Olfactory and Gustation

Chemical sense

Taste

• Depend on presence of chemical stimuli (tastants or odorants) that are present in food and drink or in the air

• Stimuli we know taste and flavors are mixtures of 5 elementary tase qualities

Odor

• More primary qualities, about 400 different odor receptors that are encoded in the human genome

• Olfactory coding resembles taste cording, the most natural odors are combinations of molecules that excite chemoreceptors of more than one odorant class

Taste buds

• Located on different types of taste papillae on tongue, palate, pharynx and larynx

  • Circumvallate

    • Large, dome shaped. V shaped row at posterior tongue. Has many taste buds

    • Innervated by glossopharyngeal

  • Foliate

    • Leaf-life folds on the posterolateral tongue

    • Innervated by glossopharyngeal and some facial

  • Fungiform

    • Mushroom shaped, scattered on the anterior of tongue. Has fewer taste buds

    • Inver vatted by the facial nerve by the chorda tympani

• Innervation by

  • Facial nerve (anterior)

  • Glossopharyngeal (posterior)

  • Vagus (larynx, esophagus)

• Tongue is made of several thousand taste buds, contains 50-100 receptor cells of different types

Distribution of flavor qualities

• Made up of 5 primary qualities uniformly distributed on tongue

• Primary taste is associated with a specific stimulus

  • Salty = sodium chloride

  • Sweet = sucrose

  • Sour = hydrochloric acid

  • Bitter - quinine

  • Umami = monosodium glutamate

• Umami = meaty flavor

Taste Bud anatomy

• Made of chemoreceptors, basal and supporting cells

  • Chemoreceptors

    •  receptor molecules on microvilli

    • Lasts for 10 days

  • Basal

    • gives rise to new chemoreceptors

  • Supporting cell

    • Support for receptors and response to sodium

    • Unofficial taste receptor for sodium

Chemoreceptor types

• Type 1

  • Most abundant and metabotropic

  • Tall microvilli

  • Dark granules near the apex

  • Supporting cells and salty taste

  • It activates another receptor for ionic current

• Type 2

  • Short microvilli and metabotropic

  • Large and round nucleus

  • Sweet, bitter and umami taste

• Type 3

  • Single tall thick micovillus, ionotropic

  • Narrow and spindle shape

  • Salty and sour taste

Type 2 cells Sweet, bitter and umami

• Sucrose, quinine and MSG will bind to GPCRs

• Activates gustducin which causes second messenger pathways to be activated

  • Gustducin acts like Gq,  phospholipase C increase DG and IP3 and increase Ca2+

• Ca2+ mediated release of ATP onto primary gustatory neurons

Type 3 cells Salty and sour

• Na+ and H+ enter through ion channels

• Depolarization causes Ca2+ opening

• Release of serotonin to excite primary gustatory neurons

Pathway

• Cell bodies for taste fibers are in cranial nerve 7, 9 and 10; facial, glossopharyngeal and vagus. Respectively the cell bodies reside in the geniculate, petrosal and nodose ganglia

• Afferent fibers from the ganglia enter the medulla and synapse on the nucleus of the solitary tract (NTS)

• Secondary neuron project from medulla to pontine taste area of pons

• Pontine neuron project to ventral posterior medial thalamus, largely uncrossed

  • Pontine taste area also project to lateral hypothalamus (homeostasis and feeding behavior)

  • Pontine taste area also projects to amygdala (emotion/reward)

• From thalamus neurons project to primary gustatory cortex (insula and frontal operculum)

  • For taste sensation and interoception

Taste quality decoding in humans

• Each taste equality produces a distinct temporal pattern after onset

  • Evokes differentiable neural response, brain encodes them into unique spatiotemporal signatures

• Higher dissimilarity = more distinct neural pattern

• Distinct cortical networks are recruited for each taste

Olfactory chemoreceptors

• Olfactory mucosa is a specialized region of nasopharynx made of many components

  • Axons project through ethmoid bone and basement membrane

  • Supporting cells and receptor cells

• Receptors are bipolar neurons with cilia with receptor proteins that detect odorant molecules that dissolve into overlaying mucus

• About 1 million olfactory chemoreceptors

  • Short lifespan and are continually replaced

  • Largest pop of GPCRs in the human genome

Signaling pathway

• The olfactory mucosa has receptor cells, olfactory nerves connect receptor cells through cribriform plate to the olfactory glomerulus

  • Axons from thousands of olfactory receptor neurons (ORNs) that express the same receptor type converge onto a single glomerulus. This convergence increases sensitivity to low odor concentrations

• Periglomerular cell allows for lateral inhibition

  • Forms inhibitory connections between neighboring glomeruli

  • Reduces activity from surround and less activated glomeruli to sharpen representation of the primary/stronger odorant

  • Granule cells is also a primary inhibitory control to sharped odor representation

Olfactory signal transduction

1. Odorant molecules bind to odorant receptor, Golf

2. Turns ATP to cAMP

3. cAMP opens receptors for Na+ and Ca2+ influx to depolarize the cell

4. Depolarization opens ANO2 to allow for Cl- efflux, this further increase Ca2+ concentration and further depolarization

5. NKCC1 allows for Na+/K+ and 2Cl- to enter the cell and allows for calcium concentration to alive

• Graded receptor potentials occurs at the receptor cilia

• Spikes occur at the axon hillock of the olfactory receptor cell, summated receptor potential

• APS occur at the olfactory bulb

Central Olfactory Pathways

1. Olfactory nerves (mucus) projects to the olfactory bulb

   1. Olfactory bulb has the glomerulus

   2. Mitral cells are the primary output neurons of OB

2. Olfactory tract projects to the amygdala

   1. Causes fear, disgust and avoidance of bad smells

3. Olfactory bulb projects to olfactory cortex by olfactory tract

   1. Doesn't require thalamic routing

   2. Does reach medial temporal lobe by other pathways

4. Projection route highlights link with physiologically related systems

   1. Hippocampus = memory (olfactory memory)

   2. Amygdala = emotion

   3. Hypothalamus = homeostasis and motivation

   4. Reticular formation = visceral responses

Smelling hand after handshake

This study supports the idea that humans:

• exchange chemical cues during social interactions

• subconsciously gather olfactory information about others

• use smell in social evaluation, even though we rarely notice it

It connects human behavior to broader mammalian social chemosignaling.