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

(a)(i) Functions of the basal ganglia and Parkinson disease

Components: caudate and putamen (striatum), globus pallidus (GPe, GPi), subthalamic nucleus and substantia nigra (SNc, SNr). Functions:

• Planning, initiation and scaling of voluntary movement through cortex–basal ganglia–thalamus–cortex loops.
• Direct pathway (striatal D1) disinhibits the thalamus and facilitates movement, indirect pathway (D2 via GPe and STN) suppresses unwanted movement. Dopamine from SNc excites the direct and inhibits the indirect pathway.
• Automatic and learned motor sequences (habits), control of posture and muscle tone.
• Cognitive (caudate loop: planning, set-shifting), emotional and motivational (limbic loop) functions, eye movements. Parkinson disease: progressive degeneration of dopaminergic neurons of the SNc with Lewy bodies (α-synuclein). Striatal dopamine loss leaves the indirect pathway overactive, so GPi output inhibits the thalamus. Features: resting pill-rolling tremor, lead-pipe or cogwheel rigidity, bradykinesia and postural instability, with mask face, shuffling festinant gait, micrographia, and non-motor features (anosmia, constipation, depression, sleep disorder, dementia). Treatment: levodopa with carbidopa, dopamine agonists, MAO-B and COMT inhibitors, anticholinergics for tremor, and deep brain stimulation of the subthalamic nucleus or GPi.

(a)(ii) Excitation–contraction coupling in skeletal muscle

1. Acetylcholine at the motor end-plate generates a muscle action potential that spreads along the sarcolemma and down the T-tubules.
2. At the triads, voltage-sensing dihydropyridine receptors (L-type channels) change shape and mechanically open ryanodine receptors (RyR1) of the sarcoplasmic reticulum (no Ca²⁺ entry needed, unlike cardiac muscle).
3. Ca²⁺ floods the sarcoplasm (about 10⁻⁷ to 10⁻⁵ M) and binds troponin C, which moves tropomyosin and exposes myosin-binding sites on actin.
4. Cross-bridge cycle: energised myosin (ADP + Pi) binds actin, releases Pi and performs the power stroke, ATP binds and detaches the head, and ATP hydrolysis re-cocks it. Filaments slide and the sarcomere shortens.
5. Relaxation: SERCA pumps Ca²⁺ back into the SR (stored on calsequestrin), troponin–tropomyosin re-covers actin. ATP is needed both for contraction and relaxation (lack causes rigor).

(b) Vitamin A: biochemical roles and deficiency

Derivatives and roles:

• Retinol: transport (on retinol-binding protein) and storage (retinyl esters in hepatic stellate cells), needed for reproduction (spermatogenesis, placenta).
• Retinal (11-cis): forms rhodopsin with opsin. Light converts it to all-trans-retinal, activating transducin and cGMP phosphodiesterase, closing cGMP channels and hyperpolarising rods (Wald visual cycle), the basis of dim-light vision.
• Retinoic acid: binds nuclear RAR/RXR receptors to regulate gene expression, maintains epithelial and mucous cell differentiation, immunity, growth and embryonic development (used therapeutically in acne and acute promyelocytic leukaemia).
• β-carotene: provitamin and antioxidant. Causes of deficiency: poor dietary intake (low green leafy vegetables, milk, eggs), protein-energy malnutrition (low RBP), fat malabsorption (coeliac disease, cholestasis, pancreatic disease), liver disease, measles and diarrhoea, increased needs in pregnancy and lactation. Clinical features (WHO xerophthalmia classification): night blindness (XN), conjunctival xerosis (X1A), Bitot spots (X1B), corneal xerosis (X2), corneal ulceration or keratomalacia (X3A, X3B), corneal scar (XS), fundus changes (XF). Also follicular hyperkeratosis (phrynoderma), growth failure, recurrent respiratory and diarrhoeal infections and increased child mortality. Management: treatment on days 0, 1 and 14 with oral vitamin A, 200,000 IU above 12 months, 100,000 IU at 6–12 months and 50,000 IU below 6 months (intramuscular water-miscible form if vomiting or malabsorption), eye care and treatment of PEM and infections. Prevention: diet rich in carotenoids, food fortification, and the national prophylaxis schedule of 1 lakh IU at 9 months with measles vaccine and 2 lakh IU every 6 months up to 5 years.

(c)(i) Facial nerve

Functional components: special visceral (branchial) efferent to muscles of facial expression, stapedius, stylohyoid and posterior belly of digastric. General visceral efferent (superior salivatory nucleus) to lacrimal, nasal, submandibular and sublingual glands. Special visceral afferent (taste, anterior two-thirds of tongue, via nucleus tractus solitarius). General somatic afferent from part of the external ear. Course: arises at the pontomedullary junction (motor root and nervus intermedius), crosses the cerebellopontine angle with VIII, enters the internal acoustic meatus, runs in the facial canal (labyrinthine segment to the geniculate ganglion, tympanic segment on the medial wall of the middle ear, mastoid segment downward), exits the stylomastoid foramen and enters the parotid gland. Branches: in the canal, the greater petrosal nerve (lacrimal secretion), nerve to stapedius and chorda tympani (taste, submandibular and sublingual glands). After exit, posterior auricular, digastric and stylohyoid branches, then in the parotid the temporal, zygomatic, buccal, marginal mandibular and cervical branches. Applied: Bell palsy (LMN, whole half of face), UMN lesion spares the forehead (bilateral cortical supply), level of lesion localised by lacrimation, stapedius reflex (hyperacusis) and taste, Ramsay Hunt syndrome (herpes zoster of the geniculate ganglion), injury in parotid surgery, temporal bone fracture or forceps delivery, exposure keratitis needing eye protection.

(c)(ii) Rotator cuff muscles

• Supraspinatus: supraspinous fossa to the superior facet of the greater tubercle, initiates abduction (suprascapular nerve).
• Infraspinatus: infraspinous fossa to the middle facet, lateral rotation (suprascapular nerve).
• Teres minor: upper lateral border of scapula to the inferior facet, lateral rotation (axillary nerve).
• Subscapularis: subscapular fossa to the lesser tubercle, medial rotation (upper and lower subscapular nerves). Their tendons fuse with the capsule and hold the humeral head in the shallow glenoid. Applied: supraspinatus tendinitis and impingement (painful arc 60–120°), rotator cuff tears in the elderly and athletes (drop-arm test), calcific tendinitis, and anterior dislocation where the inferior capsule lacks cuff support.