The sense organs connect the nervous system to the external environment. They are the sources of stimuli that cause a response in the nervous system, and they are the sources of all information to the human body.
In the human eye, the nerve cells are located in a single layer called the retina, located along the back wall of the eye. Light rays enter the eye through a curved, transparent structure called the cornea, and then pass through the pupil, an opening in the eyeball. The iris regulates the size of the pupil. Next, the lens focuses the light on the retina, which contains two types of light-sensitive cells, rod cells and cone cells, which detect light. Cone cells, which detect color, are concentrated in the central portion of the retina, while rod cells, which permit vision in dim light, are concentrated at the edge of the retina. A light-sensitive pigment called rhodopsin functions in the detection of light.
From the eye, a series of impulses is generated for transmission to the brain. The optic nerve carries these impulses. The region of keenest vision, the fovea, is located at the center of the retina. When vision is poor, light is not focusing on the fovea, and corrective lenses are prescribed.
The ear is the organ of hearing in humans. The outer ear funnels vibrations to the eardrum, or tympanic membrane, which transmits the vibrations to three inner ear bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These bones transmit the vibrations to the inner ear where the receptor of hearing, the cochlea, is located.
The cochlea is a snail-like series of coiled tubes within the skull. As the ear bones vibrate, they push and pull a membrane at one end of the cochlea, causing fluid within the tubules to vibrate. The vibrations are detected by sensitive hair cells, and nerve impulses are generated. The auditory nerve carries the impulses to the brain for interpretation.
Taste and smell
Specialized receptor cells called chemoreceptors transmit taste and smell. Chemoreceptors of the human tongue distinguish four different tastes: sweet, sour, salty, and bitter. In the human nose, chemoreceptors detect a variety of scents, including minty, floral, musky, putrid, and pungent.
In both taste and smell, chemoreceptors are stimulated by molecules and ions that reach the tongue and nose. Liquid materials affect the chemoreceptors in the taste buds of the tongue, while gaseous molecules affect the chemoreceptors in the upper reaches of the nose. The olfactory nerve carries nerve impulses from the nose to the brain for interpretation.
The other senses of the body include receptors for touch, pain, temperature, and balance. Touch and pain receptors, called Pacinian corpuscles, are located in the skin, muscles, and tendons. The sense of balance is centered in the semicircular canals of the inner ear. Visceral senses include stretch receptors in the muscles, as well as carbon dioxide receptors in the arteries.