Nerve and Muscle Physiology Practice Questions

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.

Q: Which of the following nerve fibers are responsible for conduction of fast pain (first pain) impulses?

Delta. ***Delta*** - **Aδ (A-delta) fibers** are responsible for transmitting **fast, sharp, localized pain** (first pain). - These fibers are **thinly myelinated**, allowing for rapid conduction of pain signals (12-30 m/s). - They mediate the initial sharp pain sensation when tissue is injured. *gamma* - **Gamma motor neurons** innervate intrafusal muscle fibers within muscle spindles, playing a role in **muscle tone and proprioception**, not pain conduction. - They are efferent (motor) fibers, not afferent (sensory) fibers. *Alpha* - **Alpha motor neurons** innervate extrafusal muscle fibers and are responsible for **muscle contraction** (motor function). - **Aα fibers** conduct proprioception from muscle spindles, and **Aβ fibers** transmit touch and pressure sensations, but neither are involved in pain transmission. *beta* - **Aβ fibers** transmit information related to **touch, pressure, and vibration**, not pain. - They are large, heavily myelinated fibers that conduct impulses faster than pain fibers. - **Note:** C fibers (unmyelinated) conduct slow, burning pain (second pain), but Aδ fibers specifically conduct fast pain.

Q: Which of the following is true regarding Na+ (sodium) ions?

Responsible for depolarization. ***Responsible for depolarization*** - The rapid influx of **Na+ ions** into the cell through voltage-gated sodium channels is the primary event that causes **depolarization** during an action potential. - This influx makes the inside of the cell more positive, shifting the membrane potential from negative toward positive values. *Sodium ion is responsible for Donnan effect* - The **Donnan effect** describes the unequal distribution of permeable ions across a semi-permeable membrane due to the presence of impermeant charged molecules (e.g., proteins). - **Na+ ions are small, permeable ions** - they do not create the Donnan effect. The effect is caused by large, non-diffusible charged molecules like proteins, not by sodium ions. *Does not help other ions in transport* - The **sodium-potassium pump (Na+/K+-ATPase)** actively transports Na+ out of the cell and K+ into the cell, maintaining their concentration gradients. - These Na+ gradients are crucial for **secondary active transport**, where the energy from Na+ moving down its electrochemical gradient is used to move other ions (e.g., in Na+-glucose cotransport) or molecules against their gradients. *Responsible for the resting membrane potential* - The **resting membrane potential** is primarily established by the differential permeability of the membrane to K+ ions and the activity of the Na+/K+-ATPase. - While Na+ leaking into the cell contributes slightly, the dominant factor is the efflux of **K+ ions** through leak channels, as the membrane is much more permeable to K+ than to Na+ at rest.

Q: Type I muscle fibers are

Red. ***Red*** - Type I muscle fibers are also known as **slow-twitch oxidative fibers**, characterized by their **high myoglobin content** which gives them a red appearance. - They are rich in **mitochondria** and have a large supply of capillaries, making them highly efficient at **aerobic metabolism**. *Glycolytic* - **Glycolytic fibers** (Type IIb, or fast-twitch glycolytic) primarily rely on **anaerobic glycolysis** for energy. - These fibers are generally **white** due to lower myoglobin content and fewer mitochondria. *Anaerobic* - **Anaerobic metabolism** is characteristic of **Type II (fast-twitch)** muscle fibers, which are suited for short bursts of high-intensity activity. - Type I fibers primarily use **aerobic metabolism** and are fatigue-resistant, suited for sustained activity. *Large* - **Type II (fast-twitch)** muscle fibers tend to be **larger in diameter** than Type I fibers, allowing them to generate greater force. - Type I fibers are generally **smaller** and produce less tension but are more resistant to fatigue.

Q: Pressure sensation is carried by

Aβ fibres. ***Aβ fibres*** - **Aβ fibres** are large, myelinated nerve fibres responsible for transmitting touch, vibration, and **pressure sensations**. - Their significant myelination allows for rapid conduction of these **discriminative sensory inputs**. *Aγ fibres* - **Aγ fibres** are motor efferent fibres that innervate the **intrafusal muscle fibres** of muscle spindles, regulating their sensitivity. - They are not involved in the direct transmission of **sensory information** like pressure. *Aα fibres* - **Aα fibres** are the largest and fastest conducting nerve fibres, primarily responsible for **proprioception** (muscle stretch) and **motor efferents** to skeletal muscles (extrafusal fibres). - While they carry some sensory information, they are not the primary carriers of **pressure sensation**. *Aδ fibres* - **Aδ fibres** are lightly myelinated and transmit sensations of **fast pain**, cold temperature, and crude touch. - They conduct more slowly than Aβ fibres and are not primarily responsible for **pressure sensation**.

Question 1

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 2

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 3

Inverse stretch reflex is mediated :

Accepted Answer

Golgi tendon organ

Answer

Unmyelinated C fibres

Answer

Dorsal Column

Answer

Muscle spindle

Question 4

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 5

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 6

Which of the following receptors mediate stretch reflex?

Accepted Answer

Muscle spindle

Answer

Golgi tendon organ

Answer

Meissner's corpuscles

Answer

Merkel's disc

Question 7

Which of the following nerve fibers are responsible for conduction of fast pain (first pain) impulses?

Accepted Answer

Delta

Answer

gamma

Answer

Alpha

Answer

beta

Question 8

Which of the following is true regarding Na+ (sodium) ions?

Accepted Answer

Responsible for depolarization

Answer

Does not help other ions in transport

Answer

Responsible for the resting membrane potential

Answer

Sodium ion is responsible for Donnan effect

Question 9

Type I muscle fibers are

Accepted Answer

Red

Answer

Glycolytic

Answer

Anaerobic

Answer

Large

Question 10

Pressure sensation is carried by

Accepted Answer

Aβ fibres

Answer

Aγ fibres

Answer

Aα fibres

Answer

Aδ fibres

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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