Sensory System

Structure and Function

The eye is a specialized sense organ that receives light rays and focuses them onto the back of the eyeball. From there, nerve fibers pick up the nerve impulses and carry them to the brain where they are “read” by the brain.

The outer part of the eye is covered with a tough, protective layer called the sclera, known as the “whites of the eye”. The small black opening where light enters the eye is called the pupil. The pupil can become larger to allow more light in or smaller to limit the amount of light that enters the eye. Part of the sclera covers the pupil. This clear covering is the cornea, or the “window” of the eye.

The portion of the eye that gives us eye color is called the iris. The iris surrounds the pupil and controls its size. Behind the pupil is the lens. The lens focuses light rays onto the retina at the back of the eyeball. The retina has thousands of receptors that help identify shapes, color and movement, and provide clarity of vision. The retina changes light rays into nerve impulses. These impulses are picked up by the optic nerve and carried to the brain. Light rays bend as they pass through the eye, and this is called refraction. The light bends many times until the image reaches the retina. The ability of the eyes to adjust and focus on objects near or far away is called accommodation.

A layer of mucous membrane lines the eyelids and sclera, and is called the conjunctiva. The eye is kept wet by tears produced by the lacrimal glands.

The ear is designed to receive sound waves. Sound waves are converted to nerve impulses and then carried to the brain. Sound waves are caught by the external ear and pass through the air through the auditory canal, which is about 1 inch long. The lining of the canal is kept soft with a waxy substance called cerumen or earwax. The sound waves then pass through the eardrum (tympanic membrane), which makes the eardrum vibrate. The eardrum is connected to three tiny bones in the middle ear called the malleus, incus and stapes. The vibrating eardrum makes the bones of the middle ear vibrate also. The third bone is connected to a window to the inner ear. Behind the window is a fluid-filled structure called the cochlea. The vibrating bones put the fluid in the cochlea into motion. This motion is picked up by nerve endings in the inner ear. Nerve impulses are carried to the brain where they are interpreted. The semicircular canals in the inner ear contain nerve receptors for balance.

Age-Related Changes

Many people live well into old age with good eyesight. Generally, though, elderly people need more light for reading, cooking or driving a car. Regular household light bulbs are better for older eyes than fluorescent bulbs. Many people over the age of 40 have trouble seeing objects up close, a condition called farsightedness, which happens because the eyeball loses flexibility with age. Because the eye cannot change shape quickly, light rays cannot be focused correctly. Visual acuity or the ability to see clearly diminishes with age. Peripheral vision and night vision also diminish with age. The lens becomes discolored and opaque. The fluid in the eye becomes thinner, and floaters may appear in the visual field. Eye color changes and becomes lighter. Lens accommodation is more difficult as the lens becomes less elastic.

Hearing loss may also be part of the aging process. Most people over the age of 65 have some degree of hearing loss. Many people suffer from hearing loss caused by non-age-related disorders such as noise exposure or medication reactions. Hearing loss can occur due to a problem with any part of the ear. Causes of hearing loss include impacted earwax, loss of flexibility in the middle ear and nerve damage in the inner ear. Hearing loss may be managed with ear irrigation, surgery or a hearing aid. Special communication techniques may be needed to work with residents who have hearing loss.

Drugs that Affect the Eyes and Ears

  1. Antiglaucoma medications include a variety of medications. Some of these medications control enzymes and chemicals to control muscle contraction in the eye, which acts to reduce the pressure inside the eye. Other medications, such as beta-blockers, reduce pressure in the eye by reducing the production of fluid in the eye. Beta-blocker eye drops are usually given between two and six times per day. Side effects include burning, irritation, visual changes, hypertension, heart irregularities, thirst and taste alteration.
  2. The elderly often complain of dry or itchy eyes. Lubricants for the eye are available to help provide additional moisture to the eye. Lubricants for the eye are called “artificial tears” and are available in solutions or ointments.
  3. Antibacterial agents may be applied directly to the eyes and also to the ears to treat infections. Antibiotics applied to the eye may include bacitracin, gentamicin, neomycin and tetracycline. Commonly used antibiotics for ear infections include chloramphenicol, neomycin and polymyxin B with hydrocortisone (Cortisporin). Drugs placed in the eye should be clearly marked “for ophthalmic use.” Drugs used for the ear should be marked “for otic use.”
  4. Impacted cerumen can be treated with a wax-softening agent. After the wax is softened, the ear can be irrigated. The wax-softening agent flushes the hardened wax from the ear and improves the hearing in some residents.
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