Nerve and Muscle Physiology Practice Questions

Q: Which of the following electrodes can be used to detect muscle activity without causing pain?

Surface electrode. **Explanation:** The detection of muscle electrical activity (Electromyography or EMG) is achieved through two primary types of electrodes: **Surface** and **Needle**. **1. Why Surface Electrode is correct:** Surface electrodes are **non-invasive** discs (usually made of silver-silver chloride) placed directly on the skin over the muscle belly. They detect the summation of action potentials from many underlying motor units. Because they do not penetrate the skin or the muscle fascia, they are **painless** and ideal for kinesiologic studies or monitoring general muscle activity in pediatric patients. **2. Why the other options are incorrect:** * **Needle electrode:** These are **invasive** electrodes inserted directly into the muscle tissue. While they provide superior data regarding individual motor unit action potentials (MUAPs) and denervation potentials (like fibrillations), the insertion causes significant **pain and discomfort**. * **Hook electrode:** These are a type of fine-wire electrode where the tip is bent into a hook to remain anchored within the muscle. These are invasive and used primarily for deep muscles or during dynamic movement, causing pain upon insertion. * **Round electrode:** This is a generic descriptive term for the shape of an electrode rather than a functional category. While many surface electrodes are round, "Surface electrode" is the specific medical term used to denote the non-invasive, painless method. **Clinical Pearls for NEET-PG:** * **Surface EMG:** Best for assessing global muscle timing and "on/off" patterns; cannot isolate deep muscles or single motor units. * **Needle EMG:** The "Gold Standard" for diagnosing **Lower Motor Neuron (LMN)** lesions, myopathies, and neuropathies. * **Key Finding:** Spontaneous activity (fibrillations and positive sharp waves) on needle EMG is a hallmark of **active denervation**.

Q: Where does action potential generate from?

Initial segment. **Explanation:** The generation of an action potential (AP) is determined by the density of **voltage-gated sodium (Na+) channels**. The **Initial Segment** (the region between the axon hillock and the beginning of the myelin sheath) has the highest concentration of these channels. Consequently, this area has the lowest threshold for excitation, making it the site where the "all-or-none" electrical impulse is triggered. **Why the other options are incorrect:** * **Cell Body (Soma):** While the soma contains organelles and receives signals, its membrane has a relatively low density of voltage-gated Na+ channels compared to the initial segment. It primarily conducts graded potentials, not action potentials. * **Dendrites:** These are the primary receiving stations for synaptic input. They generate local, graded potentials (EPSPs and IPSPs) that decay over distance and time. In most neurons, dendrites lack the necessary channel density to initiate a self-propagating AP. * **Axon:** While the axon is responsible for the **propagation** and conduction of the action potential toward the nerve terminal, it is not the site of **origin**. The impulse must first be generated at the initial segment before traveling down the axon. **High-Yield NEET-PG Pearls:** * **Axon Hillock vs. Initial Segment:** While often used interchangeably in casual study, the *Initial Segment* is the precise physiological "trigger zone." The Axon Hillock is the anatomical funnel-shaped region of the soma leading to it. * **Threshold:** The threshold at the initial segment is approximately **-35 to -40 mV**, whereas the soma threshold is much higher (around -10 to -20 mV). * **Nodes of Ranvier:** In myelinated axons, these are the sites of AP regeneration (saltatory conduction) due to high Na+ channel density, but the very first AP still starts at the initial segment.

Q: What is the highest equilibrium potential among the following ions?

Calcium (Ca++). ### Explanation The equilibrium potential ($E_{ion}$) of an ion is the membrane potential at which the electrical gradient exactly balances the chemical concentration gradient, resulting in no net movement of that ion. This is calculated using the **Nernst Equation**. **1. Why Calcium (Ca++) is Correct:** Calcium has the highest concentration gradient across the cell membrane. Extracellular concentration is approximately **1-2 mmol/L**, while intracellular concentration is extremely low (**~0.0001 mmol/L**). Because the gradient is so steep (10,000-fold), a very high positive electrical charge is required inside the cell to repel the influx of $Ca^{++}$. Its equilibrium potential is approximately **+120 to +130 mV**, the highest among all physiological ions. **2. Why the Other Options are Incorrect:** * **Sodium (Na+):** Sodium is higher extracellularly (142 mEq/L) than intracellularly (14 mEq/L). Its equilibrium potential is approximately **+60 to +65 mV**. While positive, it is significantly lower than Calcium. * **Potassium (K+):** Potassium is the primary intracellular cation. Its equilibrium potential is negative (approximately **-90 to -94 mV**) because a negative internal charge is needed to keep $K^+$ from leaking out. * **Chloride (Cl-):** Chloride's equilibrium potential is usually close to the Resting Membrane Potential (RMP), approximately **-70 to -85 mV**. **3. NEET-PG High-Yield Pearls:** * **Resting Membrane Potential (RMP):** In neurons, it is -70 mV; in skeletal muscle, it is -90 mV. * **Determinant of RMP:** The RMP is closest to the equilibrium potential of the ion with the highest permeability (Potassium). * **Nernst Equation Simplified:** $E = 61 \times \log([Ion]_{out} / [Ion]_{in})$ for monovalent cations at body temperature. * **Goldman-Hodgkin-Katz Equation:** Used to calculate the membrane potential when multiple ions are permeable.

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: 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 electrodes can be used to detect muscle activity without causing pain?

Accepted Answer

Surface electrode

Answer

Round electrode

Answer

Hook electrode

Answer

Needle electrode

Question 2

Where does action potential generate from?

Accepted Answer

Initial segment

Answer

Cell body

Answer

Dendrites

Answer

Axon

Question 3

What is the highest equilibrium potential among the following ions?

Accepted Answer

Calcium (Ca++)

Answer

Sodium (Na+)

Answer

Potassium (K+)

Answer

Chloride (Cl-)

Question 4

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 5

Where does the initiation of an impulse start?

Accepted Answer

Axon hillock and initial segment

Answer

Axon

Answer

Cell body

Answer

Dendritic tree

Question 6

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 7

Which of the following statements about smooth muscle is false?

Accepted Answer

The musculature of the uterus is made of multi-unit smooth muscle.

Answer

Ca2+ is the excitation-contraction coupler.

Answer

The "latch" mechanism facilitates prolonged holding of contractions of smooth muscle.

Answer

Shows the property of plasticity.

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

Which nerve fiber exhibits the slowest velocity of conduction?

Accepted Answer

C fibers

Answer

Alpha (Aa)

Answer

Alpha-delta (Aδ)

Answer

Beta (Aβ)

Question 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?