What are sense organs?

Structure and Functions

The sense organs of the body include the cutaneous sense organs, the organs of chemical reception, the organs of vision or sight, and the organs of hearing and balance. The skin is the major organ of sensation for touch, pressure, cold, warmth, and pain; the nasal epithelium and taste buds are the major organs of chemoreception; the eyes are the major organs of vision or sight; and the ears are the major organs of both hearing and balance.

There are five types of cutaneous receptors within the skin, each with a different type of sensory nerve ending and each with a different spatial pattern of distribution. Free (naked) nerve endings are sensitive to pain and are widely distributed over the body’s skin surface, especially at the base of each hair. Overstimulation of any type of nerve ending also results in a sensation of pain, but these nerve endings are so exposed that any stimulation at all is felt as an overstimulation. All the remaining cutaneous receptors are encapsulated in one of several types of end organs. Of these, the end bulbs of Krause may be temperature sensitive; they are most numerous around the conjunctiva of the eye and along the glans of the penis and the glans of the clitoris.

The Pacinian corpuscles each contain a single central nerve fiber, enclosed in many concentric layers of semitransparent tissue resembling the bulb of an onion. These structures, which are about one to four millimeters in diameter, are sensitive to deep pressure and are distributed throughout the skin, principally within the dermal papillae. They are most numerous on the palm of the hand, the sole of the foot, and the insides of many joints such as the front of the elbow or the back of the knee. The tactile (Meissner’s) corpuscles each consist of an oval, bulblike swelling in which the nerve endings run around in spiral patterns at right angles to the long axis. Meissner’s corpuscles are sensitive to light touch and are most numerous along the fingertips (and the hand in general), the tongue and lips, parts of the eye, and the skin of the mammary nipple or papilla. The corpuscles of Ruffini are enclosed in connective tissue sheaths perpendicular to the nerve that serves them. The axons of this nerve branch repeatedly within the corpuscle and these branches intertwine, each ending in a tiny knob. Corpuscles of Ruffini are sensitive to warmth and are very numerous over the fingertips, the forearm, and the skin of the face. The evidence to associate particular nerve endings with particular sensations (such as the Meissner’s corpuscles with light touch) comes largely from patterns of spatial distribution: the areas of the body most sensitive to touch are also those with the highest densities of Meissner’s corpuscles.

Neuromuscular spindles occur in most voluntary muscles and are sensitive to the state of contraction or relaxation of the muscle fibers. The spindle consists of a muscle fiber or small bundle of such fibers, around which are wrapped several turns of infrequently branching sensory nerve endings. The tendons of many muscles also frequently contain neurotendinous spindles, encapsulated structures in which a bundle of tendon fibers receive branched nerve endings. These nerve endings branch slightly just before reaching the tendon fibers but then lose their sheaths and branch profusely within the tendon. These neurotendinous spindles act as stretch receptors, sensitive to the state of stretching of the tendon.

Chemoreceptors of the body include those tissues sensitive to certain chemicals. The carotid body, a swelling within the carotid artery of the neck, contains tissue sensitive to the carbon dioxide or acid content of the blood; it stimulates the breathing reflex when the carbon dioxide level is too high. The taste buds of the tongue are sensitive to the taste of a variety of chemical substances present in moderate concentrations. Each taste bud contains gustatory cells along with nonsensitive supporting (sustentacular) cells. Experimental evidence points to five basic types of taste sensations: sweet (like sugar), bitter (like quinine), sour (like vinegar or citric acid), salty (like sodium chloride), and savory (like chicken broth). Sensitivity to each of these four basic tastes has its own characteristic pattern of distribution over the tongue and palate.

The nasal epithelium is responsible for olfaction, or smell, which is a sensitivity to chemical substances in much smaller concentrations. Most of the nasal epithelium is associated with the nose and the nasal passages, but a small part of this epithelium has become attached instead to the roof of the mouth, where it forms the vomeronasal (or Jacobson’s) organ, which “smells” the contents of the mouth (mostly food). The nasal epithelium is structurally unusual in that the cell bodies of the sensory cells originate within the epithelium and their nerve endings (axons) migrate inwardly to the brain, through the cribriform plate, forming the first cranial nerve or olfactory nerve. All other sensory nerves in the body grow outward from the central nervous system, and their cell bodies are located where their growth began.

Attempts have been made to classify smells according to a scheme similar to the classification system used for tastes, but there are many more basic smells than there are tastes—lists vary from six to twenty to more than ninety—and there is no general agreement on any of these schemes.

The eyes are the body’s principal visual receptors. (Some evidence also exists of the brain’s own ability to sense daily changes in the level of light intensity, especially in the pineal body.) The primary parts of the eye include the eyelids, cornea, lens, ciliary body, iris diaphragm, pupil, aqueous humor, vitreous humor, retina, choroid coat, scleroid coat, and optic nerve. The eyelids protect the front of the eye and prevent injury to the eye by closing. The cornea is the transparent covering of the front of the eye; the lens is the transparent, almost spherical body that focuses rays of incoming light onto the retina; and the ciliary body is a largely connective tissue structure (also containing some muscle tissue) that supports the lens. The colored part of the ciliary body is the iris diaphragm; muscle fibers within the iris diaphragm adjust the size of the pupil for different brightness levels of light. The opening in the middle of the iris diaphragm is called the "pupil." The aqueous humor is the watery fluid in front of the lens, while the vitreous humor is the thick, jellylike fluid behind the lens.

The light-sensitive portion of the eye is the retina. It is almost spherical in shape and consists of two layers: a sensory layer on the inside (closer to the front) and a pigment layer surrounding and behind the sensory layer. Within the sensory layer are contained both the rods, which are sensitive to finer details, and the cones, which are sensitive to colors. The eye’s most sensitive area is called the "area centralis"; it is centered upon a depression called the "fovea." The choroid coat is the connective tissue layer immediately surrounding the retina, which is continuous with the pia mater that surrounds the brain. The scleroid coat is the stronger connective tissue layer that surrounds the choroid coat; it is continuous with the dura mater surrounding the brain. The optic nerve fibers originate from the sensory layer of the retina, where they converge toward a spot called the "blind spot," marking the place where the nerve fibers turn inward toward the brain. A majority of the optic nerve fibers cross over to the opposite side of the brain via the optic chiasma, but a small proportion of the fibers remains on the same side without crossing over. Experiments on the physiology of vision have led researchers to conclude that there are three separate types of color receptors (cones) in the retina, sensitive principally to red, green, and blue regions of the spectrum. All other color sensations can be simulated experimentally in people with normal vision by a suitable combination of red, green, and blue stimuli.

The ears are special sense organs devoted to the two distinct functions of hearing and balance. The ear may be divided anatomically into outer, middle, and inner portions or functionally into a cochlear portion for hearing and a vestibular portion for balance. The outer (external) ear consists of a flap called the "pinna" (or "external ear flap") and a tubelike cavity called the "external acoustic meatus." Within the external ear, sound impulses exist as waves of compressed or decompressed (rarefied) air, forming a series of longitudinal waves that vibrate in the same direction in which they are transmitted. The tympanic membrane
(eardrum) is a vibrating membrane that marks the boundary between the outer and middle ears.

The middle ear consists of a cavity containing three tiny bones, the auditory ossicles. Within the middle ear, the vibrations of the tympanic membrane set up a series of vibrations within these tiny bones. The three auditory ossicles are called the "malleus" (hammer), "incus" (anvil), and "stapes" (stirrup). The malleus gets its name from its hammerlike shape, which includes a long handle (manubrium) extending across the tympanic membrane. The incus, the second of the auditory ossicles, rests against the malleus at one end and the stapes at the other. The stapes is shaped like a stirrup, in which the foot is placed when riding a horse. The flat base of the stapes is called the "footplate," in analogy to the corresponding part of a stirrup; this footplate rests against the fenestra ovalis of the inner ear. The opening in the stapes is penetrated by an artery called the "stapedial artery." The cavity of the middle ear connects to the pharynx by means of a tube, the pharyngotympanic or Eustachian tube.

The inner ear is entirely housed within the petrosal bone. It can be divided into cochlear (hearing) and vestibular (balance) portions. The cochlear portion of the inner ear begins with two windows, the fenestra ovalis (oval window) and fenestra rotundum (round window), communicating between the middle ear and the inner ear. Behind the fenestra ovalis lies a vestibule, filled with a fluid called "perilymph" and extending into a long scala vestibuli. Behind the fenestra rotundum lies another long tube, the scala tympani, also filled with perilymph and running parallel to the scala vestibuli. Between these two tubes lies a third, the scala media or cochlear duct, filled with a different fluid called "endolymph." Together, the three are prolonged into a spiral coil called the "cochlea" (Latin for “snail”), which has a bit more than three complete turns. At the end of this coil, the scala media ends, and the scala vestibuli and scala tympani join with one another by means of an intervening loop called the "helicotrema." The basilar membrane separates the scala tympani and the cochlear duct. The spiral organ (organ of Corti) runs within the cochlear duct along the basilar membrane, not far beneath a tectorial membrane that is suspended within the cochlear duct.

The outer ear receives vibrations that travel through the air and transmits these vibrations to the tympanic membrane. In the middle ear, the vibrations of the tympanic membrane are transmitted through the malleus, incus, and stapes to the oval window. These vibrations are transmitted through the perilymph of the inner ear (vestibular portion), where they cause vibrations of the basilar membrane. The vibrating basilar membrane causes vibrations within the endolymph and also in the tectorial membrane, but the tectorial membrane is less flexible than the basilar membrane, creating regions of greater and lesser pressure within the endolymph. The hair cells of the spiral organ are sensitive to these pressure differences and send out nerve impulses to the brain, where they are interpreted as sounds.

The vestibular portion of the inner ear includes two interconnected chambers called the "sacculus" and the "utriculus," both filled with endolymph. The sacculus has a downward extension called the "lagena," and it also connects into the scala media of the cochlear portion of the ear. From the utriculus emerge three semicircular ducts, approximately at right angles to one another, all filled with endolymph: an anterior vertical duct, a posterior vertical duct, and a horizontal duct. Each semicircular duct runs through a bony semicircular canal, filled with perilymph. Each duct has a bulblike swelling, the ampulla, at one end. Each ampulla has a patch, or macula, of sensory structures called "neuromasts," which are sensitive to movements in fluids such as endolymph. Other maculae, or patches of neuromasts, are located in the sacculus, the utriculus, and the lagena.

The vestibular portion of the inner ear is sensitive to movements and especially to acceleration. Normally, this acceleration is caused by gravity, but nonlinear movements (such as the swerving of a fast-moving vehicle around a curve) may also result in accelerations that cause fluid movements within the semicircular canals. These movements are perceived by the sensitive hair cells (neuromasts) within each ampulla. Spinning around or other sudden acceleration causes temporary dizziness (vertigo) and a consequent loss of balance.

Disorders and Diseases

Several types of medical specialists deal with problems of the various sense organs: ophthalmologists deal with diseases of the eye; otorhinolaryngologists deal with diseases of the ears (oto-), nose (rhino-), and throat (larynx); and neurologists deal with all the senses. All these specialists first conduct diagnostic tests in order to detect any sensory malfunction and to determine the probable cause; they then provide whatever treatment may be available for each condition.

The ability to distinguish tastes diminishes gradually with age in older persons as the number of gustatory cells declines, but complete loss of taste is rare. Persons with diminished taste are at greater risk for accidental poisoning and malnutrition. Except for the decline of taste among the elderly, other defects of smell or taste are relatively uncommon and may be indicative of more serious neurological problems such as brain damage or nerve damage or endocrine problems such as Kallman syndrome. The inability to smell is a rare condition known as "anosmia." Similarly, loss of cutaneous sensations, even over a small portion of the body, is usually indicative of nerve damage, while diminished sensitivity to touch stimuli is a common aspect of aging. Such impairment can lead to injuries, pressure ulcers, and temperature-related problems such as heat stroke.

Disorders of the eye range from easily correctable vision problems to total blindness. Impaired function of one or more of the three types of color receptors results in one of the several types of color blindness. The most common type, red-green color blindness, is inherited as a sex-linked recessive trait and is thus more common in men than in women. Blindness may result from various defects or injuries: a defective lens or cornea may limit vision to large objects, and a defective retina may limit perception to light and darkness only. Total blindness results if the optic nerve is damaged or missing. More common visual defects include myopia(nearsightedness), hyperopia (farsightedness), and astigmatism (differences in vision along different axes), all of which can be corrected with glasses or contact lenses or by surgical procedures. A failure of the mechanism that drains fluid from the interior of the eyeball may result in a buildup of ocular pressure, a condition known as "glaucoma." In times past, when treatment for glaucoma was not readily available, most cases resulted in permanent blindness. Glaucoma can now be treated, however, either surgically or with drugs. Several changes occur to the lens of the eye in older individuals. As the lens becomes more rigid with advancing age, reading and other near-vision tasks become more difficult, a condition known as "presbyopia." Also frequent among older people are cataracts, tiny opaque grains that cloud up the lens and reduce the ability to see clearly. Untreated cataracts may eventually result in blindness, but various forms of treatment are available to prevent this from occurring.

Diseases of the ear should always be treated as serious. Tinnitus, or ringing in the ears, can result from damage to the hair cells of the organ of Corti. Damage to the auditory nerve can result in deafness. Upper respiratory infections can travel up the Eustachian tube and cause a common childhood infection of the middle ear known as "otitis media." Infections of the vestibular portion of the inner ear can result in recurrent or permanent dizziness (vertigo) because the inflamed cells transmit impulses that the body wrongly interprets as resulting from accelerations in unusual directions. Some plant poisons (or the drugs derived from them, such as ipecac) can also impair the function of the inner ear and result in sensations of dizziness, often followed by nausea and by the vomiting of the plant containing the poison. This reaction may have evolved as an adaptive response to possible poisons; the same reaction also results in vomiting in other situations that cause unusual accelerations in the vestibular portion of the inner ear, as in the case of seasickness or other motion sickness
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