Gustatory System

Taste Transduction

1) Ionotropic (ion channel) and 2) Metabotropic (receptors coupled to G Protein coupled receptors). Ion channels are the more direct process.

How to classify taste cells

1) Morphological and fine structural features 2) Molecular Features 3) Functional responses to stimuli

Types of cells

- Apical, - Baso-lateral, - Basal cells. All these cells are regenerative tissues) Cells die in staggered, stochastic fashion and are replaced. Tastes are constant despite turnover and synapse re-form.

Additional paramters that cells encode for other than quality:

-Intensity -Hedonics - like-dislike -Localization on tongue -After tastes

Type IV

Basal Cells

Gustatory nerves

CN VI, IX, X Taste is conveyed by these three cranial nerves. Nucleus of the solitary tract → Thalamus, gustatory cortex (Thalamocortical; conscious perception of taste). Also goes to ventral places like amygdala, hippocampus. Emotional and memory eating.

Innervation of tip of tongue

Chorda tympani branch of mandibular branch of CN V

papillae

Contain many taste buds, each of which contains many taste cells

Salt: ENac Channels (Na+) -

Degenerin channels

Bitter Taste

EVENTS 1) Bitter substances bind to the T2R receptors activating the G-protein ---> activation of PLC. 2) The second messengers DAG and IP3 are produced activating TRPM5 ---> Release of Ca2+ from internal stores. 3) The elevated Ca2+ causes transmitter release ---> increase of firing of the primary afferent nerve.

Innervation of soft palate and tongue.

Facial Nerve

Innervation of the back of the tongue

Glossopharyngeal Nerve

IP3

IP3 → Acts on channel to allow calcium to flood out of ER, depolarize the cell

Filiform papillae

Increase friction Do NOT have taste buds

Knock out experiments

K/O of T1R1: Loss of umami taste K/O of T122/3: Loss of sweet taste K/O of T2Rs: Loss of bitter taste K/O of PKD2L1: Loss of Sour taste Taste has TRPM5 to amplify in the metabotropic pathway (knockout knocks out those three tastes)

Result of knock out of ATP (P2X) receptor

Knock out all taste in both CT and GP nerves

Result if knockout of eNac channels

Knockout of eNac channels in mouth only = Get loss of signal, animals can still taste salt. There must be something else involved.

Vallate papillae

Large, macroscopic papillae Taste buds lie in crevice anterior terminal sulcus

What controls gag, spit reflex?

Medulla

Type II Histologically: Light cells

Multiple short microvilli - more villi than type I

Type III cells Intermediate color cells

One large microvillus

Salt receptor (ionic receptor)

Salt is sodium chloride (Na+ Cl-). Na+ ions enter the receptor cells via Na-channels. These are amiloride-sensitive Na+ channel (as distinguished from TTX-sensitive Na+ channels of nerve and muscle). EVENTS 1) The entry of Na+ causes a depolarization, 2) Ca2+ enters through voltage-sensitive Ca2+ channels, 3) Transmitter release occurs and results in increased firing in the primary afferent nerve.

Foliate papillae

Slit-like papillae with taste buds Taste buds lie inside. Anterior 2/3 & posterior 1/3 have the same numbers of papillae present

Type I Histologically: Dark Cells

Small, tall microvilli (numerous)

What to taste receptors detect?

Some cells detect a) Carbohydrates (sweet), b) Some toxins (bitter), c) Some acids (sour), d) Some salts (salty), and e) Some amino acids (savory or umami). This organization represents basic nutrients or correlates of nutrients and harmful compounds or anti-nutrients that appear in our environment.

Glutamate - MSG

Sweet, sour, salty, bitter, umami

Different receptors on different Cell types

TYPE I (glial like cell) - GLAST TYPE II - T1R, - T2R, TYPE III - NCAM (neural cell adhesion) - SNAP25 (part of synaptic complex), etc. Talk to neighbor cells and communicate with neurons through synapses that are larger than normal. 5HT (serotonin) is released across this gap.

Type II cells

TYPE II CELLS - TASTE RECEPTOR These cells, once depolarized, release ATP into the pre-synpatic space. Cells that respond to taste stimuli do not release serotonin, this is the function of only the Type III cells

Type III cells

TYPE III - PRE-SYNAPTIC CELLS This cell releases serotonin to the nerve ending. Type III cells are depolarized by G protein or H+/Na+ channels.

Coding for taste sensation

Taste appears to be coded both by the: 1) The identity of the fibers that carry the information (labeled line coding) as well as by the.. 2) Pattern of fiber firing (across fiber coding). There also appears to be a temporal dimension to sensory coding.

Taste buds

Taste buds are rose bud shaped organs that contain approximately 100 elongated cells. Each taste bud has a TASTE PORE, a hole where tastants enter. The taste cells inside are electrically active neurons, derived from epithelial cell. Different taste cells detect different chemicals. Devoted cells to sweet, bitter, salty, etc. Neural fibers enter the base of the bud and innervate/communicate with cells. Taste receptor cells are not neurons, but function as neurons and are electrically active. Some of them synapse onto neural fibers and project to the brain. The cells are heterogeneous and represent different sub-specialties for detecting chemicals.

Taste bud distribution in oral cavity/tongue

Taste buds reside in specialized bumps (papillae) on the tongue. a) FUNGIFORM PALILLAE Anterior and dorsal tongue b) FOLIATE PAPILLAE: posterior dorsal lateral tongue has many folds or slits c) CIRCUMVALLATE PAPILLAE: Posterior dorsal tongue The soft-palate and pharynx have taste buds in smooth epithelium.

Taste bud impulse generation

Taste cells communicate with neurons and each other via SEROTONIN and ATP among other compounds.

Flavor

Taste combines with odor and other oral sensations to produce flavor.

Type I, II and III cells are in contact with

Taste pore

Bitternes

Tasted posteriorly (vallate, foliate) and in the throat. Hops of beer can be tasted here

Sensory nerves in oral cavity

The information from the oral cavity is transmitted to the brain via three cranial nerves: 1) FACIAL NERVE (CN V) Innervates the anterior two-thirds of the tongue and the soft palate, the 2) GLOSSOPHARYNGEAL NERVE (CN IX) Innervates the posterior 1/3 of the tongue, and the 3) VAGUS NERVE (CN X) Innervates the pharynx.

Ionotropic Recptors

The ionotropic receptors transduce acids and salts. These receptors are expressed throughout the GI tract and in other nutrient sensing organs.

Metabotropic Receptors

The metabotropic receptors comprise two classes: 1) Class I - responds to carbohydrates and amino acids and 2) Class II responds to toxins.

Molecular receptor divisions (2)

The molecular receptors are divided into two classes: 1) G-protein coupled or metabotropic 2) Ionotropic receptors.

Epithelium of nasal cavity

The nasal cavity is covered with cilia It has RESPIRATORY & OLFACTORY EPITHELIUM Cilia can be paralyzed by cold air, smoking, etc. Also activated by trigeminal reflexes (spicy foods).

Sense of taste

The sense of taste detects chemicals that enter the mouth usually for the purpose of detecting and identifying nutrients and toxins in foodstuffs. We detect taste stimuli via organs called taste buds that cover tongue, soft palate, and pharynx.

Sensory information processing from oral cavity

This information is processed first in the 1) BRAIN STEM In the nucleus of the solitary tract and bifurcates dorsally to the... 2) THALAMUS &... 3) CORTEX thalamus and corte & ventrally to the... 4) HYPOTHALAMUS 5) AMYGDALA & other structures...

Junctions in receptor

Tight junctions occlude at the tip of the cell

Fungiform papillae

Tip of tongue

Communication between cells, taste bore and nerve

Type I, II, III cells are in contact with taste pore (III, II) are in contact with the nerve Cells interact laterally and can communicate over long distances

Three GCPR receptors

UNAMI HETERODIMER (T1R1 + T1R3) Stimulated by glutamate and nucleotides. SWEET (T1R2 + T1R3) We are sensitive to high levels of sugar, no μm amounts. Sweeteners act on the same receptor act with lower concentrations. BITTER (T2Rs of 25-35 types) Evolved to detect toxins in the wild. Virtually all pharmaceuticals that are soluble taste bitter as well.

Amino Acids Receptor

Umami is the taste of certain amino acids (e.g. glutamate, aspartate and related compounds). It was first identified by Kikunae Ikeda at the Imperial University of Tokyo in 1909. It was originally shown2,3 that the metabotropic glutamate receptor (mGluR4) mediated umami taste. Binding to the receptor activates a G-protein and this elevates intracellular Ca2+. More recently it has been found that the T1R1 + T1R3 receptors mediate umami taste4 . Binding to the receptors activates the non-selective cation channel TRPM5 as for sweet and bitter receptors (i.e. via G-protein, PLC, IP3 and DAG - see above). Guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) potentiate the effect of umami tastes by binding to another site of the T1R1 receptor. Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of Soy sauce), stimulates the umami receptors. But, in addition, there are ionotropic glutamate receptors (linked to ion channels), i.e. the NMDA-receptor, on the tongue. When activated by these umami compounds or soy sauce, non-selective cation channels open, thereby depolarizing the cell. Calcium enters, causing transmitter release and increased firing in the primary afferent nerve

Innervation of Throat (phayrnx)

Vagus nerve

Type II and type III (and NOT type I) are in contact with

a nerve

Type I, Type II and Type III receptors

and a nerve