Nerve and Muscle Physiology Practice Questions

Q: Where does the initiation of an impulse start?

Axon hillock and initial segment. The initiation of a nerve impulse (action potential) occurs at the **Axon Hillock and Initial Segment** (often collectively called the **Trigger Zone**). ### Why Option B is Correct The initiation of an action potential depends on the density of **voltage-gated sodium (Na+) channels**. The initial segment of the axon has a significantly higher concentration of these channels compared to the rest of the neuron. Consequently, this region has the **lowest threshold** for excitation, meaning it requires the least amount of depolarization to trigger an all-or-none action potential. While the axon hillock funnels the graded potentials, the actual spike usually begins in the adjacent initial segment. ### Why Other Options are Incorrect * **Axon (A):** While the action potential *propagates* along the axon, it does not *initiate* there. The distal axon serves as a conduction cable. * **Cell Body / Soma (C):** The soma contains a relatively low density of voltage-gated Na+ channels. It primarily functions to integrate incoming signals (EPSPs and IPSPs) rather than generating the spike. * **Dendritic Tree (D):** Dendrites are the primary sites for receiving inputs. They generate **graded potentials** (local changes in membrane potential) which decay over distance and do not typically initiate an action potential. ### High-Yield NEET-PG Pearls * **Threshold Value:** The threshold at the initial segment is approximately **-35 to -40 mV**, whereas the rest of the soma requires depolarization to about -10 mV. * **Orthodromic vs. Antidromic:** Normal conduction from the soma to the axon terminal is "orthodromic." Experimental stimulation of an axon that travels backward to the soma is "antidromic." * **Myelination:** In myelinated neurons, the action potential "jumps" between **Nodes of Ranvier** (Saltatory conduction), where Na+ channel density is also very high.

Q: What is the approximate number of Golgi tendon organs per 100 muscle fibers?

1-10. **Explanation:** The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons (musculotendinous junction). Its primary function is to sense **muscle tension** and protect the muscle from damage due to excessive contraction via the inverse stretch reflex. **Why Option A is Correct:** In human skeletal muscle, GTOs are relatively sparse compared to the total number of muscle fibers. Anatomical studies indicate that approximately **10 to 15 muscle fibers** are usually connected in series with a single Golgi tendon organ. When calculated as a ratio per 100 muscle fibers, the number typically falls within the **1–10 range**. This distribution ensures that the GTO can sample tension from a representative group of motor units without requiring a 1:1 ratio. **Why Other Options are Incorrect:** * **Options B, C, and D:** These values (50–400) are significantly higher than the physiological reality. If there were 50–100 GTOs per 100 fibers, nearly every muscle fiber would have its own dedicated GTO, which is not the case. Such high numbers would also occupy excessive space within the tendon, interfering with structural integrity. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** GTOs are arranged **in series** with muscle fibers (unlike Muscle Spindles, which are **in parallel**). * **Innervation:** GTOs are innervated by **Type Ib sensory fibers** (fast-conducting). * **Function:** They mediate the **Inverse Stretch Reflex** (Autogenic Inhibition), causing the muscle to relax when tension becomes too high. * **Contrast:** While Muscle Spindles detect **length/stretch**, GTOs detect **force/tension**.

Q: Which examination finding is associated with lower motor neuron lesions?

Flaccid paralysis. **Explanation:** In clinical neurology, distinguishing between Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) lesions is a high-yield topic for NEET-PG. **1. Why Flaccid Paralysis is Correct:** A Lower Motor Neuron (LMN) is the "final common pathway" connecting the spinal cord to the muscle. When the LMN (located in the anterior horn of the spinal cord or cranial nerve nuclei) is damaged, the muscle loses all neural input. This results in **flaccid paralysis**, characterized by a complete loss of muscle tone (**hypotonia**) and significant muscle wasting (**atrophy**) due to denervation. **2. Why the Other Options are Incorrect:** * **A. Spasticity:** This is a hallmark of **UMN lesions**. It occurs due to the loss of inhibitory control from higher centers, leading to a velocity-dependent increase in muscle tone. * **C. Hyperactive stretch reflex:** Also known as hyperreflexia, this is seen in **UMN lesions**. In LMN lesions, reflexes are diminished (**hyporeflexia**) or absent (**areflexia**) because the efferent limb of the reflex arc is broken. * **D. Muscular incoordination:** This is typically a sign of **cerebellar lesions** (ataxia) rather than a primary motor neuron lesion. **Clinical Pearls for NEET-PG:** * **LMN Signs (The "Downs"):** Tone is down (hypotonia), Reflexes are down (hyporeflexia), Muscle mass is down (atrophy), and **Fasciculations** (spontaneous muscle twitches) are present. * **UMN Signs (The "Ups"):** Tone is up (spasticity), Reflexes are up (hyperreflexia), and the **Babinski sign** is positive (upgoing toe). * **Classic LMN Examples:** Polio, Bell’s Palsy, and Spinal Muscular Atrophy.

Q: Which of the following is not a sarcolemmal protein?

Perlecan. **Explanation:** The sarcolemma is the specialized cell membrane of a muscle fiber, which is reinforced by a complex of proteins known as the **Dystrophin-Glycoprotein Complex (DGC)**. This complex links the internal cytoskeleton (actin) to the external basal lamina, providing structural integrity during muscle contraction. * **Why Perlecan is the correct answer:** Perlecan is a large multidomain heparan sulfate proteoglycan. Crucially, it is **not** a sarcolemmal protein; rather, it is a major component of the **extracellular matrix (basal lamina)**. It interacts with dystroglycan to anchor the sarcolemma to the matrix but remains an external structural element. * **Why the other options are incorrect:** * **Dystrophin:** An essential intracellular protein that links the F-actin cytoskeleton to the sarcolemmal proteins. * **Sarcoglycan:** A group of transmembrane proteins (α, β, γ, δ) that form a sub-complex within the sarcolemma. * **Dystroglycan:** A transmembrane protein consisting of α and β subunits. The β-subunit spans the sarcolemma, while the α-subunit binds to laminin in the matrix. **High-Yield Clinical Pearls for NEET-PG:** * **Duchenne Muscular Dystrophy (DMD):** Caused by the total absence of **Dystrophin** (X-linked recessive). It is the most common and severe dystrophy. * **Becker Muscular Dystrophy (BMD):** Caused by a mutation leading to partially functional or reduced Dystrophin. * **Limb-Girdle Muscular Dystrophy (LGMD):** Often associated with mutations in the **Sarcoglycan** complex. * **Function of DGC:** It acts as a "shock absorber" during contraction; without these proteins, the sarcolemma tears, leading to muscle necrosis.

Q: In an isotonic contraction of the muscle, what occurs?

The muscle moves a load through a distance.. ### Explanation **1. Why Option A is Correct:** In an **isotonic contraction** (Greek: *iso* = same, *tonos* = tension), the muscle tension remains constant once it exceeds the weight of the load. Because the tension is sufficient to overcome the resistance, the muscle fibers shorten, resulting in the **movement of a load through a distance**. This represents external work being done ($Work = Force \times Distance$). **2. Analysis of Incorrect Options:** * **Option B:** This describes an **isometric contraction**. In isometric contractions, the load is too heavy for the muscle to move; the muscle develops tension, but its length remains constant, and no external work is performed. * **Option C:** In isotonic contraction, the **latent period is longer** than in isometric contraction. This is because, in an isotonic contraction, the muscle must first build enough tension to equal the load (the isometric phase) before shortening can begin. * **Option D:** During an isotonic contraction, the muscle **shortens** (concentric) or lengthens under controlled tension (eccentric). However, the standard definition in this context refers to shortening to move a load. The length never increases during the active contractile phase of a typical isotonic twitch. **3. NEET-PG High-Yield Pearls:** * **Isotonic vs. Isometric:** In isometric contraction, the **Elastic Component (SEC)** is stretched while the **Contractile Component (CC)** shortens, keeping the overall length the same. In isotonic, the CC shortens further after the SEC is stretched. * **Work Done:** Isotonic contraction performs external work; Isometric contraction performs zero external work (all energy is dissipated as heat). * **Velocity:** The velocity of muscle shortening is maximal when the load is zero and decreases as the load increases (**Force-Velocity Relationship**).

Question 1

Which of the following statements regarding nerve conduction is false?

Accepted Answer

None of the above statements are false.

Answer

It obeys the all-or-none phenomenon.

Answer

Myelinated nerves conduct impulses faster than unmyelinated nerves.

Answer

Nerve impulse conduction occurs at a constant velocity and amplitude.

Question 2

Where does the initiation of an impulse start?

Accepted Answer

Axon hillock and initial segment

Answer

Axon

Answer

Cell body

Answer

Dendritic tree

Question 3

All are true about local potential except?

Accepted Answer

Summation is not possible

Answer

Does not follow the all-or-none law

Answer

Can be depolarized or hyperpolarized

Answer

Is proportional to stimulus strength

Question 4

What is the approximate number of Golgi tendon organs per 100 muscle fibers?

Accepted Answer

1-10

Answer

200-400

Answer

50-60

Answer

80-100

Question 5

Salutatory conduction refers to all of the following except?

Accepted Answer

Depolarization travels in opposite directions along the axon.

Answer

It is a rapid process.

Answer

It involves the jumping of depolarization from node to node.

Answer

Current flow through the myelin sheath is negligible.

Question 6

Which examination finding is associated with lower motor neuron lesions?

Accepted Answer

Flaccid paralysis

Answer

Spasticity

Answer

Hyperactive stretch reflex

Answer

Muscular incoordination

Question 7

Which of the following is not a sarcolemmal protein?

Accepted Answer

Perlecan

Answer

Sarcoglycan

Answer

Dystrophin

Answer

Dystroglycan

Question 8

In an isotonic contraction of the muscle, what occurs?

Accepted Answer

The muscle moves a load through a distance.

Answer

The muscle is not able to move the load.

Answer

The latent period of contraction is shorter than in isometric contraction.

Answer

The length of the muscle is increased.

Question 9

Tapping the tendon of a muscle may elicit what response?

Accepted Answer

Contraction of the muscle

Answer

Relaxation of the muscle

Answer

No response

Answer

Fibrillation of the muscle

Question 10

Which of the following conditions is characterized by a decremental response on electromyography (EMG)?

Accepted Answer

Myasthenia gravis

Answer

Lambert-Eaton syndrome

Answer

Duchenne muscular dystrophy

Answer

Upper motor neuron lesion

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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