1. Chemical Senses
Chemical senses – gustation (taste) and olfaction (smell)
Their chemoreceptors respond to chemicals in aqueous solution
Taste – to substances dissolved in saliva
Smell – to substances dissolved in fluids of the nasal membranes

2. Sense of Smell
The organ of smell is the olfactory epithelium, which covers the superior nasal concha
Olfactory receptor cells are bipolar neurons with radiating olfactory cilia
Olfactory receptors are surrounded and cushioned by supporting cells
Basal cells lie at the base of the epithelium

3. Sense of Smell
Figure 15.3

4. Olfactory Mucous Membrane
-Olfactory receptor cells
-Area of 5cm2 in roof of nasal cavity near the septum
-10 to 20 million receptor cells
-Each olfactory receptor is a neuron
-Olfactory mucous membrane is the place in body where NS is closest to external world

5. Composition of Olfactory epithelium
-Each neuron has a thick dendrite with an expanded end called olfactory rod
-From rods cilia project to the mucous surface
-Each receptor neuron has cilia
-Axons of olfactory receptor neurons pierce cribriform plate of ethmoid bone and enter olfactory bulbs
-Olfactory neurons have half-time of few weeks.

6. Mucus producing Glands
-Olfactory mucous membrane is constantly covered by mucus
-Mucus is produced by Bowman’s glands, placed just under the basal lamina of the menbrane

7. Physiology of Smell
Olfactory receptors respond to several different odor-causing chemicals
When bound to ligand these proteins initiate a G protein mechanism, which uses cAMP as a second messenger
cAMP opens Na+ and Ca2+ channels, causing depolarization of the receptor membrane triggers an action potential

8. Olfactory Transduction Process
Odorant binding protein
Odorant chemical
Na+
Inactive
Active
Na+ influx causes depolarization
ATP
Adenylate cyclase
cAMP
Depolarization of olfactory receptor cell membrane triggers action potentials in axon of receptor
Cytoplasm
Figure 15.4

9. Olfactory Bulbs
-Axons of receptors contact the primary dendrites of mitral cells and tufted cells.
-Forming complex globular synapses called olfactory glomeruli.
-Periglomerular cells are inhibitory neurons connecting one glomerulus to another

10. Transmission of smell signals to CNS

11. Olfactory thresholds and discrimination
Olfactory receptors respond only to substances in contact with olfactory epithelium and need to be dissolved in mucus
Methyl marcaptan one of the substances in garlic can be smelled at very low concentration showing the remarkable sensitivity of olfactory receptors
Humans can recognize more than different odors
However determination of intensity of odor is poor
Substance
mg/L of Air
Ethyl ether
5.83
Chloroform
3.30
Pyridine
0.03
Oil of peppermint
0.02
Iodoform
Butyric acid
0.009
Propyl mercaptan
0.006
Artificial musk
Methyl mercaptan

12. Role of Pain Fibers in the Nose
Many trigeminal pain fibers are found in olfactory membrane
They are stimulated by irritating substances
Are responsible for initiating sneezing, lacrimation and other reflex responses.

13. Abnormalities Anosmia – absence of sense of smell
Hyposmia – diminished olfactory sensitivity
Dysosmia – distorted sense of smell
More than 75% of humans over the age of 80 have an impaired ability to identify smells

14. Taste Buds Most of the 10,000 or so taste buds are found on the tongue
Taste buds are found in papillae of the tongue mucosa
Papillae come in three types: filiform, fungiform, and circumvallate
Fungiform and circumvallate papillae contain taste buds

15. Taste Buds
Figure 15.1

16. Anatomy of a Taste Bud
Each taste bud consists of three major cell types
Supporting cells – insulate the receptor
Basal cells – dynamic stem cells
Gustatory cells – taste cells

17. Taste Sensations There are five basic taste sensations
Sweet – sugars, saccharin, alcohol, and some amino acids
Salt – metal ions
Sour – hydrogen ions
Bitter – alkaloids such as quinine and nicotine
Umami – elicited by the amino acid glutamate

18. Physiology of Taste In order to be tasted, a chemical:
Must be dissolved in saliva
Must contact gustatory hairs
Binding of the food chemical:
Depolarizes the taste cell membrane, releasing neurotransmitter
Initiates a generator potential that elicits an action potential

19. Taste Transduction
The stimulus energy of taste is converted into a nerve impulse by:
Na+ influx in salty tastes
H+ in sour tastes (by directly entering the cell, by opening cation channels, or by blockade of K+ channels)
Gustducin in sweet (cAMP) and bitter (Ca) tastes

20. Gustatory Pathway
Cranial Nerves VII , IX and X carry impulses from taste buds to the solitary nucleus of the medulla
These impulses then travel to the thalamus, and from there fibers branch to the:
Gustatory cortex (taste)
Hypothalamus and limbic system (appreciation of taste)

21. Gustatory Pathway
Figure 15.2

22. Influence of Other Sensations on Taste
Taste is 80% smell
Thermoreceptors, mechanoreceptors, nociceptors also influence tastes
Temperature and texture enhance or detract from taste

23. Abnormalities Ageusia: Absence of taste sensation
Dysgeusia: Disturbed taste sensation
Hypogeusia: Diminished taste sensation
Gustatory hallucination: false sensation of certain substance.