Functions of the gustatory system
This sensory system performs several critical tasks:
- Assessment of food quality and tasting.
- Regulation of homeostasis.
- Formation of food acceptance or avoidance reactions (acting as the «gateway» of the digestive system).
Peripheral division
The main sensitive element is the taste bud. Humans have up to 10,000 of them. Each bud consists of about 50 receptor cells, as well as supporting and basal cells.
Taste buds are located in specialized structures of the tongue — papillae:
- Fungiform papillae — distributed across the entire surface.
- Vallate (circumvallate) papillae — localized at the root of the tongue.
- Foliate papillae — located on the posterior borders.
Reception features: A single cell can respond to different chemical substances, but gives the maximum response only to a specific taste quality. Taste intensity is encoded by the number of excited fibers, while the quality itself (salty, sweet, etc.) is encoded by a specific pattern of impulses. With prolonged exposure to a substance, receptors adapt.
Conduction division
About 50 nerve fibers depart from each taste bud. They head to the brain as part of three cranial nerves:
- Facial nerve (n. facialis, cranial nerve VII): collects information from the anterior 2/3 of the tongue. It responds mainly to sweet, salty, and sour.
- Glossopharyngeal nerve (n. glossopharyngeus, cranial nerve IX): innervates the posterior third of the tongue, primarily responsible for the perception of bitter.
- Vagus nerve (n. vagus, cranial nerve X): serves the posteriormost part (root) of the tongue.
The cell bodies of the first neurons lie in the respective ganglia: geniculate, petrosal, and nodose ganglia.
Central pathways and cortical division
The pathway of the gustatory system includes several stages:
- 1st-order neuron — in the ganglia of cranial nerves VII, IX, and X.
- 2nd-order neuron — in the medulla oblongata, in the nucleus of the solitary tract (nucleus tractus solitarii). Here fibers form synapses and undergo a partial decussation.
- 3rd-order neuron — in the ventral nuclei of the thalamus.
The highest analysis center is located in the cerebral cortex, in the tongue projection area in the postcentral gyrus (gyrus postcentralis). A unique feature of the cortical division: pathways to the cortex do not cross over to the opposite side. Cortical neurons are multimodal, meaning they can simultaneously respond to taste, temperature, and tactile stimuli.
Briefly: answers to topical questions
What is the molecular mechanism of taste signal transduction in receptors?
The excitation process in taste receptors is indirect and involves a cascade of biochemical reactions.
Main stages of transduction:
- Primary contact — tastants bind to specific membrane protein receptors on the microvilli of receptor sensory cells.
- Enzymatic activation — binding alters the activity of membrane enzymes.
- Secondary messengers — the concentration of intracellular mediators (e.g., cAMP) changes in the cytoplasm.
- Ionic shift — changes in mediator levels affect the state of plasmalemma ion channels.
- Potential change — a shift in the cell membrane potential occurs.
- Synaptic transmission — excitation is transmitted via synapses to afferent fibers.
What basic tastes does the gustatory sensory system recognize?
The provided sources indicate the following taste qualities: sour, salty, sweet, and bitter.
The facial nerve (n. facialis) innervates the anterior part of the tongue (2/3); impulse activity occurs in response to sour, salty, and sweet. The glossopharyngeal nerve (n. glossopharyngeus) innervates the posterior part of the tongue; impulse activity occurs predominantly in response to bitter.
To memorize the nerves transmitting taste (cranial nerves 7, 9, 10), you can use the numbers: «Seven, nine, ten — weigh everything deliciously» (or equivalent mnemonic).
Class and exam questions
Where is the first neuron of the gustatory pathway located?
The cell bodies of the first neurons are located in the cranial nerve ganglia: geniculate, petrosal, and nodose.
What is the feature of the cortical representation of taste?
Its fibers do not cross over to the opposite side, maintaining ipsilateral representation. In addition, cortical neurons are multimodal.
Which papillae are located at the root of the tongue?
Vallate papillae are located at the root of the tongue.
What else to study on this topic
- Mechanisms of taste quality encoding
- The relationship between GI tract activity and functioning papillae
- The role of the gustatory system in digestive juice secretion
- Taste receptor adaptation processes
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