Nerve and Muscle Physiology

Nerve and Muscle Physiology Indian Medical PG Question 1: The activation of muscarinic receptors in bronchiolar smooth muscle is associated with:

• A. Increase in IP3 and DAG (Correct Answer)
• B. Inhibition of protein kinase C
• C. Activation of adenylyl cyclase
• D. Opening of Na+/K+ cation channels

Nerve and Muscle Physiology Explanation: ***Increase in IP3 and DAG*** - **Muscarinic receptors** on bronchial smooth muscle (M3 receptors) are **Gq protein-coupled receptors** [1]. - Activation of **Gq proteins** leads to the activation of **phospholipase C**, which hydrolyzes **PIP2** into **IP3** and **DAG** [1, 3]. *Inhibition of protein kinase C* - **DAG** (diacylglycerol), produced from the breakdown of PIP2, **activates protein kinase C (PKC)**, rather than inhibiting it [2]. - This activation of PKC contributes to downstream cellular responses, including smooth muscle contraction [1]. *Activation of adenylyl cyclase* - **Adenylyl cyclase** is typically activated by **Gs protein-coupled receptors**, leading to an increase in **cAMP**. - **Muscarinic (M3) receptors** are **Gq-coupled**, so they do not activate adenylyl cyclase; instead, they operate through the phospholipase C pathway [1, 3]. *Opening of Na+/K+ cation channels* - While some neurotransmitter receptors are **ligand-gated ion channels** (e.g., nicotinic receptors), muscarinic receptors are **G protein-coupled receptors** [1]. - Their activation does not directly lead to the opening of **Na+/K+ cation channels**; rather, they initiate intracellular signaling cascades.

Nerve and Muscle Physiology Indian Medical PG Question 2: Which of the following statements is true regarding smooth muscle contraction?

• A. None of the options.
• B. Calmodulin plays no role in smooth muscle contraction.
• C. Phosphorylation of myosin is essential for contraction. (Correct Answer)
• D. Troponin plays a significant role in smooth muscle contraction.

Nerve and Muscle Physiology Explanation: **Phosphorylation of myosin is essential for contraction.** - In **smooth muscle**, the **myosin light chain (MLC)** must be phosphorylated by **myosin light chain kinase (MLCK)** to enable interaction with actin and initiate contraction. - This phosphorylation causes a conformational change in the **myosin head**, increasing its ATPase activity and allowing cross-bridge cycling. *Calmodulin plays no role in smooth muscle contraction.* - **Calmodulin (CaM)** is crucial for smooth muscle contraction, as it binds **calcium ions (Ca²⁺)** forming a Ca²⁺-CaM complex. - This complex then activates **myosin light chain kinase (MLCK)**, which phosphorylates myosin, triggering contraction. *None of the options.* - This statement is incorrect because one of the provided options, "Phosphorylation of myosin is essential for contraction," is indeed true. *Troponin plays a significant role in smooth muscle contraction.* - Unlike **striated muscle (skeletal and cardiac)**, **smooth muscle** does not contain **troponin**. - Regulation of smooth muscle contraction is primarily **calcium-calmodulin-dependent**, with roles for **MLCK** and **MLCP**, rather than troponin.

Nerve and Muscle Physiology Indian Medical PG Question 3: Absolute refractoriness of a neuron is due to?

• A. Hyperpolarization of Cl channels
• B. Opening of rectifier K+ channels
• C. Closure of activated Na channels
• D. Inactivation of Na channels (Correct Answer)

Nerve and Muscle Physiology Explanation: ***Inactivation of Na channels*** - During the **absolute refractory period**, voltage-gated **Na+ channels** enter an inactivated state, making them unresponsive to further stimulation. - This inactivation prevents another action potential from being generated, regardless of the stimulus intensity, ensuring unidirectional propagation. *Hyperpolarization of Cl channels* - While **Cl- channels** can cause hyperpolarization, this typically leads to **inhibition** rather than absolute refractoriness. - Their activity doesn't directly prevent the generation of a new action potential by blocking Na+ channel function. *Opening of rectifier K+ channels* - The opening of **rectifier K+ channels** is involved in **repolarization** and the **relative refractory period**, by increasing K+ efflux. - While it contributes to making the neuron less excitable, it doesn't cause the absolute inability to fire associated with Na+ channel inactivation. *Closure of activated Na channels* - The **closure of activated Na+ channels** occurs as part of the repolarization process, but the critical mechanism for absolute refractoriness is their transition into an **inactivated state**, not simply closure. - **Inactivation** locks the channels in a non-responsive configuration, whereas simple closure would allow them to reopen quickly with sufficient depolarization.

Nerve and Muscle Physiology Indian Medical PG Question 4: Competitive neuromuscular blockade is enhanced by all except

• A. Acidosis
• B. Aminoglycosides
• C. Hypercalcemia (Correct Answer)
• D. Hypermagnesemia

Nerve and Muscle Physiology Explanation: ***Hypercalcemia*** - **Hypercalcemia** generally **antagonizes** the effects of neuromuscular blockers by increasing acetylcholine release at the presynaptic terminal and sensitizing the postsynaptic membrane to acetylcholine. - Therefore, it would **reduce**, rather than enhance, competitive neuromuscular blockade. *Acidosis* - **Acidosis** enhances competitive neuromuscular blockade by altering the sensitivity of the **nicotinic acetylcholine receptors** at the neuromuscular junction. - It also affects the pharmacokinetics and pharmacodynamics of neuromuscular blocking agents, leading to **prolonged blockade**. *Aminoglycosides* - **Aminoglycoside antibiotics** like gentamicin or amikacin can enhance neuromuscular blockade by **inhibiting presynaptic acetylcholine release** and blocking postsynaptic nicotinic receptors. - This effect is particularly pronounced when given concurrently with competitive neuromuscular blocking agents. *Hypermagnesemia* - **Hypermagnesemia** enhances neuromuscular blockade by **decreasing acetylcholine release** from the presynaptic nerve terminal. - It also directly **depresses muscle fiber excitability** and reduces the sensitivity of the postsynaptic membrane to acetylcholine.

Nerve and Muscle Physiology Indian Medical PG Question 5: The electromyogram (EMG) is least useful for the diagnosis of:

• A. Myasthenia gravis
• B. Charcot-Marie-Tooth disease
• C. Spinal muscular atrophy
• D. Cerebral palsy (Correct Answer)

Nerve and Muscle Physiology Explanation: ***Cerebral palsy*** - Cerebral palsy is a **disorder of movement and posture** caused by non-progressive brain damage, primarily affecting the **upper motor neurons**. - While EMG can assess muscle activity, its primary role is in evaluating **lower motor neuron diseases** and **neuromuscular junction disorders**, making it less directly useful for diagnosing a central nervous system disorder like cerebral palsy. *Myasthenia gravis* - EMG, particularly **repetitive nerve stimulation**, is highly useful for diagnosing myasthenia gravis by revealing a **decremental response** in the compound muscle action potential due to impaired neuromuscular transmission [1]. - It directly assesses the function of the **neuromuscular junction**, which is the site of pathology in myasthenia gravis [1]. *Charcot-Marie-Tooth disease* - EMG and **nerve conduction studies (NCS)** are crucial for diagnosing Charcot-Marie-Tooth disease by demonstrating **abnormal nerve conduction velocities** (demyelinating forms) or **reduced amplitude** of compound muscle action potentials (axonal forms) which indicate peripheral nerve damage. - The findings help characterize the type and severity of **peripheral neuropathy**, a hallmark of this condition. *Spinal muscular atrophy* - EMG is essential for diagnosing spinal muscular atrophy by showing **denervation and reinnervation changes** in muscles, such as **fibrillations**, **positive sharp waves**, and **large-amplitude, long-duration motor unit potentials** [1]. - These findings reflect the loss of **anterior horn cells** and subsequent attempts by surviving motor neurons to reinnervate muscle fibers.

Nerve and Muscle Physiology Flashcard 1 of 5

Question: The _____ refractory period refers to the period in which a conductive action potential cannot be generated

Answer: effective

Nerve and Muscle Physiology Flashcard 2 of 5

Question: Resting membrane potential of a nerve fiber is equal to the equilibrium potential of _____  and is _____mV.

Answer: Cl-::Ion; -70

Nerve and Muscle Physiology Flashcard 3 of 5

Question: Ligand gated ion channels are also known as _____ receptors

Answer: Ionotropic

Nerve and Muscle Physiology Flashcard 4 of 5

Question: Absolute refractory period is due to closure of _____ gates of sodium channel.

Answer: inactive

Nerve and Muscle Physiology Flashcard 5 of 5

Question: When the membrane potential is _____ mv the inactivation gate of Na+ channel is closed

Answer: between +35 to -90