Eye and ear anatomy and physiology review

Our most important sensory receptors are the eyes and the ears. The eye is the primary organ for sight, and the ear is the primary organ for sound and equilibrium. Obviously, any impairment of either of these sensory receptors can be a traumatic experience and can cause serious disability.

Eyes

The eyes contain the receptors for light stimuli and are the organs of vision. The eyes are protected within the eye orbits by surrounding bones, the eyelids, the eyelashes, the eyebrows, and the conjunctiva, or inner mucous membrane surface of the eyelids. The meibomian glands on the inner surface of the upper and lower eyelids produce lipidlike secretions to help keep the eye moist. When the eye blinks, the upper lid presses on the oil, pulling a sheet of oil upwards to coat the tear layer and keep it from evaporating. The lacrimal apparatus produces and removes tears. Tears help to keep the outer part of the eye and the conjunctiva moist. The eye is a hollow, spherical organ composed of three layers. The outer layer consists of the sclera, an opaque, white portion of the eye; and the cornea, an anterior window of the eye. The cornea bends the light rays as they pass through its convex curvature.

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The human eye and internal anatomy of the eyeball

FIGURE. (A) The human eye and (B) internal anatomy of the eyeball.

The middle layer of the eye consists of the choroid coat, the ciliary body, and the iris. The choroid coat contains the blood vessels of the eye and melanin. The melanin absorbs light within the eyeball and prevents glare. The ciliary body contains the ciliary muscles, which contract and relax to change the shape of the lens of the eye and form a ring around it.

Ligaments hold the lens in place. The lens focuses the light rays precisely on the retina. The colored portion of the eye is the iris, and it controls the amount of light entering the eye by controlling the size of the pupil. The pupil is the opening in the center of the iris through which light passes to the lens.

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The inner layer of the eye contains the retina, which lines the interior of the eye. It contains the rods (for light detection) and cones (for color detecting) and neurons. The macula is a yellow disk on the retina directly behind the lens. In the center of the macula is a small depression called the fovea centralis that contains only densely packed cones. The fovea is the area of the sharpest, bright-light vision. The sight impulses formed by the rods and cones are transmitted to ganglion neurons that converge at the optic disk forming the optic nerve. Blood vessels nourish the eye.

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The space between the cornea and the lens is called the anterior cavity. The anterior cavity is filled with aqueous humor. The aqueous humor helps to maintain the shape of the cornea and is responsible for the pressure of the eye. Behind the lens is the posterior cavity, which is filled with a clear gel-like substance called vitreous humor. The vitreous humor helps to maintain the shape of the eye by pressing firmly against the wall of the eye. Refraction is the process of bending the light rays. Refraction is produced by the cornea and lens. There is further bending by the lens (accommodation) to provide fine adjustments focusing the image on the retina. Once the light stimulus is on the retina, it must convert into impulses that are sent to the brain via the optic nerve. In the brain, the impulses are interpreted as visual images.

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Video: Anatomy and Function of the Eye

Ears

Outer, middle, and inner ear structures in a frontal section through the right temporal bone

 

FIGURE. Outer, middle, and inner ear structures in a frontal section through the right temporal bone.

The ears are the organs of hearing and equilibrium. The ear is divided into three parts: the external, the middle, and the inner parts. The external ear is the outer, funnel-like structure called the auricle or pinna, and the external auditory meatus is called the external auditory canal. Hearing begins in the external ear, where sound waves are carried through the auricle and canal. The middle ear consists of the tympanic cavity, the tympanic membrane, and three small bones called the ossicles (maleus, incus, and stapes). The tympanic membrane is a thin layer of skin on its outer surface, and on the inner surface, it is covered with mucous membrane. Sound is transmitted from the auditory canal through the auditory meatus. The sound is conducted by the change in pressure on the eardrum and then the three ossicles vibrate.

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The auditory or eustachian tube connects the middle ear to the throat or nasopharynx and mouth. The tube helps maintain equal air pressure on both sides of the eardrum, which is essential for normal hearing.

The inner ear, embedded in the temporal bone, consists of three semicircular canals and a cochlea. The canals provide a sense of equilibrium, and the cochlea contains the organ of Corti, the hearing receptors. The space between the bony and membranous labyrinths is filled with perilymph, whereas the membranous labyrinth contains endolymph. These fluids are essential in the hearing function of the inner ear. In the inner ear, sound is conducted via the organ of Corti receptor cells and nerves.

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When studying the material in this chapter, it will be helpful to refer to Figures, which indicate the major parts of the eye and ear.

Video: Ear Anatomy

Otosclerosis ICD-9: 387.2 Description Otosclerosis is a metabolic imbalance condition that causes new bone to grow over the end of the stapes, especially around the oval window, with resulting immobilization of the stapes. Eventually the bone becomes fixed and no vibration occurs, causing the elimination of transfer of sound to the inner ear and permanent hearing loss. The disease occurs primarily among women, usually appearing between ages 15 and 40. Etiology Otosclerosis is an idiopathic condition, but because the disease shows a familial pattern, genetic factors are suspected. The condition is often aggravated by pregnancy. Signs and Symptoms A gradual bilateral hearing loss of low tones or soft sounds is the first sign. Tinnitus may accompany the condition. Affected clients may turn their head to hear better or may notice they cannot use the telephone on the affected ear. Diagnostic Procedures Basic hearing tests are conducted with a tuning fork. Clients demonstrate a co...
Keratitis ICD-9: 370.xx Description Keratitis is inflammation of the cornea. The condition usually is unilateral. Trevor John Mills, MD, MPH, Chief of Emergency Medicine, Veterans Affairs Northern California Health Care System, reports that approximately 25,000 Americans develop infectious keratitis annually. Etiology Keratitis is most frequently due to infection of the cornea by herpes simplex virus (HSV), or certain bacteria, such as Staphylococcus pneumoniae and Pseudomonas aeruginosa, or fungi. The condition also may arise secondary to syphilis. Noninfectious keratitis may be caused by prolonged exposure to dry air or intense light, or it may result from corneal trauma. Signs and Symptoms Symptoms of keratitis include irritation, tearing, and photophobia. There may be redness of the eyelid and conjunctiva or mucopurulent discharge from the eye. When the cause is HSV-1, an upper respiratory infection (URI) with facial cold sores may be the precursor. When prolonged exposure ...
Motion sickness ICD-9: 994.6 Description Motion sickness consists of nausea, vomiting, and vertigo induced by irregular or rhythmic movements, such as may occur during airplane, boat, or automobile travel. Etiology This disorder is caused by any motion capable of disturbing the equilibrium of the organs of balance in the inner ear (the semicircular canals). Strong emotions, such as fear and grief, and digestive upset or offensive odors may exacerbate the problem. Signs and Symptoms An individual affected by motion sickness may experience loss of equilibrium, nausea, vomiting, dizziness, diaphoresis, pallor, headache, and anorexia. Symptoms may disappear almost immediately after the inciting motion has ceased, or they may persist for hours or days. Diagnostic Procedures Diagnosis is made by history and complaints made by the client. Treatment Ongoing attacks of motion sickness usually are successfully treated with antihistamines, antiemetics, or sedatives. A transdermal me...
Glaucoma ICD-9: 365.xx Description Glaucoma is a condition in which accumulating fluid pressure within the eye damages the retina and optic nerve, often causing blindness. The buildup of pressure occurs because more fluid, called aqueous humor, is produced than can be drained from the eye. The most common form of this condition, called open-angle glaucoma, results from obstruction of passages within the eye that form the trabecular meshwork, which drains the aqueous humor into the lymphatic system. In the United States, glaucoma affects 2% of the population older than age 40. The condition may be unilateral or bilateral. Etiology Primary forms of the condition, such as open-angle glaucoma, are idiopathic; however, a strong familial tendency toward developing this condition suggests that unknown genetic factors may be involved. Glaucoma also may arise secondary to a wide variety of other diseases, or it may be induced by certain drugs or toxins. Glaucoma most often occurs in adults o...
Refractive errors ICD-9: 365.9 FIGURE. Errors of refraction compared with the normal eye. Corrective lenses are shown for nearsightedness and farsightedness. (Adapted from Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 208, with permission.) Description Refractive errors are defects in visual acuity resulting from the inability of the eye to effectively focus light on the surface of the retina. Four common refractive errors follow. Hyperopia. This condition occurs when light entering the eye comes to a focus behind the retina so that vision is better for distant objects. For this reason, the condition is commonly called farsightedness, which causes difficulties in seeing objects that are close. Hyperopia often results when the globe of the eye is abnormally short in length from front to back. Presbyopia. This refractive error is a form of farsightedness that causes the eye to lose its ability to focus. Unlike hyperopia, however, pres...