Which of the following statements is true regarding post-ganglionic parasympathetic fibers?
Which of the following fiber types is classically categorized as Group B nerve fibers?
Which of the following is classified as a pain receptor?
Spinal pathway mainly regulating fine motor activity?
What is the normal cerebral blood flow in milliliters per minute for a healthy adult?
What is the primary action observed in the withdrawal reflex?
Which of the following substances has the same concentration in cerebrospinal fluid (CSF) and plasma?
Cell bodies of orexigenic neurons are present in?
Cushing reflex is associated with all except?
Which of the following does not have sympathetic noradrenergic fibers?
NEET-PG 2013 - Physiology NEET-PG Practice Questions and MCQs
Question 31: Which of the following statements is true regarding post-ganglionic parasympathetic fibers?
- A. They originate from the spinal cord.
- B. They are part of the sympathetic nervous system.
- C. They are responsible for 'fight or flight' responses.
- D. They release acetylcholine at the target organs. (Correct Answer)
Explanation: ***They release acetylcholine at the target organs.*** - Post-ganglionic parasympathetic fibers are **cholinergic**, meaning they release the neurotransmitter **acetylcholine** at their effector organs. - This action mediates the characteristic "rest and digest" responses of the parasympathetic nervous system. *They originate from the spinal cord.* - **Pre-ganglionic parasympathetic fibers** originate from the **brainstem** (cranial nerves III, VII, IX, X) and the **sacral spinal cord** (S2-S4). - Post-ganglionic fibers originate in ganglia located near or within their target organs, not the spinal cord directly. *They are part of the sympathetic nervous system.* - Post-ganglionic parasympathetic fibers are a component of the **parasympathetic nervous system**, not the sympathetic nervous system. - The sympathetic and parasympathetic systems are distinct divisions of the autonomic nervous system with generally opposing functions. *They are responsible for 'fight or flight' responses.* - The **'fight or flight' response** is characteristic of the **sympathetic nervous system**, which prepares the body for stressful situations. - The parasympathetic nervous system is responsible for **'rest and digest' functions**, promoting energy conservation and maintenance activities.
Question 32: Which of the following fiber types is classically categorized as Group B nerve fibers?
- A. Sympathetic postganglionic
- B. Parasympathetic preganglionic
- C. Parasympathetic post ganglionic
- D. Sympathetic preganglionic (Correct Answer)
Explanation: ***Sympathetic preganglionic*** - **Group B nerve fibers** are **myelinated preganglionic autonomic fibers** with intermediate diameter (3-15 μm) and moderate conduction velocity (3-15 m/s) - Both **sympathetic and parasympathetic preganglionic fibers** are classified as Group B fibers - **Sympathetic preganglionic** neurons are the classical example, originating from T1-L2 spinal segments and synapsing in paravertebral or prevertebral ganglia *Sympathetic postganglionic* - These are **unmyelinated Group C fibers** with slow conduction velocity (0.5-2 m/s) - They extend from ganglia to target organs *Parasympathetic preganglionic* - These are also **Group B fibers** (myelinated preganglionic) - However, **sympathetic preganglionic** is the more commonly cited classical example in standard classifications - They originate from cranial nerves (III, VII, IX, X) and sacral segments (S2-S4) *Parasympathetic postganglionic* - These are **unmyelinated Group C fibers** with the slowest conduction velocities - Short fibers extending from ganglia near or within target organs to effector cells
Question 33: Which of the following is classified as a pain receptor?
- A. Free nerve endings (Correct Answer)
- B. Meissner's corpuscle
- C. Pacinian corpuscle
- D. Merkel disc
Explanation: ***Free nerve endings*** - **Free nerve endings** are the most common type of sensory receptor in the skin and are responsible for detecting **pain**, temperature, and crude touch. - They lack specialized structures and are found throughout the epidermis and dermis. *Meissner's corpuscle* - **Meissner's corpuscles** are encapsulated nerve endings that detect **light touch** and **vibration**. - They are rapidly adapting and are abundant in sensitive, hairless skin areas like fingertips and lips. *Pacinian corpuscle* - **Pacinian corpuscles** are large, encapsulated receptors that detect **deep pressure** and **vibration**. - They are rapidly adapting and respond to high-frequency stimuli. *Merkel disc* - **Merkel discs** are specialized epithelial cells associated with nerve endings that detect **sustained pressure** and **texture**. - They are slowly adapting receptors crucial for tactile discrimination.
Question 34: Spinal pathway mainly regulating fine motor activity?
- A. Lateral corticospinal tract (Correct Answer)
- B. Vestibulospinal tract
- C. Anterior corticospinal tract
- D. Reticulospinal tract
Explanation: ***Lateral corticospinal tract*** - This pathway contains **85-90% of corticospinal fibers** that cross at the medullary pyramids and descend in the **lateral funiculus** of the spinal cord - It is the **primary pathway for fine, precise, voluntary movements** of **distal extremities**, particularly the hands, fingers, feet, and toes - Enables intricate skilled movements like writing, buttoning, and fine manipulation due to direct monosynaptic connections to motor neurons - Damage results in loss of fine motor control and skilled movements *Anterior corticospinal tract* - Contains only **10-15% of corticospinal fibers** that descend uncrossed in the anterior spinal cord - Controls **bilateral movements of axial and proximal muscles** (neck, trunk, shoulders) - Not specialized for fine motor control of distal limbs *Vestibulospinal tract* - Regulates **posture and balance** by modulating extensor muscle tone - Coordinates head position and maintains upright posture - Does not control fine voluntary movements *Reticulospinal tract* - Modulates **muscle tone, posture, and locomotion** - Provides general motor control and autonomic regulation - Not specialized for precise, intricate fine motor movements
Question 35: What is the normal cerebral blood flow in milliliters per minute for a healthy adult?
- A. 55 ml/min
- B. 150 ml/min
- C. 750 ml/min (Correct Answer)
- D. 1000 ml/min
Explanation: ***750 ml/min*** - The brain receives approximately **15% of the cardiac output**, which for an average adult with a cardiac output of 5 L/min (5000 ml/min) translates to about **750 ml/min**. - This flow rate is essential to meet the high metabolic demands of the brain, which consumes about **20% of the body's total oxygen**. - For reference, this corresponds to approximately **50-55 ml/100g/min** when normalized to brain tissue weight. *55 ml/min* - This value represents the **cerebral blood flow per 100 grams of brain tissue** (50-55 ml/100g/min), not the **total cerebral blood flow**. - As a total flow value, 55 ml/min would be severely **inadequate** for the entire brain (~1400g) and would lead to immediate **ischemia** and neurological dysfunction. *150 ml/min* - While higher than 55 ml/min, this rate is still **grossly insufficient** to maintain the metabolic needs of the entire adult brain. - Such a low total flow would result in widespread **cerebral hypoperfusion** and severe neurological deficits. *1000 ml/min* - Although the brain has significant blood flow, 1000 ml/min is generally **higher than the normal average** for a healthy adult at rest. - The normal range is typically **750-800 ml/min**; sustained flow at 1000 ml/min might be seen in hyperemia or certain physiological states but is not the typical baseline.
Question 36: What is the primary action observed in the withdrawal reflex?
- A. Extension
- B. Flexion (Correct Answer)
- C. Flexion followed by extension
- D. Not applicable
Explanation: ***Flexion*** - The **withdrawal reflex** is a protective reflex that causes the affected limb to **flex** and withdraw from a painful stimulus. - This **flexion** is mediated by the contraction of flexor muscles and relaxation of extensor muscles, moving the limb away from danger. *Extension* - **Extension** is the opposite of flexion and would move the limb closer to or maintain its position relative to the painful stimulus. - This action is typically observed in the **crossed extensor reflex**, where the contralateral limb extends to support the body, not in the direct withdrawal of the stimulated limb. *Flexion followed by extension* - While **flexion** is the primary action, it is not typically followed immediately by extension within the same limb in a simple withdrawal reflex. - If a coordinated movement were to occur, such as shifting weight, the **crossed extensor reflex** would involve extension in the opposite limb. *Not applicable* - The withdrawal reflex involves a clear and defined muscle action which is **flexion**, making "not applicable" incorrect. - This reflex is a fundamental component of the nervous system's response to noxious stimuli.
Question 37: Which of the following substances has the same concentration in cerebrospinal fluid (CSF) and plasma?
- A. Glucose
- B. Ca
- C. HCO3
- D. Cl (Correct Answer)
Explanation: ***Cl*** - **Chloride ions (Cl-)** have the **closest concentration** between CSF and plasma among the listed options, with a CSF-to-plasma ratio of approximately 1.1-1.15. - CSF chloride is **slightly higher** than plasma chloride (CSF: ~120-130 mEq/L; Plasma: ~100-110 mEq/L) because chloride ions freely cross the **blood-brain barrier** and help maintain **electroneutrality** in CSF due to the low protein content. - The elevated chloride compensates for the absence of negatively charged proteins in CSF, making it the **best answer** among the given options. *Glucose* - **Glucose** concentration in CSF is approximately **60-70%** of plasma glucose concentration (CSF: 50-80 mg/dL; Plasma: 70-110 mg/dL). - Transport across the **blood-brain barrier** occurs via **GLUT1 transporters**, which are tightly regulated to meet brain metabolic demands. *Ca* - **Calcium (Ca2+)** concentration in CSF is **significantly lower** than in plasma (CSF: ~2.1-2.5 mg/dL; Plasma: ~8.5-10.5 mg/dL). - Only the **ionized, unbound fraction** can cross the blood-brain barrier, as protein-bound calcium cannot pass through. *HCO3* - **Bicarbonate (HCO3-)** concentration in CSF is typically **slightly lower** than in plasma (CSF: ~20-25 mEq/L; Plasma: ~22-28 mEq/L). - Active regulation maintains **CSF pH** and CO2 buffering capacity independent of plasma bicarbonate levels.
Question 38: Cell bodies of orexigenic neurons are present in?
- A. Dorsal raphe
- B. Locus coeruleus
- C. Lateral hypothalamic area (Correct Answer)
- D. Hippocampus
Explanation: ***Lateral hypothalamic area*** - The **lateral hypothalamic area** (LHA) contains neurons that produce **orexin (hypocretin)**, a neuropeptide critical for promoting appetite and wakefulness. - Stimulation of the LHA leads to increased food seeking and consumption, earning it the moniker "**feeding center**." *Dorsal raphe* - The **dorsal raphe nucleus** is a key source of **serotonin** in the brain, involved in mood, sleep-wake cycles, and appetite regulation (often promoting satiety). - It does not primarily house orexigenic neurons that directly stimulate appetite. *Locus coerulus* - The **locus coeruleus** is the primary source of **norepinephrine** in the brain, playing a significant role in arousal, attention, and stress response. - While it modulates appetitive behaviors indirectly, its neurons are not the primary orexigenic cell bodies. *Hippocampus* - The **hippocampus** is crucial for **learning, memory formation**, and spatial navigation. - It is not directly involved in the primary neural circuits that control hunger and satiety through orexigenic neuropeptides.
Question 39: Cushing reflex is associated with all except?
- A. Irregular respiration
- B. Hypotension (Correct Answer)
- C. Increased intracranial pressure
- D. Bradycardia
Explanation: ***Hypotension*** - The **Cushing reflex** is a compensatory response to increased intracranial pressure (ICP) aiming to maintain cerebral perfusion, which typically involves **hypertension**, not hypotension. - While prolonged or severe ICP can lead to decompensation and eventual hypotension, it is not a direct component of the reflex itself. *Increased intracranial pressure* - The **Cushing reflex** is triggered by an elevation in **intracranial pressure (ICP)**, as the body attempts to maintain blood flow to the brain. - This increased ICP reduces cerebral perfusion pressure, prompting a systemic response to raise mean arterial pressure. *Bradycardia* - **Bradycardia** is a classic component of the **Cushing reflex**, occurring as a compensatory response to the reflex hypertension. - The increased arterial blood pressure stimulates carotid and aortic baroreceptors, leading to a vagal response that slows the heart rate. *Irregular respiration* - **Irregular respiration** is another key component of the **Cushing reflex**, often manifesting as **Cheyne-Stokes breathing** or **ataxic breathing**. - This respiratory dysregulation is due to direct compression and dysfunction of the brainstem, specifically the medullary respiratory centers, caused by increased ICP.
Question 40: Which of the following does not have sympathetic noradrenergic fibers?
- A. Heart
- B. Eye
- C. Sweat gland (Correct Answer)
- D. Blood vessels
Explanation: ***Sweat gland*** - While sweat glands are innervated by the **sympathetic nervous system**, their postganglionic fibers are **cholinergic**, releasing **acetylcholine** rather than noradrenaline. - This is an important exception where sympathetic stimulation leads to acetylcholine release, causing sweating. *Blood vessels* - Most blood vessels, particularly resistance vessels such as **arterioles**, receive substantial **sympathetic noradrenergic innervation** that causes vasoconstriction. - This sympathetic tone is crucial for regulating **blood pressure** and distributing blood flow. *Heart* - The heart is richly innervated by **sympathetic noradrenergic fibers** that increase **heart rate**, **contractility**, and **conduction velocity** via beta-1 adrenergic receptors. - This makes noradrenaline a key neurotransmitter in the sympathetic regulation of cardiac function. *Eye* - The eye receives sympathetic noradrenergic innervation primarily to the **dilator pupillae muscle**, causing **mydriasis** (pupil dilation) upon activation. - These fibers also contribute to the sympathetic control of the **tarsal muscle** (Müller's muscle) in the eyelid.