Hearing is the ability to detect sound waves or vibrations. These vibrations enter the auditory canal and travel to the eardrum. The eardrum vibrates, transmitting the vibrations to three bones, or ossicles, in the middle ear, which magnify the vibrations. Tiny hair cells trigger impulses that reach the brain through the auditory nerve.
Three tissue layers comprise the human eye. The sclerotic coat contains the cornea, which allows light to pass into the eye's interior and bends the light for focusing. The choroid coat contains pigments and, in addition to providing eye color, these pigments reduce the amount of stray light that is reflected inside the eye. The retina is the part of the eye that contains the cones and rods, or light receptors, that process signals and then transmit them to the brain. The normal human eye can distinguish the color of an object and can change its focus for objects that are nearby and those that are far away.
In humans, the sense of smell depends on the body's ability to sense chemicals in the air. Odorant molecules are those that humans can smell. Odorants dissolve in the mucus of the nasal passages and sinuses, triggering a reaction that causes the olfactory signals to travel to the brain. The brain evaluates the signals it receives to classify the odor.
Taste buds are clusters of taste receptor cells on the tongue. Pores on the taste buds open onto the tongue's surface and allow stimuli access to the receptor cells. The human taste receptor cells can distinguish among five tastes, which are salty, sweet, sour, bitter and umami. Umami is the signal from glutamic acids such as monosodium glutamate, which is found in many Asian foods as well as processed cheeses and meats. Like other senses, the brain is responsible for processing the information it receives from the taste buds.
The sense of touch allows a person to feel physical pressure and the temperature of nearby objects. Receptors are located in the skin, although some parts of the body contain more receptors than others. For example, the tongue and fingertips can contain 10 times as many receptors per square centimeter as the back of the hand. Pacinian corpuscles, located in some internal organs as well as the skin, are special receptors that respond to pressure. The sense of touch enables a human to detect the number, shape, size, texture and temperature of items he cannot see, hear, smell or taste.