(a) Using a well labelled diagram, trace the taste pathway from circumvallate papillae of tongue to the cerebral cortex. 15…

Taste pathway. The taste pathway from circumvallate papillae is a special visceral afferent (SVA) sequence. Taste buds in circumvallate papillae contain type III gustatory receptor cells that transduce sweet, umami, bitter, sour and salty stimuli. Afferent fibres from these buds enter the glossopharyngeal nerve (CN IX), whose cell bodies lie in the petrosal (otic) ganglion. In the medulla, fibres ascend in the solitary tract and synapse in the rostral nucleus solitarius. Second-order neurons decussate in the tegmentum and project to the parvocellular part of the ventral posteromedial (VPM) nucleus of the thalamus. Third-order thalamocortical fibres reach the primary gustatory area in the anterior insular cortex and, for flavour integration, the orbitofrontal cortex. A labelled diagram would show: circumvallate papilla → taste bud → type III cell → CN IX → petrosal ganglion → solitary tract → nucleus solitarius → decussation → VPM → insular cortex → orbitofrontal cortex.

Renal tubular tests. Five tests assess tubular function by measuring what tubules reabsorb, secrete or acidify. (1) Urine pH and PCO2 gap test: acetazolamide causes bicarbonaturia; a fall in urine pH with a widened PCO2 gap indicates intact distal acidification, while a high urine pH and small gap suggest distal tubular acidification failure. (2) Urine concentrating ability: Uosm/Posm and free-water clearance (CH2O) test ADH-dependent water reabsorption in collecting ducts; low Uosm in dehydration indicates impaired aquaporin-2/urea handling. (3) Urinary acidification: NH4+ excretion and titratable acid measure distal H+ secretion and buffer capacity; low NH4+ with metabolic acidosis indicates impaired distal acid secretion. (4) Renal glycosuria/aminoaciduria: glucose or amino acids in urine despite normal plasma levels indicate proximal tubular reabsorptive defects, e.g. SGLT2 or amino acid transporter failure. (5) PAH extraction ratio: assessed with inulin/PAH clearance, it estimates GFR and effective renal plasma flow; a low PAH extraction ratio reflects reduced tubular secretion or renal plasma flow.

Biotin. Biotin is a water-soluble vitamin covalently attached to lysine residues of carboxylases as biocytin; the biotin moiety carries activated CO2. It is essential for pyruvate carboxylase in gluconeogenesis, acetyl-CoA carboxylase in fatty acid synthesis, and propionyl-CoA carboxylase in odd-chain fatty acid/propionate metabolism to succinyl-CoA. Deficiency impairs these pathways. Avidin, a biotin antagonist in raw egg white, binds biotin with Kd ~10^-15 M, outcompetes carboxylase biotin and causes deficiency. The avidin/streptavidin-biotin interaction is used in ELISA, immunohistochemistry and molecular diagnostics; biotinidase deficiency screening uses biotinidase activity and biotin metabolites to detect recycling defects.

Cardiac output. Cardiac output (CO) is the volume of blood pumped by the ventricle per minute, CO = HR × SV. In normal adults it is regulated by intrinsic Frank-Starling mechanism: increased end-diastolic volume/pressure stretches myocardium, increasing stroke volume via length-tension relationship. Extrinsic regulation includes sympathetic and parasympathetic autonomic control via baroreceptors, changes in contractility, heart rate, preload, afterload, and hormones such as angiotensin II, ADH and catecholamines. Cardiac Index is CO divided by body surface area, normally 2.5–4.0 L/min/m². Cardiac Reserve is the ratio of maximum to resting CO, about 4–5 in healthy adults.

Systemic vascular resistance. SVR is the resistance to flow in the systemic circulation, SVR = (MAP − CVP)/CO × 80 dyn·s·cm⁻⁵. Its determinants are vessel radius, vessel length and blood viscosity. Radius is dominant because resistance varies inversely with radius to the fourth power; it is regulated by local metabolites, endothelial factors and sympathetic tone. Length is fixed in adults, while viscosity depends on haematocrit and plasma proteins. Thus, taste is a sequential SVA relay, tubular tests index transport defects, biotin-dependent carboxylation links metabolism, and CO/SVR regulation maintains perfusion.