Gustatory cortex

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The primary gustatory cortex is a brain structure that is responsible for taste perception . It consists of two substructures: the anterior insula of the islet cortex and the operculum frontale on the inferior frontal gyrus . Because of this composition, the primary gustatory cortex is sometimes referred to in the literature as AI / FO (en: Anterior Insula / Frontal Operculum). By using single cell recording , scientists were able to show that neurons in the AI ​​/ FO react to sweet, salty, bitter and sour and encode the intensity of taste stimuli.

Sensory path of taste

Like smell perception , taste perception is also defined by its specialized peripheral receptors and central transmission channels that process and pass on taste information. Peripheral taste receptors are located on the surface of the tongue , the soft palate , the pharynx, and the upper part of the esophagus . Gustatory cells form synapses with the axons of sensory nerves of the chorda tympani , the larger superficial petrous branches of the facial nerve (VII cranial nerve ), the tongue branches ( rami linguales ) of the glossopharyngeal nerve (IX cranial nerve) and the superior laryngeal nerve of the vagus nerve (Xth cranial nerve) . Cranial nerve) to stimulate the taste buds on the tongue, palate, epiglottis and esophagus. The central axons of these primary sensory neurons in respective cranial nerve ganglia project to rostral and lateral regions of the nucleus of the solitary tract in the medulla oblongata , which is also known as the gustatory nucleus of the solitary tract. Axons from the rostral (gustatory) part of the nucleus of the solitary tract project to the posterior nucleus of the thalamus , where they terminate in the medial half. The nucleus in turn projects into several regions of the neocortex , which contains the gustatory cortex (operculum frontale and the anterior insula).

Function and stimulation

Studies on the observation of the functionality of the GC with different chemical and electrical stimulations as well as examinations of patients with lesions and convulsions in the gustatory cortex have taken place. It has been found that electrical stimulation of the lingual nerve , the notochord, and a lingual branch of the glossopharyngeal nerve produces evoked field potential in the frontal operculum. Electrical stimulation of the human insula induces gustatory sensations. Gustatory information is conveyed to the orbitofrontal cortex, the secondary gustatory cortex. Studies have shown that 7.9% of the neurons in the orbitofrontal cortex respond to taste stimuli, and some of these neurons are finely tuned to specific stimuli. It has also been shown in monkeys that the responses of orbitofrontal neurons to stimuli decrease when a monkey has eaten to fullness. In addition, neurons in the orbitofrontal cortex respond to visual and / or olfactory stimuli. These results suggest that gustatory neurons in the orbitofrontal cortex may play an important role in the identification and selection of food. One patient study found that damage to the rostral part of the insula caused gustatory disorders, as well as taste-recognition deficits in patients with lesions in the insular cortex. In addition, it was found that patients with an epileptic focus in the frontal operculum and epileptic activity in said focus perceived unpleasant tastes. Activation in the insula also occurs when the subject is exposed to pictorial representations of food. Studies compared the active regions of subjects shown pictures of food to the active regions of subjects shown landscape pictures. Images from Essen activated the right insula / operculum and right orbitofrontal cortex.

Chemosensory neurons

Chemosensory neurons are neurons that distinguish between flavors and sense the presence and absence of a flavor. In a study on rats, the responses in these neurons to touching the tongue with gustatory-stimulating flavors were greater than to touching without flavors. 34.2% of the neurons of the gustatory cortex showed chemosensory reactions. The other neurons differentiated between the presence and absence of a flavoring substance or processed task-related information.

Flavor concentration-dependent neuronal activity

Chemosensory neurons in the GC show concentration-dependent reactions. In a study of reactions in the GC of rats during licking, an increase in MNG ( monosodium glutamate ) was found. Lingual exposure resulted in an increased neural firing rate in the neurons of the gustatory cortex of the rats, while an increased concentration of sucrose led to a decrease in the firing rate. The neurons have a rapid and selective response to flavors. Sodium chloride and sucrose caused the largest reactions in the rats' GC, while citric acid only caused a moderate increase in activity in a single nerve cell. Chemosensory neurons of the gustatory cortex are broadly attuned, which means that a greater proportion of them respond to many flavors (4 and 5) while the smaller proportion responds to fewer flavors (1 and 2). The number of neurons that respond to a particular taste stimulus varies. The study on the gustatory cortex of rats showed that more neurons respond to MSG, sodium chloride, sucrose and citric acid (all of which activated roughly the same percentage of neurons) than to quinine and water.

Ability to react to changes in concentration

Studies using the rat gustatory cortex as a model have shown that neurons in the GC exhibit complex responses to changes in the concentration of a flavor. A neuron that increases its rate of fire in response to a flavor may only respond to moderate levels of concentration. In these studies it became clear that few chemosensory neurons in the GC respond to changes in the concentration of flavors with a monotonically increased or reduced firing rate, but the vast majority responded in a more complex manner. Some neurons respond most strongly to a medial concentration while others only respond to the highest and lowest concentrations.

Coherence and interaction between neurons during gustatory perception

During the tasting, the neurons of the GC interact with each other. Interactions happen in milliseconds, are flavor-specific and delimit different, overlapping neural ensembles that react to the presence of a flavor by experiencing coupled changes in the firing rate. These couplings are used to differentiate between flavors. Coupled changes in the firing rate are the actual source of interactions in the GC. Subsets of neurons in the GC couple upon the presentation of certain flavors, and the responses of neurons in this ensemble change in accordance with others.

Already known flavors

Units of the GC signal familiar tastes in a delayed temporal phase of the reaction. An analysis suggests that specific neural ensembles are working on the processing of already known flavors. In addition, the neural signature of the familiar is correlated with the habituation to a specific flavor. This signature can be seen for up to 24 hours after the initial exposure. This persistent cortical representation of taste familiarity requires post-acquisition processing for development. This process could be linked to the activation of neurotransmitter receptors, the modulation of gene expression, and post-translational modifications that can be detected in the insular cortex in the first few hours after exposure to an unknown flavoring agent.

Individual evidence

  1. ↑ Elaine N. Marieb, Hoehn Katja: Anatomy & Physiology . 3. Edition. Benjamin Cummings / Pearson, Boston 2008, ISBN 978-0-8053-0094-9 , pp. 391-395 .
  2. ^ Thomas C. Pritchard, David A. Macaluso, Paul J. Eslinger: Taste perception in patients with insular cortex lesions. In: Behavioral Neuroscience . tape 113 , no. 4 , p. 663-671 , doi : 10.1037 / 0735-7044.113.4.663 .
  3. ^ Masayuki Kobayashi: Functional Organization of the Human Gustatory Cortex . In: J. Oral Biosci . tape 48 , no. 4 , 2006, p. 244-260 , doi : 10.1016 / S1349-0079 (06) 80007-1 .
  4. ↑ Dale Purves (Ed.): Neuroscience . 2nd Edition. Sinauer Association, Sunderland, Mass 2001, ISBN 0-87893-742-0 .
  5. ↑ Hisashi Ogawa, Shin-ichi Ito, Tomokiyo Nomura: Two distinct projection areas from tongue nerves in the frontal operculum of macaque monkeys as revealed with evoked potential mapping . In: Neuroscience Research . tape 2 , no. 6 , August 1, 1985, pp. 447-459 , doi : 10.1016 / 0168-0102 (85) 90017-3 ( sciencedirect.com [accessed August 4, 2016]).
  6. ^ SJ Thorpe, ET Rolls, S. Maddison: The orbitofrontal cortex: Neuronal activity in the behaving monkey . In: Experimental Brain Research . tape 49 , no. 1 , January 1983, ISSN  0014-4819 , p. 93-115 , doi : 10.1007 / BF00235545 .
  7. ^ ET Rolls, S. Yaxley, ZJ Sinkiewicz: Gustatory responses of single neurons in the caudolateral orbitofrontal cortex of the macaque monkey . In: J. Neurophysiol . tape 64 , 1990, pp. 1055-1066 ( physiology.org ).
  8. ^ ET Rolls: Information processing in the taste system of primates. In: J. Exp. Biol. 146, 1989, pp. 141-164.
  9. ^ TC Pritchard, DA Macaluso, PJ Eslinger: Taste perception in patients with insular cortex lesions. In: Behav. Neurosci. 113, 1999, pp. 663-671.
  10. ↑ WK Simmons, A. Martin, LW Barsalou: Pictures of appetizing foods activate gustatory cortices for taste and reward. In: Cereb. Cortex. 15, 2005, pp. 1602-1608.
  11. ↑ a b c d J. R. Stapleton: Rapid Taste Responses in the Gustatory Cortex during Licking . In: Journal of Neuroscience . tape 26 , no. 15 , April 12, 2006, pp. 4126-4138 , doi : 10.1523 / jneurosci.0092-06.2006 .
  12. ^ DB Katz, SA Simon, MA Nicolelis: Taste-Specific Neuronal Ensembles in The Gustatory Cortex of Awake Rats. In: J Neuroscience. 22 (5), 2002, pp. 1850-1857.
  13. ^ A. Bahar, Y. Dudai, E. Ahissar: Neural Signature Of Taste Familiarity in the Gustatory Cortex of The Freely Behaving Rat. In: J. Neurophysiol. 92, 2004, pp. 3298-3308.