Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Sensory systems
Gustatory sensory system

The gustatory and olfactory Sensory systems allow humans to evaluate The chemical composition of food and the surrounding air. For this reason, they are grouped together under the term chemosensory systems. This also includes visceral chemoreceptors (of the carotid sinus, digestive tract, and others). Chemoreception is one of the phylogenetically oldest forms of communication between an Organism and its environment.

The receptor division of the gustatory sensory system is located in the Oral Cavity and is represented by taste receptor Cells. They are clustered into taste buds, which are primarily located in the papillae on the dorsal surface of the Tongue—fungiform, foliate, and circumvallate. Solitary taste buds are scattered throughout the mucosa of the soft palate, Tonsils, posterior pharyngeal wall, and epiglottis. In children, their distribution is wider than in adults; their number decreases with age.

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Fig. 3.73. Structure of a taste bud:

1 — epithelial cells; 2 — supporting cells; 3 — receptor cells; 4 — basal Cell; 5 — nerve fibers; 6 — surrounding epithelial cells; 7 — taste pore

In humans, the taste buds of the circumvallate papillae exhibit the most typical structure (Fig. 3.73). Each bud is an oval structure that spans the entire thickness of the epithelium and opens onto its surface via a taste pore. The bud is approximately 70 μm in height, 40 μm in diameter, and is composed of 40–60 elongated cells arranged like orange segments. Among the Cells of the taste buds, receptor, supporting, and basal cells are distinguished. The first Two Types of cells span the entire length of the bud, from its basal part to the taste pore. The receptor function of these cells is still a subject of debate. It is assumed that supporting cells may also participate in the receptor process.

Taste receptor cells are secondary sensory cells. Receptor molecules are embedded in their apical membrane, which faces the taste pore, and bind with various chemical substances. As a result, The Cell membrane becomes excited. Through synaptic contacts in the basolateral part of the cell, the excitation is transmitted to a nerve fiber, and then to the Brain. Humans distinguish four basic tastes (sweet, salty, bitter, sour) and several additional ones (metallic, alkaline, etc.). The reception of tastants becomes possible when these substances reach The surface of the tongue, dissolve in saliva, pass through the taste pore, and reach the apical membrane of the receptor cells.

The lifespan of receptor and supporting cells is short—about 10 days. Their renewal occurs through mitotic division of cells in the basal part of the bud.

Conductive and central Divisions of the gustatory sensory system.

Fig. 3.74. Innervation of the taste buds of the tongue and Salivary Glands (dashed line — cranial nerve fibers; solid line — Autonomic Nervous system fibers):

1 — trigeminal, 2 — geniculate, and 3 — otic ganglia; 4 — parotid gland; 5 — lingual nerve; 6 — tongue; 7 — submandibular gland; 8 — stellate and 9 — petrosal ganglia; 10 — chorda tympani; 11 — Medulla Oblongata; 12 — salivary center

Development of The ORGAN OF TASTE in the Prenatal period of ontogeny. In a 4-week-old human embryo, the facial region is just beginning to form. At this stage, the oral cavity is represented by an ectodermal invagination adjacent to, but not yet connected with, the foregut. A thin plate consisting of ectoderm and endoderm later ruptures, connecting the oral cavity with other PARTS OF THE digestive tract. On the sides of the oral cavity, the primordia of the upper and lower jaws are visible, which grow toward the midline of the Mouth to form the jaws. The increase in the relative size of the midface region occurs throughout the prenatal period and continues after birth.

At the beginning of its formation, the tongue is a hollow outgrowth of the mucosa of the posterolateral parts of the oral cavity, which becomes filled with growing Muscles. Most of the tongue's mucosa is of ectodermal origin; however, in the region of the ROOT of the tongue, it develops from the endoderm. The muscles and Connective Tissue are derivatives of the mesodermal layer. Outgrowths—gustatory and tactile papillae—form On the surface of the tongue. Taste buds containing receptor cells develop within the gustatory papillae. In humans, they first appear at the 7th week of Embryogenesis as a result of the interaction between the fibers of sensory Cranial Nerves (VII and IX) and the lining epithelium of the tongue. There is evidence that the fetus is capable of tasting. It is assumed that this function may be used by the fetus to monitor the surrounding Amniotic Fluid.

Fig. 3.75. Pathways of the gustatory sensory system:

1 — somatosensory area of the tongue (Touch, Temperature); 2 — thalamus; 3 — gustatory Nucleus of the Pons; 4 — nucleus of the solitary tract; 5 — lateral Hypothalamus; 6 — insular cortex (taste, touch, temperature); 7 — somatosensory gustatory area (tactile, temperature sensitivity); VA — visceral afferents; VII, IX, X — cranial nerves

Fibers of the Trigeminal nerve (cranial nerve V) also terminate in the mucosa of the anterior part of the tongue. They reach this area as part of the lingual nerve (Fig. 3.74). These fibers transmit tactile, thermal, pain, and Other Sensations from the surface of the tongue, supplementing the information about The properties of the stimulus in the oral cavity.