Lecture #24: Overview of Olfaction and Taste

What is olfaction?

Olfaction is the sensory system responsible for the detection and perception of odorants, contributing to smell and flavor perception.

What is the difference between orthonasal and retronasal olfaction?

Orthonasal olfaction occurs during sniffing through the nose, while retronasal olfaction occurs when odorants from the oral cavity reach the nasal epithelium during eating and drinking and contribute to flavor.

What is neccessary for flavor perception?

Flavor perception requires the integration of taste and retronasal olfaction.

Where are olfactory receptor neurons located?

Olfactory receptor neurons are located in the olfactory epithelium of the superior nasal cavity.

What type of neurons are olfactory receptor neurons?

They are bipolar primary sensory neurons with dendrites that end in specialized cilia.

Why is the olfactory epithelium unique among sensory systems?

Olfactory receptor neurons are short-lived and undergo continuous turnover, being replaced approximately every 7 days.

What role do basal cells play in the olfactory epithelium?

Basal cells serve as stem cells that generate new olfactory receptor neurons.

What are sustentacular cells?

Sustentacular cells provide metabolic and structural support to olfactory neurons and contribute to mucus secretion.

What is the function of Bowman’s glands?

Bowman’s glands secrete mucus containing glycoproteins that dissolve odorants and protect the olfactory epithelium.

How do odorants activate olfactory receptor neurons?

Odorants bind G protein–coupled receptors on olfactory cilia, triggering a second messenger cascade that depolarizes the neuron.

What second messenger pathway is activated in olfactory transduction?

G protein activation leads to opening of cation channels and calcium influx, which opens chloride channels and amplifies depolarization.

Why does chloride efflux amplify olfactory signals?

The high intracellular chloride concentration causes chloride to exit the cell, increasing depolarization.

How do olfactory receptor neuron axons reach the brain?

Their unmyelinated axons form bundles called olfactory fila that pass through the cribriform plate to the olfactory bulb.

Where do olfactory receptor neurons synapse?

They synapse in glomeruli within the olfactory bulb.

What is a glomerulus?

A glomerulus is a spherical structure in the olfactory bulb where axons from olfactory neurons expressing the same receptor converge.

How are odors encoded in the olfactory bulb?

Odors are encoded by unique patterns of glomerular activation rather than by a single receptor.

Which cells transmit olfactory output from the bulb?

Mitral and tufted cells serve as second-order neurons and transmit olfactory information to the brain.

What role do interneurons play in the olfactory bulb?

Granule and periglomerular cells modulate mitral and tufted cell activity through inhibition and refinement of signals.

What is the olfactory tract?

The olfactory tract contains axons of mitral and tufted cells that carry information from the bulb to cortical regions.

Where does the olfactory tract project?

It projects to the piriform cortex, olfactory tubercle, amygdala, entorhinal cortex, and anterior olfactory nucleus.

Why is olfaction unique among sensory systems?

Olfactory information bypasses an initial thalamic relay and projects directly to primary cortical areas.

What is the role of the mediodorsal thalamic nucleus in olfaction?

It relays olfactory information to the orbitofrontal cortex for higher-order processing and integration.

What brain regions integrate smell and taste?

The insular cortex and orbitofrontal cortex integrate olfactory and gustatory information.

What is anosmia?

Anosmia is the complete loss of smell sensation.

What is hyposmia?

Hyposmia is a decreased sensitivity to odors.

What is parosmia?

Parosmia is a distortion of smell perception in which odors are perceived incorrectly, often unpleasantly.

What is phantosmia?

Phantosmia is the perception of odors in the absence of any external stimulus.

What neurological diseases are associated with early olfactory deficits?

Parkinson’s disease and Alzheimer’s disease often present with early hyposmia or anosmia.

What is Foster Kennedy syndrome?

A condition caused by an olfactory groove meningioma resulting in ipsilateral anosmia and optic nerve atrophy with contralateral papilledema.

What is Kallmann syndrome?

A congenital disorder characterized by anosmia and hypogonadotropic hypogonadism due to failed migration of olfactory and GnRH neurons.

How can head trauma cause anosmia?

Fracture of the cribriform plate can shear olfactory fila, leading to loss of smell and possible CSF rhinorrhea.

What are uncinate fits?

Seizure auras involving foul or unusual smells caused by temporal lobe epilepsy affecting the uncus.

What are the five basic taste modalities?

Sweet, salty, sour, bitter, and umami.

Where are taste receptors located?

Taste receptors are located within taste buds on the tongue and oral cavity.

Which papillae contain taste buds?

Fungiform, foliate, and circumvallate papillae contain taste buds, while filiform papillae do not.

How is salty taste transduced?

Salty taste is mediated by sodium influx through amiloride-sensitive sodium channels.

How is sour taste transduced?

Sour taste is mediated by proton detection and inhibition of potassium channels.

How are sweet tastes transduced?

Sweet tastes activate GPCRs that increase cAMP, leading to depolarization.

How are bitter tastes transduced?

Bitter substances activate GPCRs that stimulate phospholipase C and intracellular calcium release.

Why are there many bitter receptors?

Bitter taste detects a wide variety of potentially toxic compounds.

Which cranial nerves transmit taste sensation?

Taste is transmitted by CN VII, CN IX, and CN X.

Which nerve carries taste from the anterior two-thirds of the tongue?

The facial nerve via the chorda tympani branch.

Which nerve carries taste from the posterior one-third of the tongue?

The glossopharyngeal nerve.

Where do gustatory afferents first synapse centrally?

In the rostral nucleus of the solitary tract in the medulla.

What is ageusia?

Ageusia is the complete loss of taste sensation.

How does smoking affect taste?

Smoking raises taste detection thresholds and reduces sensitivity to multiple taste modalities.

How does Bell’s palsy affect taste?

Damage to the facial nerve can impair taste on the anterior two-thirds of the tongue.

How do chemotherapy and radiation affect taste?

They damage rapidly dividing taste receptor cells, leading to taste disturbances.

How are smell and taste clinically linked?

Loss of smell often leads to impaired flavor perception even when taste buds are intact.

Why is olfaction important for safety?

It helps detect hazards such as smoke, gas leaks, and spoiled food.