Smell, Taste and Touch
Taste
Most mammals, including humans, have a far better sense of smell than of taste. In fact, smell helps refine taste. Cold sufferers know that their ability to taste is seriously diminished by a blocked nose. Like scent, taste involves the detection of chemicals, but ones found in solids and fluids, not in the air. Groups of taste receptors, known as taste buds, are spread over the tongue. Humans have about 9,000 taste buds on their tongue and can detect five basic tastes: salty, sour, sweet, bitter and glutamate. A canid tongue has only about 2,000 taste buds.
Unlike scent receptors, the pear-shaped taste buds of mammals are not connected directly to the brain. They rely on indirect nerve links to carry their messages. The cells that cluster to form taste buds are short-lived. Most die after about seven to 10 days, while others last a mere 24 hours. New taste cells are continually replacing dying ones.
Touch
Touch receptors, often called mechanoreceptors, are also being continually replaced in the skin's epidermis. These cells, which consist of nerve-fiber terminals embedded in the skin, undergo electrochemical changes whenever their membranes are deformed, resulting in the transmission of information about pressure to the brain. Each type responds to a particular kind of pressure. For example, Merkel corpuscles are cells that react to changing deformation of the skin and Meissner corpuscles react to steady pressure on the skin.
As well as touch receptor cells, most mammals have at least some form of whiskers, or vibrissae, on their body. Typically found on the face, muzzle and lower legs, these long, stiff, modified hairs serve to extend a mammal's touch capabilities beyond its epidermis. Nocturnal mammals use vibrissae to help them navigate in the dark. The whiskers of felines are said to pick up the slightest variation in air currents, and those of various aquatic mammals can detect subtle changes in water currents as well as the presence of anything moving, such as fish, in dark waters.
