Adam Quick MD Department of Neurology The Ohio State University College of Medicine The Chemical Senses – Taste and Olfaction

Objectives Describe which sensory systems comprise the chemical senses Differentiate between “flavor” and taste Recognize the pathological states affecting the chemical senses and the consequences of abnormalities in these systems. Describe the basic anatomy of the gustatory and olfactory systems. Describe the anatomy and physiology of the gustatory and olfactory systems At the end of the module, you will learn to:

WHAT ARE THE CHEMICAL SENSES? Senses that respond to chemicals.TasteOlfaction Chemesthesis (Common Chemical Sense- often response to noxious chemical stimuli)

WHERE ARE THE CHEMICAL SENSES?  NASAL CAVITY  Olfaction olfactory nerve  Vomeronasal vomeronasal nerve  Chemesthesis trigeminal  ORAL CAVITY  Gustation facial, glossopharngeal, vagal  Chemesthesis trigeminal

Normal Function  Reward  Enjoyment of a meal  Smell of the ocean  Cool feeling of eucalyptus vapors  Warning  Spoiled milk  Noxious fumes like gas or smoke  Burning quality of ammonia

What are chemosensory disorders ?  GUSTATION  Aguesia  Hypoguesia  Dysguesia  Hyperguesia  Gustatory Agnosia  OLFACTION  Anosmia  Hyposmia  Dysosmia  Hyperosmia  Olfactory Agnosia

How important is “TASTE”? What is the definition of taste? What happens when we lose “taste”?

“FLAVOR” is a more accurate descriptor of chemosensory perception.  oral cavity “TASTE”  gustation  tactile  thermal  pain  nasal cavity “SMELL”  olfaction  common chemical sense

Consequences of Loss of Flavor Indiscriminate Eating Reduced drive to consume food because food is “bland” Results in weight loss and malnutrition Compensatory Eating Increased use of non-olfactory sensory components Often leads to high salt, high sugar diets

CLINICAL ETIOLOGY  Data of complaints from 750 patients 77% complain of TASTE loss

CLINICAL ETIOLOGY 77% complain of some taste loss 3% have some taste loss

Causes of Olfactory Dysfunction  Nasal/Sinus Disease (NSD)  Not completely understood; chronic inflammation and changes in mucous probably important  Nasal polyps  Upper Respiratory Infection (URI)  Not completely understood; degeneration of olfactory receptor neurons noted  Head Trauma- cribiform plate  Neurodegenerative disorders  Neoplasms  (Specific Anosmias- born without specific odorant receptor)

Possible Causes of Taste Dysfunction Common  Infections  Bell’s Palsy  Drugs-including diffusion from blood  Oral Appliances  Dental procedures  Periodontal disease  Salivary disturbance Less Common  Vitamin, trace metal deficiency  CNS Tumor  Head Trauma  Toxic chemical exposure  Radiation treatment Uncommon  Psychiatric disorder  Epilepsy  Migraine  Sjogren’s  Endocrine disorder

Olfaction The nasal cavity Flavor is mostly olfaction Even in visually oriented species many thousands of different odorants can be detected 10,000 in most people

 Occupies about 1-2 cm2 of the roof of the nasal cavity  Olfactory receptor neurons are continually replenished throughout life, ½ life of about 60 days Olfactory epithelium

Olfactory Receptor Neurons

The seven transmembrane olfactory receptors  Only one receptor gene expressed per ORN easily explains prevalence of specific anosmias  Each receptor responds to >1 odorant  Human ~ 300s; mouse ~ 1,000  Transduction similar to visual system

ORNs expressing the same receptor project to the same glomeruli

The olfactory bulb  The first processing station of olfactory information  Different odorants activate different collections of glomeruli

Central Olfaction  Ant. Olfactory Nucleus  Pyriform Cortex  Olfactory tubercle  Amygdala  Entorhinal Cortex Does not include a thalamus-cortex relay, but has direct projections to limbic system Olfactory stimuli can easily evoke emotional responses

 soft palate  epiglottis  cheeks (rare)  lips (rare) LINGUAL EXTRALINGUAL Filiform papillae sense tactile, thermal and chemical stimuli A quick look at gustation Taste Buds- located on papillae

The Taste Bud Highly variable in number from person to person (average of about 5000)

Taste Receptors SWEET T1R T1R1/T1R2 BITTER T2R 33 members Mostly co- expressed SALTY Ion Channel SOUR Ion Channel UMAMI T1R family T1R2/T1R3 T1R “Sweet” T2R “Bitter Epithelial Sodium Channel

Cranial Nerves Carrying Taste 1) Facial nerve: chorda tympani and greater superficial petrosal branches from anterior tongue and palate 2) Glossopharyngeal nerve: posterior tongue 3) Vagus nerve: epiglottis

Gustatory Cortex Hypothalamus and amygdala Taste Central Projections

Summary  Chemical senses consist of olfaction, gustation and chemethesis  Taste is a neurological sensation, but flavor is a multimodality integrated sensory experience  People often complain of inability to “taste” subjectively, but most often it is their experience of “flavor” that is diminished. This is frequently related to problems with olfaction.  Many disorders of the upper airways and central nervous system can affect the senses of olfaction and gustation and abnormalities in these sensory modalities can have significant physical consequences.

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