Nerve and Muscle Physiology Practice Questions

Q: What are the main types of muscle cells?

All of the above. **Explanation:** The human body contains three distinct types of muscle tissue, categorized based on their structure (striated vs. non-striated) and control mechanism (voluntary vs. involuntary). 1. **Skeletal Muscle:** These are striated, multinucleated fibers under **voluntary** control (somatic nervous system). They are primarily attached to bones and responsible for locomotion. 2. **Cardiac Muscle:** Found exclusively in the heart wall (myocardium), these are striated, branched cells with intercalated discs. They are **involuntary** and possess inherent rhythmicity (autorhythmicity). 3. **Smooth Muscle:** These are non-striated (spindle-shaped) cells found in the walls of hollow organs (e.g., GI tract, blood vessels). They are **involuntary** and regulated by the autonomic nervous system. **Why Option D is Correct:** Since skeletal, cardiac, and smooth muscles are the three fundamental types of muscle cells in the human body, options A, B, and C are only partially correct. "All of the above" is the most comprehensive and accurate choice. **Analysis of Other Options:** Options A, B, and C are incorrect because they are **incomplete**. Selecting any one of them would exclude the third essential muscle type, leading to an inaccurate classification of human muscular histology. **High-Yield NEET-PG Pearls:** * **Troponin:** Present in skeletal and cardiac muscle, but **absent** in smooth muscle (which uses **Calmodulin** instead). * **Gap Junctions:** Present in cardiac and single-unit smooth muscle; absent in skeletal muscle. * **Regeneration:** Skeletal muscle has limited regeneration via **satellite cells**; cardiac muscle has virtually no regenerative capacity; smooth muscle can undergo hypertrophy and hyperplasia. * **T-tubules:** Largest in cardiac muscle (located at Z-lines), smaller in skeletal muscle (located at A-I junctions), and absent in smooth muscle (replaced by **caveolae**).

Q: Which of the following is an example of an electrical synapse?

Gap junctions. **Explanation:** **1. Why Gap Junctions are the Correct Answer:** Electrical synapses are specialized connections between neurons where transmission occurs via the direct flow of ions through low-resistance channels. These channels are formed by **Gap Junctions**. A gap junction consists of two hemichannels called **connexons** (each made of six **connexin** proteins) that bridge the pre- and post-synaptic membranes. Unlike chemical synapses, electrical synapses are characterized by **minimal synaptic delay**, **bidirectional flow**, and the ability to synchronize the activity of a group of neurons. **2. Analysis of Incorrect Options:** * **A. Tight Junctions (Zonula Occludens):** These function as barriers that seal the space between epithelial cells to prevent the paracellular leakage of molecules. They do not facilitate electrical communication. * **C. Anchoring Junctions (e.g., Desmosomes):** These provide mechanical stability by tethering the cytoskeletons of adjacent cells together. They are essential for structural integrity (especially in skin and heart) but do not transmit electrical signals. * **D. Neuromuscular Junction (NMJ):** This is a classic example of a **chemical synapse**. It involves the release of a neurotransmitter (Acetylcholine) into a synaptic cleft, resulting in a significant synaptic delay (approx. 0.5 ms), which is absent in electrical synapses. **3. High-Yield Facts for NEET-PG:** * **Location:** In humans, electrical synapses are found in the **retina**, **cerebral cortex**, and **olfactory bulb**. * **Cardiac Muscle:** Gap junctions are a key component of **intercalated discs**, allowing the heart to function as a functional syncytium. * **Comparison:** Chemical synapses are more common, unidirectional, and allow for signal amplification/modulation, whereas electrical synapses are faster and primarily used for rapid, synchronized firing.

Q: Which muscle is the chief mover of the mandible towards the left?

Right lateral pterygoid. The movement of the mandible is a high-yield topic in head and neck anatomy and physiology. To understand side-to-side (lateral) movement, one must grasp the specific action of the **Lateral Pterygoid** muscle. ### 1. Why the Right Lateral Pterygoid is Correct The lateral pterygoid is the only muscle of mastication that assists in opening the mouth by depressing the mandible. However, its most unique feature is its role in **lateral deviation**. * **Mechanism:** When the right lateral pterygoid contracts, it pulls the condyle of the mandible and the articular disc forward (protrusion) and medially. * **Result:** Because the right side moves forward while the left side remains relatively fixed, the chin is pushed toward the **opposite (contralateral) side**. Therefore, to move the mandible to the **left**, the **right** lateral pterygoid must contract. ### 2. Analysis of Incorrect Options * **Left Medial/Lateral Pterygoid (Options A & B):** Contraction of these muscles on the left side would result in the mandible moving toward the **right**. * **Right Medial Pterygoid (Option C):** While the medial pterygoid does assist in lateral deviation to the opposite side, it is primarily an elevator of the mandible (closing the mouth). The lateral pterygoid is considered the **chief mover** or primary initiator of this horizontal displacement. ### 3. NEET-PG High-Yield Pearls * **Unilateral Contraction:** Moves the jaw to the opposite side. * **Bilateral Contraction:** Results in protrusion (protrusion) and depression of the mandible. * **Nerve Supply:** All muscles of mastication are supplied by the **Mandibular Nerve (V3)**. * **Clinical Correlation:** In cases of **Trigeminal Nerve palsy**, when the patient is asked to protrude the jaw, it deviates **toward the side of the lesion** because the healthy contralateral lateral pterygoid acts unopposed.

Q: Which one of the following is a regulatory protein of the muscle?

Troponin. **Explanation:** The contractile machinery of skeletal muscle is composed of three functional categories of proteins: contractile, regulatory, and structural. **1. Why Troponin is Correct:** **Troponin** and **Tropomyosin** are the primary **regulatory proteins**. In a resting state, tropomyosin covers the myosin-binding sites on the actin filament. Troponin consists of three subunits: * **Troponin C:** Binds to Calcium ions. * **Troponin I:** Inhibits the actin-myosin interaction. * **Troponin T:** Tethers the troponin complex to tropomyosin. When $Ca^{2+}$ binds to Troponin C, a conformational change occurs that moves tropomyosin away, allowing the "power stroke" to occur. **2. Why the Other Options are Incorrect:** * **Myosin (Option B):** This is a **contractile protein** (thick filament). It possesses ATPase activity and forms cross-bridges with actin. * **Actin (Option C):** This is a **contractile protein** (thin filament). It provides the binding sites for myosin heads. * **Protein-C (Option D):** This is a physiological **anticoagulant** synthesized in the liver (Vitamin K dependent). It is not a structural or regulatory component of the muscle sarcomere. **High-Yield NEET-PG Pearls:** * **Sarcomere:** The functional unit of contraction, defined as the segment between two Z-lines. * **Dystrophin:** A vital structural protein; its deficiency leads to Duchenne Muscular Dystrophy. * **Titin:** The largest protein in the body; it acts as a molecular spring providing passive elasticity to the muscle. * **Clinical Marker:** Cardiac Troponin I and T are highly specific gold-standard biomarkers for diagnosing Myocardial Infarction (MI).

Q: Which type of nerve fibers are the fastest?

A fibers. **Explanation:** The speed of nerve impulse conduction is determined by two primary factors: **myelination** and **fiber diameter**. According to the **Erlanger-Gasser classification**, nerve fibers are categorized into types A, B, and C based on these characteristics. **Why A fibers are correct:** Type A fibers are the **fastest** because they are both **heavily myelinated** and have the **largest diameter** (up to 20 μm). Myelin allows for saltatory conduction, where the action potential "jumps" between Nodes of Ranvier, significantly increasing velocity. Within this group, **Type A-alpha (Aα)** fibers are the fastest of all, conducting at speeds of 70–120 m/s (primarily serving motor neurons and proprioception). **Why other options are incorrect:** * **B fibers:** These are preganglionic autonomic fibers. While they are myelinated, their diameter is much smaller than Type A fibers, resulting in intermediate conduction velocities (3–15 m/s). * **C fibers:** These are the **slowest** nerve fibers. They are **unmyelinated** and have the smallest diameter (0.4–1.2 μm), conducting at speeds of only 0.5–2 m/s. They carry slow pain, temperature, and postganglionic autonomic signals. * **All nerve fibers:** This is incorrect as conduction velocity varies significantly across different fiber types to suit their physiological functions. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility to Local Anesthetics:** B > C > A (Small myelinated > Small unmyelinated > Large myelinated). * **Order of Susceptibility to Pressure:** A > B > C (Large fibers are affected first). * **Order of Susceptibility to Hypoxia:** B > A > C. * **Fastest Fiber:** A-alpha (Proprioception/Somatic motor). * **Slowest Fiber:** C fibers (Dull/Slow pain, Olfaction).

Q: Initiation of nerve impulse occurs at the axon hillock because:

It has a lower threshold than the rest of the axon. The **axon hillock** (specifically the initial segment) is known as the **"Trigger Zone"** of the neuron. ### Why Option A is Correct The initiation of an action potential depends on the density of **voltage-gated sodium (Na+) channels**. The axon hillock has the highest concentration of these channels per unit area compared to the cell body or the rest of the axon. Because there are more channels available to open in response to depolarization, the **threshold for excitation is significantly lower** (approx. -45 mV) compared to the soma (approx. -30 mV). This makes it the site with the highest excitability, where graded potentials are summed to trigger an all-or-nothing impulse. ### Why Other Options are Incorrect * **Option B:** While the axon hillock is unmyelinated, this is not the functional reason for impulse *initiation*. Many parts of a neuron (like the dendrites and soma) are unmyelinated but cannot initiate an action potential as easily because they lack the necessary density of Na+ channels. * **Option C:** Neurotransmitter release occurs at the **axon terminals** (presynaptic boutons), not the hillock. The hillock is responsible for signal *generation*, while the terminal is responsible for signal *transmission*. ### High-Yield Facts for NEET-PG * **Initial Segment:** The actual site of impulse generation is the unmyelinated segment between the axon hillock and the first myelin sheath. * **Refractory Period:** This period is determined by the inactivation gate (h-gate) of the voltage-gated Na+ channels. * **Accommodation:** If a nerve is subjected to a slowly rising subthreshold stimulus, the threshold for firing increases; this is called accommodation. * **Safety Factor:** The ratio of action potential strength to the excitability threshold. In normal nerve fibers, this is usually greater than 1.

Q: As a neuron’s diameter increases, what effect manifests?

Conduction velocity increases. ***Conduction velocity increases*** - Increasing the diameter of an axon decreases the **internal (axial) resistance** ($R_i$) to passive current flow longitudinal to the axon. - Reduced internal resistance allows local current loops to spread further and faster, significantly increasing the **length constant**, thereby increasing conduction velocity. *Conduction velocity decreases* - This is incorrect, as larger diameter decreases internal resistance, leading directly to a **faster electrotonic spread** of depolarization and a higher conduction speed. - Decreased conduction velocity is typically observed in **small-diameter** or **demyelinated** axons where internal resistance is higher or membrane capacitance is altered. *Membrane resistance increases* - Membrane resistance ($R_m$) is determined by the density and activity of **leak ion channels** within the cell membrane, which is independent of the overall axon diameter. - While the total membrane area increases, the **specific membrane resistance** (resistance per unit area) does not change with diameter. *Action potential amplitude increases* - Action potential (AP) conduction is an **all-or-none** phenomenon, meaning the amplitude is fixed and determined by the electrochemical gradient of **voltage-gated sodium channels**. - Changes in axon diameter influence the **speed** of propagation (conduction velocity), but they do not alter the required peak voltage (amplitude) of the action potential.

Question 1

What are the main types of muscle cells?

Accepted Answer

All of the above

Answer

Skeletal and cardiac

Answer

Smooth and cardiac

Answer

Smooth and skeletal

Question 2

Which of the following is an example of an electrical synapse?

Accepted Answer

Gap junctions

Answer

Tight junction

Answer

Anchoring junction

Answer

Neuromuscular junction

Question 3

Which muscle is the chief mover of the mandible towards the left?

Accepted Answer

Right lateral pterygoid

Answer

Left medial pterygoid

Answer

Left lateral pterygoid

Answer

Right medial pterygoid

Question 4

Which one of the following is a regulatory protein of the muscle?

Accepted Answer

Troponin

Answer

Myosin

Answer

Actin

Answer

Protein-C

Question 5

Which type of nerve fibers are the fastest?

Accepted Answer

A fibers

Answer

B fibers

Answer

C fibers

Answer

All nerve fibers

Question 6

Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction is/are due to?

Accepted Answer

IP3-DAG

Answer

CAMP

Answer

CGMP

Answer

CCMP

Question 7

Initiation of nerve impulse occurs at the axon hillock because:

Accepted Answer

It has a lower threshold than the rest of the axon

Answer

It is unmyelinated

Answer

Neurotransmitter release occurs here

Answer

None of the above

Question 8

Which type of nerve injury has the best prognosis?

Accepted Answer

Neuropraxia

Answer

Axonotemesis

Answer

Neurotemesis

Answer

Complete transaction

Question 9

The thin filament of muscle consists of all the following except:

Accepted Answer

Myosin

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Question 10

As a neuron’s diameter increases, what effect manifests?

Accepted Answer

Conduction velocity increases

Answer

Conduction velocity decreases

Answer

Membrane resistance increases

Answer

Action potential amplitude increases

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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