Chemical sensation includes not only the special chemical senses described below, but also internal sensory receptor functions that monitor the concentrations of gases and other chemical substances dissolved in the blood or other body fluids. Since we are seldom aware of these internal chemical sensations, they are treated throughout this book as needed,- the discussion here covers only taste and smell.
Gustatory Sensation. The sense of taste is mediated by multicellular receptors called taste buds, several thousand of which are located on folds and projections on the dorsal tongue, called papillae. Taste buds are located mainly on the tops of the numerous fungiform papillae but are also located on the sides of the less numerous foliate and vallate papillae. The filiform papillae, which cover most of the tongue, usually do not bear taste buds. An individual taste bud is a spheroid collection of about 50 individual cells that is about 70 ^m high and 40 |xm in diameter (Fig. 4.27). The cells of a taste bud lie mostly buried in the surface of the tongue, and materials access the sensory cells by way of the taste pore.
Most of the cells of a taste bud are sensory cells. At their apical ends, they are connected laterally by tight junctions, and they bear microvilli that greatly increase the surface area they present to the environment. At their basal ends, they form synapses with the facial (VII) and glossopharyngeal (IX) cranial nerves. This arrangement indicates that the sensory cells are actually secondary receptors (like the hair cells of the ear), since they are anatomically separate from the afferent sensory nerves. About 50 afferent fibers enter each taste bud, where they branch so that each axon synapses with more than one sensory cell. Among the sensory cells are elongated supporting cells that do not have synaptic connections. The sensory cells typically have a lifespan of 10 days. They are continually replenished by new sensory cells formed from the basal cells of the lower part of the taste buds. When a sensory cell is replaced by a maturing basal cell, the old synaptic connections are broken, and new ones must be formed.
From the point of view of their receptors, the traditional four modalities of taste—sweet, sour, salty, and bitter— are well defined, and the areas of the tongue where they are located are also rather specific, although the degree of localization depends on the concentration of the stimulating substance. In general, the receptors for sweetness are located just behind the tip of the tongue, sour receptors are located along the sides, the salt sensation is localized at the tip, and the bitter sensation is found across the rear of the tongue. (The two "accessory qualities" of taste sensation are alkaline [soapy] and metallic.) The broad surface of the tongue is not as well supplied with taste buds. Most taste experiences involve several different sensory modalities, including taste, smell, mechanoreception (for texture), and temperature, artificially confining the taste sensation to only the four modalities found on the tongue (e.g., by
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.