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AIIMS Rajkot - 1st MBBS Physiology Paper I - Complete Answers

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SECTION A

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QUESTION 1 (10 Marks)

Name & Introduction

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Actions of Insulin

A. Carbohydrate Metabolism

B. Fat Metabolism

C. Protein Metabolism

D. Other Actions

• Promotes K+ uptake into cells (hypokalemia with excess insulin)
• Promotes cell growth and proliferation
• Stimulates gene transcription

Mechanism of Action

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Physiological Basis for Symptoms of Deficiency (Diabetes Mellitus)

• Costanzo Physiology 7th Ed., p.447-450

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QUESTION 2 - Short Notes (4 × 5 marks)

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2a) Cushing Syndrome - Causes and Features (5 marks)

Definition

Causes

1. Exogenous (most common): Prolonged pharmacologic use of corticosteroids
2. Cushing's Disease: Pituitary adenoma secreting excess ACTH → drives adrenal cortex to overproduce cortisol
3. Primary adrenal tumor: Adrenal adenoma/carcinoma independently overproduces cortisol (ACTH is LOW due to negative feedback)
4. Ectopic ACTH syndrome: ACTH-secreting tumors outside pituitary (e.g., small cell lung carcinoma)

Clinical Features (due to excess cortisol and androgens)

• Central (truncal) obesity
• Moon face (round face)
• Buffalo hump (supraclavicular and dorsocervical fat)
• Thin extremities (muscle wasting)

• Hyperglycemia (steroid diabetes)
• Hypertension (cortisol has weak mineralocorticoid activity + sensitizes vessels to catecholamines)
• Osteoporosis

• Muscle weakness and wasting
• Purple/violaceous striae (stretch marks from loss of connective tissue)
• Poor wound healing
• Thin skin

• Hirsutism and virilization in females
• Menstrual irregularities

• Cushing's disease (pituitary): ACTH elevated; suppressed by HIGH-dose dexamethasone
• Cushing's syndrome (adrenal/ectopic): ACTH low; not suppressed by dexamethasone
• Costanzo Physiology 7th Ed., p.442

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2b) Types of Contraceptives and Mechanism of Action (5 marks)

Hormonal Contraceptives

• Contains: Estrogen + Progestin
• Mechanism:
  • Inhibit GnRH → suppress FSH and LH surge → prevent ovulation (primary mechanism)
  • Thicken cervical mucus → prevent sperm penetration
  • Alter endometrium → prevent implantation

• Mechanism:
  • Thicken cervical mucus
  • Suppress LH surge (less reliable ovulation suppression)
  • Thin endometrium

• IM injection every 3 months
• Mechanism: Strong suppression of LH surge → no ovulation

• Contains etonogestrel; releases continuously
• Mechanism: Same as progestin-only

• High-dose levonorgestrel within 72 hours
• Mechanism: Delays or inhibits ovulation; may impair implantation

Non-Hormonal Contraceptives

• Copper IUD: Copper ions are spermicidal (inhibit sperm motility)
• Hormonal IUD (Mirena): releases levonorgestrel locally

• Condoms, diaphragm: Physical block to sperm

• Tubectomy (tubal ligation): Blocks passage of egg
• Vasectomy: Blocks sperm release

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2c) Sequence of Events in Neuromuscular Transmission (5 marks)

• ACh is synthesized from Acetyl CoA + Choline by choline acetyltransferase.

• Choline is taken back into the presynaptic terminal by an Na+-choline cotransporter (recycling).

Mnemonic: AP → Ca²+ → ACh release → Binds NnR → EPP → Muscle AP → Contraction → AChE terminates

• Costanzo Physiology 7th Ed., p.34

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2d) Functional Differences Between Skeletal and Smooth Muscle (5 marks)

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QUESTION 3 - Short Answer Questions (4 × 2 marks)

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3a) Myasthenia Gravis (2 marks)

• Pathophysiology: Autoantibodies against nicotinic ACh receptors (NnAChR) on the motor end plate → receptor destruction and complement-mediated damage → reduced EPP → failure to trigger muscle AP
• Also: Anti-MuSK (muscle-specific kinase) antibodies in seronegative cases
• Features: Fluctuating muscle weakness, worse with activity (fatigable weakness), improves with rest; ptosis, diplopia, dysphagia, respiratory muscle weakness
• Diagnosis: Edrophonium (Tensilon) test - AChE inhibitor rapidly reverses weakness
• Association: Thymoma/thymic hyperplasia
• Treatment: AChE inhibitors (neostigmine, pyridostigmine), immunosuppressants, thymectomy

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3b) LH Surge (2 marks)

• Trigger: Rising estrogen levels from the dominant follicle cause a positive feedback on the anterior pituitary (switch from negative to positive feedback above a threshold ~200 pg/mL for >36 hours)
• Result: LH surge triggers:
  1. Ovulation - final maturation and rupture of the Graafian follicle (~36-40 hours after LH surge peak)
  2. Resumption of meiosis I in the oocyte
  3. Luteinization - conversion of ruptured follicle into corpus luteum
• FSH also surges simultaneously (smaller)
• Clinical use: LH surge detection is used to time ovulation in fertility treatments

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3c) Major Role of Parathormone (PTH) (2 marks)

1. Bone: Activates osteoclasts → bone resorption → releases Ca²+ and PO₄³- into blood
2. Kidney (proximal tubule): Increases Ca²+ reabsorption; increases PO₄³- excretion (phosphaturia)
3. Kidney (activates 1α-hydroxylase): Converts 25-OH-D₃ to active 1,25-(OH)₂-D₃ (calcitriol) → which increases intestinal Ca²+ absorption
4. Net effect: Blood Ca²+ ↑, Blood PO₄³- ↓ (phosphate is excreted)

• Receptor: PTH binds to Gs-protein-coupled receptor → activates adenylyl cyclase → ↑cAMP

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3d) Role of Gap Junctions in Cellular Transport (2 marks)

1. Ion transport: Allow free passage of small ions (Na+, K+, Ca²+) - enabling electrical coupling between cells (e.g., cardiac myocytes, smooth muscle)
2. Small molecule transfer: Pass molecules <1000 Da - cAMP, IP₃, ATP, glucose, amino acids
3. Electrical syncytium: In cardiac muscle and single-unit smooth muscle, gap junctions allow action potentials to spread directly cell-to-cell without neurohumoral transmission
4. Metabolic coupling: Share nutrients and signaling molecules between adjacent cells
5. Embryonic development: Coordinate cell differentiation via chemical signaling
6. NOT permeable to: Large proteins, nucleic acids, organelles

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SECTION B

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QUESTION 4 (Case-Based: Hemiplegia with UMN lesion)

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4a) Pathway Affected and Diagram (4 marks)

Motor Cortex (Primary - Precentral gyrus, Area 4)
         ↓
Internal Capsule (Posterior limb, Genu)
         ↓
Cerebral Peduncle (Midbrain)
         ↓
Pons (Corticobulbar fibers diverge to cranial nerve nuclei here)
         ↓
Pyramids of Medulla
         ↓
DECUSSATION of Pyramids (at medullary-spinal cord junction)
         ↓
Lateral Corticospinal Tract (contralateral spinal cord)
         ↓
Anterior Horn Cells (Lower Motor Neurons)
         ↓
Muscle

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4b) Functions of the Corticospinal Tract (2 marks)

1. Voluntary movement: Initiates and controls precise, skilled voluntary movements, especially of distal limb muscles (fine finger movements)
2. Speed and dexterity: Fast-conducting pathway for fractionated (individual finger) movements
3. Tone: Carries inhibitory signals to reduce tone in antigravity muscles (damage leads to spasticity)
4. Reflexes: Modulates spinal cord reflex arcs; damage leads to hyperreflexia (release of inhibition)
5. Somatosensory: Some fibers end in dorsal horn and modify sensory processing

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4c) Differences: UMN Lesion vs LMN Lesion (2 marks)

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4d) Commonest Site of Lesion Causing Hemiplegia (1 mark)

• Here all corticospinal and corticobulbar fibers are tightly packed together
• A small lesion (e.g., lacunar infarct) causes complete contralateral hemiplegia with facial involvement
• This is the site affected in the most common type of stroke (hypertensive small vessel disease)

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QUESTION 5 - Short Notes (4 × 5 marks)

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5a) Auditory Pathway (5 marks)

1. Cochlea (receptor) - Hair cells of the Organ of Corti transduce sound vibrations to electrical signals
2. Cochlear nerve (CN VIII - vestibulocochlear nerve) - 1st order neurons (cell bodies in Spiral Ganglion)
3. Cochlear nucleus (dorsal and ventral) - in pons/medulla junction (1st synapse)
4. Trapezoid body - Major decussation point; most fibers cross to the contralateral side but bilateral representation begins here
5. Superior olivary nucleus - 2nd synapse (sound localization, binaural integration)
6. Lateral lemniscus - Ascends through pons
7. Inferior colliculus (midbrain) - Important relay for auditory reflexes (e.g., startle reflex)
8. Medial Geniculate Nucleus (MGN) of thalamus - Final relay before cortex
9. Primary Auditory Cortex (A1) - Located in Heschl's gyri (transverse temporal gyri), Brodmann's areas 41 and 42, in the superior temporal gyrus

• Bilateral representation at all levels above the cochlear nucleus (so unilateral central lesions rarely cause complete deafness)
• Tonotopic organization maintained throughout - each frequency has a spatial map
• The crossed pathway (contralateral ear) is dominant

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5b) Types of Aphasia (5 marks)

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5c) Role of Hypothalamus in Thermoregulation (5 marks)

Structure

• Anterior hypothalamus / Preoptic area: Contains heat-sensitive neurons; controls heat LOSS mechanisms
• Posterior hypothalamus: Controls heat PRODUCTION/conservation mechanisms

Sensors

• Central thermoreceptors: Neurons in preoptic area respond to blood temperature changes
• Peripheral thermoreceptors: Warm and cold receptors in skin send afferent signals

Responses to HIGH temperature (heat)

1. Cutaneous vasodilation - shunts blood to skin → heat loss by radiation/conduction
2. Sweating - evaporative heat loss (most effective mechanism in humans)
3. Decreased metabolic rate - reduce heat production
4. Behavioral responses - seeking cool environment, removing clothing

Responses to LOW temperature (cold)

1. Cutaneous vasoconstriction - reduces heat loss from skin
2. Shivering - involuntary skeletal muscle contractions → heat production
3. Non-shivering thermogenesis - catecholamines and thyroid hormone increase metabolic rate; brown adipose tissue activation
4. Piloerection - minimal in humans
5. Behavioral - huddling, adding clothing

Fever

• Pyrogens (bacteria, LPS) stimulate macrophages to release interleukin-1 (IL-1), IL-6, TNF-α
• These trigger prostaglandin E₂ (PGE₂) synthesis in hypothalamus
• PGE₂ raises the set point → body feels cold → shivering, vasoconstriction until new set point reached
• Aspirin/NSAIDs: Inhibit cyclooxygenase → block PGE₂ → reduce fever

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5d) Refractive Errors of the Eye and Their Correction (5 marks)

1. Myopia (Near-sightedness)

• Defect: Eyeball too long or corneal/lens curvature excessive → light focuses in front of retina
• Vision: Near objects clear; distant objects blurred
• Correction: Concave (diverging) lens - diverges light rays before entering eye

2. Hypermetropia/Hyperopia (Far-sightedness)

• Defect: Eyeball too short or insufficient refractive power → light focuses behind the retina
• Vision: Distant objects clearer; near objects blurred (requires constant accommodation effort)
• Correction: Convex (converging) lens - converges light before entering eye

3. Astigmatism

• Defect: Unequal curvature in different meridians of cornea (non-spherical cornea) → light focuses at two different points instead of one
• Vision: Blurring and distortion at all distances; may cause headaches
• Correction: Cylindrical lens - corrects in one meridian only

4. Presbyopia

• Defect: Age-related loss of elasticity of lens → reduced accommodation (cannot focus on near objects)
• Typically begins after age 40
• Correction: Convex (reading) glasses for near work; bifocals

5. Aphakia

• Defect: Absence of lens (post-cataract surgery)
• Strong convex lens needed

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QUESTION 6 - Short Answer Questions (4 × 2 marks)

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6a) Referred Pain - Definition, Examples, Mechanism (2 marks)

1. Cardiac ischemia (heart attack): Pain felt in left chest, left arm, jaw, left shoulder (T1-T4 segments)
2. Diaphragmatic irritation: Pain referred to the shoulder tip (C3,4,5 - phrenic nerve)
3. Appendicitis: Initial pain around umbilicus (T10 dermatome), later shifts to right iliac fossa
4. Ureteric colic: Flank pain radiating to groin/inner thigh (L1-L2)
5. Gallbladder: Right shoulder tip and right scapula

• Afferent pain fibers from viscera and from skin converge on the SAME 2nd order neuron in the dorsal horn (same spinal segment)
• The brain misinterprets the source of pain as coming from the skin (because somatic pain is far more common and has been experienced before)
• The cortex "projects" the pain to the familiar (somatic/skin) location

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6b) Graded Potential (2 marks)

• End plate potential (EPP) at NMJ
• Receptor potential (generator potential)
• EPSP and IPSP at synapses
• Slow waves in smooth muscle

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6c) Effect of Autonomic Stimulation on Intestinal Motility (2 marks)

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6d) Abnormal Taste Sensations (2 marks)

• COVID-19: Ageusia/hypogeusia due to damage to taste receptor cells or olfactory neurons
• Zinc deficiency: Hypogeusia (zinc required for taste receptor turnover)
• Chorda tympani damage (during middle ear surgery): Ipsilateral taste loss on anterior 2/3 of tongue
• Glossopharyngeal nerve (CN IX) damage: Taste loss from posterior 1/3 of tongue

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