Smell and taste

Olfactory receptors

• The olfactory epithelium lies in the roof of the nasal cavity and upper septum. It contains olfactory receptor neurons (bipolar cells with cilia carrying odorant receptors), supporting cells, Bowman glands (mucus) and basal stem cells.
• Olfactory neurons are replaced throughout life (roughly every 1–2 months) from basal stem cells, which is unusual for neurons. Each neuron expresses one of about 400 receptor types.
• Odorants dissolve in mucus and bind G protein-coupled receptors coupled to G_olf, which raises cAMP and opens cyclic nucleotide-gated cation channels; Ca²⁺ then opens Cl⁻ channels and Cl⁻ efflux adds to depolarisation.
• Smell adapts rapidly, partly centrally.

Olfactory pathway

• Unmyelinated axons gather into about 20 olfactory filaments that pass through the cribriform plate of the ethmoid to the olfactory bulb, synapsing with mitral and tufted cells in glomeruli.
• The olfactory tract runs to the primary olfactory cortex (piriform cortex and uncus), amygdala and entorhinal cortex, then to the orbitofrontal cortex.
• Smell is the only sense that reaches the cortex without a thalamic relay, and its direct limbic connections explain why smells evoke strong emotions and memories.

Disorders of smell

• Head injury: a blow to the back of the head jolts the brain forward and back, shearing the olfactory filaments where they cross the cribriform plate (contrecoup injury); anterior cranial fossa fractures do the same and may leak CSF. Because flavour depends largely on smell, the patient often complains that food tastes bland.
• Kallmann syndrome: GnRH neurons fail to migrate from the olfactory placode to the hypothalamus with the olfactory axons, giving anosmia with hypogonadotrophic hypogonadism (delayed puberty, low LH, FSH and sex steroids); MRI shows absent or small olfactory bulbs and sulci.
• Others: viral upper respiratory infection, nasal polyps, ageing, early Parkinson and Alzheimer disease, olfactory groove meningioma (with Foster Kennedy syndrome).
• Uncinate fits: brief unpleasant olfactory hallucinations from temporal lobe (uncus) epilepsy.

Taste buds and taste qualities

• Taste buds sit on fungiform papillae (anterior tongue), foliate papillae (sides), circumvallate papillae (in front of the sulcus terminalis), and on the soft palate, epiglottis and pharynx. Filiform papillae are the most numerous but have no taste buds; they give the tongue its rough texture.
• Each bud has 50–100 taste receptor cells, replaced every 10–14 days.

• Every part of the tongue can detect every quality; the old "tongue map" is wrong. Bitter sensitivity is somewhat greater over the posterior tongue (circumvallate papillae), which protects against swallowing toxins and is why bitter quinine is felt most at the back.
• Salt appetite rises with adrenal insufficiency or after bilateral adrenalectomy without replacement (loss of aldosterone causes sodium wasting and salt craving).

Nerves carrying taste

• General sensation of the anterior two-thirds is carried by the lingual nerve (V3); of the posterior third by IX. Motor supply of the tongue is hypoglossal (XII): a lesion makes the tongue deviate towards the side of the lesion on protrusion, with taste intact.
• Glossopharyngeal, vagus and accessory nerves all leave through the jugular foramen, so a lesion there causes loss of taste on the posterior third with weakness of shoulder elevation (trapezius) and sternocleidomastoid.

Central taste pathway

All taste fibres end in the rostral nucleus of the tractus solitarius (gustatory nucleus) in the medulla. Second-order fibres ascend ipsilaterally to the ventral posteromedial (VPM) nucleus of the thalamus, then to the primary gustatory cortex in the insula and frontal operculum (around Brodmann area 43). The NTS also feeds reflex salivation and gastric secretion.

Disorders of taste

• Bell palsy (lesion of VII in the facial canal above the chorda tympani branch): loss of taste on the anterior two-thirds, with weakness of the whole half face, including the forehead, and often hyperacusis.
• Lingual nerve injury (third molar extraction, tongue surgery) damages chorda tympani fibres travelling with it: loss of taste and touch on the anterior two-thirds.
• Middle ear disease or surgery can damage the chorda tympani.
• Drugs: captopril and penicillamine contain a sulfhydryl (thiol) group that causes metallic or salty taste disturbance and taste loss; lisinopril, which lacks it, usually does not. Metronidazole gives a metallic taste; lithium and some chemotherapy drugs also alter taste.
• Zinc deficiency reduces taste; radiotherapy, smoking and ageing also blunt it.
• Frey syndrome (gustatory sweating after parotidectomy) is misdirected regrowth of parasympathetic fibres, not a taste disorder.

Remember Smell skips the thalamus; taste goes to the NTS and then VPM. Anterior tongue VII, posterior IX, epiglottis X.

Exam trap Loss of smell often presents as "loss of taste" because flavour depends on smell.

Exam trap The chorda tympani carries taste, but general sensation of the anterior tongue comes from the lingual nerve (V3).

How FCPS asks about this

• Loss of smell after a fall on the back of the head: shearing of olfactory filaments at the cribriform plate.
• Unusual feature of olfactory neurons: continuous replacement from basal stem cells.
• Anosmia with hypogonadism: Kallmann syndrome; small or absent olfactory bulbs.
• Taste from the anterior two-thirds: chorda tympani (VII); from the posterior third: glossopharyngeal (IX).
• First relay of all taste fibres: nucleus of the tractus solitarius, then VPM thalamus.
• Papillae without taste buds: filiform.
• Captopril taste disturbance: its sulfhydryl group.