CHEMICAL CHEMICAL SENSESSENSES

Sense of SmellSense of Smell

THE SENSES OF TASTE AND SMELL ALLOW US TO SEPARATE UNDESIRABLE OR EVEN LETHAL FOODS FROM THOSE THAT ARE PLEASANT TO

AND NUTRITIOUS

The Chemical Senses Taste and Smell

Introduction: Smell & TasteSmell and taste are generally classified as chemical senses . Physiologically, they are related to each other. The flavors of various foods are in large part a combination of their taste and smell. Consequently, food may taste "different" if one has a cold that depresses the sense of smell. Both taste and smell receptors are chemoreceptors that are stimulated by molecules in solution in mucus in the nose and saliva in the mouth. The smell receptors are distance receptors (teleceptors), and the smell pathways have no relay in the thalamus.

Smell is the least understood of our senses.

A. Stimulus – Odorant or Odoriferous substances

Physical Factors that affect the degree of stimulation

1. only volatile substances that can be sniffed into the nostril can be smelled.

2. the stimulating substances must be slightly water soluble. 3. substances must be at least slightly lipid soluble.

Olfactory Mucous MembraneThe olfactory receptor cells are located in a specialized portion of the nasal mucosa, the yellowish pigmented olfactory mucous membrane. In dogs and other animals in which the sense of smell is highly developed (macrosmatic animals), the area covered by this membrane is large; in microsmatic animals such as humans, it is small. In humans, it covers an area of 5 cm2 in the roof of the nasal cavity near the septum

Smell (olfaction)

Olfactory apparatus consists of receptor cells, supporting cells and basal (stem) cells. Basal cells generate new receptor cells every 1-2 months. Supporting cells contain enzymes that oxidize hydrophobic

volatile odorants. Bipolar sensory neurons located within olfactory

epithelium are pseudostratified. Axon projects directly up into olfactory bulb of cerebrum.

Olfactory bulb projects to olfactory cortex, hippocampus, and amygdaloid nuclei. Synapses with 2nd order neuron.

Dendrite projects into nasal cavity where it terminates in cilia. Neuronal glomerulus receives input from 1 type of olfactory

receptor.

Stimulation of the Olfactory Cells

Mechanism of Excitation of the Olfactory Cells

1. Activation of the receptor protein by the odorant substance activates the G-protein complex

2. This, in turn activates multiple molecules of adenylyl cyclase inside the olfactory cell membrane

3. This causes the formation of many times more of molecules of cAMP

4. cAMP opens still many times more sodium channels

Olfaction

Olfactory receptors

Mitral cell

Olfactorygland

Olfactorytract

Olfactoryepithelium

Filaments of olfactory nerve

Cribriform plateof ethmoid bone

Lamina propria connective tissue

Basal cell

Supporting cell

DendriteOlfactory cilia

Glomeruli

Axon

Olfactory receptorcell

Mucus

Route of inhaledair containing odormolecules

Frontal lobeof cerebrum

Nasalconchae

Route ofinhaled air

Olfactoryepithelium

Olfactory tractOlfactorybulb

Olfactory Receptors Bipolar sensory neurons located within olfactory epithelium

Dendrite projects into nasal cavity, terminates in cilia Axon projects directly up into olfactory bulb of cerebrum Olfactory bulb projects to olfactory cortex, hippocampus, and

amygdaloid nuclei

Olfactory Epithelium and Olfactory BulbOlfactory Epithelium and Olfactory Bulb Olfactory Epithelium and Olfactory BulbOlfactory Epithelium and Olfactory Bulb

OlfactoryOlfactory epitheliumepithelium

Olfactory BulbOlfactory Bulb

GlomerulusGlomerulus Mitral cellMitral cell Tufted cellTufted cell periglomerular periglomerular cellcell Olfactory tractOlfactory tract

Modality: Modality: Olfaction Olfaction

Receptor: Receptor: Olfactory Cell of Olfactory EpitheliumOlfactory Cell of Olfactory Epithelium

Cranial Nerve: I (Olfactory Nerve)Cranial Nerve: I (Olfactory Nerve)

1st Neuron: 1st Neuron: Olfactory Bulb --- Mitral & Tufted CellOlfactory Bulb --- Mitral & Tufted Cell

olfactory tractolfactory tract

olfactory striaeolfactory striae

lateral & intermediate olfactory striaelateral & intermediate olfactory striae

Termination:Termination: Primary Olfactory Area (Rhinencephalon)Primary Olfactory Area (Rhinencephalon)

piriform lobe (prepiriform cortex, entorhinal cortex) piriform lobe (prepiriform cortex, entorhinal cortex)

corticomedial amygdala corticomedial amygdala

anterior perforated substance (olfactory tubercle)anterior perforated substance (olfactory tubercle)

Modality: Modality: Olfaction Olfaction

Receptor: Receptor: Olfactory Cell of Olfactory EpitheliumOlfactory Cell of Olfactory Epithelium

Cranial Nerve: I (Olfactory Nerve)Cranial Nerve: I (Olfactory Nerve)

1st Neuron: 1st Neuron: Olfactory Bulb --- Mitral & Tufted CellOlfactory Bulb --- Mitral & Tufted Cell

olfactory tractolfactory tract

olfactory striaeolfactory striae

lateral & intermediate olfactory striaelateral & intermediate olfactory striae

Termination:Termination: Primary Olfactory Area (Rhinencephalon)Primary Olfactory Area (Rhinencephalon)

piriform lobe (prepiriform cortex, entorhinal cortex) piriform lobe (prepiriform cortex, entorhinal cortex)

corticomedial amygdala corticomedial amygdala

anterior perforated substance (olfactory tubercle)anterior perforated substance (olfactory tubercle)

Olfactory PathwayOlfactory Pathway Olfactory PathwayOlfactory Pathway

A. olfactory epitheliumA. olfactory epitheliumB. olfactory bulbB. olfactory bulbC. nucleus of diagonal bandC. nucleus of diagonal band of Brocaof BrocaD. periamygdaloid cortexD. periamygdaloid cortexE. cirticomedial amygdalaE. cirticomedial amygdalaF. entorhinal areaF. entorhinal areaG. septal nucleiG. septal nuclei

I. olfactory nerveI. olfactory nerve1. olfactory tract1. olfactory tract2. lateral olfactory stria2. lateral olfactory stria3. intermediate olfactory stria3. intermediate olfactory stria4. medial olfactory stria4. medial olfactory stria

Olfactory System - Olfactory PathwaysOlfactory System - Olfactory PathwaysOlfactory System - Olfactory PathwaysOlfactory System - Olfactory Pathways

Olfactory PathwaysOlfactory Pathways

I

A. olfactoryA. olfactory epitheliumepithelium

B. olfdactory bulbB. olfdactory bulbC. anterior C. anterior olfactoryolfactory nucleus &nucleus & olfactory olfactory tubercletubercleD. periamygdaloidD. periamygdaloid areaareaE. corticomedialE. corticomedial amygdalaamygdalaF. entorhinal areaF. entorhinal area

II. olfactory nerve . olfactory nerve 2. lateral olfactory striae2. lateral olfactory striae1. olfactory tract 1. olfactory tract 3. intermediate olfactory striae3. intermediate olfactory striae

A. olfactory epitheliumA. olfactory epitheliumB. olfactory bulbB. olfactory bulbC. nucleus of diagonal bandC. nucleus of diagonal band of Brocaof BrocaD. periamygdaloid cortexD. periamygdaloid cortexE. cirticomedial amygdalaE. cirticomedial amygdalaF. entorhinal areaF. entorhinal areaG. mediodorsal thalamicG. mediodorsal thalamic nucleus (MD)nucleus (MD)

I. olfactory nerveI. olfactory nerve1. olfactory tract1. olfactory tract2. lateral olfactory stria2. lateral olfactory stria3. intermediate olfactory stria3. intermediate olfactory stria4. thalamocortical radiation4. thalamocortical radiation

Olfactory System Olfactory System - Connections of Primary Olfactory Cortex- Connections of Primary Olfactory CortexOlfactory System Olfactory System - Connections of Primary Olfactory Cortex- Connections of Primary Olfactory Cortex

Membrane Potentials and Action Potentials in Olfactory Cells

Adaptation – Large numbers of centrifugal nerve fibers pass from the olfactory regions of the brain backward along the olfactory tract and terminate on special inhibitory cell s in the olfactory bulb, the granule cells

Primary Sensations of Smell 1. Camphoraceous 2. Musky

3. Floral 4. Pepperminty 5. Ethereal 6. Pungent 7. Putrid

Olfactory Cells > Glomerulus > Mitral Cells > Olfactory Bulb > Olfactory Tract

D. Center

Olfactory Area 1. Medial Olfactory Area – Very Old Olfactory System *subserves the basic olfactory reflexes such as licking of the lips, salivation, and other feeding responses caused by smell

of food 2. Lateral Olfactory Area –

A.. Less Old Olfactory system

provides automatic but partially learned control of food intake and aversion to toxic and unhealthy foods

B. Newer Pathway Used for conscious perception and analysis of olfaction

Adaptation (Centrifugal Control of Activity in the Olfactory Bulb by the Central Nervous System)

Abnormalities of Olfaction

1. Anosmia – absence of the sense of smell

2. Hyposmia – diminished olfactory sensitivity

3. Dysosmia – distorted sense of smell

AdaptationIt is common knowledge that when one is continuously exposed to even the most disagreeable odor, perception of the odor decreases and eventually ceases. This sometimes beneficent phenomenon is due to the fairly rapid adaptation, or desensitization, that occurs in the olfactory system. It is mediated by Ca2+ acting via calmodulin on cyclic nucleotide-gated (CNG) ion channels. When CNG A4 is knocked out, adaptation is slowed.

Smell (Olfaction) Olfactory epithelium with olfactory receptors, supporting cells, basal

cells Olfactory receptors are modified neurons Surfaces are coated with secretions from olfactory glands Olfactory reception involves detecting dissolved chemicals as they

interact with odorant binding proteins

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