Calcium Signaling — MCQs
Sequence the events in neuromuscular action potential conduction: 1. Sodium channels open in the end plate 2. Calcium enters at the nerve terminal 3. Release of acetylcholine
Source of ATP in RBCs is
All the following mediate their action using cAMP as second messenger except:
Which one of the following substances increases the release of Ca2+ from the endoplasmic reticulum?
A small Ca+2 binding protein that modifies the activity of many enzymes and other proteins in response to changes of Ca+2 concentration is known as:
Which cytokine activates eosinophils?
Some cells secrete chemicals into the extracellular fluid that act on cells in the same tissue. Which of the following refers to this type of regulation?
Equilibrium potential of calcium is
Which hormone primarily inhibits gastric acid secretion in response to acidic chyme?
During which phase of the cell cycle is the cellular content of DNA doubled?
Calcium Signaling Indian Medical PG Practice Questions and MCQs
Question 1: Sequence the events in neuromuscular action potential conduction: 1. Sodium channels open in the end plate 2. Calcium enters at the nerve terminal 3. Release of acetylcholine
- A. $1 \rightarrow 2 \rightarrow 3$
- B. $1 \rightarrow 3 \rightarrow 2$
- C. $3 \rightarrow 2 \rightarrow 1$
- D. $2 \rightarrow 3 \rightarrow 1$ (Correct Answer)
Explanation: ***Correct: $2 \rightarrow 3 \rightarrow 1$*** - **Calcium entry at the nerve terminal** is the initial trigger - when an action potential reaches the presynaptic nerve terminal, voltage-gated calcium channels open, allowing Ca²⁺ influx - **Acetylcholine release** follows - the increased intracellular calcium causes synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft - **Sodium channels open in the end plate** last - acetylcholine binds to nicotinic receptors on the motor end plate, opening ligand-gated sodium channels, which depolarizes the muscle membrane and triggers muscle contraction *Incorrect: $1 \rightarrow 2 \rightarrow 3$* - Places sodium channel opening first, which is physiologically impossible - Sodium channels at the motor end plate only open in response to acetylcholine binding - Cannot occur before acetylcholine is released from the nerve terminal *Incorrect: $1 \rightarrow 3 \rightarrow 2$* - Incorrectly sequences sodium channel opening before calcium entry - Violates the fundamental principle that calcium influx is required for neurotransmitter release - Acetylcholine cannot be released without prior calcium entry *Incorrect: $3 \rightarrow 2 \rightarrow 1$* - Places acetylcholine release before calcium entry, which is impossible - Calcium-triggered exocytosis is an absolute requirement for neurotransmitter release - Without calcium influx, vesicles cannot fuse with the presynaptic membrane
Question 2: Source of ATP in RBCs is
- A. Glucose (Correct Answer)
- B. Fatty acid
- C. Amino acid
- D. Ketone body
Explanation: ***Glucose*** - Red blood cells (RBCs) lack mitochondria, so they cannot perform **aerobic respiration**, **fatty acid oxidation**, or utilize **ketone bodies** for ATP production. - Their sole source of ATP is through **anaerobic glycolysis**, which metabolizes glucose to produce a small amount of ATP. *Fatty acid* - Fatty acid oxidation, or **beta-oxidation**, occurs in the mitochondria. - Since mature **RBCs lack mitochondria**, they cannot metabolize fatty acids to produce ATP. *Amino acid* - Amino acid metabolism for energy primarily involves the **Krebs cycle** and **oxidative phosphorylation**, which also take place in the mitochondria. - **RBCs lack the necessary enzymatic machinery** and organelles for this process. *Ketone body* - Ketone bodies are metabolized for energy in mitochondria, particularly in tissues like the brain and muscle, through the **Krebs cycle**. - **RBCs do not have mitochondria** and thus cannot utilize ketone bodies as an energy source.
Question 3: All the following mediate their action using cAMP as second messenger except:
- A. Glucagon
- B. Dopamine
- C. Corticotropin
- D. Vasopressin (Correct Answer)
Explanation: ***Vasopressin (ADH)*** - Vasopressin has **dual signaling mechanisms** depending on receptor type: - **V2 receptors** (kidney collecting duct): Use **Gs-protein → cAMP pathway** for water reabsorption via aquaporin-2 insertion - **V1 receptors** (vascular smooth muscle): Use **Gq-protein → IP3/DAG pathway** for vasoconstriction - In the context of this question, vasopressin is considered the exception because it has **significant non-cAMP mediated actions** through V1 receptors, unlike the other hormones listed which **predominantly or exclusively** use cAMP - **Note**: This is a teaching point about receptor subtypes; vasopressin DOES use cAMP at V2 receptors *Glucagon* - **Exclusively uses cAMP pathway** in hepatocytes and adipocytes - Binds to **glucagon receptor** (GPCR) → **Gs-protein** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Promotes glycogenolysis, gluconeogenesis, and lipolysis *Dopamine* - **D1 and D5 receptors** are **Gs-coupled** → **stimulate adenylyl cyclase** → **increase cAMP** - Important for neurotransmission (motor control, reward) and renal vasodilation - D2-family receptors (D2, D3, D4) inhibit cAMP but D1-family predominates in many physiological contexts *Corticotropin (ACTH)* - Binds to **melanocortin-2 receptor (MC2R)** on adrenal cortex - **Gs-protein coupled** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Stimulates steroidogenesis and cortisol secretion - **Exclusively cAMP-dependent mechanism**
Question 4: Which one of the following substances increases the release of Ca2+ from the endoplasmic reticulum?
- A. Inositol triphosphate (Correct Answer)
- B. 1,25 - dihydroxy cholecalciferol
- C. Diacylglycerol
- D. Parathyroid hormone
Explanation: ***Inositol triphosphate*** - **Inositol triphosphate (IP3)** is a secondary messenger that binds to specific receptors on the **endoplasmic reticulum (ER)**, triggering the release of stored **Ca2+** into the cytoplasm. - This calcium release plays a crucial role in various cellular processes, including **muscle contraction**, **neurotransmission**, and **hormone secretion**. *1,25 - dihydroxycholecalciferol* - This is the active form of **vitamin D**, primarily involved in **calcium absorption** from the gut and **calcium reabsorption** in the kidneys. - It does not directly increase Ca2+ release from the endoplasmic reticulum. *Diacylglycerol* - **Diacylglycerol (DAG)** is another secondary messenger, also produced from the cleavage of **PIP2** along with IP3. - DAG primarily activates **protein kinase C (PKC)**, which is involved in signal transduction pathways, but it does not directly trigger Ca2+ release from the ER. *Parathyroid hormone* - **Parathyroid hormone (PTH)** is a hormone that primarily regulates **extracellular calcium levels** by promoting bone resorption, increasing renal calcium reabsorption, and stimulating the synthesis of 1,25-dihydroxycholecalciferol. - While it ultimately increases blood calcium, it does not directly cause Ca2+ release from the endoplasmic reticulum.
Question 5: A small Ca+2 binding protein that modifies the activity of many enzymes and other proteins in response to changes of Ca+2 concentration is known as:
- A. Cyclin
- B. Kinesin
- C. Collagen
- D. Calmodulin (Correct Answer)
Explanation: ***Calmodulin*** - **Calmodulin** is a highly conserved, small **calcium-binding protein** that acts as a central mediator of calcium signaling pathways. - It undergoes a conformational change upon binding **Ca2+**, enabling it to interact with and regulate the activity of numerous target enzymes and proteins, thereby orchestrating various cellular processes. *Cyclin* - **Cyclins** are a family of proteins that activate **cyclin-dependent kinases (CDKs)**, playing a crucial role in regulating the progression through the cell cycle. - While essential for cell division, cyclins do not primarily function as **Ca2+-binding regulatory proteins**. *Kinesin* - **Kinesin** is a motor protein that moves along **microtubules**, transporting vesicles, organelles, and other cellular components within the cell. - Its primary function is in intracellular transport and cell division, not in **Ca2+-dependent enzymatic regulation**. *Collagen* - **Collagen** is the most abundant protein in mammals, forming the main structural component of connective tissues like skin, bone, and cartilage. - It provides structural integrity and tensile strength to tissues and is not involved in **calcium-dependent enzyme modulation**.
Question 6: Which cytokine activates eosinophils?
- A. IL-1
- B. IL-4
- C. IL-5 (Correct Answer)
- D. IL-6
Explanation: ***IL-5*** - **Interleukin-5 (IL-5)** is the primary cytokine responsible for activating **eosinophils**, stimulating their **production, maturation, and release** from the bone marrow. - It also enhances eosinophil survival and function at sites of inflammation, particularly in **allergic reactions** and **parasitic infections**. *IL-1* - **Interleukin-1 (IL-1)** is a **pro-inflammatory cytokine** primarily involved in fever, acute phase responses, and activating lymphocytes, but not directly in eosinophil activation. - It plays a role in general immune responses and inflammation by activating various cell types, including **macrophages and endothelial cells**. *IL-4* - **Interleukin-4 (IL-4)** is crucial for **Th2 differentiation** and isotype switching to IgE in B cells, promoting allergic responses. - While involved in allergic reactions, it primarily affects B cell and T cell function rather than direct activation of eosinophils, though it can support their development indirectly. *IL-6* - **Interleukin-6 (IL-6)** is a **pro-inflammatory cytokine** involved in acute phase reactions, hematopoiesis, and immune responses. - It is critical for B cell differentiation and T cell activation but does not directly activate eosinophils.
Question 7: Some cells secrete chemicals into the extracellular fluid that act on cells in the same tissue. Which of the following refers to this type of regulation?
- A. Neural
- B. Endocrine
- C. Neuroendocrine
- D. Paracrine (Correct Answer)
Explanation: ***Paracrine*** - **Paracrine signaling** involves chemical messengers, or **paracrine factors**, that act on **neighboring cells** within the **same tissue** without entering the bloodstream. - This type of regulation is crucial for local communication and coordination, such as in wound healing or immune responses. *Neural* - **Neural regulation** involves communication via **neurons** that transmit **electrical signals** (action potentials) and release **neurotransmitters** at synapses. - Neurotransmitters act on target cells, which can be distant from the neuron, for rapid and precise responses throughout the body. *Endocrine* - **Endocrine regulation** involves glands that secrete **hormones** directly into the **bloodstream**, which then travel to distant target cells in other tissues or organs. - This form of signaling leads to widespread and long-lasting effects, such as growth regulation or metabolic control. *Neuroendocrine* - **Neuroendocrine regulation** is a hybrid system where specialized **neurons** (neurosecretory cells) release **hormones** into the **bloodstream**, rather than releasing neurotransmitters into a synapse. - An example is the hypothalamus secreting ADH and oxytocin, which act on distant target organs.
Question 8: Equilibrium potential of calcium is
- A. +130mV (Correct Answer)
- B. -32mV
- C. +65mV
- D. -95mV
Explanation: ***+130mV*** - The **equilibrium potential** for an ion is the **membrane potential** at which the net movement of that ion across the membrane is zero, even if there is a concentration gradient. - Due to the significantly higher extracellular concentration of **calcium ions (Ca2+)** relative to the intracellular concentration, a large positive membrane potential is required to prevent Ca2+ influx. *-32mV* - This value does not represent the typical **equilibrium potential** for any major physiological ion like sodium, potassium, chloride, or calcium in mammalian cells. - Equilibrium potentials are highly dependent on the **concentration gradients** of the specific ion. *+65mV* - This value is close to the typical **equilibrium potential for sodium (Na+)**, which is approximately +60 to +70 mV in many cells, due to its outward concentration gradient. - **Calcium's equilibrium potential** is much more positive than sodium's due to its larger concentration gradient and its divalent charge. *-95mV* - This value is close to the typical **equilibrium potential for potassium (K+)**, which is approximately -90 to -95 mV, reflecting the movement of potassium out of the cell. - The **equilibrium potential for calcium** is highly positive, whereas this negative value suggests an inward current for a positively charged ion.
Question 9: Which hormone primarily inhibits gastric acid secretion in response to acidic chyme?
- A. Secretin
- B. Somatostatin (Correct Answer)
- C. Insulin
- D. Gastrin
Explanation: ***Somatostatin*** - **Somatostatin** is the **primary hormone** that inhibits gastric acid secretion in response to acidic chyme. - Released by D cells in the stomach and duodenum when pH drops below 3.0. - **Direct inhibitory effects:** Inhibits parietal cells directly, suppresses gastrin release from G cells, and blocks histamine release from ECL cells. - Acts as the main **negative feedback mechanism** to prevent excessive gastric acidification. *Secretin* - **Secretin** is released by S cells in the duodenum in response to acidic chyme (pH < 4.5). - Its **primary function** is to stimulate pancreatic bicarbonate secretion to neutralize duodenal acid. - While it does have a **secondary effect** of inhibiting gastric acid secretion, this is not its primary role. *Gastrin* - **Gastrin** is a hormone that **stimulates** gastric acid secretion, not inhibits it. - Released by G cells in the gastric antrum in response to peptides, amino acids, and gastric distension. - Promotes acid secretion by stimulating parietal cells and ECL cells (which release histamine). *Insulin* - **Insulin** is a pancreatic hormone primarily involved in **glucose metabolism** and cellular glucose uptake. - It has **no significant role** in the regulation of gastric acid secretion.
Question 10: During which phase of the cell cycle is the cellular content of DNA doubled?
- A. Mitotic phase
- B. G1 phase
- C. G2 phase
- D. S phase (Correct Answer)
Explanation: **Explanation:** The cell cycle is a highly regulated sequence of events divided into Interphase and the Mitotic (M) phase. **Why S phase is correct:** The **S phase (Synthesis phase)** is the specific period during interphase when **DNA replication** occurs. During this stage, the DNA content of the cell doubles (from 2n to 4n in terms of DNA mass, though the chromosome number remains the same). This ensures that when the cell eventually divides, each daughter cell receives a complete and identical set of genetic material. Key enzymes like DNA polymerase are most active during this phase. **Why other options are incorrect:** * **G1 phase (Gap 1):** This is the pre-synthetic phase characterized by cell growth and RNA/protein synthesis. The DNA content remains at the diploid level (2n). * **G2 phase (Gap 2):** This is the post-synthetic phase. While the DNA content has already doubled by this stage, no further synthesis occurs here; the cell focuses on synthesizing proteins like tubulin for the mitotic spindle. * **Mitotic (M) phase:** This is the phase of actual nuclear and cytoplasmic division. The doubled DNA is distributed equally into two daughter cells, returning the DNA content back to the original level. **High-Yield NEET-PG Pearls:** * **G1 Phase** is the most variable in duration and determines the overall length of the cell cycle. * **Quiescent stage (G0):** Cells that stop dividing (like neurons or mature muscle cells) exit the cycle at the G1 checkpoint. * **Checkpoints:** The transition from G1 to S is the most critical regulatory checkpoint (Restriction point), primarily regulated by **Cyclin D and CDK4**. * **Vincristine/Vinblastine** (anti-cancer drugs) act specifically on the **M phase** by inhibiting microtubule formation.
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