Paper I — Q1
Describe the radial nerve under the following headings: (i) Origin and course (ii) Branches and muscles supplied (iii) Applied…
Describe the radial nerve under the following headings: Origin and course
Branches and muscles supplied
Applied aspects
Describe the development of ventricles of the heart. Add a note on Fallot's tetralogy.
Explain the 'Mucosal Block Theory' for iron absorption. Discuss briefly the complications of excess of iron deposition in the body.
Give the sequence of events at the neuromuscular junction during the transmission of a nerve impulse. Name the muscles most commonly affected by myasthenia gravis.
Describe the components of the nervous system which are concerned with 'conscious alert state' that makes perception possible.
हिंदी में प्रश्न पढ़ें
बहिःप्रकोष्ठिका तंत्रिका का निम्नलिखित शीर्षकों के अंतर्गत वर्णन कीजिए : उद्गम तथा मार्ग
शाखाएँ तथा पेशियाँ जिनकी यह तंत्रिका आपूर्ति करती है
अनुप्रयुक्त पहलू
हृदय में निलयों के विकास का वर्णन कीजिए। फैलो चतुष्क (टेट्रालॉजी) पर एक टिप्पणी लिखिए।
लोह अवशोषण का 'श्लेष्मकलारोध सिद्धांत' समझाइए। शरीर में अत्यधिक लोह निक्षेप से उत्पन्न होने वाली जटिलताओं की संक्षेप में व्याख्या कीजिए।
तंत्रिका आवेग के संचरण के दौरान तंत्रिका-पेशी संगम पर होने वाली घटनाओं को क्रमवत उल्लिखित कीजिए। गंभीर पेशी-दुर्बलता (मायस्थेनिया ग्रेविस) में सर्वाधिक प्रभावित होने वाली पेशियों के नाम बताइए।
तंत्रिका तंत्र के उन अवयवों का वर्णन कीजिए जो 'संजाग्रीत सतर्क अवस्था' से संबंध रखते हैं, जिससे बोध संभव हो पाता है।
Model answer
Drafted with AI by UPSC Answer Check against this question's marking rubric, to the expected length. UPSC does not publish answers for Mains — this is one way to score well, not an official key. How we build rubrics and answers.
(a) Radial nerve. The radial nerve arises from the posterior cord (C5–T1), passes posterior to the third part of the axillary artery, leaves the axilla through the triangular interval, and runs in the spiral groove of the humerus. At the elbow it divides into a superficial sensory branch and a deep branch; the deep branch supplies the supinator and extensor carpi radialis brevis before piercing the supinator to become the posterior interosseous nerve. It supplies triceps, anconeus, the lateral part of brachialis, brachioradialis, extensor carpi radialis longus, and forearm extensors. Cutaneous branches include posterior cutaneous nerve of arm, lower lateral cutaneous nerve of arm, posterior cutaneous nerve of forearm, and superficial radial nerve to the dorsum of hand and radial digits. Mid-shaft humeral fracture or Saturday-night palsy in the spiral groove causes wrist drop, impaired finger extension, and dorsal hand sensory loss.
(b) Ventricles and Fallot’s tetralogy. The primitive ventricle is divided into right and left ventricles by a muscular interventricular septum growing from the floor; the membranous part forms by fusion of endocardial cushions, the aorticopulmonary septum, and the muscular septum. The bulbus cordis partitions into the outflow tracts, and the aorticopulmonary septum spirals to separate the truncus arteriosus into aorta and pulmonary trunk. Fallot’s tetralogy results from anterior and cephalad deviation of the infundibular septum, producing a VSD, overriding aorta, pulmonary stenosis/right ventricular outflow obstruction, and right ventricular hypertrophy.
(c) Mucosal block theory. In the duodenum, ferric iron is reduced to ferrous iron and enters enterocytes via DMT1. The classic mucosal block occurs when intracellular iron binds apoferritin to form ferritin; adequate stores trap iron, limit further absorption, and the iron is lost when the enterocyte sloughs. Hepcidin reinforces this by inhibiting ferroportin export. Excess deposition causes haemochromatosis/haemosiderosis: liver cirrhosis, hepatocellular carcinoma, cardiomyopathy, bronze diabetes, hypogonadism, arthropathy; transfusional overload in thalassaemia is a common Indian setting.
(d) Neuromuscular junction. A nerve action potential depolarises the presynaptic axon terminal, opening voltage-gated Ca2+ channels. Calcium influx releases acetylcholine into the cleft; ACh binds nicotinic receptors, opening Na+/K+ channels and producing an end-plate potential. If threshold is reached, a muscle action potential follows; acetylcholinesterase hydrolyses ACh. Myasthenia gravis, due to anti-ACh receptor antibodies, commonly affects extraocular, facial, bulbar, and proximal limb muscles.
(e) Conscious alert state. The conscious alert state depends on the reticular activating system in the brainstem reticular formation, especially midbrain and pons, projecting to the thalamus and cortex. Posterior hypothalamus, orexin/hypocretin neurons, and cholinergic/monoaminergic pathways—locus coeruleus, raphe nuclei, ventral tegmental area—maintain arousal; thalamocortical activation makes perception possible. Damage to these circuits produces coma or altered sensorium.
How this answer will be evaluated
Framework: Anatomical & Physiological Description
What earns full marks: Comprehensive, accurate, and well-structured answers with diagrams and clinical correlations.
Evaluation rubric
Each sub-part is marked on its own, against the marks and word limit printed on the paper.
- (a) Radial nerve: origin and course, branches and muscles supplied, applied aspects. 10 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Origin: posterior cord (C5–T1); course behind axillary artery, lower triangular space, spiral groove, lateral intermuscular septum, division in front of lateral epicondyle
- Muscular branches: triceps, anconeus, brachioradialis, ECRL, (lateral brachialis); deep branch/PIN to supinator, ECRB and forearm extensors
- Cutaneous branches: posterior cutaneous of arm and forearm, lower lateral cutaneous of arm, superficial radial to dorsum of hand
- Applied: crutch palsy, Saturday-night palsy, mid-shaft humerus fracture (wrist drop), PIN injury (no wrist drop), sensory loss at first dorsal web space
Loses marks
- Confusing with ulnar/median nerve supply
Earns more
- Sketch of course
Extra mark
- Level-wise injury table as list
- (b) Development of ventricles; note on Fallot's tetralogy. 10 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Primitive ventricle and bulbus cordis; muscular interventricular septum
- Membranous septum from endocardial cushions, conotruncal ridges and muscular septum
- Outflow tract separation by spiral aorticopulmonary septum
- Fallot: anterosuperior deviation of infundibular septum ⇒ pulmonary stenosis, VSD, overriding aorta, RVH; cyanosis, boot-shaped heart
Loses marks
- Listing ASD as part of tetralogy
Earns more
- Clinical features and surgical correction
Extra mark
- Tet spells explanation
- (c) Mucosal block theory of iron absorption; complications of iron overload. 10 marks
explain— definition/context → points in order → small example → short close
Must cover
- Fe³⁺ reduced to Fe²⁺ (duodenal cytochrome b), DMT1 uptake
- Ferritin trapping in enterocytes; loss with shedding; ferroportin/hephaestin export
- Hepcidin regulation of ferroportin
- Iron overload: haemochromatosis/haemosiderosis — cirrhosis, HCC, cardiomyopathy, diabetes (bronze), hypogonadism, arthropathy, skin pigmentation
Loses marks
- No complications
Earns more
- Transfusional overload in thalassaemia
Extra mark
- HFE mutation
- (d) Sequence of events at the NMJ; muscles commonly affected in myasthenia gravis. 10 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Nerve AP → voltage-gated Ca²⁺ influx → ACh release by exocytosis
- ACh binds nicotinic receptors → end-plate potential → muscle AP
- ACh hydrolysed by acetylcholinesterase
- Myasthenia gravis: extraocular (ptosis, diplopia), facial, bulbar, neck and proximal limb muscles
Loses marks
- Muscarinic receptors at NMJ
Earns more
- Anti-AChR/anti-MuSK antibodies
Extra mark
- Edrophonium test / thymus link
- (e) Components of the nervous system concerned with the conscious alert state. 10 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Ascending reticular activating system (brainstem reticular formation)
- Thalamic (intralaminar) relay to cortex
- Neuromodulators: cholinergic, noradrenergic (locus coeruleus), serotonergic (raphe), histaminergic, orexinergic hypothalamus
- Cerebral cortex integration for perception; effect of lesions (coma)
Loses marks
- Only naming cerebellum
Earns more
- EEG arousal correlates
Extra mark
- Sleep–wake regulation
What "Describe" is asking you to do
Give a full, ordered account of the thing named — its parts, stages or mechanism — in the sequence in which it actually exists or occurs. Most describe questions come from the science optionals, where the marks sit in correct technical detail and, where the stem says so, a labelled diagram.
Structure that answers it
One-line identification of the subject → the parts or stages in their real order, each with its defining detail → labelled diagram where the subject is structural → closing line on function or significance
Where marks are lost
Loose general prose where the examiner is ticking named parts, correct terminology and their sequence; and in the General Studies papers, turning to evaluation before the description is finished.
Practice this exact question
Write your answer and it is marked point by point against the model answer above — what you covered, what you missed, what you got wrong.
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