Nerve and Muscle Physiology Practice Questions

Q: What covers the binding sites for myosin heads on actin in skeletal muscles?

Tropomyosin. ***Tropomyosin*** - **Tropomyosin** is a two-stranded alpha-helical coiled coil protein that lies in the grooves of the **actin** filaments. - In a resting muscle, it physically blocks the **myosin-binding sites** on **actin**, preventing contraction. *Troponin* - **Troponin** is a complex of three proteins (troponin C, troponin I, and troponin T) that binds to **tropomyosin**. - It plays a crucial role in pulling **tropomyosin** away from the **myosin-binding sites** on **actin** when calcium binds to troponin C, initiating muscle contraction. *Calcium* - **Calcium ions (Ca2+)** do not directly cover the binding sites but rather bind to **troponin C**. - This binding causes a conformational change in **troponin**, which in turn shifts **tropomyosin** to expose the binding sites. *None of the above* - This option is incorrect because **tropomyosin** specifically performs the function of covering the myosin-binding sites on actin. - The other components (troponin and calcium) are involved in regulating the position of tropomyosin, not directly covering the sites themselves.

Q: Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

25 Hz. ***25 Hz*** - **Tetanizing frequency** is the minimum stimulation frequency required to produce tetanus (sustained muscle contraction without complete relaxation between stimuli). - For **incomplete tetanus** to occur, the next stimulus must arrive during the relaxation phase, before the muscle fully relaxes. - The critical time window is the **latent period + contraction period** = 10 ms + 40 ms = 50 ms. However, to ensure summation occurs reliably during relaxation, stimuli typically arrive at a slightly higher frequency. - **Practical tetanizing frequency** = approximately 1/(40 ms) = **25 Hz**, which ensures stimuli arrive during the latter part of contraction or early relaxation phase, producing sustained tension. - This frequency allows sufficient overlap for tetanic fusion while accounting for the physiological requirements of the muscle twitch cycle. *50 Hz* - This frequency (one stimulus every 20 ms) would produce a **complete tetanus** with no visible relaxation between stimuli. - This is higher than the minimum tetanizing frequency required for this muscle with its 100 ms total twitch duration. - While this would produce tetanus, it exceeds the minimum frequency needed. *75 Hz* - This very high frequency (one stimulus every 13.3 ms) would produce a **smooth, complete tetanus**. - This is approximately 3 times the minimum tetanizing frequency and represents excessive stimulation. - Such high frequencies are well beyond what is needed to prevent relaxation in this muscle. *100 Hz* - This extremely high frequency (one stimulus every 10 ms, equivalent to the latent period alone) would produce **maximal tetanic fusion**. - This is 4 times the minimum tetanizing frequency needed for this muscle. - While physiologically possible, this represents supramaximal stimulation frequency for tetanus production in this scenario.

Q: What mediates the response observed in Lewis's triple response?

Axon reflex. ***Axon reflex*** - The **axon reflex** is the characteristic **neural mechanism** that mediates the spreading **flare** (the distinctive third component) of Lewis's triple response. - This antidromic reflex occurs when sensory nerve endings are stimulated, causing impulses to travel along branches of the same axon and release **substance P and CGRP** (calcitonin gene-related peptide), producing vasodilation in surrounding areas. - The axon reflex is what makes Lewis's triple response a **neurovascular** phenomenon, distinguishing it from simple inflammatory responses. - While histamine initiates the cascade, the **axon reflex** is considered the key mediating mechanism for the complete triple response pattern. *Arteriolar dilation* - Arteriolar dilation is the **effect** or result seen in the flare, not the mediating mechanism itself. - This is what happens as a consequence of axon reflex activation, not what mediates the response. *Histamine release* - **Histamine** from mast cells is the initial **chemical trigger** that causes the red line (direct capillary dilation) and wheal (increased permeability). - However, histamine alone cannot explain the **spreading flare** beyond the site of injury, which requires the neurogenic mechanism of the axon reflex. - The question asks for the **mediating mechanism** of the observed response pattern, where the axon reflex is the distinguishing feature. *Local hormones* - This term is too vague and non-specific to describe the precise mechanism. - While bradykinin and other mediators may contribute, the **axon reflex** is the specific neural mechanism that characterizes Lewis's triple response.

Q: Upstroke of the action potential would lead to:

All of the options. ***All of the options*** - The **upstroke** or **depolarization phase** of an action potential is characterized by a rapid influx of **positive ions** (primarily sodium) into the cell. - This influx causes the **cell's membrane potential** to become less negative and then positive relative to the outside, representing an inward net current and making the cell interior more positive. *Net current in an inward direction* - The **upstroke** is driven primarily by the rapid opening of **voltage-gated sodium channels**, allowing **Na+ ions** to rush into the cell. - This movement of positive charge into the cell constitutes an **inward current**, making the inside of the cell more positive. *Cell interior becomes more positive* - As **positively charged sodium ions** flow into the cell during the upstroke, the accumulation of these charges within the cell leads to a change in membrane potential. - This change shifts the membrane potential from a **negative resting state** towards a **positive value**. *Cell interior becomes less negative* - The initial phase of the action potential upstroke involves the **depolarization** of the membrane, meaning the potential moves from its resting negative value (e.g., -70mV) towards zero. - Before becoming positive, the cell interior first becomes **less negative** as it approaches the threshold potential and then rapidly depolarizes.

Q: Which of the following conditions does not involve neuronal degeneration?

Temporary conduction block (Neuropraxia). ***Temporary conduction block (Neuropraxia)*** - **Neuropraxia** is the mildest form of nerve injury, involving a **temporary conduction block** without structural damage to the axon. - It causes a transient loss of function but does not involve **neuronal degeneration** or Wallerian degeneration. *Growth and differentiation in the fetal nervous system* - During fetal development, a significant amount of **neuronal apoptosis** (programmed cell death) occurs. - This process is crucial for sculpting the developing nervous system by eliminating redundant or improperly connected neurons. *Aging process of neurons* - The aging process is associated with some degree of **neuronal loss** and **degeneration**, particularly in specific brain regions. - This can lead to cognitive decline and other neurological changes common in older adults. *Crush injury to a nerve* - A **crush injury** to a nerve typically results in significant **axonal disruption** and **Wallerian degeneration**. - This involves the breakdown of the axon distal to the injury site, which is a form of neuronal degeneration.

Q: Decrease in reflex response after repetitive stimulation

Fatigue. ***Fatigue*** - **Fatigue** in the context of reflex arcs refers to a temporary decline in the responsiveness of a reflex following repeated or prolonged stimulation. - This decrease is often attributed to the depletion of **neurotransmitter** stores at the presynaptic terminals or changes in the postsynaptic membrane excitability. *Spatial summation* - **Spatial summation** is the process where multiple **presynaptic neurons** simultaneously release neurotransmitters onto a **postsynaptic neuron**, and their combined effects reach the threshold for an action potential. - It involves the integration of signals from different locations at the same time, not a decrease in response due to repetitive stimulation. *Occlusion* - **Occlusion** occurs when the combined effect of stimulating two separate afferent nerves simultaneously is **less than the sum** of their individual effects. - This happens because some interneurons or motoneurons are common to both pathways, and once activated by one, they cannot be activated further by the other, leading to a smaller than expected overall response. *Temporal summation* - **Temporal summation** refers to the rapid firing of a **single presynaptic neuron** that causes successive graded potentials in the postsynaptic neuron to add up, eventually reaching the threshold for an action potential. - This mechanism involves repeated stimulation from a single source over time to *increase* the likelihood of an action potential, which is the opposite of a decrease in reflex response.

Q: How is the rate of skeletal muscle relaxation related to calcium ion reuptake by the sarcoplasmic reticulum?

When free Ca++ is removed from the sarcoplasm.. ***When free Ca++ is removed from the sarcoplasm*** - Skeletal muscle relaxation occurs as **calcium ions (Ca++)** are actively pumped back into the **sarcoplasmic reticulum (SR)** by **SERCA pumps**. - This removal of Ca++ from the **sarcoplasm** reduces Ca++ concentration around the myofibrils, causing **troponin to return to its original conformation**, which **blocks myosin-binding sites** on actin, leading to muscle relaxation. - The **rate of muscle relaxation is directly proportional to the rate of Ca++ reuptake** by the SR. *When phosphocreatine is metabolized* - **Phosphocreatine** is primarily involved in quickly regenerating **ATP** for muscle contraction, especially during the initial phases of intense activity. - Its metabolism does not directly regulate the rate of calcium reuptake or muscle relaxation. *During ATP hydrolysis* - **ATP hydrolysis** by **myosin heads** is essential for muscle contraction, providing the energy for the power stroke. - It also powers the **SERCA pumps** for calcium reuptake, but the hydrolysis itself is not the rate-determining factor for relaxation—the Ca++ removal is. *During acetylcholine resynthesis* - **Acetylcholine resynthesis** occurs at the **neuromuscular junction** to replenish neurotransmitter stores after nerve stimulation. - This process is upstream of muscle fiber excitation and does not directly regulate the rate of calcium reuptake or muscle relaxation within the muscle cell.

Q: Which of the following represents the site of reversal of polarity of myosin molecules in thick muscle filaments?

M line. ***M line*** - The **M line** is the central anchor point for thick filaments in a sarcomere, where myosin molecules are arranged in a **tail-to-tail fashion**. - This **tail-to-tail arrangement** at the M line means that the myosin heads on either side point outwards, creating a reversal of polarity where the molecules meet. *Z line* - The **Z line** defines the boundaries of a sarcomere and is where **actin (thin) filaments** are anchored. - It does not involve the arrangement or polarity reversal of myosin molecules. *I band* - The **I band** is the region of a sarcomere that contains only **thin (actin) filaments**. - Myosin molecules are not present in this region, thus no reversal of their polarity occurs here. *H zone* - The **H zone** is the central region of the A band that contains only **thick (myosin) filaments** when the muscle is relaxed. - While it contains myosin, the M line runs through its center, and it's at the M line where the polarity reversal occurs, not uniformly throughout the H zone.

Question 1

What covers the binding sites for myosin heads on actin in skeletal muscles?

Accepted Answer

Tropomyosin

Answer

Troponin

Answer

Calcium

Answer

None of the above

Question 2

Latent period of muscle twitch is 10 milliseconds, contraction period is 40 milliseconds, and the relaxation time is 50 milliseconds. What would be the tetanizing frequency?

Accepted Answer

25 Hz

Answer

50 Hz

Answer

75 Hz

Answer

100 Hz

Question 3

What mediates the response observed in Lewis's triple response?

Accepted Answer

Axon reflex

Answer

Arteriolar dilation

Answer

Histamine release

Answer

Local hormones

Question 4

Upstroke of the action potential would lead to:

Accepted Answer

All of the options

Answer

Cell interior becomes less negative

Answer

Net current in an inward direction

Answer

Cell interior becomes more positive

Question 5

Which of the following functions is primarily associated with neuroactive substances such as substance P, 5-hydroxytryptamine, vasoactive intestinal peptide, somatostatin, and prostaglandins?

Accepted Answer

Both modify excitability of nerve endings and affect vascular tone.

Answer

None of the above.

Answer

Affect vascular tone only.

Answer

Modify excitability of nerve endings only.

Question 6

Which of the following conditions does not involve neuronal degeneration?

Accepted Answer

Temporary conduction block (Neuropraxia)

Answer

Aging process of neurons

Answer

Growth and differentiation in the fetal nervous system

Answer

Crush injury to a nerve

Question 7

Decrease in reflex response after repetitive stimulation

Accepted Answer

Fatigue

Answer

Occlusion

Answer

Spatial summation

Answer

Temporal summation

Question 8

How is the rate of skeletal muscle relaxation related to calcium ion reuptake by the sarcoplasmic reticulum?

Accepted Answer

When free Ca++ is removed from the sarcoplasm.

Answer

When phosphocreatine is metabolized.

Answer

During ATP hydrolysis.

Answer

During acetylcholine resynthesis.

Question 9

What is the primary cause of the plateau phase in the cardiac muscle action potential?

Accepted Answer

The calcium channels remaining open longer than the sodium channels

Answer

The movement of fewer sodium ions across the cell membrane

Answer

The increased membrane permeability to potassium ion

Answer

A decrease in the amount of calcium diffusing across the membrane

Question 10

Which of the following represents the site of reversal of polarity of myosin molecules in thick muscle filaments?

Accepted Answer

M line

Answer

Z line

Answer

I band

Answer

H zone

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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