Dr. Mohanad R. Alwan

Several types of chemoreceptors are present:

Taste buds - receptors of taste (gustatation) Olfactory receptors- receptors of smell (olfaction) Cutaneous nociceptors -Irritating chemicals on

skin Muscle sensors - burning during heavy exercise,

acidity receptors Circulatory sensors - oxygen & CO2 receptors Digestive tract sensors - receptors for various

ingested substances

Taste and smell (chemical receptors)tend to influence each other

Taste (gustation)

Taste buds- epithelial cells that act likeneurotransmitters

Innervated by cranial nerves

Taste (‘gustation’) and smell (‘olfaction’) are often referred

to as the chemical senses

They are mediated in the first instance by receptors that

are stimulated by chemical substances. Receptors are

called ‘chemoreceptors’

Taste: Chemicals dissolve in our mouth (must be water

soluble) and stimulate the taste buds in the oral cavity

(tongue, soft palate, cheek, etc.).

Gustation = Taste

Basic tastes:◦ Sweetness - outer tip of tongue◦ Saltiness - lateral edge of tongue behind tip◦ Sourness - lateral edge of tongue behind

saltiness region◦ Bitterness - back edge of tongue toward throat◦ Umami - taste of glutamate - not localized

Tastes triggers are not unique◦ many chemicals can trigger each receptor type

Loss of taste is ageusia

Four major sensations of taste

Sweet- G-protein activates second messengersystem

Bitter- also G-protein activated specific receptor

Sour- proton channel (acids taste sour)

Salty- sodium channels

Tastes seem to be enhanced in different partsof the tongue

The tongue is the major organ of gustationThere are also some taste buds on the

palate, pharynx, and epiglottis

Papillae - contain taste buds◦Fungiform papillae (mushrooms) - top surface of

tongue◦Foliate papillae (ridges) - rear, lateral margin of

tongue◦Vallate papillae (pimples) - rear, medial margin of

tongue

Only 1% of the epithelial cells on tongue’s surface are taste buds

Contain 50 to 75 sensitive cells eachTaste-sensitive cells have a limited lifetime,

and are constantly being replaced. Taste pore permits substances to enterThree cell types in each taste bud:◦Taste receptors – chemoreceptors◦Basal cells - source of new taste

receptors◦Gustatory afferent fibers - conduct

action potentials to brain

Come from the division and differentiation of the basal cells

Not neurons in the strict sense, but have synapses with gustatory afferent fibers

Arrival of chemical molecules on surface leads to change in membrane potential - receptor potential

Most taste receptors respond to at least two of the basic tastes

Sometimes taste receptors generate action potentials

Transduction differs for each type of taste receptor

Afferents◦Anterior 2/3 of tongue - VII (facial nerve)◦Posterior 1/3 of tongue - IX

(glossopharyngeal nerve)◦glottis, epiglottis, pharynx, palate - X (vagus

nerve)All afferent fibers end in gustatory nucleus

in the medulla

Taste fibers proceed along several pathways to the

medulla oblongata or brain stem, then to the

thalamus, and finally to the taste area on the anterior

cortex.

For taste sensation, gustatory nucleus neurons send

fibers to the ventral posterior medial nucleus (VPM) of

thalamus

VPM sends fibers to the primary gustatory cortex

(Brodmann's area 43 in ventral parietal lobe

For autonomic functions, gustatory nucleus sends

fibers to the many brainstem regions involved in

swallowing, salivation, gagging, vomiting, digestion,

respiration and to the hypothalamus and amygdala

• appetite and food preferences

receptor cells in olfactory epithelium

sense of smell transmitted directly to

cerebral cortex (all others go through

thalamus)

receptors in medial temporal lobesand in limbic system (emotionalresponse to odors)

Molecular basis for smell is complex-

humans can distinguish up to 10,000

different odors

Smell (olfaction)

Inside each side of the nose is an air chamber, the nasal cavity.

Air comes in through the nostril and flows down, around the rear of the roof of the mouth, into the throat.

When you sniff, air swirls up into the top of the cavity.

A small patch of about 10 million specialized olfactory (smelling) cells are in the cavity

Loss of smell is anosmia

Smell: Volatile (gaseous) chemicals are inhaled into the nasal

passages (or enter via the mouth) where olfactory receptors

line the membranes

Taste and smell are closely linked in that they are both usually

involved in many activities such as food seeking and sampling

(flavour involves both)

VOMERONASAL SYSTEM

Smell also conveys important non-nutritive information

such as the presence of prey, predators and in some

species mates (pheromones regulate sexual activity)

- Taste aids in the regulation of nutrients and enables the

organism to ‘test’ substances prior to ingestion (important

for identifying both nutrious foods and harmful

substances)

Olfactory epithelium in roof of nasal cavity

Olfactory receptors – chemoreceptorsSupporting cells - secrete mucousBasal cells - generate new olfactory

receptorsSize of the olfactory epithelium is a

measure of keenness of smell humans have about 10 sq. cm dogs have 170 sq. cm and dogs may also

have 100 times the olfactory receptor density

The olfactory sensors are located on each side of the inner nose.

True neurons: Dendrite ends in knob with multiple cilia

(containing receptors) at surface of epithelium Odor particles in the air stick to the cilia Unmyelinated axon leaves base of epithelium

thru cribiform plate and ends in olfactory bulb Born, live, and die with a 4-8 week cycle - only

neurons regularly replaced throughout life The olfactory cells produce nerve signals, which

travel to the olfactory bulb

Odorant binds to G-protein coupled receptor protein◦ There are 500 - 1000 different olfactory receptor

proteins coded genetically Effector protein is adenylyl cyclase cAMP produced and binds to cation channel

(sodium + calcium) Calcium influx opens calcium-sensitive

chloride channel Calcium and chloride cause membrane

depolarization (receptor potential) Action potentials in olfactory receptor fibers

A pre-processing center that sorts the signals before they travel along the olfactory tract to the brain

Axons from receptor cells project to the olfactory bulbs

Here the glomeruli, receive signals from 26,000 receptors

The olfactory bulbs on either side are cross-connected.

Nerve fibers project from the olfactory bulb to

the olfactory tubercle

Olfactory tubercle neurons project to medial

dorsal nucleus of thalamus

Thalamic neurons project to orbitofrontal cortex

reach the olfactory areas in the neocortex for

the sensation of smell

29