **Option B (Uptake into nerve ending):** Unlike other neurotransmitters (like Norepinephrine or Serotonin), **ACh itself is not reuptaken**. Only the byproduct **Choline** is transported back into the presynaptic terminal via a Na⁺-choline symporter to synthesize new ACh. * **Option C (Uptake into muscle):** Muscle cells do not possess transport mechanisms to "uptake" intact ACh molecules. **3. NEET-PG High-Yield Clinical Pearls:** * **Myasthenia Gravis:** Caused by antibodies against post-synaptic ACh receptors. It is treated with **AChE inhibitors** (e.g., Neostigmine, Pyridostigmine) to increase the concentration and duration of ACh in the cleft. * **Organophosphate Poisoning:** These compounds irreversibly inhibit AChE, leading to a "cholinergic crisis" (SLUDGE syndrome) due to persistent ACh action. * **Pseudocholinesterase (Butyrylcholinesterase):** Found in the plasma (not the NMJ). It is responsible for the metabolism of drugs like **Succinylcholine**. Deficiency leads to prolonged apnea after anesthesia. * **Hemicholinium:** A drug that blocks the reuptake of Choline into the nerve terminal, depleting ACh stores.

Q: Which of the following muscles has the maximum density of white muscle fibers?

Extraocular muscles. **Explanation:** The classification of muscle fibers into **Red (Type I)** and **White (Type II)** is based on their metabolic profile and contraction speed. White muscle fibers (Type II) are characterized by high myosin ATPase activity, low myoglobin content, and a reliance on anaerobic glycolysis. They are designed for **rapid, short bursts of activity** but fatigue easily. **Why Extraocular Muscles are correct:** The extraocular muscles (EOMs) require the fastest contraction speeds in the human body to perform saccadic eye movements. Consequently, they possess the highest density of **Type II (White) fibers**. These muscles are specialized for precision and extreme velocity rather than sustained weight-bearing or posture, making them the classic example of white muscle predominance. **Analysis of Incorrect Options:** * **Calf Muscle (Gastrocnemius/Soleus):** The Soleus is a classic "Red" muscle (Type I) used for maintaining standing posture. While the Gastrocnemius has mixed fibers, it does not match the white fiber density of the EOMs. * **Back Muscles (Erector Spinae):** These are "Antigravity" muscles. They consist predominantly of Type I (Red) fibers because they must remain contracted for long periods to maintain posture without fatiguing. * **Hip Muscles (Gluteus Maximus):** These are large, powerful muscles used for locomotion and posture. They contain a high proportion of Type I fibers to support sustained activity. **High-Yield Clinical Pearls for NEET-PG:** * **Type I (Red):** "One Slow Red Ox" — Type **I**, **Slow**-twitch, **Red** (high myoglobin), **Ox**idative phosphorylation (high mitochondria). Found in marathon runners. * **Type II (White):** Fast-twitch, glycolytic, low mitochondria. Found in sprinters and extraocular muscles. * **Laryngeal muscles** also have a high density of fast-twitch fibers, second only to extraocular muscles.

Q: Energy in muscles is stored as:

Phosphocreatine. **Explanation:** The primary form of energy **storage** in skeletal muscle is **Phosphocreatine (Creatine Phosphate)**. While ATP is the direct source of energy for muscle contraction, its concentration within the muscle cell is very low—sufficient only to sustain maximal contraction for about 1 to 2 seconds. To maintain activity, muscles store high-energy phosphate bonds in the form of Phosphocreatine, which is present at concentrations **5 to 6 times higher than ATP**. * **Why Phosphocreatine is correct:** It acts as a "buffer" or a reservoir of high-energy phosphate. When ATP is hydrolyzed to ADP during contraction, the enzyme **Creatine Kinase (CK)** rapidly transfers a phosphate group from Phosphocreatine back to ADP to reconstitute ATP. This system (the Phosphagen system) provides energy for short bursts of maximal effort (approx. 5–8 seconds). * **Why ATP is incorrect:** ATP is the immediate energy *currency*, not the storage form. Because ATP is a potent allosteric effector of many enzymes, storing it in high concentrations would disrupt cellular metabolism. * **Why ADP is incorrect:** ADP is a metabolic byproduct of energy utilization. High levels of ADP signal a low-energy state in the cell and trigger pathways like glycolysis to produce more ATP. **High-Yield Clinical Pearls for NEET-PG:** 1. **Lohmann’s Reaction:** The reversible transfer of phosphate between creatine and ATP catalyzed by Creatine Kinase. 2. **Creatinine:** A waste product formed by the non-enzymatic breakdown of phosphocreatine. Its excretion rate is relatively constant and proportional to total muscle mass. 3. **Order of Energy Source Utilization:** ATP (1-2s) → Phosphocreatine (5-8s) → Muscle Glycogen (Anaerobic glycolysis) → Oxidative phosphorylation.

Question 1

Increased velocity of conduction in a nerve is caused by:

Accepted Answer

Increased capacitance

Answer

Decreased capacitance

Answer

Increased resistance

Answer

All

Question 2

Which of the following best describes an attribute of visceral smooth muscle that is not shared by skeletal muscle?

Accepted Answer

Contracts in response to stretch

Answer

Contraction is ATP dependent

Answer

Does not contain actin filaments

Answer

High rate of cross-bridge cycling

Question 3

Calcium initiates skeletal muscle contraction by which mechanism?

Accepted Answer

Binding to troponin

Answer

Binding to tropomyosin

Answer

Being released primarily from the sarcolemma during an action potential

Answer

Covering actin binding sites

Question 4

Which muscles are primarily used during the stance and swing phases of normal walking?

Accepted Answer

Gastrocnemius

Answer

Popliteus

Answer

Tibialis anterior

Answer

Iliopsoas

Question 5

Two electrodes are placed 4.5 cm apart. It takes 1.5 ms for current to propagate along the nerve from one electrode to the other. What is the velocity of nerve conduction?

Accepted Answer

30 m/s

Answer

60 m/s

Answer

45 m/s

Answer

80 m/s

Question 6

The action of acetylcholine at the neuromuscular junction is terminated primarily by?

Accepted Answer

Enzymatic breakdown by acetylcholinesterase

Answer

Diffusion into the surrounding extracellular fluid

Answer

Uptake into the nerve ending

Answer

Uptake into the muscle

Question 7

What is the nerve impulse that initiates muscle contraction?

Accepted Answer

It causes the opening of calcium channels, leading to an increase in calcium ion concentration within the sarcomere.

Answer

It causes both the plasma membrane and the T-tubules to undergo hyperpolarization.

Answer

It inhibits sodium entry into the sarcomere.

Answer

It is inhibited by the binding of acetylcholine to receptors in the sarcoplasmic reticulum.

Question 8

Which of the following muscles has the maximum density of white muscle fibers?

Accepted Answer

Extraocular muscles

Answer

Calf muscle

Answer

Back muscles

Answer

Hip muscles

Question 9

When energy is derived from creatine phosphate to cause muscle contraction, what is the first step in this transfer of energy?

Accepted Answer

The energy of the creatine phosphate is used to convert ADP to ATP

Answer

The creatine phosphate transfers its energy to the cross bridges to cause them to become locked

Answer

The creatine phosphate causes the power stroke of the cross bridges

Answer

The creatine phosphate transfers its energy to the actin filament

Question 10

Energy in muscles is stored as:

Accepted Answer

Phosphocreatine

Answer

ATP

Answer

ADP

Answer

None of the above

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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