The Chemical Senses Taste and Smell

The senses of taste and smell allow us to separate undesirable or even lethal foods from

those that are pleasant to and nutritious

Sense of Taste

A.Stimulus – Sapid substance or substances with taste

Primary Sensations of Taste

1. Sour Taste – caused by acid (hydrogen ion concentration. Intensity of sensation is proportional to the logarithm of the hydrogen ion concentration

2. Salty taste – elicited by ionized salts, mainly by sodium ion concentration .The cations of salt especially sodium cations, are mainly responsible for the salty

3. Sweet Taste – is not caused by any single class of chemicals. Includes sugars, glycols, alcohols, aldehydes, ketones, amines, esters, some amino acids etc.

4. Bitter Taste – Like bitter taste is not caused by any single type of chemical agent. Long chain organic substances that contain nitrogen and alkaloids are likely to causes bitter taste sensation

5. Umami Taste – Japanese work (meaning “delicious”) designating a pleasant taste sensation . It is dominant in food containing L-glutamate

A. Receptors - Taste Buds ( Sustentacular cells and Taste cells)

Location of Taste Buds – found on three types of papillae in the tongue (Guyton)

1. Walls of the troughs that surrounds the Circumvallate papilla which form V line on the posterior tongue

2. Fungiform papillae over the flat anterior surface of the tongue (moderate number)

3. Foliate papillae located in the folds along the lateral surfaces of the tongue (moderate number)

Additional taste buds are located on the palate, tonsillar pillars, epiglottis and proximal esophagus

Also in the mucosa of the epiglottis, palate, and pharynx

(Ganong) 1. Fungiform papilla – numerous at the tip of the tongue

2. Vallate (circumvalate ) papilla – arrange on the back of the tongue

3. Filiform papilla – cover the dorsum of the tongue , do not usually contain taste buds

Specificity of Taste Buds for a Primary Taste Stimulus

Each taste bud usually responds mostly to one of the five primary taste stimuli when taste substance is in low concentration. At high concentration, most buds can be excited by two or more of the primary taste stimuli

Mechanisms of Stimulation of Taste BudsReceptor Potential

* For sodium ions and hydrogen ions, which elicit salty and sour sensations, respectively, the receptor proteins open specific ion channels in the apical membranes of the taste cells, thereby activating the receptors

* For sweet and bitter taste sensation the receptor protein molecules activate the second -messenger transmitter substances inside the taste cells and these second messengers cause intracellular chemical changes that elicit the taste signals

Transmission of Signals into the Central Nervous System (Conducting Pathway)

* Taste buds from the anterior 2/3 of the tongue > Chorda tympani branch of Facial Nerve. * From posterior third of the tongue > Glossopharyngeal nerve * From tonsillar areas and back of the tongue > Vagus Nerve

Tractus Solitarius (medulla oblongata) > Medial Lemniscus > Thalamus

Center – Postcentral Gyrus

Adaptation of Taste – Extreme degree of adaptation that occurs in the sensation of taste almost certainly occurs in the central nervous system

Abnormalities of Taste

1. Ageusia – absence of taste sensitivity

2. Hypogeusia – diminished taste sensitivity

3. Dysgeusia – disturbed sense of taste

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.

Receptors – Olfactory Cells ( olfactory hairs or cilia)Guyton Fig. 53-3

, Fig. 10-2

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

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

C. Transmission of Smell signals into the Central Nervous System (Conducting Pathway)Guyton Fig. 53-4

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

Modified Source: Victoria DiGano