Taste buds and the nose are special sense organs. Special sense organs contain groups of specialized sensory receptors. Taste and smell are called the chemical senses because the receptors for these senses are sensitive to certain chemical substances in food and in the air.
There are four types of tastes (bitter, sour, salty, and sweet), and the taste buds for each are concentrated on the tongue in particular regions. Most taste buds lie along the walls of papillae, the small elevations visible to the naked eye. The taste buds are pockets of taste cells in the tongue epithelium with microvilli that project out of a pore. Nerve impulses, generated when chemicals bind to the microvilli, go to the brain where the sensation of taste occurs.
The olfactory cells are in the superior aspect of a nasal cavity. These cells, which are specialized endings of the fibers that make up the olfactory nerve, lie among supporting epithelial cells. It is likely that there are at least 1,000 different types of olfactory cells. Each cell ends in a tuft of six to eight cilia, and when chemicals bind to these cilia, nerve impulses are generated. The particular aroma that a person perceives results from the combination of olfactory cells that have been stimulated. For example, a rose may stimulate one combination of olfactory cells, while a daffodil stimulates another. The olfactory cells stimulate interneurons in the olfactory bulb, which pass this information via the olfactory tract to the olfactory areas of the cerebral cortex where the perception of the odor occurs.
The senses of taste and smell supplement each other, creating a combined effect when interpreted by the cerebral cortex. For example, when a person has a cold, food seems tasteless, but actually, the person has temporarily lost the ability to sense the food's smell. When a person smells
something, some of the molecules move from the nose down into the mouth region and stimulate the taste buds. Therefore, part of what is referred to as smell may actually be taste.
The sensory receptors for taste (taste buds) and for smell (olfactory cells) work together to produce the senses of taste and smell.
Photoreceptors
The photoreceptors are in the eyes. The eyes are located in orbits formed by seven of the skull's bones (frontal, lacrimal, ethmoid, zygomatic, maxilla, sphenoid, and palatine). The supraorbital ridge protects the eye from blows, and the eyebrow diverts sweat around the eye. The eye's accessory organs are discussed first.
