Smooth Muscle Physiology — MCQs
Which of the following statements is true regarding smooth muscle contraction?
What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?
Motor protein in organ of Corti -
Which of the following neurotransmitters is primarily released from the sympathetic nervous system to increase heart rate in response to a DECREASE in blood pressure?
Slow wave potential originates in which of the following structures in the intestine:
Hyperpolarization is caused by which ions?
The most distinguishing feature between skeletal and smooth muscle is the absence of ------ in smooth muscle.
In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?
The most significant immediate result of lowered serum calcium is
Protein connecting Z-lines to M-lines is:
Smooth Muscle Physiology Indian Medical PG Practice Questions and MCQs
Question 1: 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.
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.
Question 2: What is the second messenger responsible for smooth muscle relaxation mediated by nitric oxide (NO)?
- A. Calcium
- B. Cyclic AMP
- C. Cyclic GMP (Correct Answer)
- D. Magnesium
Explanation: ***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.
Question 3: Motor protein in organ of Corti -
- A. Kinesin
- B. Myosin (Correct Answer)
- C. Albumin
- D. Dynein
Explanation: **Myosin** - **Myosin I** and **Myosin VIIa** are key motor proteins found in the **hair cells** of the organ of Corti. - They play crucial roles in **stereocilia bundle stiffness** and the **adaptation** process of mechanotransduction, which is essential for hearing. *Kinesin* - **Kinesin** is primarily involved in **anterograde axonal transport** of vesicles and organelles along microtubules in neurons. - It is not the primary motor protein responsible for the unique mechanical properties of hair cells in the organ of Corti. *Albumin* - **Albumin** is a major **plasma protein** primarily involved in maintaining **osmotic pressure** and transporting various substances in the bloodstream. - It is not a motor protein and does not have a direct role in the mechanical function of the organ of Corti. *Dynein* - **Dynein** is a microtubule-associated motor protein responsible for **retrograde axonal transport** and the movement of cilia and flagella. - While important for intracellular transport in many cells, it is not the primary motor protein driving the mechanical processes within the hair cells of the organ of Corti.
Question 4: Which of the following neurotransmitters is primarily released from the sympathetic nervous system to increase heart rate in response to a DECREASE in blood pressure?
- A. Norepinephrine (Correct Answer)
- B. Dopamine
- C. Acetylcholine
- D. Epinephrine
Explanation: ***Norepinephrine*** - **Norepinephrine** is the primary neurotransmitter released by **postganglionic sympathetic neurons** directly onto the heart to increase heart rate and contractility in response to a drop in blood pressure. - It acts on **beta-1 adrenergic receptors** in the sinoatrial (SA) node, atria, and ventricles, leading to increased chronotropy (heart rate) and inotropy (contractility). *Dopamine* - While **dopamine** can have cardiovascular effects, particularly at high doses, it is not the primary neurotransmitter released by the sympathetic nervous system for direct heart rate regulation. - Dopamine is a precursor to norepinephrine and epinephrine, but its main physiological roles involve **renal blood flow regulation** and central nervous system functions. *Acetylcholine* - **Acetylcholine** is the primary neurotransmitter of the **parasympathetic nervous system**, which generally acts to **decrease heart rate** (bradycardia) through muscarinic receptors. - It is also released by **preganglionic sympathetic fibers**, but these do not directly innervate the heart to produce the desired effect of increasing heart rate. *Epinephrine* - **Epinephrine** (adrenaline) is primarily a **hormone** released from the **adrenal medulla** into the bloodstream, not directly from postganglionic sympathetic nerve terminals to the heart. - Although it has strong effects on beta-1 receptors in the heart, its release is more generalized and slower than the direct neuronal release of norepinephrine.
Question 5: Slow wave potential originates in which of the following structures in the intestine:
- A. Interstitial cells of Cajal (Correct Answer)
- B. Smooth muscle cells
- C. Myenteric plexus
- D. Parasympathetic neurons
Explanation: ***Interstitial cells of Cajal*** - The **interstitial cells of Cajal (ICCs)** are specialized **pacemaker cells** located throughout the gastrointestinal tract. - They generate spontaneous rhythmic depolarizations and repolarizations of the cell membrane, known as **slow waves**, which set the pace for smooth muscle contractions. *Smooth muscle cells* - While smooth muscle cells are the target of slow waves and contract in response to them, they do not **spontaneously generate** the slow waves themselves. - Their contractions are primarily regulated by the **electrical activity** transmitted from the ICCs. *Myenteric plexus* - The **myenteric plexus (Auerbach's plexus)** is a network of neurons that primarily controls **gastrointestinal motility**. - It modulates the frequency and amplitude of contractions but does not originate the fundamental rhythm of slow waves. *Parasympathetic neurons* - **Parasympathetic neurons** regulate various gastrointestinal functions, including motility and secretion, by releasing **neurotransmitters** like acetylcholine. - They can modulate the activity of ICCs and smooth muscle cells but are not the source of the electrical slow wave potentials themselves.
Question 6: Hyperpolarization is caused by which ions?
- A. K+ (Correct Answer)
- B. Na+
- C. HCO3-
- D. Ca2+
Explanation: ***K+*** - **Efflux of K+ ions** out of the cell makes the inside of the cell more negative, leading to **hyperpolarization**. - This efflux is typically mediated by **voltage-gated potassium channels** opening, or by activation of **GABA-A** or **glycine receptors** that increase K+ conductance. *Na+* - **Influx of Na+ ions** into the cell makes the inside of the cell more positive, causing **depolarization**, not hyperpolarization. - This influx is responsible for the **rising phase of an action potential**. *Ca2+* - **Influx of Ca2+ ions** into the cell also contributes to **depolarization** and can trigger various intracellular processes. - Ca2+ influx is crucial for **neurotransmitter release** and muscle contraction, but not for hyperpolarization. *HCO3-* - Bicarbonate ions (**HCO3-**) play a significant role in **maintaining pH balance** in the body and are involved in various physiological processes. - While ion channels can conduct HCO3-, their movement is not typically the primary cause of cell membrane hyperpolarization.
Question 7: The most distinguishing feature between skeletal and smooth muscle is the absence of ------ in smooth muscle.
- A. Actin
- B. Troponin (Correct Answer)
- C. Tropomyosin
- D. Myosin
Explanation: ***Troponin*** - **Smooth muscle** lacks the **troponin complex** (troponin I, C, and T) that is essential for initiating contraction in skeletal and cardiac muscle. - Instead of troponin, smooth muscle uses **calmodulin** to bind calcium, which then activates **myosin light chain kinase** to regulate contraction. *Tropomyosin* - **Tropomyosin** is present in both **skeletal** and **smooth muscle**, though it plays a different regulatory role in smooth muscle. - In smooth muscle, tropomyosin does not block myosin binding sites, but rather modulates the interaction between actin and myosin. *Myosin* - **Myosin** is a fundamental motor protein found in all types of muscle, including both **skeletal** and **smooth muscle**. - It forms thick filaments and interacts with actin to generate force and muscle contraction. *Actin* - **Actin** is a primary component of thin filaments and is universally present in all muscle types, including **skeletal** and **smooth muscle**. - It provides the framework along which myosin heads slide to produce muscle shortening.
Question 8: In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?
- A. Sarcomere
- B. Sarcolemma
- C. Myofibril (Correct Answer)
- D. Myofilament
Explanation: ***Myofibril*** - A **myofibril** is a cylindrical organelle that runs longitudinally inside a muscle fiber and contains **contractile proteins**. - Myofibrils are composed of repeating units called **sarcomeres**, which are the fundamental units of muscle contraction. *Sarcomere* - A **sarcomere** is the basic contractile unit of a muscle fiber, extending from one Z-disc to the next. - While it is a key component for muscle contraction, it is a **segment within a myofibril**, not the threadlike component that extends the entire length of the fiber. *Sarcolemma* - The **sarcolemma** is the cell membrane of a muscle fiber, responsible for transmitting nerve impulses to the muscle cell. - It encloses the muscle fiber but is not an internal, threadlike contractile component. *Myofilament* - **Myofilaments** are the individual protein filaments (actin and myosin) that make up a sarcomere within a myofibril. - They are the **smallest contractile elements**, but they are not the threadlike structure that extends along the entire muscle fiber.
Question 9: The most significant immediate result of lowered serum calcium is
- A. Decalcification of bones
- B. Decalcification of teeth
- C. Weakened heart action
- D. Hyperirritability of nerves and muscles (Correct Answer)
Explanation: ***Hyperirritability of nerves and muscles*** - Lowered serum calcium (hypocalcemia) decreases the threshold potential of excitable cells, leading to **increased neuronal and muscular excitability**. - This can manifest as **tetany**, muscle cramps, paresthesias, and in severe cases, seizures. *Decalcification of bones* - **Chronic hypocalcemia** can lead to secondary hyperparathyroidism, which may eventually cause bone decalcification. - This is a **long-term effect**, not an immediate significant result of acutely lowered serum calcium. *Decalcification of teeth* - Tooth decalcification is primarily associated with **fluoride deficiency**, poor oral hygiene, or acidic erosion, not directly with acute systemic hypocalcemia. - The calcium in teeth is **highly stable** and less readily mobilized than bone calcium in response to acute serum calcium changes. *Weakened heart action* - While severe **hypocalcemia can impair myocardial contractility** and lead to a weakened heart action, it is often preceded or accompanied by significant neuromuscular symptoms. - **Hyperkalemia** is more classically associated with immediate life-threatening cardiac dysfunction, while hypocalcemia primarily affects nerve and muscle excitability first.
Question 10: Protein connecting Z-lines to M-lines is:
- A. Kinin
- B. Desmin
- C. Titin (Correct Answer)
- D. Actin
Explanation: ***Titin*** - **Titin** is a giant protein that extends from the **Z-disc to the M-line** in the sarcomere, acting as a molecular spring. - It maintains the **structural integrity** of the sarcomere and provides passive elasticity to muscles. *Kinin* - **Kinin** is a protein involved in **inflammation and blood pressure regulation**, not a structural component of muscle sarcomeres. - Examples include **bradykinin**, which mediates pain and vasodilation. *Desmin* - **Desmin** is an **intermediate filament protein** that forms a scaffold around the Z-discs, linking myofibrils together. - While it connects myofibrils, it does not directly span between the Z-line and M-line within a single sarcomere. *Actin* - **Actin** is a primary component of **thin filaments** in the sarcomere and participates in muscle contraction, but it does not connect the Z-line to the M-line. - Thin filaments are anchored at the **Z-disc** but only extend partway into the A-band.
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