Nerve and Muscle Physiology Practice Questions

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**.

Q: In skeletal muscle, which step of excitation-contraction coupling requires ATP?

Cross-bridge cycling. ***Cross-bridge cycling*** - ATP is essential for two key actions in **cross-bridge cycling**: the **detachment of myosin heads from actin** and the **re-cocking of the myosin heads** for the next power stroke. - Without ATP, myosin heads remain attached to actin, leading to **rigor mortis**. *Action potential propagation* - This process involves the flow of **ions (Na+ and K+)** across the sarcolemma through voltage-gated channels, which is a passive event down their electrochemical gradients. - While ion pumps (like the Na+/K+ pump) maintain these gradients over time, the **propagation itself is not a direct ATP-dependent step** in the immediate sense of the action potential. *Troponin binding to calcium* - The binding of **calcium to troponin C** is a passive chemical interaction driven by the *concentration gradient* of calcium ions. - This binding triggers a **conformational change** in the troponin-tropomyosin complex, exposing actin binding sites, and does not directly consume ATP. *Calcium release from SR* - The release of calcium from the **sarcoplasmic reticulum (SR)** into the sarcoplasm occurs through **ryanodine receptors**, which are mechanically or voltage-gated channels. - This is a passive efflux down the **calcium concentration gradient**, and does not directly consume ATP.

Q: During a nerve conduction study, which of the following ions is primarily responsible for the rapid upstroke of the action potential?

Sodium. ***Sodium*** - The rapid upstroke of an **action potential** (depolarization) in nerves is primarily due to the rapid influx of **sodium ions** (Na+) into the cell. - This influx occurs through **voltage-gated sodium channels** that open in response to a threshold stimulus. *Calcium* - **Calcium ions** (Ca2+) play a significant role in neurotransmitter release at the **synaptic terminals** and in cardiac and smooth muscle action potentials. - However, they are not the primary ion responsible for the initial rapid **depolarization** in peripheral nerve conduction. *Chloride* - **Chloride ions** (Cl-) are generally involved in maintaining the resting membrane potential and mediating **inhibitory postsynaptic potentials** (IPSPs) by causing hyperpolarization or preventing depolarization. - They do not contribute to the rapid upstroke of an **action potential**. *Potassium* - **Potassium ions** (K+) are primarily responsible for the **repolarization phase** of the action potential. - The efflux of K+ through **voltage-gated potassium channels** causes the membrane potential to return to its resting state.

Q: Inverse stretch reflex is mediated :

Golgi tendon organ. ***Golgi tendon*** - The **Golgi tendon organ (GTO)** is a **proprioceptor** located at the junction of muscle fibers and tendons, sensitive to changes in muscle tension. - When muscle tension becomes excessive, the GTO is activated, inhibiting the alpha motor neurons innervating that muscle, leading to muscle relaxation, which is the **inverse stretch reflex**. *Unmyelinated C fibres* - These fibers are primarily involved in transmitting **slow, dull pain** and **temperature sensations**, but not proprioceptive reflexes. - Their conduction velocity is much slower than that required for rapid protective reflexes. *Dorsal Column* - The dorsal column-medial lemniscus pathway is responsible for transmitting **fine touch, vibration, and proprioception** to the brain, but it is an ascending sensory pathway and does not directly mediate spinal reflexes. - This pathway is involved in conscious perception, not the direct arc of a reflex. *Muscle spindle* - The **muscle spindle** is responsible for the **stretch reflex** (myotatic reflex), which causes muscle contraction in response to stretch. - It detects changes in **muscle length and rate of change of length**, which is distinct from the inverse stretch reflex mediated by the GTO.

Q: Delivery of stimulus above threshold intensity leads to a constant amplitude of AP and is known as:

All or none law. ***All or none law*** - The **all-or-none law** states that if a stimulus reaches or exceeds the **threshold intensity**, a neuron will fire an action potential of a constant, maximal amplitude. - If the stimulus is below the threshold, no action potential will fire, meaning there is no partial or submaximal action potential. *Electrotonic potential* - **Electrotonic potentials** are subthreshold, local changes in membrane potential that decay with distance and time. - They are **graded**, meaning their amplitude is proportional to the stimulus intensity, unlike the fixed amplitude of an action potential. *Absolute refractory period* - The **absolute refractory period** is the time during an action potential when the membrane is completely unresponsive to further stimulation, no matter how strong. - This period is due to the **inactivation of voltage-gated sodium channels**, preventing another action potential from being generated. *Relative refractory period* - The **relative refractory period** is the time following the absolute refractory period when a **larger-than-normal stimulus** is required to elicit another action potential. - This occurs because some potassium channels are still open, and the membrane is hyperpolarized, making it harder to reach the threshold.

Q: Resting membrane potential of nerve fibre is close to isoelectric potential of:

Potassium ions. ***Potassium ions*** - The **resting membrane potential** is primarily determined by the **equilibrium potential of potassium ions** because the membrane is far more permeable to potassium than to other ions at rest. - Due to the high **permeability to K+**, a significant outward flow of potassium ions occurs, making the inside of the cell negative relative to the outside, approaching the **Nernst potential for K+**. *Sodium ions* - The membrane has very low permeability to **sodium ions** at rest, so **Na+ influx** only slightly affects the resting potential. - The **Nernst potential for Na+** is positive, which is opposite to the negative resting membrane potential. *Chloride ions* - While chloride ions contribute to the **resting membrane potential**, their contribution is typically less significant than potassium due to varying membrane permeability in different neurons. - In many cells, chloride ions follow the electrical gradient set by other ions and do not actively determine the resting potential. *Magnesium ions* - **Magnesium ions** play crucial roles as cofactors for enzymes and in neurotransmission but have minimal direct influence on establishing the **resting membrane potential**. - The membrane is largely **impermeable to Mg2+** at rest, and their concentration gradients do not establish the baseline voltage.

Q: Which of the following receptors mediate stretch reflex?

Muscle spindle. ***Muscle spindle*** - Muscle spindles are **stretch-sensitive receptors** located within the muscle belly that detect changes in muscle length and the rate of change in length. - When a muscle is stretched, the muscle spindles are activated, sending signals via **afferent neurons** to the spinal cord, which then initiates a reflex contraction of the same muscle to counteract the stretch—this is the basis of the stretch reflex. *Golgi tendon organ* - **Golgi tendon organs** are located in the tendons and respond to changes in **muscle tension**, not muscle length. Its primary role is to prevent excessive muscle contraction. - When activated by high tension, Golgi tendon organs inhibit the muscle, leading to relaxation (inverse stretch reflex), which is opposite to the stretch reflex. *Meissner's corpuscles* - **Meissner's corpuscles** are **mechanoreceptors** located in the superficial layers of the skin, primarily responsible for detecting **light touch** and **vibrations**. - They are not involved in the regulation of muscle length or tension and therefore do not mediate the stretch reflex. *Merkel's disc* - **Merkel's discs** are **mechanoreceptors** found in the basal layer of the epidermis, specialized for detecting **sustained pressure** and **texture**. - These receptors contribute to fine tactile discrimination but are unrelated to the proprioceptive mechanisms of the stretch reflex.

Question 1

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 2

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 3

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

Accepted Answer

Late repolarization

Answer

Peak

Answer

Hyperpolarization

Answer

Rising phase

Question 4

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

Question 5

In skeletal muscle, which step of excitation-contraction coupling requires ATP?

Accepted Answer

Cross-bridge cycling

Answer

Action potential propagation

Answer

Troponin binding to calcium

Answer

Calcium release from SR

Question 6

During a nerve conduction study, which of the following ions is primarily responsible for the rapid upstroke of the action potential?

Accepted Answer

Sodium

Answer

Calcium

Answer

Chloride

Answer

Potassium

Question 7

Inverse stretch reflex is mediated :

Accepted Answer

Golgi tendon organ

Answer

Unmyelinated C fibres

Answer

Dorsal Column

Answer

Muscle spindle

Question 8

Delivery of stimulus above threshold intensity leads to a constant amplitude of AP and is known as:

Accepted Answer

All or none law

Answer

Electrotonic potential

Answer

Absolute refractory period

Answer

Relative refractory period

Question 9

Resting membrane potential of nerve fibre is close to isoelectric potential of:

Accepted Answer

Potassium ions

Answer

Sodium ions

Answer

Chloride ions

Answer

Magnesium ions

Question 10

Which of the following receptors mediate stretch reflex?

Accepted Answer

Muscle spindle

Answer

Golgi tendon organ

Answer

Meissner's corpuscles

Answer

Merkel's disc

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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