Nerve and Muscle Physiology Practice Questions

Q: Which of the following statements is true regarding type 2b muscle fibers?

White, glycolytic, fast contracting. ***White, glycolytic, fast contracting*** - **Type 2b muscle fibers** (also known as fast glycolytic fibers) are characterized by their **fast contraction speed** and high capacity for **anaerobic glycolysis**. - They appear **white** due to a lower myoglobin content and fewer mitochondria, relying on glycolytic metabolism for quick, powerful bursts of activity. - These are the classic fast-twitch white muscle fibers optimized for explosive power. *White, glycolytic, slow contracting* - While **Type 2b fibers** are indeed **white** and **glycolytic**, this option inaccurately describes them as **slow contracting**. - Their primary characteristic is their **fast contraction speed**, which allows for rapid, forceful movements. *Red, oxidative, fast contracting* - This describes **Type 2a fibers** (fast oxidative-glycolytic), not Type 2b fibers. - **Type 2b fibers** are predominantly **white** and **glycolytic**, not red and oxidative. - Type 2b fibers have lower myoglobin content and fewer mitochondria compared to Type 2a. *Red, glycolytic, slow contracting* - **Red, oxidative, slow contracting** fibers refer to **Type 1 (slow-twitch) fibers**, optimized for endurance. - This option incorrectly combines characteristics from different fiber types. - Type 2b fibers are neither red nor slow contracting.

Q: When the tension becomes great enough, contraction suddenly ceases and the muscle relaxes, this phenomenon is known as:

Autogenic inhibition. ***Autogenic inhibition*** - This reflex is mediated by **Golgi tendon organs (GTOs)**, which are proprioceptors located within the muscle tendons. - When muscle tension becomes excessively high, GTOs are activated and send inhibitory signals to the alpha motor neurons supplying that same muscle, causing it to relax and preventing injury. *Autocrine innervation* - **Autocrine signaling** refers to a form of cell signaling in which a cell secretes a hormone or chemical messenger that binds to receptors on the same cell, leading to changes in the cell. - This term does not describe a reflex mechanism involving muscle tension and relaxation. *Converse stretch reflex* - There is no recognized physiological reflex termed the "converse stretch reflex." - The **stretch reflex** (or myotatic reflex) is a muscle contraction in response to stretching within the muscle, acting to maintain a constant muscle length, which is the opposite of muscle relaxation due to high tension. *Reciprocal innervation* - **Reciprocal innervation** (or reciprocal inhibition) is a reflex where the contraction of one muscle is accompanied by the simultaneous relaxation of its antagonist muscle. - While it involves coordinated muscle activity, it does not explain the sudden cessation of contraction and relaxation of a single muscle due to high tension.

Q: Hyperpolarization is caused by which ions?

K+. ***K+*** - **Efflux of K+ ions** out of the cell makes the inside of the cell more negative, leading to **hyperpolarization**. - This efflux is typically mediated by **voltage-gated potassium channels** opening, or by activation of **GABA-A** or **glycine receptors** that increase K+ conductance. *Na+* - **Influx of Na+ ions** into the cell makes the inside of the cell more positive, causing **depolarization**, not hyperpolarization. - This influx is responsible for the **rising phase of an action potential**. *Ca2+* - **Influx of Ca2+ ions** into the cell also contributes to **depolarization** and can trigger various intracellular processes. - Ca2+ influx is crucial for **neurotransmitter release** and muscle contraction, but not for hyperpolarization. *HCO3-* - Bicarbonate ions (**HCO3-**) play a significant role in **maintaining pH balance** in the body and are involved in various physiological processes. - While ion channels can conduct HCO3-, their movement is not typically the primary cause of cell membrane hyperpolarization.

Q: What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?

Hyperpolarization. ***Hyperpolarization*** - During **hyperpolarization**, the membrane potential becomes more negative than the **resting membrane potential**, primarily due to the outflow of **potassium (K+)** ions or influx of **chloride (Cl-)** ions. - This increased K+ efflux or Cl- influx signifies a state where potassium permeability is maximal relative to sodium permeability, making the membrane less excitable. *Action Potential* - An **action potential** involves a rapid **depolarization** phase due to a massive influx of **sodium (Na+)** ions, causing the PNa/PK ratio to be high, followed by repolarization where K+ efflux restores the resting potential. - Therefore, during an action potential, the ratio of PK/PNa is at its lowest during the rising phase when sodium channels are open. *Depolarization* - **Depolarization** is characterized by a decrease in the absolute value of the membrane potential, making it less negative or even positive, primarily due to the influx of **sodium (Na+)** ions. - During depolarization, the permeability to sodium is significantly higher than to potassium, thus the PK/PNa ratio is low. *Resting Membrane Potential* - At **resting membrane potential**, potassium permeability is already much higher than sodium permeability due to **leak potassium channels**, but it is not maximized to the extent seen during hyperpolarization. - The resting potential is established by a balance of ion movements, primarily K+ efflux and limited Na+ influx, maintained by the **Na+/K+-ATPase pump**.

Q: Which of the following components is primarily found in thick filaments?

Heavy chain of Myosin and Light chain of Myosin. ***Heavy chain of Myosin and Light chain of Myosin*** - **Myosin** is the primary component of **thick filaments** in muscle tissue, consisting of heavy chains that form the rod-like tail and heads, and light chains that regulate the myosin head function. - The **myosin heads** bind to **actin** during muscle contraction, driven by ATP hydrolysis. *Dystrophin, Titin, and Actin* - **Actin** is the main component of **thin filaments**, not thick filaments, and interacts with myosin during contraction. - **Dystrophin** is a structural protein linking actin to the sarcolemma, and **titin** is involved in muscle elasticity, but neither are primary components of the thick filament itself, although titin is associated with it. *Titin, Actin, and Nebulin* - **Actin** and **nebulin** are components of **thin filaments**, with nebulin regulating the length of actin filaments. - **Titin** is a large protein associated with thick filaments, providing elasticity and maintaining their position, but not forming the filament's structural core. *Nebulin and Actin* - Both **nebulin** and **actin** are integral components of **thin filaments**. - **Nebulin** helps regulate the length of **actin filaments**, while actin forms the backbone of the thin filament where myosin heads bind.

Q: Unidirectional flow of a nerve impulse is characterized by

Synaptic junction. ***Synaptic junction*** - The **synapse** ensures that neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron, enforcing a **unidirectional flow** of information. - This structural and functional arrangement prevents the impulse from traveling backward to the transmitting neuron. *Nerve fiber* - While a nerve fiber (axon) conducts impulses, it does not inherently prevent backward propagation; the **synapse** is crucial for establishing directionality between neurons. - An isolated nerve fiber can conduct in both directions if artificially stimulated mid-axon, but the synaptic structure prevents this in vivo. *Neuronal branches* - **Neuronal branches** (dendrites and axon terminals) increase the surface area for receiving and transmitting signals but do not inherently establish or maintain the **unidirectional flow** of the impulse itself. - They allow for complex integration and divergence of signals but rely on the synapse for directionality. *Myelin sheath* - The **myelin sheath** insulates the axon and increases the speed of impulse conduction through **saltatory conduction**. - It does not, however, determine the direction of the impulse; that function is primarily regulated by the **synapse**.

Q: Which of the following pathways is the major energy-providing pathway for fast-twitch muscle?

Glycolysis. ***Glycolysis*** - **Fast-twitch muscle fibers** (Type II) are designed for rapid, powerful contractions over short periods and rely primarily on **anaerobic metabolism**. - **Glycolysis** is the major energy-providing pathway under these conditions, quickly converting **glucose** into ATP without the need for oxygen, leading to lactate production. *β oxidation of fatty acids* - This pathway is the primary energy source for **slow-twitch muscle fibers** (Type I) which are adapted for sustained activity and rely on **aerobic respiration**. - **Fatty acid oxidation** is slower and requires oxygen, making it less suitable for the rapid ATP demands of fast-twitch muscles. *Utilisation of ketone bodies* - **Ketone bodies** are typically used as an alternative fuel source by the **brain** and **muscle** during prolonged fasting or starvation, when glucose availability is low. - While muscles can utilize ketone bodies, they are not the primary or major energy source for fast-twitch muscle activity, especially during immediate, intense exertion. *Amino acid breakdown* - **Amino acid breakdown** (protein catabolism) is primarily used for energy during conditions of severe calorie restriction or prolonged exercise when other fuel sources are depleted, or for glucose synthesis via **gluconeogenesis**. - It is not a major or rapidly accessible energy source for the immediate, high-demand ATP requirements of fast-twitch muscle.

Q: What protein anchors actin filaments to the Z-line in muscle fibers?

Alpha actinin. ***Alpha actinin*** - **Alpha-actinin** is a protein that cross-links **actin filaments** and anchors them to the **Z-disc** in skeletal muscle. - It plays a crucial role in maintaining the **structural integrity** of the sarcomere. *Titin* - **Titin** is a giant protein that extends from the **Z-disc to the M-line** and acts as a molecular spring. - It maintains the **passive elasticity** of muscle and helps in sarcomere assembly. *Nebulin* - **Nebulin** is a large protein that co-extends with **actin filaments** from the Z-disc. - It acts as a **molecular ruler** and regulates the length of actin filaments. *Dystrophin* - **Dystrophin** connects the **intracellular cytoskeleton to the extracellular matrix** via the dystrophin-glycoprotein complex. - It is located at the **sarcolemma**, not at the Z-line, and its deficiency causes **Duchenne muscular dystrophy**.

Q: Sarcomere is the area between which two Z lines?

Two adjacent Z lines. ***Two adjacent Z lines*** - A **sarcomere** is defined as the fundamental contractile unit of muscle, extending from one **Z line** to the next. - The **Z lines** anchor the **actin (thin) filaments**, and their proximity helps delineate the functional sarcomere unit. *Two consecutive I bands* - The **I band** contains only **thin (actin) filaments** and is bisected by a Z line. It is not a boundary that defines a sarcomere. - A sarcomere encompasses parts of two I bands, plus an A band in the middle. *A band and I band* - The **A band** contains **thick (myosin) filaments** and overlapping thin filaments, while the **I band** contains only thin filaments. - These bands are components within a sarcomere, not boundaries that define its extent. *Two H zones* - The **H zone** is a region within the A band that contains only **thick (myosin) filaments** and is visible in relaxed muscle. - It is located in the center of the A band and does not serve as a boundary for the entire sarcomere.

Q: Which of the following proteins prevents contraction by covering binding sites on actin, thereby preventing its interaction with myosin?

Tropomyosin. ***Tropomyosin*** - **Tropomyosin** is a regulatory protein that wraps around the **actin filament**, covering the **myosin-binding sites** in a resting muscle. This physically blocks the myosin heads from attaching to actin. - When **calcium ions** bind to troponin, it causes a conformational change that shifts tropomyosin away, exposing the binding sites and allowing muscle contraction. *Thymosin* - **Thymosin** is a protein involved in the regulation of **actin polymerization**, primarily by sequestering **G-actin monomers**. - It plays a role in the formation and regulation of the **actin cytoskeleton** but is not directly involved in blocking myosin binding sites on actin filaments during muscle contraction. *Troponin* - **Troponin** is a complex of three proteins (troponin C, I, and T) that is attached to **tropomyosin**. - Its primary role is to **bind calcium ions**, which then causes a conformational change that moves tropomyosin, *exposing* the myosin-binding sites on actin, rather than *covering* them. *Calmodulin* - **Calmodulin** is a ubiquitous calcium-binding messenger protein that plays a key role in various cellular processes, including smooth muscle contraction. - In smooth muscle, **calmodulin binds calcium** and activates myosin light chain kinase, but it does not directly cover myosin-binding sites on actin in skeletal muscle.

Question 1

Which of the following statements is true regarding type 2b muscle fibers?

Accepted Answer

White, glycolytic, fast contracting

Answer

White, glycolytic, slow contracting

Answer

Red, oxidative, fast contracting

Answer

Red, glycolytic, slow contracting

Question 2

When the tension becomes great enough, contraction suddenly ceases and the muscle relaxes, this phenomenon is known as:

Accepted Answer

Autogenic inhibition

Answer

Autocrine innervation

Answer

Converse stretch reflex

Answer

Reciprocal innervation

Question 3

Hyperpolarization is caused by which ions?

Accepted Answer

K+

Answer

Na+

Answer

HCO3-

Answer

Ca2+

Question 4

What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?

Accepted Answer

Hyperpolarization

Answer

Depolarization

Answer

Action Potential

Answer

Resting Membrane Potential

Question 5

Which of the following components is primarily found in thick filaments?

Accepted Answer

Heavy chain of Myosin and Light chain of Myosin

Answer

Dystrophin, Titin, and Actin

Answer

Titin, Actin, and Nebulin

Answer

Nebulin and Actin

Question 6

Unidirectional flow of a nerve impulse is characterized by

Accepted Answer

Synaptic junction

Answer

Nerve fiber

Answer

Neuronal branches

Answer

Myelin sheath

Question 7

Which of the following pathways is the major energy-providing pathway for fast-twitch muscle?

Accepted Answer

Glycolysis

Answer

Utilisation of ketone bodies

Answer

Amino acid breakdown

Answer

β oxidation of fatty acids

Question 8

What protein anchors actin filaments to the Z-line in muscle fibers?

Accepted Answer

Alpha actinin

Answer

Titin

Answer

Nebulin

Answer

Dystrophin

Question 9

Sarcomere is the area between which two Z lines?

Accepted Answer

Two adjacent Z lines

Answer

A band and I band

Answer

Two H zones

Answer

Two consecutive I bands

Question 10

Which of the following proteins prevents contraction by covering binding sites on actin, thereby preventing its interaction with myosin?

Accepted Answer

Tropomyosin

Answer

Thymosin

Answer

Troponin

Answer

Calmodulin

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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