Nerve and Muscle Physiology Practice Questions

Q: Disruption of the myelin sheath without interruption in the axons is called:

Neuropraxia. ### Explanation This question refers to the **Seddon Classification of Nerve Injury**, which categorizes nerve damage based on the severity of structural disruption. **1. Why Neuropraxia is correct:** Neuropraxia is the mildest form of nerve injury. It is characterized by a **localized conduction block** due to focal demyelination (disruption of the myelin sheath). Crucially, the **axon and the connective tissue sheaths (endoneurium, perineurium, and epineurium) remain intact**. Because the axon is not severed, Wallerian degeneration does not occur, and recovery is typically complete within days to weeks once the myelin repairs. **2. Why the other options are incorrect:** * **Axonotmesis:** This involves the **disruption of the axon** and its myelin sheath, but the supporting connective tissue framework (endoneurium) remains intact. Wallerian degeneration occurs distal to the injury. * **Neurotmesis:** This is the most severe grade, involving **complete transection** of both the axon and all surrounding connective tissue layers. Spontaneous recovery is impossible without surgical intervention. * **Transient ischemia:** While ischemia can cause temporary numbness (e.g., "Saturday Night Palsy"), it is a mechanism of injury rather than a structural classification of nerve damage. **3. High-Yield Clinical Pearls for NEET-PG:** * **Wallerian Degeneration:** Occurs in Axonotmesis and Neurotmesis, but **NOT** in Neuropraxia. * **Recovery Sequence:** Neuropraxia (Fastest/Complete) > Axonotmesis (Slow/Partial) > Neurotmesis (Requires Surgery). * **Tinel’s Sign:** Usually **absent** in Neuropraxia (since the axon is intact) but **present** in Axonotmesis as regenerating axonal sprouts advance. * **Sunderland Classification:** An expansion of Seddon’s; Neuropraxia corresponds to **Grade 1**, Axonotmesis to **Grades 2-4**, and Neurotmesis to **Grade 5**.

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: Which motor unit is recruited last?

Fast fatigable motor unit. This question tests your understanding of the **Henneman Size Principle**, a fundamental concept in neurophysiology regarding motor unit recruitment. ### **Explanation of the Correct Answer** According to the **Henneman Size Principle**, motor units are recruited in order of increasing size of the alpha motor neuron. 1. **Small motor neurons** have a lower threshold for excitation and are recruited first. These innervate **Type I (Slow Oxidative)** fibers. 2. **Large motor neurons** have a higher threshold and are recruited only when high force is required. These innervate **Type IIb (Fast Fatigable)** fibers. Therefore, **Fast Fatigable (FF) units** are the last to be recruited because they require the highest level of neural drive. ### **Analysis of Incorrect Options** * **A. Slow oxidative motor unit:** These are Type I fibers. They are small, fatigue-resistant, and recruited **first** for low-intensity, aerobic activities (e.g., maintaining posture). * **C & D. Active phase/Relaxation:** These options describe the *timing* of a muscle twitch rather than the *type* of motor unit. Recruitment is a process of increasing the number of active motor units during the buildup of contractile force, not a specific phase of a single twitch. ### **High-Yield Facts for NEET-PG** * **Recruitment Order:** Type I (Slow) → Type IIa (Fast-Resistant) → Type IIb (Fast-Fatigable). * **De-recruitment:** This occurs in the exact **reverse order**; the largest units (FF) are the first to stop firing as force decreases. * **Physiological Basis:** Smaller neurons have a higher membrane resistance ($R$), so a given excitatory postsynaptic current ($I$) produces a larger voltage change ($V=IR$), reaching the threshold faster than in large neurons. * **Type IIb Fibers:** High glycogen content, low myoglobin (White muscle), and high ATPase activity.

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.

Q: Nuclear bag fibers are related to which of the following?

Length and velocity. **Explanation:** The muscle spindle is a complex sensory receptor responsible for detecting changes in muscle length. It consists of two types of intrafusal fibers: **Nuclear Bag fibers** and **Nuclear Chain fibers**. **Why the correct answer is right:** Nuclear bag fibers are further divided into **Dynamic bag fibers** and **Static bag fibers**. 1. **Dynamic bag fibers** are highly sensitive to the **rate of change** in muscle length (Velocity). This is known as the dynamic response. 2. **Static bag fibers** (along with nuclear chain fibers) detect the **actual change** in muscle length. This is known as the static response. Therefore, nuclear bag fibers collectively provide information regarding both the **length** and the **velocity** of muscle stretch. **Why incorrect options are wrong:** * **A. Force:** Force or tension is sensed by the **Golgi Tendon Organ (GTO)**, not the muscle spindle. * **B. Length:** While bag fibers do sense length, this option is incomplete because it ignores their unique role in sensing velocity (dynamic response). * **C. Tone:** Muscle tone is a clinical state maintained by the stretch reflex (monosynaptic reflex arc), but the primary physiological parameter sensed by the bag fiber itself is length/velocity. **High-Yield Clinical Pearls for NEET-PG:** * **Innervation:** Nuclear bag fibers are primarily supplied by **Primary (Type Ia) afferents**, which are responsible for the dynamic stretch reflex (e.g., Knee jerk). * **Motor Supply:** Dynamic bag fibers are innervated by **Dynamic Gamma Motor Neurons**, while static bag and chain fibers are innervated by **Static Gamma Motor Neurons**. * **Function:** Muscle spindles prevent overstretching (protection) and maintain posture through the stretch reflex.

Q: Heart muscles act as a functional syncitium because:

Gap junctions. **Explanation:** The heart functions as a **functional syncytium**, meaning that when a single atrial or ventricular cell is excited, the action potential spreads to all muscle cells, causing them to contract as a single unit. **1. Why Gap Junctions are correct:** The primary structural basis for this syncytial behavior is the **Gap Junction**. These are low-resistance protein channels (formed by connexins) located within the **intercalated discs**. They allow the rapid flow of ions and electrical impulses directly from the sarcoplasm of one cardiomyocyte to the next. This electrical coupling ensures synchronized contraction, which is essential for effective pumping. **2. Why other options are incorrect:** * **Striations (A):** While cardiac muscle is striated (like skeletal muscle) due to the organized arrangement of actin and myosin, striations relate to the mechanism of contraction, not the electrical connectivity between cells. * **Long Action Potential (C):** Cardiac muscle has a long action potential (due to the plateau phase/calcium influx), which prevents tetany and allows for filling time, but it does not facilitate the spread of impulses between cells. * **Voluntary (D):** Cardiac muscle is **involuntary** and regulated by the autonomic nervous system, unlike skeletal muscle. **High-Yield Facts for NEET-PG:** * **Two Syncytia:** The heart has two separate functional syncytia—the atrial and the ventricular—separated by the fibrous AV ring. * **Intercalated Discs:** These contain two types of junctions: **Gap junctions** (for electrical communication) and **Desmosomes** (mechanical junctions that hold cells together during contraction). * **Connexin 43:** This is the most abundant gap junction protein in the ventricles.

Question 1

Disruption of the myelin sheath without interruption in the axons is called:

Accepted Answer

Neuropraxia

Answer

Axonotmesis

Answer

Neurotmesis

Answer

Transient ischemia

Question 2

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 3

Conduction velocity of nerve is reduced in all of the following conditions, except?

Accepted Answer

Acute Motor Axonal Neuropathy (AMAN)

Answer

Acute Inflammatory Demyelinating Neuropathy (AIDP)

Answer

Hereditary Sensory Motor Neuropathy (HSMN)

Answer

Multifocal Motor Neuropathy

Question 4

Which of the following statements is TRUE regarding the excitation-contraction coupling mechanism in smooth muscles?

Accepted Answer

Presence of intracellular calcium is essential to cause muscle contraction

Answer

Presence of troponin is essential

Answer

Sustained contraction occurs with high calcium concentration

Answer

Phosphorylation of actin is required for contraction

Question 5

Wallerian degeneration is seen in which part of a nerve?

Accepted Answer

The part of the nerve that is not attached to the cell body.

Answer

The part of the nerve attached to the cell body when the nerve is cut peripherally.

Answer

The cell body that is not attached to the cut nerve.

Answer

The cell body that is attached to the cut nerve.

Question 6

Which motor unit is recruited last?

Accepted Answer

Fast fatigable motor unit

Answer

Slow oxidative motor unit

Answer

During active phase of contraction

Answer

During relaxation

Question 7

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 8

Nuclear bag fibers are related to which of the following?

Accepted Answer

Length and velocity

Answer

Force

Answer

Length

Answer

Tone

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

Heart muscles act as a functional syncitium because:

Accepted Answer

Gap junctions

Answer

Striations

Answer

Long action potential

Answer

It is voluntary

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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