Senses of Taste and Smell
Taste and smell are called chemical senses because their receptors are sensitive to molecules in the food we eat and the air we breathe. The body also has other chemoreceptors. Chemoreceptors in the carotid arteries and in the aorta are primarily sensitive to the pH of the blood. These bodies communicate via sensory nerve fibers with the respiratory center in the medulla oblongata. When the pH drops, they signal this center, and immediately thereafter the breathing rate increases. The expiration of CO2 raises the pH of the blood.
Sense of Taste
The sensory receptors for the sense of taste are located in taste buds. Taste buds are embedded in epithelium primarily on the tongue (Fig. 9.3). Many lie along the walls of the papillae, the small elevations on the tongue that are visible to the naked eye. Isolated taste buds are also present on the hard palate, the pharynx, and the epiglottis. We have at least four primary types of taste, but the taste buds for each are located throughout the tongue (Fig. 9.3a). Even so, certain regions of the tongue are most sensitive to particular tastes: The tip of the tongue is most sensitive to sweet tastes; the margins to salty and sour tastes; and the rear of the tongue to bitter tastes.
Figure 9.3 Taste buds. a. Papillae on the tongue contain taste buds that are sensitive to sweet, sour, salty, and bitter tastes. b. Enlargement of papillae. c. Taste buds occur along the walls of the papillae. d. Taste cells end in microvilli that bear receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve impulses are generated that go to the brain, where the sensation of taste occurs.
How the Brain Receives Taste Information
Taste buds open at a taste pore. They have supporting cells and a number of elongated taste cells that end in microvilli. The microvilli of taste cells project through the taste pore.
These microvilli have receptor proteins for molecules that cause the brain to distinguish between sweet, sour, salty, and bitter tastes. When these molecules bind to receptor proteins, nerve impulses are generated in associated sensory nerve fibers. These nerve impulses go to the brain, including the cortical areas, which interpret them as tastes. Since we can respond to a range of sweet, sour, salty, and bitter tastes, the brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste. Again, we can note that even though our senses are dependent on sensory receptors, the brain integrates the incoming information and gives us our sense perceptions.
Sense of Smell
Our sense of smell is dependent on olfactory cells located within olfactory epithelium high in the roof of the nasal cavity (Fig. 9.4). Olfactory cells are modified neurons. Each cell ends in a tuft of about five olfactory cilia, which bear receptor proteins for odor molecules. The brain distinguishes odors after odor molecules bind to the receptor proteins.
How the Brain Receives Odor Information
Each olfactory cell has only one type out of 1,000 different types of receptor proteins. Nerve fibers from like olfactory cells lead to the same neuron in the olfactory bulb, an extension of the brain. An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose might stimulate olfactory cells, designated by purple and green in Figure 9.4, while a hyacinth might stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled a rose or a hyacinth. Have you ever noticed that a certain aroma vividly brings to mind a certain person or place? A person’s perfume may remind you of someone else, or the smell of boxwood may remind you of your grandfather’s farm. The olfactory bulbs have direct connections with the limbic system and its centers for emotions and memory. One investigator showed that when subjects smelled an orange while viewing a painting, they not only remembered the painting when asked about it later, but they also had many deep feelings about it.
Sense of Taste and Sense of Smell
Actually, the sense of taste and the sense of smell work together to create a combined effect when interpreted by the cerebral cortex. For example, when you have a cold, you think food has lost its taste, but most likely you have lost the ability to sense its smell. This method works in reverse also. When you smell something, some of the molecules move from the nose down into the mouth region and stimulate the taste buds there. Therefore, part of what we refer to as smell may in fact be taste.
Figure 9.4 Olfactory cell location and anatomy. a. The olfactory epithelium in humans is located in the nasal cavity. b. Olfactory cells end in cilia that bear receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). For example, if a rose causes olfactory cells sensitive to “purple” and “green” odor molecules to be stimulated, then neurons designated by purple and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.