Nerve and Muscle Physiology Practice Questions

Q: In excitable cells, repolarization is closely associated with one of the following events:

K+ efflux. ***K+ efflux*** - Repolarization in excitable cells is primarily caused by the **outward movement of potassium ions (K+)** through voltage-gated potassium channels. - This **efflux of positive charge** makes the inside of the cell more negative, returning the membrane potential to its resting state. *Na+ efflux* - **Na+ efflux** is primarily mediated by the **Na+/K+ ATPase pump**, which is crucial for maintaining the resting membrane potential but does not directly cause repolarization during an action potential. - The pump expels 3 Na+ ions for every 2 K+ ions taken in, slowly contributing to the negative resting membrane potential. *Na+ influx* - **Na+ influx** is responsible for the **depolarization phase** of an action potential, where the membrane potential becomes more positive. - This occurs when voltage-gated sodium channels open rapidly, allowing sodium ions to rush into the cell. *K+ influx* - **K+ influx** occurs during the **resting membrane potential** and is maintained by the Na+/K+ ATPase pump, which brings K+ ions back into the cell. - This influx helps to establish the potassium concentration gradient, which is critical for K+ efflux during repolarization.

Q: Post-tetanic potentiation is due to what?

Increased availability of Ca++. ***Increased availability of Ca++*** - **Post-tetanic potentiation (PTP)** is a short-term enhancement of synaptic efficacy that occurs after a brief period of high-frequency stimulation (tetanus). - This phenomenon is primarily due to the **accumulation of residual intracellular calcium ions (Ca++)** in the presynaptic terminal, which leads to increased neurotransmitter release upon subsequent action potentials. *Hyperpolarization of muscle fibers* - **Hyperpolarization** makes the muscle fiber less excitable, thereby *reducing* its response to subsequent stimuli rather than enhancing it. - This effect would decrease muscle contractility, which is opposite to what is observed in PTP. *Rapid K+ efflux* - **Rapid K+ efflux** from a cell typically causes repolarization or hyperpolarization, which would decrease neuronal excitability and thus *reduce* neurotransmitter release. - This process is essential for repolarizing the neuron after an action potential but does not directly cause PTP. *Rapid Na+ influx* - **Rapid Na+ influx** is responsible for the depolarization phase of an action potential, triggering nerve impulse propagation. - While essential for neural activity, it doesn't directly explain the *potentiation* or enhanced neurotransmitter release following tetanic stimulation, which is primarily calcium-dependent.

Q: Which of the following parameters decreases from its peak after the overshoot of an action potential?

Membrane conductance for sodium. ***Membrane conductance for sodium*** - During the **peak and overshoot phase** of an action potential, voltage-gated sodium channels are maximally open, resulting in **peak sodium conductance**. - **After the overshoot**, these sodium channels rapidly undergo **inactivation**, leading to a sharp decrease in **sodium conductance**. - This decrease in sodium conductance is essential for terminating sodium influx and initiating the **repolarization phase** of the action potential. *Membrane conductance for potassium* - **Potassium conductance** actually *increases* after the overshoot as **voltage-gated potassium channels** open in response to depolarization. - This increased **potassium conductance** facilitates potassium efflux, which drives membrane repolarization back toward the resting potential. *Transference for sodium* - **Transference** refers to the fraction of total current carried by a specific ion, a concept more relevant to solutions and ionic equilibria. - While sodium's contribution to membrane current decreases as channels inactivate, the primary physiological parameter is **membrane conductance**, which directly reflects channel activity. *Transference for potassium* - **Potassium transference** (or its contribution to current) would increase as potassium channels open after the overshoot. - The most physiologically precise parameter for describing rapid ion channel dynamics during action potentials is **membrane conductance**, not transference.

Q: Single-unit smooth muscles are seen in which of the following?

Ureter. ***Ureter*** - The ureter contains **single-unit smooth muscle** which exhibits spontaneous electrical activity and contraction, allowing for peristaltic movement of urine. - In single-unit smooth muscle, cells are connected by **gap junctions**, enabling them to contract as a coordinated unit. *Iris* - The iris contains **multi-unit smooth muscle**, which allows for fine, independent control of each muscle cell for precise pupil dilation and constriction. - Multi-unit smooth muscle cells are not connected by gap junctions and require individual neural stimulation. *Ductus deferens* - The ductus deferens primarily consists of **multi-unit smooth muscle**, which is necessary for the strong, rapid contractions required for sperm expulsion during ejaculation. - This type of muscle allows for graded contractions depending on the intensity of nervous stimulation. *Trachea* - The smooth muscle in the trachea (trachealis muscle) is primarily **multi-unit smooth muscle**, facilitating independent regulation of airway diameter. - Contraction of the trachealis muscle can reduce the tracheal lumen, aiding in coughing or regulating airflow.

Q: Golgi tendon organs are innervated by which type of nerve fibre?

Ib. ***Ib*** - **Golgi tendon organs (GTOs)** are encapsulated sensory receptors located in the musculoskeletal junction that monitor **muscle tension**. - They are innervated by **Ib afferent nerve fibers**, which are large diameter, myelinated nerve fibers with a high conduction velocity that transmit information to the central nervous system. *Ia* - **Ia afferent nerve fibers** innervate **muscle spindles**, which detect changes in **muscle length** and the rate of change of muscle length. - While both Ib and Ia fibers are involved in proprioception, their specific sensory receptors and functions differ. *II* - **Type II afferent nerve fibers** also innervate **muscle spindles**, primarily sensing sustained changes in **muscle length** (static stretch). - They do not innervate Golgi tendon organs; their role is distinct in providing information about muscle position. *III* - **Type III afferent nerve fibers** are smaller, thinly myelinated fibers that respond mainly to **nociceptive (pain)** and **temperature stimuli** in muscles and joints. - They are not involved in sensing muscle tension or length and do not innervate Golgi tendon organs.

Q: Gamma motor neurons (fusimotor neurons/Leksell efferents) primarily innervate which structures?

Spindle. ***Spindle*** - **Gamma motor neurons** specifically innervate the **intrafusal muscle fibers** within the **muscle spindles** [1], [2]. - This innervation regulates the **sensitivity** of the muscle spindle to stretch, thereby contributing to the precision of muscle control [1]. *Interneurons* - **Interneurons** are primarily involved in transmitting signals between neurons within the central nervous system, not directly innervated by **gamma motor neurons** [3]. - They integrate sensory and motor information, playing a crucial role in spinal reflexes and complex motor patterns [4]. *Golgi tendon organ* - The **Golgi tendon organ** is a sensory receptor located at the junction of muscle and tendon, primarily sensing muscle tension [4]. - It is innervated by **Ib afferent fibers**, not **gamma motor neurons** [4]. *Golgi bottle cells* - **Golgi bottle cells (Golgi epithelial cells)** are located in the **cerebellum** and are involved in granule cell inhibition, not direct innervation by **gamma motor neurons** [5]. - These are a type of inhibitory interneuron in the cerebellum that modulate motor learning and coordination [5].

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: 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: 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: 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**.

Question 1

In excitable cells, repolarization is closely associated with one of the following events:

Accepted Answer

K+ efflux

Answer

Na+ efflux

Answer

Na+ influx

Answer

K+ influx

Question 2

Post-tetanic potentiation is due to what?

Accepted Answer

Increased availability of Ca++

Answer

Hyperpolarization of muscle fibers

Answer

Rapid K+ efflux

Answer

Rapid Na+ influx

Question 3

Which of the following parameters decreases from its peak after the overshoot of an action potential?

Accepted Answer

Membrane conductance for sodium

Answer

Transference for potassium

Answer

Membrane conductance for potassium

Answer

Transference for sodium

Question 4

Single-unit smooth muscles are seen in which of the following?

Accepted Answer

Ureter

Answer

Iris

Answer

Ductus deferens

Answer

Trachea

Question 5

Golgi tendon organs are innervated by which type of nerve fibre?

Accepted Answer

Ib

Answer

Ia

Answer

II

Answer

III

Question 6

Gamma motor neurons (fusimotor neurons/Leksell efferents) primarily innervate which structures?

Accepted Answer

Spindle

Answer

Interneurons

Answer

Golgi tendon organ

Answer

Golgi bottle cells

Question 7

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 8

Hyperpolarization is caused by which ions?

Accepted Answer

K+

Answer

Na+

Answer

HCO3-

Answer

Ca2+

Question 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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