Nerve and Muscle Physiology Practice Questions

Q: Calculate the tetanizing frequency based on the contraction dynamics of gastrocnemius muscle of frog shown in the image?

20-25 Hz. **20-25 Hz** - Tetanizing frequency (or fusion frequency) is the stimulation rate at which individual muscle twitches fuse to produce a **smooth, sustained contraction** (tetanus). - For the **frog gastrocnemius muscle**, a common model in physiology, this frequency typically falls within the **20-25 Hz range**. *10-15 Hz* - At this lower frequency, the muscle would likely exhibit **incomplete tetanus** or summation, where individual twitches are still discernible, but tension is increasing. - This range is generally insufficient to achieve a **smooth, fused tetanic contraction** in the frog gastrocnemius. *15-20 Hz* - This range might produce **treppe** or early stages of incomplete tetanus, where successive contractions are slightly stronger, but the relaxation phase is still partially visible between stimuli. - While closer to the tetanizing frequency, it's generally not high enough to achieve **complete fusion** for the frog gastrocnemius. *30-35 Hz* - While this frequency would certainly result in a **fused tetanic contraction**, it's higher than the minimum required for the frog gastrocnemius, which means the muscle is already in complete tetanus at a lower frequency. - Using excessively high frequencies beyond the fusion frequency does not significantly increase tension and can lead to **faster fatigue**.

Q: In the process shown below stretch stimulus is mediated by which of the following receptors?

Muscle spindle. ***Muscle spindle*** - The image depicts a **muscle**, and the "Steady stretch" stimulus clearly shows an increase in muscle tension followed by sustained neural firing, characteristic of a **stretch reflex**. - **Muscle spindles** are proprioceptors located within skeletal muscles that detect changes in muscle length and the rate of change of length, playing a crucial role in the stretch reflex. *Merkel's disc* - **Merkel's discs** are mechanoreceptors located in the basal layer of the epidermis, primarily responsible for detecting sustained light touch and pressure. - They are not involved in sensing muscle stretch. *Meissner's corpuscle* - **Meissner's corpuscles** are rapidly adapting mechanoreceptors found in the dermal papillae, specialized for detecting light touch and low-frequency vibration. - They are cutaneous receptors and do not mediate muscle stretch. *Pacinian corpuscle* - **Pacinian corpuscles** are rapidly adapting mechanoreceptors located deep in the dermis and subcutaneous tissue, sensitive to deep pressure and high-frequency vibration. - They are not responsible for detecting muscle stretch.

Q: Which ion plays a role in the process shown below? (Recent NEET Pattern 2018)

Calcium. ***Calcium*** - **Calcium ions (Ca2+)** are the primary trigger for **exocytosis**, binding to **synaptotagmin** proteins on synaptic vesicles to initiate membrane fusion. - Ca2+ influx activates the **SNARE complex** formation, allowing vesicles to fuse with the plasma membrane and release neurotransmitters or other cellular contents. *Potassium* - **Potassium ions (K+)** are primarily responsible for maintaining the **resting membrane potential** and **repolarization** during action potentials. - While essential for neuronal function, K+ does not directly trigger **vesicle fusion** or exocytosis processes. *Sodium* - **Sodium ions (Na+)** are crucial for **action potential depolarization** and maintaining **electrochemical gradients** across cell membranes. - Na+ influx initiates nerve impulses but does not serve as the direct trigger for **vesicle release** during exocytosis. *Magnesium* - **Magnesium ions (Mg2+)** function as essential **cofactors** for numerous enzymes and play roles in **ATP metabolism** and **protein synthesis**. - Although Mg2+ can modulate some cellular processes, it is not the primary ion responsible for triggering **exocytotic release**.

Q: Sequence the events in neuromuscular action potential conduction: 1. Sodium channels open in the end plate 2. Calcium enters at the nerve terminal 3. Release of acetylcholine

$2 \rightarrow 3 \rightarrow 1$ . ***Correct: $2 \rightarrow 3 \rightarrow 1$*** - **Calcium entry at the nerve terminal** is the initial trigger - when an action potential reaches the presynaptic nerve terminal, voltage-gated calcium channels open, allowing Ca²⁺ influx - **Acetylcholine release** follows - the increased intracellular calcium causes synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft - **Sodium channels open in the end plate** last - acetylcholine binds to nicotinic receptors on the motor end plate, opening ligand-gated sodium channels, which depolarizes the muscle membrane and triggers muscle contraction *Incorrect: $1 \rightarrow 2 \rightarrow 3$* - Places sodium channel opening first, which is physiologically impossible - Sodium channels at the motor end plate only open in response to acetylcholine binding - Cannot occur before acetylcholine is released from the nerve terminal *Incorrect: $1 \rightarrow 3 \rightarrow 2$* - Incorrectly sequences sodium channel opening before calcium entry - Violates the fundamental principle that calcium influx is required for neurotransmitter release - Acetylcholine cannot be released without prior calcium entry *Incorrect: $3 \rightarrow 2 \rightarrow 1$* - Places acetylcholine release before calcium entry, which is impossible - Calcium-triggered exocytosis is an absolute requirement for neurotransmitter release - Without calcium influx, vesicles cannot fuse with the presynaptic membrane

Q: Arrange the following parts of sarcomere from periphery to center. 1. Z line 2. M line 3. A band 4. H zone

1,3,4,2. ***1,3,4,2*** - The **Z line** is found at the **periphery** of the sarcomere, defining its boundaries and anchoring the **actin filaments**. - Moving inwards, the **A band** is next, representing the entire length of the **myosin filament**, which may also overlap with actin. - The **H zone** is located within the A band, comprising only **myosin filaments** without actin overlap. - Finally, the **M line** is at the **center** of the sarcomere, bisecting the H zone and anchoring the myosin filaments. *2,3,4,1* - This sequence is incorrect because the **M line** is at the **center** and the **Z line** is at the **periphery**, which is the reverse of the expected order for from periphery to center. - Such an arrangement would place the innermost structure first and outermost last, not reflecting the correct spatial organisation. *4,2,3,1* - This order is incorrect as the **H zone** and **M line** are more central, while the **Z line** is peripheral. - Placing structures like the H zone and M line at the beginning does not align with arrangement from periphery to center. *3,1,4,2* - This option is incorrect because the **A band** includes both actin and myosin filaments, while the **Z line** is at the periphery of the sarcomere. - The given order does not represent a progression from the periphery to the center of the sarcomere.

Q: What is the order of bands in a sarcomere from the Z-disc toward the center?

Z-A-H-M. ***Z-A-H-M*** - This sequence accurately represents the arrangement of bands within a **sarcomere** when moving from the **Z-disc** towards the central **M-line**. - The **Z-disc** anchors **actin (thin) filaments**, which extend into the **A-band**, partially overlapping with myosin (thick) filaments. The **H-zone** is within the A-band, and the **M-line** bisects the H-zone. *Z-M-A-H* - This order incorrectly places the **M-line** immediately after the **Z-disc** and before the A and H bands. - The **M-line** is located at the very center of the sarcomere, a significant distance from the Z-disc. *Z-H-A-M* - This sequence incorrectly places the **H-zone** before the entire **A-band**. - The **H-zone** is a region *within* the **A-band**, specifically where only myosin (thick) filaments are present without actin (thin) overlap. *Z-H-M-A* - This order incorrectly places the **H-zone** and **M-line** before the **A-band**. - The **A-band** encompasses the entire length of the myosin (thick) filaments and includes the **H-zone** and **M-line** centrally.

Q: Which ion flux is primarily responsible for the depolarization phase of smooth muscle action potentials?

Calcium influx. ***Calcium influx*** - The depolarization phase in smooth muscle action potentials is primarily driven by the opening of **voltage-gated calcium channels**, allowing **calcium ions** to flow into the cell. - This influx of positive charge causes the membrane potential to become more positive, leading to depolarization and subsequent muscle contraction. *Chloride efflux* - While chloride ions can influence membrane potential, **chloride efflux** typically contributes to hyperpolarization or stabilization of the resting potential, not depolarization. - In many excitable cells, chloride channels open during repolarization or to inhibit excitability. *Potassium efflux* - The efflux of **potassium ions** is primarily responsible for the **repolarization phase** of action potentials, as it carries positive charge out of the cell, returning the membrane potential to its resting state. - It hyperpolarizes the membrane, rather than depolarizing it. *Sodium influx* - In many excitable tissues, such as skeletal muscle and neurons, **sodium influx** is the primary driver of depolarization during an action potential. - However, in smooth muscle cells, voltage-gated **calcium channels** play a more prominent role than sodium channels for depolarization.

Q: During which phase of the action potential is the membrane most excitable?

Late repolarization. ***Correct: Late repolarization*** - During **late repolarization** (also called the **supernormal period**), the membrane is **most excitable**. - At this phase, **Na+ channels have recovered from inactivation** and are available to open again. - However, the membrane potential is still slightly depolarized compared to resting potential, meaning it is **closer to threshold**. - Therefore, a **smaller stimulus** than normal is required to reach threshold and trigger another action potential. - This represents the period of **increased excitability** during the relative refractory period. *Incorrect: Hyperpolarization* - During hyperpolarization (afterhyperpolarization), the membrane potential becomes **more negative** than the resting potential. - This occurs due to continued **K+ efflux** through slow-closing potassium channels. - The membrane is **farther from threshold**, requiring a **stronger stimulus** to elicit an action potential. - This represents a period of **decreased excitability**, not increased. *Incorrect: Peak* - At the **peak** of the action potential, the membrane potential is at its most positive value. - All voltage-gated **Na+ channels are inactivated** at this point. - This corresponds to the **absolute refractory period**, during which the membrane is **completely unexcitable**. - No stimulus, regardless of strength, can trigger another action potential. *Incorrect: Rising phase* - The **rising phase** involves rapid **depolarization** due to Na+ influx through open voltage-gated Na+ channels. - The membrane is already undergoing an action potential during this phase. - Na+ channels are either **opening or becoming inactivated**, and the membrane enters the absolute refractory period. - The membrane cannot respond to another stimulus during this phase.

Q: Which ion channel type is primarily responsible for the resting membrane potential in neurons?

Leak K+ channels. ***Leak K+ channels*** - **Leak potassium channels** are constitutively open, allowing K+ ions to flow down their concentration gradient out of the cell. - This efflux of positive charge creates the **negative resting membrane potential** in neurons, as the cell becomes more negative inside relative to the outside. *Ca2+ channels* - **Calcium channels** are primarily involved in synaptic transmission, muscle contraction, and intracellular signaling. - While they can influence membrane potential, they are not the primary determinant of the **resting state**. *Voltage-gated Na+ channels* - **Voltage-gated sodium channels** are essential for the **rising phase of the action potential** by allowing a rapid influx of Na+. - They are mostly closed at rest and thus do not significantly contribute to the **resting membrane potential**. *Voltage-gated K+ channels* - **Voltage-gated potassium channels** are crucial for the **repolarization phase of the action potential**, opening in response to membrane depolarization. - They remain largely closed at rest and play a minor role in establishing the **resting potential**.

Question 1

Calculate the tetanizing frequency based on the contraction dynamics of gastrocnemius muscle of frog shown in the image?

Accepted Answer

20-25 Hz

Answer

10-15 Hz

Answer

15-20 Hz

Answer

30-35 Hz

Question 2

In the process shown below stretch stimulus is mediated by which of the following receptors?

Accepted Answer

Muscle spindle

Answer

Merkel's disc

Answer

Meissner's corpuscle

Answer

Pacinian corpuscle

Question 3

Which ion plays a role in the process shown below? (Recent NEET Pattern 2018)

Accepted Answer

Calcium

Answer

Potassium

Answer

Sodium

Answer

Magnesium

Question 4

Sequence the events in neuromuscular action potential conduction:
1. Sodium channels open in the end plate
2. Calcium enters at the nerve terminal
3. Release of acetylcholine

Accepted Answer

$2 \rightarrow 3 \rightarrow 1$

Answer

$1 \rightarrow 2 \rightarrow 3$

Answer

$1 \rightarrow 3 \rightarrow 2$

Answer

$3 \rightarrow 2 \rightarrow 1$

Question 5

Assertion: RMP depends on proteins and phosphate ions.
Reason: Diffusion potential can be calculated using nernst equation.
Choose the best statement regarding the assertion and reason.

Accepted Answer

Both true, Reason is not the explanation of assertion

Answer

Assertion false, Reason true

Answer

Both true, Reason is the explanation of assertion

Answer

Assertion true, Reason false

Question 6

Arrange the following parts of sarcomere from periphery to center.

1. Z line
2. M line
3. A band
4. H zone

Accepted Answer

1,3,4,2

Answer

2,3,4,1

Answer

4,2,3,1

Answer

3,1,4,2

Question 7

What is the order of bands in a sarcomere from the Z-disc toward the center?

Accepted Answer

Z-A-H-M

Answer

Z-M-A-H

Answer

Z-H-A-M

Answer

Z-H-M-A

Question 8

Which ion flux is primarily responsible for the depolarization phase of smooth muscle action potentials?

Accepted Answer

Calcium influx

Answer

Chloride efflux

Answer

Potassium efflux

Answer

Sodium influx

Question 9

During which phase of the action potential is the membrane most excitable?

Accepted Answer

Late repolarization

Answer

Peak

Answer

Hyperpolarization

Answer

Rising phase

Question 10

Which ion channel type is primarily responsible for the resting membrane potential in neurons?

Accepted Answer

Leak K+ channels

Answer

Ca2+ channels

Answer

Voltage-gated Na+ channels

Answer

Voltage-gated K+ channels

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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