Taste
The sensory organs that mediate the sense of taste are the taste buds. These pear-shaped organs are found on the tongue, soft palate, pharynx, larynx, epiglottis, uvula, the upper third of the esophagus, and (especially in infants) the lips and cheeks. Taste buds on the tongue are contained in small spccialized structures called papillae. There are three types of papillae: fungiform, circumvallate; and foliate. Fungiform papillae are elevated structures located on the anterior two thirds of the tongue; each fungiform papilla contains an average of 1 to 18 taste buds, although many fungiform papillae contain no buds at all. Circumvallate papillae are large mushroom-shaped structures arranged in a V shape on the posterior tongue and are surrounded by a "moat." Foliate papillae are vertical folds on the lateral border of the tongue, just
anterior to the circumvallate papillae. Taste buds consist of about 50 cells that have a life span of approximately 10 to 10 1 2 days. Taste cells are constantly replaced by division of epithelial cells that surround the bud.
Three cranial nerves transmit taste signals from taste buds. The taste buds on the fungiform papillae, the anterior foliate papillae, and most buds on the soft palate are innervated by the seventh cranial nerve. Buds on the circumvallate papillae and posterior foliate papillae are innervated by the ninth cranial nerve. Buds on the pharynx, larynx, epiglottis, and uvula are innervated by the tenth cranial nerve. Taste information is transmitted to the cortex (cortical taste area) via the nucleus of the solitary tract and thalamus. Some taste information is also transmitted to the hypothalamus, which is integral to the feeding system in the brain. The three cranial nerves also contain some axons that terminate in the spinal trigeminal nucleus, and it is presumed that these fibers convey thermal and tactile information from the oral cavity.
The taste system is stimulated by a wide range of chemicals including organic and inorganic compounds. The range of taste sensations is broad and includes not only sweet, sour, salty, and bitter qualities but "umami" (glutamate), astringent, and other tastes that are difficult to describe in words.

Smell
The receptors for smell are located in the olfactory epithelium in the pigmented upper part of the superior turbinate, the nasal septum, and the roof in between these regions. These receptors are specialized bipolar neurons with cilia that protrude into the mucus that covers the olfactory epithelium. Like taste cells, the receptors of the bipolar olfactory cells is constantly renewed from basal cells, but the turnover time is three times longer, approximately 30 days (191).
The very thin axons of bipolar neurons are aggregated in bundles that traverse small holes in the cribriform plate to reach the olfactory bulb where they form small bushy masses called glomeruli. With age, the glomeruli deteriorate and assume a moth-eaten appearance as the fibers disappear. Projections from the olfactory bulb then project to the primitive cortex including the pyriform lobe and hippocampal formation. The areas of primitive cortex not only process olfactory information but also process emotional information. The neurons in the hippocampus and pyriform cortex degenerate with age sooner than other parts of the brain. Like the taste system, olfactory information also projects to the hypothalamic feeding centers.
 

By the end of this lesson, students should be able to learn about 

1. Receptors involved in taste perception 

2. Receptors involved in smell perception 

3. Olfactory pathway

4. primary taste sensations

5. taste buds and functions

6. mechanism of excitation of olfactory cells