Mean arterial pressure is calculated as:
What is the definition of preload in the context of cardiac physiology?
Neurotransmitter involved in nigrostriatal pathway is?
What is a key difference between smooth muscle and skeletal muscle physiology?
When the tension in a muscle fibre is maximum, its length is called?
Which of the following is the MOST accurate statement about CSF?
What triggers the cephalic phase of gastric secretion?
When differentiated cells transform to form cells characteristic of other tissues, the process is called as -
Cells most sensitive to hypoxia are?
Which structure of the eye has the maximum refractive power?
NEET-PG 2012 - Physiology NEET-PG Practice Questions and MCQs
Question 71: Mean arterial pressure is calculated as:
- A. (DBP+3SBP)/2
- B. (SBP+3DBP)/2
- C. (DBP+2SBP)/3
- D. (SBP+2DBP)/3 (Correct Answer)
Explanation: ***(SBP+2DBP)/3*** - This formula accurately calculates **mean arterial pressure (MAP)**, emphasizing the longer duration of diastole compared to systole in the cardiac cycle. - The diastolic blood pressure (**DBP**) is weighted twice as much as the systolic blood pressure (**SBP**) to reflect this physiological difference. *(DBP+2SBP)/3* - This formula incorrectly weighs the diastolic pressure less and the systolic pressure more, which does not reflect the **physiological duration of the cardiac cycle**. - While it attempts to average pressures, it does not correctly represent the **mean perfusion pressure**. *(SBP+3DBP)/2* - This formula is inaccurate for calculating MAP as the **denominator should be 3**, not 2, to account for the three components being averaged (one SBP and two DBP). - It also disproportionately weights **DBP** too high relative to the standard physiological formula. *(DBP+3SBP)/2* - This formula is incorrect as it applies an **excessive weighting to SBP** and uses an incorrect denominator. - It would yield a significantly higher and inaccurate value for **mean arterial pressure**.
Question 72: What is the definition of preload in the context of cardiac physiology?
- A. Volume of blood in the ventricles at the end of systole
- B. Volume of blood in the ventricles at the end of diastole (Correct Answer)
- C. Amount of blood pumped by the heart per beat
- D. Resistance to blood flow in the arteries
Explanation: ***Volume of blood in the ventricles at the end of diastole*** - Preload represents the **initial stretching** of the cardiac myocytes prior to contraction, largely determined by the **volume of blood filling the ventricles** at the end of relaxation (diastole). - This **end-diastolic volume** directly correlates with the ventricular muscle fiber length at the start of systole, influencing the force of contraction according to the **Frank-Starling mechanism**. *Volume of blood in the ventricles at the end of systole* - This describes the **end-systolic volume**, which is the amount of blood remaining in the ventricle after it has contracted and ejected blood. - End-systolic volume is a determinant of the **ejection fraction** but does not define preload. *Amount of blood pumped by the heart per beat* - This refers to the **stroke volume**—the volume of blood ejected from the left ventricle with each heartbeat. - While preload influences stroke volume, stroke volume itself is not the definition of preload. *Resistance to blood flow in the arteries* - This describes **afterload**, which is the pressure or resistance the ventricle must overcome to eject blood during systole. - Afterload primarily affects the *force* needed for contraction, rather than the initial stretch or filling volume of the heart.
Question 73: Neurotransmitter involved in nigrostriatal pathway is?
- A. Acetylcholine
- B. Serotonin
- C. Dopamine (Correct Answer)
- D. Norepinephrine
Explanation: ***Dopamine*** - The **nigrostriatal pathway** is a major dopaminergic pathway in the brain, originating in the **substantia nigra pars compacta** and projecting to the striatum. - It is crucial for the control of voluntary movement, and its degeneration is a hallmark of **Parkinson's disease**. *Serotonin* - Serotonin (5-HT) is primarily involved in mood, sleep, appetite, and cognition, and is not the primary neurotransmitter of the **nigrostriatal pathway**. - Serotonergic pathways originate in the **raphe nuclei** and project widely throughout the brain. *Acetylcholine* - Acetylcholine is a key neurotransmitter in the periphery (neuromuscular junction, autonomic nervous system) and in the central nervous system, involved in learning and memory. - Cholinergic neurons in the **basal forebrain** project to the cortex and hippocampus, but acetylcholine is not the neurotransmitter of the **nigrostriatal pathway**. *Norepinephrine* - Norepinephrine (noradrenaline) is involved in arousal, attention, and the fight-or-flight response, with pathways originating in the **locus coeruleus**. - While it plays a role in modulating motor circuits, it is not the main neurotransmitter of the **nigrostriatal pathway**.
Question 74: What is a key difference between smooth muscle and skeletal muscle physiology?
- A. Calcium is required for contraction.
- B. Troponin is absent in smooth muscle. (Correct Answer)
- C. Myosin is essential for contraction.
- D. Potassium is required for contraction.
Explanation: ***Troponin is absent in smooth muscle.*** * Smooth muscle contraction is regulated by **calcium-calmodulin complex** and subsequent activation of **myosin light chain kinase (MLCK)**, in contrast to skeletal muscle's reliance on the troponin-tropomyosin system. * **Troponin** is a calcium-binding protein found in skeletal and cardiac muscle, which plays a critical role in regulating muscle contraction by initiating the movement of tropomyosin, thereby exposing myosin-binding sites on actin. *Calcium is required for contraction.* * While calcium is indeed required for contraction in both smooth and skeletal muscle, the **mechanism of its action** differs, making this statement insufficiently discriminative as a *key difference*. * In both muscle types, an increase in intracellular **calcium** initiates the contractile process, but the downstream signaling pathways diverge significantly. *Myosin is essential for contraction.* * **Myosin** is a fundamental motor protein essential for contraction in *all* muscle types, including skeletal, cardiac, and smooth muscle. * This statement highlights a similarity, not a key difference, as **actin-myosin cross-bridge cycling** is the basis of force generation in all muscle tissues. *Potassium is required for contraction.* * **Potassium ions** are crucial for maintaining the resting membrane potential and for repolarization following an action potential, which is necessary for muscle excitability, but they do not directly trigger muscle contraction. * The influx of calcium (or release from intracellular stores) is the direct trigger for contraction, not potassium.
Question 75: When the tension in a muscle fibre is maximum, its length is called?
- A. None of the options
- B. Initial length
- C. Equilibrium length
- D. Optimum length (Correct Answer)
Explanation: ***Optimum length*** - This is the muscle length at which the **maximum number of cross-bridges** can form between actin and myosin filaments. - At this length, the sarcomere allows for the **greatest overlap** of thick and thin filaments without excessive stretching or compression, leading to peak tension generation. *Equilibrium length* - This term usually refers to the **resting length** of a muscle fiber when no external forces are acting upon it. - At equilibrium, the tension generated by the muscle may not necessarily be at its maximum. *Initial length* - This is a general term that refers to the **starting length** of a muscle fiber before it contracts or is stretched. - It does not specifically denote the length at which maximum tension is achieved. *None of the options* - This option is incorrect because **optimum length** accurately describes the muscle length yielding maximum tension.
Question 76: Which of the following is the MOST accurate statement about CSF?
- A. Formed by the choroid plexus in the ventricles. (Correct Answer)
- B. Normally contains no neutrophils
- C. pH is less than that of plasma
- D. Removal of CSF during dural tap can cause a headache due to the change in pressure.
Explanation: ***Formed by the choroid plexus in the ventricles.*** * The **choroid plexus**, located in the ventricles of the brain, is primarily responsible for the production of **cerebrospinal fluid (CSF)**. * Specialized epithelial cells of the choroid plexus filter blood plasma to produce CSF, which then circulates through the central nervous system. *Normally contains no neutrophils* * Normal CSF should contain **virtually no neutrophils**; their presence typically indicates an inflammatory or infectious process, such as **bacterial meningitis**. * While normal CSF doesn't have neutrophils, this option isn't as broadly accurate as the choroid plexus statement because the presence of other cell types like lymphocytes in small numbers is normal. *pH is less than that of plasma* * The pH of CSF is typically **slightly lower than that of plasma** (around 7.31 compared to 7.40), but the statement "less than" is broad and the degree of difference can be variable and is a less defining characteristic than its formation site. * This slight difference in pH is important for regulating **respiration** through chemoreceptors, but it's not the most accurate or fundamental statement about CSF properties. *Removal of CSF during dural tap can cause a headache due to the change in pressure.* * A **post-dural puncture headache** (PDPH) is a well-known complication of a dural tap (lumbar puncture), caused by the leakage of CSF from the puncture site, leading to **intracranial hypotension**, not simply a change in pressure. * This decrease in CSF volume and pressure causes a traction on pain-sensitive structures within the cranium, resulting in a headache that is typically **worse when upright** and relieved by lying down.
Question 77: What triggers the cephalic phase of gastric secretion?
- A. On food entering stomach
- B. On food entering intestine
- C. On seeing food (Correct Answer)
- D. On stress
Explanation: ***On seeing food*** - The **cephalic phase** of gastric secretion is initiated by sensory input such as the sight, smell, taste, or even the thought of food. - This phase is mediated by the **vagus nerve**, stimulating gastric acid and enzyme secretion in anticipation of food arrival. *On food entering stomach* - This describes the initiation of the **gastric phase** of digestion, where mechanical stretch and chemical presence of food in the stomach stimulate further secretions. - The gastric phase primarily involves local reflexes and hormonal mechanisms (like **gastrin** release), rather than purely sensory input from the head. *On food entering intestine* - This marks the beginning of the **intestinal phase** of digestion, which involves both stimulatory and inhibitory signals for gastric secretion. - The primary role of the intestinal phase is to regulate the rate at which chyme enters the small intestine and to coordinate bile and pancreatic enzyme release. *On stress* - While stress can impact digestive function, it typically affects the **autonomic nervous system** in a generalized way, often leading to inhibition of digestion or altered motility. - Stress does not specifically trigger the cephalic phase of gastric secretion, which is a physiological response linked to nutrient anticipation.
Question 78: When differentiated cells transform to form cells characteristic of other tissues, the process is called as -
- A. De-differentiation
- B. Re-differentiation
- C. Trans-differentiation (Correct Answer)
- D. Sub-differentiation
Explanation: ***Trans-differentiation*** - **Trans-differentiation** refers to the direct conversion of one differentiated cell type into another differentiated cell type without entering a pluripotent stem cell state. - This process is achieved by altering the **gene expression profile** of existing cells to adopt the characteristics of a different lineage. *De-differentiation* - **De-differentiation** is the process where a specialized cell loses its specific characteristics and reverts to a more primitive or stem cell-like state. - This is often observed in certain disease processes, like cancer, or in response to injury where cells regain limited proliferative capacity. *Re-differentiation* - **Re-differentiation** typically describes a cell that has undergone de-differentiation and then differentiates again into a new or its original cell type. - This process is often seen in tissue repair, where progenitor cells proliferate and then re-differentiate to replace damaged tissue. *Sub-differentiation* - **Sub-differentiation** is not a standard or recognized term in cell biology or developmental biology to describe the transformation of stem cells into other tissue types. - The term does not have a defined meaning within the context of cellular lineage alterations.
Question 79: Cells most sensitive to hypoxia are?
- A. Myocardial cells
- B. Neurons (Correct Answer)
- C. Hepatocytes
- D. Renal tubular epithelial cells
Explanation: ***Neurons*** - Neurons have a very high metabolic rate and an **absolute requirement for oxygen** and glucose to maintain their complex electrochemical functions and ionic gradients. - Due to their lack of significant energy reserves and high metabolic demand, they can sustain **irreversible damage within minutes** (typically 3-5 minutes) of complete oxygen deprivation. *Myocardial cells* - While myocardial cells are highly susceptible to hypoxia and can undergo **ischemic necrosis** (e.g., in a myocardial infarction), they can often tolerate oxygen deprivation for somewhat longer periods than neurons due to some anaerobic metabolic capacity. - Significant damage to myocardial cells usually occurs after **20-30 minutes of severe ischemia**. *Hepatocytes* - Hepatocytes (liver cells) are relatively **resilient to hypoxia** compared to neurons, possessing significant metabolic flexibility and capacity for regeneration. - They can endure **longer periods of oxygen deprivation** before irreversible damage occurs, often hours. *Renal tubular epithelial cells* - Renal tubular epithelial cells are generally **sensitive to hypoxia**, especially those in the medulla, due to their high metabolic activity for reabsorption and secretion. - They are a common target for **acute tubular necrosis** in ischemic injury but generally have a **higher tolerance than neurons**, with damage becoming widespread after tens of minutes to an hour of severe ischemia.
Question 80: Which structure of the eye has the maximum refractive power?
- A. Anterior surface of lens
- B. Posterior surface of lens
- C. Anterior surface of cornea (Correct Answer)
- D. Posterior surface of cornea
Explanation: ***Anterior surface of cornea*** - The **cornea** accounts for approximately two-thirds of the eye's total refractive power due to the large difference in refractive index between air and the corneal tissue. - The **anterior surface of the cornea** is the primary site of light refraction as light enters the eye from the air. *Anterior surface of lens* - The **lens** contributes significantly to accommodation, but its overall refractive power is less than that of the cornea in the unaccommodated state. - The change in refractive index between the aqueous humor and the lens is less pronounced compared to the air-cornea interface. *Posterior surface of lens* - The **posterior surface of the lens** also contributes to the focusing power of the lens, but its curvature and refractive index difference are typically less than the anterior surface of the cornea. - Its contribution to total refractive power is secondary to the anterior corneal surface and the anterior lens surface. *Posterior surface of cornea* - The **posterior surface of the cornea** has a much smaller refractive power compared to the anterior surface due to the smaller difference in refractive index between the cornea and the aqueous humor. - This interface does contribute to refraction but is not the primary focusing component.