Chapter 8 The Chemical Senses

Introduction Animals depend on the chemical senses to identify nourishment, poison, potential mate Chemical sensation –Oldest and most common sensory system Chemical senses –Gustation –Olfaction –Chemoreceptors

in 1907 he conducted experiments to identify the fifth taste category…. umami

Taste The Basics Tastes –Saltiness, sourness, sweetness, bitterness, and umami –Examples of correspondence between chemistry Sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame) Bitter—ions like K + and Mg 2+, quinine, and caffeine –Advantage – Survival Poisonous substances - often bitter

Taste The Basic Tastes –Steps to distinguish the countless unique flavors of a food Each food activates a different combination of taste receptors Distinctive smell Other sensory modalities

Taste The Organs of Taste –Tongue, mouth, palate, pharynx, and epiglottis

Taste The Organs of Taste –Areas of sensitivity on the tongue Tip of the tongue –Sweetness Back of the tongue – Bitterness Sides of tongues –Saltiness and sourness

Taste The Organs of Tastes –Papillae Foliate papillae Vallate papillae Fungiform papillae –Threshold concentration Just enough exposure of single papilla to detect taste

Taste Tastes Receptor Cells –Apical ends  Microvilli  Taste pore –Receptor potential: Voltage shift

Taste Taste Receptor Cells

Taste Mechanisms of Taste Transduction –Transduction process Taste stimuli (tastants) –Pass directly through ion channels –Bind to and block ion channels –Bind to G-protein-coupled receptors

Taste Mechanisms of Taste Transduction –Saltiness Salt-sensitive taste cells – Special Na + selective channel Blocked by the drug amiloride

Taste Mechanisms of Taste Transduction –Sourness Sourness- acidity – low pH Protons causative agents of acidity and sourness

Taste Mechanisms of Taste Transduction –Bitterness Families of taste receptor genes - TIR and T2R

Taste Mechanisms of Taste Transduction –Sweetness Sweet tastants natural and artificial Sweet receptors –T1R2+T1R3 –Expressed in different taste cells

Taste Mechanisms of Taste Transduction –Umami Umami receptors: –Detect amino acids –T1R1+T1R3

Taste Central Taste Pathways

Taste Central Taste Pathways (Cont’d) –Localized lesions Ageusia- the loss of taste perception –Gustation Important to the control of feeding and digestion –Hypothalamus –Basal telencephalon

Taste The Neural Coding of Taste –Labeled line hypothesis Individual taste receptor cells for each stimuli In reality, neurons broadly tuned Population coding –Roughly labeled lines –Temperature –Textural features of food

Smell Pheromones –Smell— a mode of communication –Important signals Reproductive behavior Territorial boundaries Identification Aggression –Role of human pheromones

Caspar Berthelsen Bartholin - Danish physician and theologian who who in 1619 wrote one of the most widely read Renaissance manuals of anatomy. He was first to describe the olfactory nerve (the first identified cranial nerve).

Smell The Organs of Smell –Olfactory epithelium Olfactory receptor cells, supporting cells, and basal cells

Smell The Organs of Smell –Odorants: Activate transduction processes in neurons –Olfactory axons constitute olfactory nerve –Cribriform plate: A thin sheet of bone through which small clusters of axons penetrate, coursing to the olfactory bulb –Anosmia: Inability to smell –Humans: Weak smellers Due to small surface area of olfactory epithelium

Smell Olfactory Receptor Neurons –Olfactory Transduction

Smell Olfactory Receptor Neurons (Cont’d) –Olfactory Transduction

Olfactory Transduction –Adaptation: Decreased response despite continuous stimulus Smell

Central Olfactory Pathways (Cont’d) Smell

Central Olfactory Pathways (Cont’d) Smell

Central Olfactory Pathways (Cont’d) –Axons of the olfactory tract: Branch and enter the forebrain –Neocortex: Reached by a pathway that synapses in the medial dorsal nucleus Smell

Spatial and Temporal Representations of Olfactory Information –Olfactory Population Coding –Olfactory Maps (sensory maps) Smell

Concluding Remarks Transduction mechanisms – Gustation and olfaction Similar to the signaling systems used in every cell of the body Common sensory principles - broadly tuned cells –Population coding –Sensory maps in brain Timing of action potentials –May represent sensory information in ways not yet understood