Chemical senses

In vertebrates, the senses of smell (olfaction) and taste (gustation) plus the so-called common chemical sense constitute the external chemical senses (as contrasted with internal chemoreceptors). The olfactory cells of vertebrates, usually located in the olfactory mucosa of the upper nasal passages, are specialized neural elements that are responsive to chemicals in the vapor phase. Taste buds of the oral cavity, especially the tongue, are composed of modified epithelial cells responsive to chemicals in solution. The common chemical senses are composed of free nerve endings in the mucous membrane of the eye, nose, mouth, and digestive tract and are responsive to irritants or other chemicals in either the vapor or liquid phase. See Chemoreception

Among invertebrates, sense organs occur as specialized hairs and sensilla, or minute cones supplied with sensory nerves and nerve cells. Characteristic of male moths, for example, are their distinctive bushy antennae, by which they detect and locate females by sex pheromones. Rodents, ungulates, carnivores, and other mammals also show sexual attraction to female odors produced by specialized glands. Whether humans in general are susceptible to pheromonal influences from other humans is debatable. See Pheromone

Taste plays an important role in selection and acceptance of food. Besides the protective, inborn aversion to bitter (many poisons, but not all, are bitter), a single experience with the particular taste of a toxic substance which caused illness may establish a strong and persistent learned taste aversion. By contrast, a compensatory salt hunger may occur in persons or animals suffering salt deficiency.

The limbic system of the brain, which modulates appetitive and emotional behavior and hedonic (pleasant vs. unpleasant) experiences, has both taste and olfactory neural pathways to it, providing the neural substrate for the pleasure or displeasure of sensations. See Neurobiology, Olfaction, Sensation, Taste