Taste

     The structures responsible for taste are the taste buds.  The taste buds are receptors that lay mostly on the tongue.  Of all the taste buds, about 10,000 lie on the tongue, while others rest on the soft plate of the mouth.  The taste buds secrete certain chemicals that interact with the food being processed.  These chemicals then send back nervous signals back to the taste buds.  When a new impulse has reached the taste buds, they then send an impulse to the facial nerve.  The facial nerve then sends a impulse to the brain.  There are four main taste sensations, sweet, bitter, sour, and salty.  The salty receptors lie at the tip of the tongue, the sweet receptors lie behind the salty receptors.  In the mid range of the tongue lie the sour receptors, and at the posterior of the tongue lie the bitter receptors.

Smell

     The main reason why the human body is able to understand such a wide variety of smells is due to the olfactory cells.  Olfactory cells are receptor cells, receptor cells receive an impulse from outside the body and change it to a nerve signal.  An example of a receptor cell is the rods and cones in the eye.  The rods and cones receive an impulse from the outside the body, i.e. an image, and change it to a nervous signal.  The olfactory cells, also known as the olfactory receptors, lay in about a 2' by 2' area in the roof of the nasal cavity.  The olfactory cells are neurons, brain cells, that have olfactory hairs.  Olfactory hairs, also known as nose hairs, are cilia, hair like protrusions that can receive an impulse from outside the body and change it to a nerve impulse, that protrude to the center of the nasal cavity.  When a chemical enters the nasal passage the olfactory cells are stimulated.  These cells then send an impulse to the olfactory nerve.  The olfactory nerve then send an impulse to the brain, where the impulse is interrogated and deciphered.

Touch

     Though touch is considered a sense, anatomically speaking the sense of touch would be characterized under the nervous system.  There are billions of neurons throughout the epidermis.  The neurons are constantly "talking" to the brain via electrical impulses.  The collection of all of the neurons in the epidermis would make up the sense of touch.  When someone touches an object, such as a cd, the neurons immediately send signals to the brain, describing the cd's surface.  When the impulse arrives in the brain, it is deciphered, and kept in the brains memory.  Thus, when someone touches two things that have the same surfaces, the brain is able to recognize that the two surfaces are similar.