Nerve and Muscle Physiology Practice Questions

Q: Which of the following contains the site of attachment for calcium ions that initiates muscle contraction?

Troponin C. **Explanation:** Muscle contraction is initiated by the **Excitation-Contraction (E-C) coupling** process, where an increase in intracellular calcium ions ($Ca^{2+}$) acts as the primary trigger. **1. Why Troponin C is Correct:** The troponin complex is a regulatory protein associated with the actin (thin) filament. It consists of three subunits, each with a specific function. **Troponin C (TnC)** is the specific subunit that contains four binding sites for calcium. When $Ca^{2+}$ binds to Troponin C, it induces a conformational change in the entire troponin-tropomyosin complex. This shift moves tropomyosin away from the active sites on the actin filament, allowing the myosin heads to bind and initiate the cross-bridge cycle. **2. Analysis of Incorrect Options:** * **Troponin I (TnI):** The "I" stands for **Inhibitory**. It binds to actin and inhibits the interaction between actin and myosin by physically blocking the binding site. * **Troponin T (TnT):** The "T" stands for **Tropomyosin**. Its primary role is to anchor the troponin complex to the tropomyosin molecule. * **Myosin:** This is the thick filament. While it has binding sites for ATP and Actin, it does not have the primary regulatory binding site for calcium to initiate contraction. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Biomarkers:** Troponin I and T are highly specific markers for myocardial infarction (MI). Troponin C is not used clinically because it is identical in both skeletal and cardiac muscle. * **The "Power Stroke":** This occurs when ADP and inorganic phosphate are released from the myosin head, not when calcium binds. * **Relaxation:** Muscle relaxation occurs when $Ca^{2+}$ is pumped back into the Sarcoplasmic Reticulum via the **SERCA pump** (an active process requiring ATP).

Q: Which of the following steps is NOT involved in muscle contraction following a nerve impulse?

Dopamine release at the NMJ. **Explanation:** The process of muscle contraction, known as **Excitation-Contraction Coupling**, relies on a specific sequence of electrochemical events. **Why Option B is Correct:** The neurotransmitter responsible for signal transmission at the Neuromuscular Junction (NMJ) is **Acetylcholine (ACh)**, not dopamine. When an action potential reaches the motor nerve terminal, voltage-gated calcium channels open, leading to the exocytosis of ACh into the synaptic cleft. Dopamine is a neurotransmitter primarily involved in the Central Nervous System (e.g., basal ganglia) and does not play a role in peripheral skeletal muscle contraction. **Why Other Options are Incorrect:** * **Option A:** An action potential must cross the NMJ via ACh binding to nicotinic receptors to initiate depolarization of the sarcolemma. * **Option C:** Depolarization travels down **T-tubules**, activating DHP receptors, which triggers the **Ryanodine receptors** to release stored Calcium from the **Sarcoplasmic Reticulum (SR)**. * **Option D:** Once released, Calcium binds to **Troponin C**, causing a conformational change in Tropomyosin. This exposes the active sites on actin, allowing the "attractive forces" (cross-bridge formation) between actin and myosin to occur. **High-Yield Clinical Pearls for NEET-PG:** * **Lambert-Eaton Syndrome:** Antibodies against *pre-synaptic* voltage-gated calcium channels. * **Myasthenia Gravis:** Antibodies against *post-synaptic* NMJ nicotinic ACh receptors. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine receptor (RYR1)**, leading to excessive calcium release. * **Rigor Mortis:** Occurs because ATP is required for the *detachment* of myosin from actin; without ATP, the cross-bridge remains locked.

Q: Rigor mortis results after death is due to which of the following?

Failure of ATP supply. **Explanation:** Rigor mortis is the post-mortem stiffening of muscles caused by the depletion of **Adenosine Triphosphate (ATP)**. **Why Option B is Correct:** In a living muscle, ATP is required for two critical steps in the cross-bridge cycle: 1. **Dissociation:** ATP binding to the myosin head is essential to break the bond between actin and myosin. 2. **Relaxation:** ATP powers the SERCA pump (Sarcoplasmic Endoplasmic Reticulum Calcium ATPase) to pump calcium back into the sarcoplasmic reticulum. After death, cellular respiration ceases, leading to a **failure of ATP supply**. Without ATP, the myosin heads remain permanently attached to actin filaments in a "latched" state, and calcium remains in the cytosol, maintaining a state of rigid contraction. **Why Other Options are Incorrect:** * **Option A:** Acetylcholine (ACh) is responsible for initiating depolarization at the neuromuscular junction. While ACh release stops after death, it is the lack of ATP for detachment, not the absence of ACh diffusion, that causes the physical rigidity. * **Option C:** This is a distractor. While calcium bridges (troponin-tropomyosin interactions) remain active because calcium cannot be sequestered, the fundamental "failure" is the lack of the energy molecule (ATP) required to break the actin-myosin cross-bridge. **High-Yield Clinical Pearls for NEET-PG:** * **Sequence:** Rigor mortis typically starts in the small muscles (eyelids, jaw) and follows a **proximal-to-distal** (Nysten’s Law) progression. * **Timeline:** It usually begins 2–3 hours after death, peaks at 12 hours, and disappears after 36–48 hours due to **autolysis** (proteolytic enzyme action). * **Temperature:** High ambient temperatures accelerate the onset of rigor mortis.

Q: Gamma neurons innervate which type of muscle fibers?

Intrafusal muscle fibers. **Explanation:** The correct answer is **B. Intrafusal muscle fibers.** **1. Why it is correct:** The muscle spindle is a sensory receptor located within skeletal muscles that detects changes in muscle length. It consists of specialized fibers called **intrafusal fibers**. These fibers are innervated by **Gamma ($\gamma$) motor neurons**, which originate in the ventral horn of the spinal cord. When gamma neurons fire, they cause the poles (ends) of the intrafusal fibers to contract, which stretches the central sensory portion, thereby maintaining the sensitivity of the muscle spindle even when the muscle itself is shortened. **2. Why the other options are incorrect:** * **Option A:** **Extrafusal muscle fibers** are the standard contractile fibers responsible for muscle bulk and force generation. These are innervated by **Alpha ($\alpha$) motor neurons**, not gamma neurons. * **Option C:** The **primary motor innervation** to skeletal muscle is provided by Alpha motor neurons, which form the "final common pathway" for motor control. * **Option D:** Inhibition of antagonistic muscles is achieved via **Ia inhibitory interneurons** during the reciprocal inhibition reflex, not directly by gamma neurons. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Alpha-Gamma Co-activation:** During voluntary movement, both alpha and gamma neurons are activated simultaneously. This ensures the muscle spindle remains "taut" and sensitive to further changes in length during contraction. * **Gamma Loop:** This refers to the pathway: Gamma motor neuron $\rightarrow$ Intrafusal fiber contraction $\rightarrow$ Ia afferent stimulation $\rightarrow$ Alpha motor neuron activation $\rightarrow$ Extrafusal fiber contraction. * **Clinical Correlation:** Overactivity of gamma motor neurons is a primary contributor to **spasticity** and increased muscle tone in Upper Motor Neuron (UMN) lesions.

Question 1

All of the following functions are associated with myelination except?

Accepted Answer

Decreases the release of neurotransmitter from the nerve endings

Answer

Decreases energy expenditure

Answer

Increases speed of conduction

Answer

Provides protective covering for the axon

Question 2

Which of the following contains the site of attachment for calcium ions that initiates muscle contraction?

Accepted Answer

Troponin C

Answer

Troponin I

Answer

Troponin T

Answer

Myosin

Question 3

Which of the following steps is NOT involved in muscle contraction following a nerve impulse?

Accepted Answer

Dopamine release at the NMJ

Answer

Action potential across the neuromuscular junction (NMJ)

Answer

Release of calcium from the sarcoplasmic reticulum

Answer

Calcium initiates attractive forces between actin and myosin filaments

Question 4

What is the chronaxie minimum?

Accepted Answer

Myelinated nerve

Answer

Mixed nerves

Answer

Unmyelinated nerve

Answer

Sensory nerves

Question 5

Rigor mortis results after death is due to which of the following?

Accepted Answer

Failure of ATP supply

Answer

Failure of acetylcholine to diffuse

Answer

Failure of breakdown of calcium bridges

Answer

None of the above

Question 6

Determine the chronaxie and rheobase from the provided graph.

Accepted Answer

Chronaxie 0.2 msec; rheobase 0.3V

Answer

Chronaxie 0.3V; rheobase 0.2 msec

Answer

Chronaxie 0.3V; rheobase 0.6V

Answer

Chronaxie 0.2 msec; rheobase 0.6V

Question 7

Which of the following statements is NOT true regarding cardiac and skeletal muscle fibers?

Accepted Answer

Both cardiac and skeletal muscles exhibit graded contraction.

Answer

Excitation-contraction coupling in both depends upon increased intracellular Calcium.

Answer

Excitation-contraction coupling is not dependent on extracellular Ca2+ in skeletal muscle.

Answer

The cardiac action potential has a plateau and a longer refractory period.

Question 8

Gamma neurons innervate which type of muscle fibers?

Accepted Answer

Intrafusal muscle fibers

Answer

Extrafusal muscle fibers

Answer

Primary motor innervations to skeletal muscles

Answer

Inhibit antagonistic muscles

Question 9

Clasp knife rigidity is also known as:

Accepted Answer

Inverse stretch reflex

Answer

Withdrawal reflex

Answer

Lengthening reaction

Answer

Crossed extensor reflex

Question 10

What characterizes the inverse stretch reflex?

Accepted Answer

It is a disynaptic reflex activated on change in muscle tension.

Answer

It occurs when muscle length changes.

Answer

It is a monosynaptic reflex initiated by activation of the Golgi tendon organ.

Answer

It uses type II afferent fibers from the Golgi tendon organ.

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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