Nerve and Muscle Physiology Practice Questions

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.

Q: Which of the following proteins is primarily responsible for muscle relaxation?

SERCA (Sarcoplasmic Reticulum Ca2+ ATPase). ***SERCA (Sarcoplasmic Reticulum Ca2+ ATPase)*** - **SERCA** pumps calcium ions from the **cytosol** back into the **sarcoplasmic reticulum**, reducing cytosolic calcium levels and allowing muscle relaxation. - This active transport of **Ca2+** away from the contractile proteins (actin and myosin) is essential for stopping muscle contraction. *Both Actin and SERCA* - While **SERCA** is critical for relaxation, **actin** is a structural component of the thin filament involved in muscle contraction, not relaxation. - **Actin** interacts with myosin during the cross-bridge cycle to shorten the sarcomere, a process reversed by **Ca2+** removal. *Actin* - **Actin** is a primary component of the **thin filaments** and is directly involved in muscle **contraction** when bound by myosin. - It does not play a direct role in the active process of muscle relaxation; its interaction with myosin ceases when **calcium** is removed. *None of the options* - This option is incorrect because **SERCA** is a specific protein with a well-defined role in muscle relaxation by actively sequestering **calcium**. - The other options either include incorrect proteins or misattribute functionality for muscle relaxation.

Q: Which of the following statements is NOT true regarding red muscle fibers?

Increased muscle fiber length. ***Increased muscle fiber length*** - The length of muscle fibers is generally determined by the muscle's anatomical structure and function, not by whether they are red or white fibers. - While red muscle fibers (slow-twitch) are optimized for **endurance** and **sustained contractions**, this optimization does not involve an inherent increase in the length of individual muscle fibers. *Decreased glycolytic enzymes* - Red muscle fibers, also known as slow-oxidative fibers, primarily rely on **aerobic metabolism** for ATP production. - They have a lower content of glycolytic enzymes compared to white muscle fibers, which are specialized for **anaerobic glycolysis**. *Increased blood flow* - Red muscle fibers have a rich capillary supply, leading to **increased blood flow**, which is essential for delivering oxygen and nutrients for sustained aerobic activity. - This extensive vascularization contributes to their characteristic red appearance and their ability to resist fatigue. *Used for aerobic activity* - Red muscle fibers are well-suited for prolonged, low-intensity activities due to their high concentration of **mitochondria**, myoglobin, and oxidative enzymes. - They are primarily responsible for maintaining posture and performing **endurance activities** such as long-distance running.

Q: What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?

Cyclic GMP. ***Cyclic GMP*** - **Nitric oxide (NO)** activates **guanylyl cyclase**, an enzyme that converts **GTP to cGMP**. - Elevated **cGMP** levels activate **protein kinase G (PKG)**, leading to smooth muscle relaxation through various mechanisms, including reduced intracellular calcium and altered sensitivity of contractile proteins. *Calcium* - **Calcium** is primarily a key second messenger for **smooth muscle contraction**, not relaxation. - An increase in intracellular **calcium** promotes the binding of **calcium to calmodulin**, activating myosin light chain kinase and leading to contraction. *Cyclic AMP* - While **cyclic AMP (cAMP)** can cause smooth muscle relaxation (e.g., via beta-2 adrenergic stimulation), it is not the direct second messenger for **nitric oxide (NO)**-mediated relaxation. - **cAMP** is produced by **adenylyl cyclase** and primarily activates **protein kinase A (PKA)**. *Magnesium* - **Magnesium** is an important cofactor for many enzymes and can influence muscle contraction and relaxation, but it does not serve as a primary second messenger for **nitric oxide (NO)**. - High concentrations of **magnesium** can directly induce muscle relaxation by competing with **calcium** and modulating various channels and enzymes.

Q: Excitatory postsynaptic potentials are due to

Na+ movement into the cell. ***Na+ movement into the cell*** - **Excitatory postsynaptic potentials (EPSPs)** are caused by the **depolarization** of the postsynaptic membrane. - This depolarization primarily occurs when **neurotransmitters** open **ligand-gated ion channels** that allow a net influx of **positively charged sodium (Na+) ions** into the cell. *K+ movement into the cell* - **K+ influx** would make the inside of the cell more positive, but the resting membrane potential is already close to the **equilibrium potential for K+**; its movement into the cell under normal circumstances does not cause the characteristic depolarization of an EPSP. - While **Na+/K+ ATPase** pumps K+ into the cell, this process is responsible for maintaining the resting potential, not generating rapid EPSP depolarization. *Na+ movement out of the cell* - **Na+ movement out of the cell** would lead to **hyperpolarization** or repolarization, making the inside of the cell more negative, which is characteristic of an inhibitory postsynaptic potential (IPSP) or the repolarization phase of an action potential. - The **Na+/K+ ATPase** pumps Na+ out of the cell, but this is a metabolic pump, not the ion channel mechanism responsible for EPSP generation. *Ca++ movement into the cell* - While **Ca++ influx** can also be excitatory and lead to depolarization, especially in certain types of neurons or during specific synaptic events (e.g., neurotransmitter release), it is not the primary mechanism responsible for most fast **EPSPs** in the central nervous system. - The influx of **Ca++** is more commonly associated with signaling pathways, muscle contraction, and sustained depolarization rather than the rapid, transient depolarization seen in typical EPSPs from ionotropic receptors.

Q: The reflex originating from the Golgi tendon organ to relax the responding muscle is a

Disynaptic reflex. ***Disynaptic reflex*** - The reflex arc originating from the **Golgi tendon organ** involves an afferent neuron synapsing with an inhibitory interneuron, which then synapses with the alpha motor neuron. - This two-synapse pathway (Golgi tendon organ sensory neuron to interneuron, then interneuron to alpha motor neuron) makes it a **disynaptic reflex**. *Monosynaptic reflex* - This type of reflex involves only **one synapse** between the afferent sensory neuron and the efferent motor neuron, such as the **stretch reflex**. - The Golgi tendon reflex requires an **interneuron** to inhibit the motor neuron, thus making it more complex than monosynaptic. *Polysynaptic reflex* - This term describes reflexes involving **multiple interneurons** and more than two synapses. - While the Golgi tendon reflex involves an interneuron, the primary direct inhibition of the motor neuron is achieved through a **single inhibitory interneuron**, making disynaptic a more precise description. *Reflex with center in medulla oblongata* - Reflexes centered in the **medulla oblongata** typically regulate vital functions such as breathing, heart rate, and blood pressure. - The **Golgi tendon reflex** is a spinal reflex with its neural circuitry located within the spinal cord segments associated with the involved muscle.

Q: A person is able to sense fast, sharp pain and temperature normally. Which type of nerve fiber is primarily involved in carrying these fast pain and temperature sensations?

A-delta. ***A-delta*** - **A-delta fibers** are **myelinated**, medium-diameter fibers responsible for transmitting **fast, sharp pain** and **cold temperature** sensations. - Their myelination allows for **rapid conduction** of nerve impulses, leading to the immediate perception of acute pain. *A-beta* - **A-beta fibers** are large-diameter, highly myelinated fibers primarily involved in transmitting **touch** and **pressure** sensations. - While they can transmit some non-painful signals from the skin, they are not the primary carriers of sharp pain or temperature. *C fibers* - **C fibers** are **unmyelinated**, small-diameter fibers that transmit **slow, dull, burning pain** and **warm temperature** sensations. - Their lack of myelination results in **slower conduction velocity**, which is why the duller pain is perceived after the sharp pain. *A-alpha* - **A-alpha fibers** are the **largest diameter**, most heavily myelinated fibers in the periphery. - They are primarily responsible for **proprioception** (sense of body position) and **motor control** to skeletal muscles, not pain or temperature.

Question 1

Decrease in reflex response after repetitive stimulation

Accepted Answer

Fatigue

Answer

Occlusion

Answer

Spatial summation

Answer

Temporal summation

Question 2

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 3

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 4

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

Question 5

Which of the following proteins is primarily responsible for muscle relaxation?

Accepted Answer

SERCA (Sarcoplasmic Reticulum Ca2+ ATPase)

Answer

Both Actin and SERCA

Answer

Actin

Answer

None of the options

Question 6

Which of the following statements is NOT true regarding red muscle fibers?

Accepted Answer

Increased muscle fiber length

Answer

Decreased glycolytic enzymes

Answer

Used for aerobic activity

Answer

Increased blood flow

Question 7

What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?

Accepted Answer

Cyclic GMP

Answer

Calcium

Answer

Cyclic AMP

Answer

Magnesium

Question 8

Excitatory postsynaptic potentials are due to

Accepted Answer

Na+ movement into the cell

Answer

K+ movement into the cell

Answer

Na+ movement out of the cell

Answer

Ca++ movement into the cell

Question 9

The reflex originating from the Golgi tendon organ to relax the responding muscle is a

Accepted Answer

Disynaptic reflex

Answer

Polysynaptic reflex

Answer

Reflex with center in medulla oblongata

Answer

Monosynaptic reflex

Question 10

A person is able to sense fast, sharp pain and temperature normally. Which type of nerve fiber is primarily involved in carrying these fast pain and temperature sensations?

Accepted Answer

A-delta

Answer

C fibers

Answer

A-beta

Answer

A-alpha

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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