Chemical Senses: Olfaction and Taste

• Dedicated to detection of chemicals in our environment.

• Differ from other modalities in that the stimuli they detect cannot be easily classified by a quantitative scale.

Olfaction

• Monitors chemical environment at a distance, unlike taste.

• Humans distinguish 1,000’s of different odors.• Mediates food selection, avoidance of ingesting

toxins• Important in social cues including reproductive

behavior and mother—infant relationship

Vomeronasal organ is rudimentary in humans

Vomeronasal nerve and organ

Olfactory bulb and tract

Vomeronasal organ detects pheromones in many animals where it projects to accessory olfactory bulb and then to hypothalamus. It is apparently not functional in humans and is present bilaterally in only 8% of adults.

Pheromones?

Nolte, p. 332

Olfactory epithelium

Olfactory epithelium 1-2 cm2

Olfactory bulb & tract

Olfactory epithelium contains olfactory receptor neurons and supporting cells.

Bowman’s glands secrete mucus that covers surface of sensory epithelium

Nolte, p. 330

Human olfactory epithelium showing chemosensory cilia

Short microvilli on supporting cells

• Only visual system has more receptor cells

• Total surface area of cilia is estimated to be:

– 22 cm2 in human– 7 m2 in German

shepard!• Olfactory neurons replaced

every 60 days

Cilia on olfactory receptorNolte p. 333

Olfactory receptorsThere are about 950 odorant receptor genes in humans, but about 60% are not transcribed, leaving ~400 functional odorant receptors. There are odorant receptor genes on each human chromosome except chromosomes 20, 22 and the Y chromosome.

How olfactory information is coded is still poorly understood.

In mammals each receptor neuron probably only expresses a single receptor type, but some receptors respond to more than one odorant molecule. Ligand specificity is known for only 1 mammalian odorant receptor. It is the aldehyde n-octanal, which smells like freshly cut grass.

Ligand depolarizes receptor, causing action potentials travel along receptor axon to glomerulus in olfactory bulb

Cilia

Dendrite

Axon

Nolte, p. 334

Olfactory transduction

Odorants bind to receptors with 7 transmembrane domains characteristic of G-protein receptors.

Domains 3 – 5 are highly variable and probably odorant binding sites.

Binding may be direct or via odorant binding proteins in mucus that sequester odorant and may shuttle it to receptors

Odorant binding to receptor activates G protein > activates adenylate cyclase > increases intracellular [cAMP] > opens cyclic nucleotide gated channels > Na+ & Ca2+ enter > Ca2+ opens Cl- channel that further depolarizes receptor

http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html

Recognition of chemicals by olfactory system

• Nose can distinguish very similar compounds as different smells

• For example, the two stereoisomers of carvone smell like spearmint and caraway

• This implies there are stereoisomer-specific carvone receptors

• Schemes classifying odorants in categories such as ‘pungent, floral, musky’ etc. are empirical and not based on known receptor coding.

• Bell pepper odorant can be detected at concentration of 0.01 nM

• Threshold is lower for lipid soluble odorants than for water soluble odorants. CH3 CH2

O

CH3

Carvone

Each glomerulus gets axons from one receptor type

Olfactory epithelium

Glomerulus

Mitral cell

Olfactory tractMitral cells are principal projection neurons of the olfactory bulb

Receptor cell

Nolte, p. 336

Central olfactory pathwaysLateral olfactory tract projects directly to the piriform cortex (= primary olfactory cortex = paleocortex) adjacent to lateral olfactory tract in temporal lobe. This is only sense that does not have relay in thalamus on way from receptors to cerebral cortex. From piriform cortex there are projections to hypothalamus, the thalamus, amygdala, entorhinal cortex. From thalamus there is a projection to orbitofrontal cortex where odor perception and discrimination takes place. Electrical stimulation of piriform cortex causes olfactory sensations. People with lesions of orbitofrontal cortex are unable to discriminate odors. Pathways through amygdala and hypothalamus mediate emotional, motivational and many physiological effects of odors.

Fix, p. 326

Olfactory hallucinations and uncinate seizures

• Olfactory hallucinations can be the result of temporal lobe seizures; they are often part of the “aura” that precedes a seizure. May be accompanied by chewing or lip smacking.

• Odors are potent contextual cues for memory formation, emotional conditioning, olfactory flashbacks. Trauma-related smells precipitate flashbacks in patients with post-traumatic stress disorder.

• Identification of odors likely involves matching them to memory templates stored in brain. A smell is categorized based on one’s previous experiences of it and on the other sensory stimuli that correlate with its appearance.

Olfactory memories

AnosmiaTemporary or permanent loss of sense of smell

• Conductive: nasal polyps, septal deviation, inflammation• Sensorineural: head trauma, toxic chemicals, allergic

reactions, neuro- degenerative diseases such as Alzheimer’s or Parkinson’s

• Tumor beneath orbital surface of frontal lobe can produce unilateral anosmia

• Age: Reduced sensitivity due to altered vascularity, mucus composition, changes in innervation.

Taste

Taste provides information about quality, quantity and safety of ingested food.

Papillae distributed over tongue. Types:

Fungiform: anterior tongue; ~200. Facial nerve.

Foliate: lateral edge; mainly sour; glossopharyngeal n.

Circumvallate: sour, bitter; back of tongue

Receptor cells in taste buds on papillae and also epiglottis, soft palate, pharynx

Circumvallate papilla Taste buds

Nolte, p 324-325

Papillae Taste Buds, Receptor Cells

• Taste buds have 3 cells types: receptor, supporting, and basal cells• Receptor cells do not have axons, innervated by nerve fibers• Receptor cells live only about 2 weeks. Basal cells differentiate into new receptor

cells.

There are five basic tastes with different transduction mechanisms

• Salt: Na ions enter receptor via sodium channels, depolarize, transmitter release

• Sour: H+ blocks potassium channels

• Sweet and Bitter: involve G proteins & second messengers

• Umami: Amino acids, e.g., glutamate, aspartate; MSG. Metabotropic glutamate G protein receptors and ionotropic glutamate receptors.

Artificial sweetenersAll taste sweet but structures differ

Sugars and artificial sweeteners activate different second messenger systems in same receptor cell. Sugars activate cyclic nucleotide cascade leading to increase in cAMP. Artificial sweeteners activate IP3 (inositol-1, 4, 5-trisphosphate) system.

Supertasters

% of population Density of taste papillae at tip of tongue

cm-2

________________________________________________________________________________________

Supertasters 25 165

Normal tasters 50 127

Non-tasters 25 117

__________________________________________________________

Bitter compound: phenylthiocarbamide (PTC) contains N—C=S group.

Single autosomal gene.

Innervation of taste buds

Blue: VII.Facial n. geniculate ganglion

Green: IX. glossopharyngeal n. inferior ganglion

Red: X. Vagus n. inferior ganglion

Nucleus of solitary tract

• Taste receptors in tongue mucosa, few on epiglottis and pharynx

• Ganglion neurons in cranial nerve ganglia of VII, IX, and X

• Relay neurons in medulla

Nolte p 327

Taste pathways in CNS

Nucleus of the solitary tract projects to VPM of thalamus and to parabrachial nucleus in the pons.

Parabrachial nucleus projects to hypothalamus (H) [and amygdala (A) in most mammals, but maybe not in primates.] DMN X = dorsal motor nucleus of vagus.

Nolte p. 329

How are different tastes coded in the brain? The “Across Fiber Pattern theory”

• There are many more taste qualities than the traditional ‘primary’ tastes

• Taste quality is coded by a pattern of simultaneous activity of a number of nerve fibers.

• There are many types of nerve fibers• The same nerve fiber may respond to several

different tastants