THE CHEMICAL SENSES

THE CHEMICAL SENSES

Animals depend on the chemical senses to identify nourishment

Chemical sensationOldest and most common sensory system with the aim to detect environmental chemicals

Chemical sensesGustation & Olfaction (separate but processed in parallel)Chemoreceptors

TASTE

The Basics TastesSaltiness, sourness, sweetness, bitterness, and umami.

Innate preferences and rejections for particular tastes (sweet and bitter) have a survival reasons

Usually there is correspondence between chemical ingredients andtaste:

Sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame)Bitter—ions like K+ and Mg2+, quinine, and caffeineSalty—saltsSour—acids

How to distinguish the countless unique flavors of a food1) Each food activates a different combination of taste receptors2) Distinctive smell (it combines with taste to give the flavor)3) Other sensory modalities (texture and temperature)

TASTE

The Organs of TasteTongue, mouth, palate, pharynx, and epiglottisNasal cavity for smell

TASTEAreas of sensitivity on the tongue (but most of the tongue is sensitive to all basics tastes)

Tip of the tongue: SweetnessBack of the tongue : BitternessSides of tongues: Saltiness and sourness

Papillae (taste receptors)FoliateVallateFungiform

At threshold concentration (just enough exposure of single papilla to detect taste) they respond to only one taste. More concentrations lead to less selectivity

TASTETastes Receptor Cells

Apical end is the chemically sensitive part. It has small extensions called microvillithat project into the taste pore.Receptor potential: Voltage shift – depolarization of the membrane cause CA++ entering the cell and release of transmitter

TASTE

Transduction: process by an environmental stimulus cause an electrical response in a sensory receptor.

In the case of taste, chemical stimuli (tastants) may:

1)Pass directly through ion channels2)Bind to and block ion channels3)Bind to G-protein-coupled receptors

Slightly different mechanisms for saltiness, sourness, bitterness, sweetness and umami (amino acids)

TASTE

SournessSourness- acidity – low pHH + binds to and block ion channelscausing deporalization

SaltinessSpecial Na+ selective channel. The ion pass directly through channelcausing deporalization

TASTE

BitternessBitter substances are detected by different types T1R and T2R receptor. They work as G-protein coupled receptors

SweetnessIt also detected by receptors T1R2+T1R that have the same signaling mechanism (cf. bitter taste)The expressed in different taste cells allow the system not to be confused about the taste

UmamiUmami receptors T1R1+T1R3 detect amino acids

TASTE

Bitterness Sweetness Umami

TASTE

VII Facial nerveIX Glossopharyngeal nerveX Vagus nerveThey carry primary gustatory axons

Gustatory nucleusPoint where taste axons bundle and synapse

Ventral posterior medial nucleus (VPM)Deals with sensory information from the head

Primary gustatory cortex (Insula)Receives axons from VPM taste neurons

Lesion in VPM and Gustatory cortex can cause ageusia- the loss of taste perception

SMELL

Smell is not only important for taste but also for social communication

Pheromones are important signals• Reproductive behavior• Territorial boundaries• Identification• Aggression

SMELL

The Organs of Smell1)Olfactory epithelium: contains olfactory receptor cells, supporting cells (produce mucus), and basal cells (source of new receptor cells)2)Olfactory axons constitute olfactory nerve3)Cribriform plate: A thin sheet of bone through which small clusters of axons penetrate, coursing to the olfactory bulb

Anosmia: Inability to smell

SMELL

Olfactory Transduction

Receptor potential: if strong enough generates APs in the cell body and spikes will propagate along the axon

SMELL

Adaptation: decreased response despite continuous stimulus. Common features of sensory receptors across modalities

Each receptor cell express a single olfactory receptor protein.

They responds to different odours but with preferences.

Many different cells are scattered into the epithelium

SMELL

Central Olfactory Pathways

Mapping of receptor cell into glomeruli is extremely precise

SMELL

Axons of the olfactory tract branch and enter the forebrain (unconscious perception) bypassing the thalamus

Neocortex (conscious perception) is reached by a pathway that synapses in the medial dorsal nucleus of the thalamus