(a) (i) Discuss the functions of basal ganglia. Add a note on Parkinson's disease. (5+5=10 marks) (ii) Discuss the mechanism of…

Basal Ganglia and Parkinson’s Disease The basal ganglia (caudate, putamen, globus pallidus, substantia nigra, and subthalamic nucleus) regulate voluntary motor execution, motor learning, and cognitive-emotional loops. Motor control relies on a balance between two striatocortical circuits: the direct pathway (striatal D1 receptors projecting to GPi/SNpr) which disinhibits the thalamus to initiate movement, and the indirect pathway (striatal D2 receptors via GPe and STN) which increases GPi/SNpr output to inhibit unnecessary motor programs. Parkinson’s disease arises from the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), marked histopathologically by Lewy bodies containing aggregated α-synuclein. The resulting loss of striatal dopamine shifts the pathway balance toward excessive indirect pathway hyperactivity. Clinically, this manifests as the classic "TRAP" tetrad: resting pill-rolling Tremor, lead-pipe or cogwheel Rigidity, Akinesia or bradykinesia, and Postural instability.

Excitation-Contraction Coupling in Skeletal Muscle Excitation-contraction coupling translates a sarcolemmal action potential into mechanical tension. Depolarization propagates along the transverse tubules (T-tubules), activating voltage-gated L-type calcium channels, the dihydropyridine receptors (DHPR). Mechanical coupling between DHPR and ryanodine receptors (RyR1) on the sarcoplasmic reticulum (SR) terminal cisternae triggers massive Ca²⁺ release into the sarcoplasm. Cytoplasmic Ca²⁺ binds to troponin C on the thin filament, inducing a conformational shift in the troponin-tropomyosin complex that exposes the myosin-binding sites on actin. Cross-bridge cycling ensues: energized myosin heads bind actin, release inorganic phosphate, perform the power stroke, bind ATP to detach, and hydrolyze ATP to re-cock. Relaxation occurs when Ca²⁺ is actively pumped back into the SR lumen by the sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA) pump and buffered by calsequestrin.

Biochemical Role and Deficiency of Vitamin A Vitamin A derivatives perform distinct functions: Retinal (11-cis-retinal) combines with opsin in retinal rod cells to form rhodopsin. Light isomerizes 11-cis-retinal to all-trans-retinal, triggering a transducin-mediated cGMP phosphodiesterase cascade that hyperpolarizes the photoreceptor, enabling scotopic (dim-light) vision (Wald's visual cycle). Retinoic acid functions as a ligand for nuclear receptors (RAR and RXR), acting as a transcription factor to maintain normal epithelial differentiation, mucus secretion, and immune competence. Retinol represents the primary transport (bound to retinol-binding protein) and storage form (retinyl palmitate in hepatic stellate cells), essential for reproductive function.

Deficiency results from primary dietary lack (addressed in India via the National Prophylaxis Programme against Nutritional Blindness and ICDS), fat malabsorption syndromes (celiac disease, cholestasis), or advanced liver dysfunction. Clinical manifestations progress according to WHO staging: night blindness (nyctalopia), conjunctival xerosis, Bitot’s spots (keratinized triangular conjunctival plaques), corneal xerosis, and irreversible keratomalacia (liquefactive corneal necrosis leading to blindness). Management involves oral retinol therapy (200,000 IU for age >1 year on days 0, 1, and 14; intramuscular formulations in malabsorption) alongside prophylactic biannual mega-dose Vitamin A Supplementation (VAS).

Facial Nerve and Rotator Cuff Anatomy The facial nerve (cranial nerve VII) contains four functional components: Special Visceral Efferent (SVE to muscles of facial expression, stapedius, stylohyoid, posterior digastric), Special Sensory (SVA taste from anterior two-thirds of tongue via chorda tympani), General Visceral Efferent (GVE parasympathetic to lacrimal, sublingual, and submandibular glands via greater petrosal and chorda tympani), and General Somatic Afferent (GSA sensation from the external acoustic meatus). Its course traverses the cerebellopontine angle, enters the internal acoustic meatus, travels the Z-shaped facial canal (giving off the greater petrosal nerve, nerve to stapedius, and chorda tympani), and exits the stylomastoid foramen. It then pierces the parotid gland to divide into five terminal motor branches: temporal, zygomatic, buccal, marginal mandibular, and cervical. Lower motor neuron lesion (Bell’s palsy) causes ipsilateral flaccid paralysis of facial muscles, assessed clinically via the House-Brackmann scale; inability to close the eye (lagophthalmos) risks exposure keratitis, mandating immediate corneal lubrication and tarsorrhaphy if severe.

The rotator cuff consists of the SITS muscles which dynamic-stabilize the glenohumeral joint: Supraspinatus (supraspinous fossa to upper facet of greater tubercle; initiates abduction 0–15°). Infraspinatus (infraspinous fossa to middle facet of greater tubercle; lateral rotator). Teres minor (lateral scapular border to lower facet of greater tubercle; lateral rotator). Subscapularis (subscapular fossa to lesser tubercle; medial rotator). Clinically, subacromial impingement of the supraspinatus tendon beneath the coracoacromial arch causes a "painful arc" between 60° and 120° of active abduction, frequently progressing to degenerative cuff tears in manual laborers and overhead athletes.

Addressing these neurological, musculoskeletal, and metabolic conditions requires early diagnostic recognition (such as testing the painful arc or identifying Bitot's spots) combined with evidence-based interventions like dopamine replacement therapy, surgical tendon repair, and sustained national public-health nutritional supplementation.