Gustation Domina Petric, MD

Gustation Gustation (sense of taste) begins in the oropharynx. Taste buds on the tongue allow for the presentation of ingested molecules: their function is sensory transduction. At the base of the sensory cells there is a synaptic connection with an afferent axon. Axon comes from one of these cranial nerves: VII., IX. and X. nerve. There are also taste buds on the epiglottis and posterior parts of the pharynx.

Gustation Axons from the VII., IX. and X. cranial nerve synapse in the Nucleus of the solitary tract (gustatory nucleus). Next station is in ventral and posterior part of the thalamus: third order neurons of the gustatory pathway. Third order neurons provide inputs for Insular cortex: primary gustatory cortex. Another part of the insular cortex is Frontal taste cortex in the frontal operculum.

Insular and frontal taste cortices Hypothalamus Amygdala Thalamus Nucleus of the solitary tract Cranial nerve X: taste buds from epiglottis Cranial nerve IX: taste buds from posterior 1/3 of the tonuge Cranial nerve VII: taste buds from anterior 2/3 of the tongue

Studyblue.com Nucleus solitarius: looks like bull´s eye Central dark region is the solitary tract: central process of the first order afferents (VII., IX., X.) Nucleus solitarius surrounds the solitary tract. The part of the nucleus solitarius that is recieving gustatory information is the rostral part. The caudal part of the nucleus solitarius is recieving inputs from the viscera: visceral sensory integrator. Taste and visceral sensations are intimately integrated.

Central processing Taste and visceral signals are processed and integrated in the insular cortex. From the insular cortex informations go to the orbitofrontal cortex (hedonic value of food).

Sensory transduction At the back of the tongue there is a row of circumvallate papillae: nerve IX (glossopharyngeal nerve). At both sides of the tongue (laterally) there are foliate papillae. At the anterior part of the tongue there are fungiform papillae: nerve VII (facialis).

Sensory transduction Taste cells have at the appical surface the taste pore where ingested molecules interact with taste cells. When the taste cell is depolarised, it releases the neurotransmitter in the synapse between the base of the taste cell and afferent gustatory axon.

Sensory transduction Taste categories are: SALT ACID (SOUR) SWEET BITTER UMAMI (taste of monosodium glutamate)

Sensory transduction Salty and acidic tastants interact with ion channels that have binding place on the extracellular part of the channels. When salty and acidic tastants bind to the channels, positive cations influx the taste cell and depolarise it. Depolarisation causes opening of the voltage gated channels at the base of the taste cell that allow calcium to influx into the taste cell. Then happens the calcium dependent exocytosis of the vesicles and neurotransmitter SEROTONIN is released.

Sensory transduction With sufficient release of the serotonin there will be action potential in the afferent gustatory axon. Sweet, bitter and umami tastants interact with G-protein coupled receptors. G-protein activation causes interaction with several second messenger systems: some of those systems cause depolarisation of the taste cell, other systems cause release of the calcium from intracellular stores. Increase of the calcium levels (wheter from voltage gated channels or intracellular stores) causes the release of neurotransmitter.

Topography of taste receptors There are taste cells that have different types of receptors and taste cells with only one type of receptor. The most of taste cells sensitive to the salty tastants are in the anterior and lateral parts of the tongue. Majority of taste cells sensitive to umami and sweet tastants are also in the anterior and lateral parts of the tongue. Salty and sweet is going to be represented primarily via the facial nerve (VII. nerve).

Topography of taste receptors Majority of taste cells sensitive to bitter tastants are in the posterior part of the tongue. Bitter is going to be represented primarily via the glossopharyngeal nerve (IX. nerve). Bitter taste cells are the most sensitive of all taste cells and can detect harmfull molecules in food even in nanomolar concentrations. Majority of taste cells sensitive to sour are distributed in the lateral flanks of the tongue.

Topography of taste receptors Sweet and salty taste cells are sensitive to milimolar concentrations.

Coding of the gustatory system is labeled-line coding.

Trigeminal chemoreception II.

Trigeminal nerve At the endings of three divisions of trigeminal nerve (ophtalmic, maxillary and mandibular nerve) there are receptors that belong to transient receptor potential family of receptors: TRP channels. TRP channels are sensitive to heat or cold or acids or protons or to organic molecules (for example capsaicin). Capsaicin is found in spicy food. Capsaicin binds to the cytoplasmic binding spot on the TRP channel and causes opening of the TRP channel: influx of calcium and sodium ions, depolarisation.

Trigeminal nerve This activates nociceptive division of trigeminal nerve and its branches. Pain and temperature information from face is processed via the spinal trigeminal tract. Second order neurons are in the spinal trigeminal nucleus. Second order axons cross the midline and ascend through the tegmentum of the midbrain. Third order neurons are in the ventral posterior medial nucleus of the thalamus.

Trigeminal nerve Trigeminal chemoreception allows humans to recognise potentially harmfull substances in, for example, food in our mouth, irritating particles in our eyes... This will cause coordinated visceral and motor response like spitting the bad food or crying to wash the irritating particles from our eyes.

Literature https://www.coursera.org/learn/medical-neuroscience/lecture: Leonard E. White, PhD Studyblue.com