PT22 LEC: Smell & Taste

main function of smell & taste

separate undesirable or even lethal foods from pleasant & nutritious ones

Taste sensation

- Mainly function of the taste buds, but smell also contributes to taste perception

- Texture of food & presence of substances that stimulate pain endings greatly alter taste experience

primary taste sensations

Sour

Salty

Sweet

Bitter

"Umami"

sour elicited by

hydrogen ions; acidity

salty elicited by

sodium ions

sweet elicited by

sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids, and inorganic salts of lead & beryllium

bitter elicited by

long-chain organic substances containing nitrogen & alkaloids; very sensitive & low threshold d/t protective mechanism from alkaloid toxins in plants

umami elicited by

L-glutamate; delicious

people are taste blind for?

thiourea compounds

taste buds

- Diameter of about 1/30 mm and length of about 1/16 mm

- Composed of 50 modified epithelial cells; sustentacular cells & taste cells

Taste cells

continually being replaced by mitotic division of surrounding epithelial cells

Taste pore

structure that outer tips of taste cells are arranged around

Microvilli or taste hairs

receptor surface for taste; protrude from tip of each taste cell

Taste nerve fibers

branching terminal network stimulated by taste receptor cells; interwoven around bodies of the taste cells

Vesicles

contain an NTM substance that is released through the cell membrane to excite the nerve fibers in response to taste stimulation

CircumVallate papillae

V-line on the surface of the posterior tongue; large number of taste buds

FungifOrm papillae

flat anteriOr surface of the tongue; moderate number of taste buds

FoLiate papillae

Lateral surface of the tongue; moderate number of taste buds

Adults

have 3,000 to 10,000 taste buds

Children

have more taste buds than adults

Taste sensitivity in old age

degenerates after 45 years

Low concentration of taste substance

taste buds response to one of primary stimuli

High concentration of taste substance

taste buds excited by 2 or more of primary stimuli, as well as stimuli that do not fit into the primary categories

Receptor potential for taste

change in electrical potential in the taste cell

Salty & sour taste depolarization

sodium & hydrogen ions; receptor proteins open specific ion channels in the apical membranes of the taste cells, thereby activating the receptors

Sweet & bitter taste depolarization

activate second-messenger system

Saliva

gradually washes away taste chemical

Taste bud stimulus

initially strong then gets weaker

Taste preference

CNS function rather than receptors

Taste aversion

negative perception developed after unpleasant food experience (such as getting sick)

Taste pathways into CNS

- From anterior ⅔ of tongue (fungiform papillae) → lingual nerve → chorda tympani → facial nerve → tractus solitarius (solitary nucleus of the medulla)

- Circumvallate papillae → glossopharyngeal nerve → tractus solitarius

- From base of tongue & other parts of pharyngeal region → vagus nerve → tractus solitarius

Second-order neurons

ventral posterior medial nucleus of thalamus

Third-order neurons

lower tip of the postcentral gyrus in the parietal cerebral cortex

improves control of saliva secretion

From tractus solitarius → superior & inferior salivatory nuclei (brainstem) → submandibular, sublingual, & parotid glands (secretomotor

BA 43

primary gustatory center

Smell sensation

- least understood

- presence/absence rather than intensity

primary smell sensations

Camphoraceous

Musky

Floral

Pepperminty

Ethereal

Pungent

Putrid

Olfactory membrane

lies in the superior part of each nostril; surface area of about 2.4 cm2

Olfactory cells

bipolar nerve cells derived originally from CNS

Olfactory hairs

also called olfactory cilia; project into the mucus that coats the inner surface of the nasal cavity & responds to olfactory chemical stimuli

Bowman glands

secrete mucus onto the surface of the olfactory membrane

Olfactory bulb

bulbous enlargement; has several thousand glomeruli & 25,000 axons from olfactory cells

Glomeruli

globular structures within the olfactory bulb; terminus for for dendrites from about 25 large mitral cells and about 60 smaller tufted cells

Threshold for smell

very low threshold; maximum intensity evoked only at 10-50x the lowest threshold

Odor blindness

lack of appropriate receptor protein in olfactory cells for a particular substance

factors affecting degree of stimulation

Chemical stimuli/odorants

Only volatile substances that can be sniffed into the nostrils can be smelled

Stimulating substance must be at least slightly water soluble so that it can pass through the mucus to reach the olfactory cilia

Slightly lipid soluble substance better than non-lipid soluble due to weak lipid barrier in the cilia

Primary olfactory cortex

periamygdaloid & prepyriform areas of the temporal lobe

Secondary olfactory cortex

pyriform lobe; uncus of the parahippocampal gyrus

Olfactory receptor adaptation

adapts about 50% within first second

Psychological adaptation

greater degree than receptor adaptation

Method of olfactory inhibition

- inhibitory granule cells

- CNS develops strong feedback inhibition to suppress relay of smell signals through olfactory bulb

Mechanism of olfactory stimulation

- After stimuli is detected, odorant substance diffuses into mucus that covers the cilia → binds with receptors proteins in each cilium membrane

- Second-messenger system → gated sodium ion channel opens → excitation of olfactory nerve → mitral cells in olfactory bulb → olfactory tract → primary & secondary olfactory cortex

Principle of transduction

Rate of impulses directly proportional to stimulus strength

Transduction

process wherein the brain is given information about the chemical composition of the objects/substances via smell or taste system

medial olfactory area

- PRIMITIVE

- ant to hypothalamus

- basic olfactory reflexes

lateral olfactory area

less old & newer systems

less old lat olfactory system

- prepyriform & pyriform cortex and cortical portion of amygdaloid nuclei

- COMPLICATED olfactory conditioned reflexes like food aversion

paleocortext

older part of the cerebral cortex in the anteromedial portion of the temporal lobe; only area where sensory signals pass directly to the cortex without passing first through the thalamus

newer lat olfactory system

- thalamus → dorsomedial thalamic nucleus → lateroposterior quadrant of the orbitofrontal cortex

- conscious analysis of odor

Centrifugal fibers

pass from the brain in the outward direction into the olfactory tract to the olfactory bulb; terminate on a large number of small granule cells located among mitral & tufted cells in the olfactory bulb

Granule cells

send inhibitory signals to mitral & tufted cells

Inhibitory feedback in olfactory bulb is?

a means for sharpening one's specific ability to distinguish one odor from another