Nerve and Muscle Physiology Practice Questions

Q: Which of the following statements about smooth muscles is true?

Presence of Caveolae. ***Presence of Caveolae*** - **Caveolae** are small invaginations of the plasma membrane in smooth muscle cells that function in **calcium handling** and cell signaling. - They increase the surface area and contain receptors and ion channels crucial for smooth muscle contraction. *Ca2+ binds to troponin C* - In **smooth muscle**, calcium ions (Ca2+) bind to **calmodulin**, not troponin C. - **Troponin C** is the calcium-binding protein found in **striated muscle** (skeletal and cardiac muscle). *The anchorage for actin filaments is given by Z-lines* - In **smooth muscle**, **dense bodies** serve as the anchor points for **actin filaments**, analogous to Z-lines in striated muscle. - **Z-lines** are characteristic structures of **sarcomeres** in skeletal and cardiac muscle. *Voltage-gated L-type Ca2+ channels are present in T-tubules* - While L-type Ca2+ channels are present in some smooth muscle cells, their distribution in **T-tubules** is primarily characteristic of **cardiac muscle**. - **Smooth muscle** generally lacks **T-tubules**, and Ca2+ entry occurs mainly through channels on the plasma membrane or from the sarcoplasmic reticulum.

Q: Which of the following types of nerve fibers are primarily responsible for transmitting slow, dull, and chronic pain sensations?

C fibers. ***C fibers*** - These are **unmyelinated**, small-diameter nerve fibers that conduct impulses slowly (0.5-2 m/s). - They are primarily responsible for transmitting **slow, dull, burning, or aching pain** (second pain or chronic pain), as well as temperature sensations and itch. - Their slow conduction velocity results in the delayed, poorly localized pain sensation that persists after initial injury. *A-alpha fibers* - These are the **largest and fastest-conducting** myelinated nerve fibers (70-120 m/s). - They are primarily involved in transmitting **proprioception** (sense of body position) and **motor information** to skeletal muscles. - They do **not transmit pain** signals. *A-beta fibers* - These are **large, myelinated** fibers with a fast conduction velocity (30-70 m/s). - They primarily transmit **touch and pressure sensations**, and can modulate pain perception through the gate control theory. - They are **not nociceptors** and do not directly transmit pain. *A-delta fibers* - These are **small, myelinated** nerve fibers that conduct impulses at 12-30 m/s. - They transmit **fast, sharp, well-localized pain** (first pain or acute pain) and cold sensations. - While they do transmit pain, they are responsible for the **initial sharp pain**, not the slow, dull, chronic pain that defines C fiber function.

Q: Which among the following is a feature of denervation of smooth muscle?

Increased sensitivity to chemical mediators. ***Increased sensitivity to chemical mediators*** - Denervation of smooth muscle leads to **denervation supersensitivity**, meaning the muscle becomes more reactive to agonists due to an increase in receptor numbers or changes in post-receptor signaling pathways. - This increased sensitivity specifically applies to circulating or locally released chemical mediators, even at low concentrations. *Atrophy of the muscle.* - While denervation of **skeletal muscle** often leads to significant atrophy, **smooth muscle cells** are less prone to severe atrophy following denervation. - Smooth muscle tone is significantly influenced by both nervous and intrinsic myogenic activity, so loss of innervation alone does not typically cause complete incapacitation or severe atrophy. *No change in the number of receptors for neurotransmitters.* - Denervation often leads to an **increase in the number of receptors** on the smooth muscle cell surface, a phenomenon known as **upregulation**, contributing to denervation supersensitivity. - This adaptive change helps the muscle respond more strongly to any available neurotransmitters or circulating hormones. *Decreased neurotransmitter release At NMJ* - This option describes a potential cause of denervation (e.g., nerve damage leading to reduced release) rather than a feature of the denervated smooth muscle itself. - Denervation refers to the state where the nerve supply to the muscle is lost, not a change in neurotransmitter release from the now-absent nerve endings.

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: 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: 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: 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].

Question 1

Which of the following statements about smooth muscles is true?

Accepted Answer

Presence of Caveolae

Answer

Ca2+ binds to troponin C

Answer

The anchorage for actin filaments is given by Z-lines

Answer

Voltage-gated L-type Ca2+ channels are present in T-tubules

Question 2

Which of the following types of nerve fibers are primarily responsible for transmitting slow, dull, and chronic pain sensations?

Accepted Answer

C fibers

Answer

A-alpha fibers

Answer

A-beta fibers

Answer

A-delta fibers

Question 3

Which among the following is a feature of denervation of smooth muscle?

Accepted Answer

Increased sensitivity to chemical mediators

Answer

Atrophy of the muscle

Answer

Decreased neurotransmitter release at NMJ

Answer

No change in the number of receptors for neurotransmitters

Question 4

What is the primary reason for the unidirectional nature of neuronal synaptic conduction?

Accepted Answer

Chemical mediators are released from the presynaptic terminal.

Answer

Dendrites can be depolarized and repolarized.

Answer

An area can be depolarized again after repolarization.

Answer

Antidromic impulses are less effective than orthodromic impulses.

Question 5

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 6

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 7

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 8

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

Accepted Answer

Ib

Answer

Ia

Answer

II

Answer

III

Question 9

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

Accepted Answer

Ureter

Answer

Iris

Answer

Ductus deferens

Answer

Trachea

Question 10

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

Accepted Answer

Spindle

Answer

Interneurons

Answer

Golgi tendon organ

Answer

Golgi bottle cells

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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