Taste Receptors

taste buds (bulbs, calyculi), organs by which taste stimulation is perceived. The taste buds are the peripheral part of the taste analysor, which consists of special sensory cells (gustatory receptors). In most invertebrates the gustatory and olfactory organs are not yet separate and are a single organ for chemical perception, common for taste and smell. The taste receptors of insects consist of special chitinous pili, or sensilla, situated on the palpi, in the oral cavity, and elsewhere. The pilus contains supporting cells that surround receptor cells, from which emerge two delicate processes—the peripheral, which is supplied with a modified cilium that ends near a pore and comes into direct contact with tasteable substances, and the central, which travels to the central nervous system. In lower animals, such as fish, the taste receptors may be located over the entire body, especially on the lips, on the cirri, in the oral cavity, and on the gill arches. In amphibians the taste receptors are to be found mainly in the oral cavity and sometimes in the nasal cavity. In mammals and man the taste receptors are situated mainly on the lingual papillae and sometimes on the soft palate and the posterior wall of the pharynx.

The taste buds are most developed in animals that chew food slowly and thoroughly. There are several types of papillae formed by the lingual mucosa. Circumvallate papillae (man has from six to 16 of them), each of which contains 300 to 5,000 taste buds, are arranged in two symmetrical rows converging at the root of the tongue. Foliate papillae (absent in man, except for infants) are found on either side of the tongue, with one papilla on each side. These two types of papillae are supplied with mucous glands; their secretions help to dissolve solid food, which makes possible a chemical interaction with the taste receptors. On the tip and back of the tongue are 350-400 fungiform papillae, each of which has two or three taste buds. In all vertebrates oval-shaped taste buds are found in the stratified epithelium of the mucous membrane. They communicate by a short gustatory pore with the surface of the mucous membrane. Each taste bud consists of ten to 15 receptor cells and several supporting cells. Slightly striated, slender processes (“gustatory hairs”) emerge from the cells that form the bottom of the gustatory pore and branching out, give off microvilli, which line the bottom of the taste pit. The gap between the microvilli is filled with a “cement” substance rich in amino acids and mucopolysaccharides.

Enzymes, the activity of which is altered by tasteable substances, and a protein capable of forming specific complexes with sugars are found in the taste buds. This is the basis for the hypothesis that when tasteable matters are diffused through the “cement” substance and come into contact with the microvilli they combine with the molecules of certain “gustatory” proteins. This leads to the excitation of the receptor cells that is transmitted via the gustatory nerve to the central nervous system. The nerve endings of the gustatory nerve approach the base of the taste cells, where they form synapses. The region of the synapses is characterized by high acetylcholinesterase activity, an indication of the cholinergic mechanism of transmission of excitation of the taste cell to the central nervous system.

The taste fibers of the facial nerve originate in the taste buds of the anterior two-thirds of the tongue. They initially form part of the lingual nerve and then enter the chorda tympani, from where they penetrate into the facial nerve. The fibers that innervate the taste buds in the posterior third of the tongue, the palate, and the epiglottis start from the cells of the petrosal ganglion of the glossopharyngeal nerve. The vagus nerve fibers that innervate the gustatory organs origi-nate in the cells of its sensory ganglia and proceed to the medulla oblongata, where they, like the other taste fibers, terminate in the nucleus of the solitary tract. The ascending pathways from the nucleus of the solitary tract pass through the medical lemniscus into the optic thalamus, where the fibers ending in the gustation center of the cortex originate.