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THE SENSORY SYSTEM


Learning Objectives

  1. Identify the senses of the body.
  2. Describe the physical characteristics of the senses.

The sensory system informs areas of the cerebral cortex of changes that are taking place within the body or in the external environment. The special sensory receptors respond to special individual stimuli such as sound waves, light, taste, smell, pressure, heat, cold, pain, or touch. Positional changes, balance, hunger, and thirst sensations are also detected and passed on to the brain.

SMELL

Odor is perceived upon stimulation of the receptor cells in the olfactory membrane of the nose. The olfactory receptors are very sensitive, but they are easily fatigued. This tendency explains why odors that are initially very noticeable are not sensed after a short time. Smell is not as well developed in man (350 odorant receptors) as it is in other mammals such as mice, which have 1,000 receptors.

TASTE

The taste buds are located in the tongue (Fig. 6-30). The sensation of taste is limited to sour, sweet, bitter, savory, and salty. It does not matter where on the tongue an object is placed; it can detect different tastes everywhere on the tongue. Many foods and drinks tasted are actually smelled, and their taste depends upon their odor. (This interdependence between taste and smell can be demonstrated by pinching the nose shut when eating onions.) Sight can also affect taste. Several drops of green food coloring in a glass of milk will make it all but unpalatable, even though the true taste has not been affected.

SIGHT

The eye, the organ of sight, is a specialized structure for the reception of light. It is assisted in its function by accessory structures, such as the eye brows, eyelashes, eyelids, and lacrimal apparatus. The lacrimal apparatus consists of structures that produce tears and drains them from the surface of the eyeball (Fig. 6-54).

Figure 6-54.—Lacrimal apparatus. Fluid produced by lacrimal glands (tears) streams across the eye surface, enters the canals, and then passes through the lacrimal sac and nasolacrimal duct to enter the nose.

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & physiology (6th ed.). St. Louis: Elsevier Health Sciences.

Structure of the Eye

Approximately five-sixths of the eyeball lies recessed in the orbit, protected by a bony socket. Only the small anterior surface of the eyeball is exposed. The eye is not a solid sphere but contains a large interior cavity that is divided into two cavities, anterior and posterior. The anterior cavity is further subdivided into anterior and posterior chambers (Fig. 6-55).

The anterior cavity of the eye lies in front of the lens. The anterior chamber of the anterior cavity is the space anterior to the iris, but posterior to the cornea. The posterior chamber of the anterior cavity consists of a small space directly posterior to the iris, but anterior to the lens.

Both chambers of the anterior cavity are filled with a clear, watery fluid called aqueous humor. Aqueous humor helps to give the cornea its curved shape (Fig. 6-55). The aqueous humor drains out of the anterior chamber at the same rate it enters the posterior chamber. When there is a pressure increase inside the eye, and the level exceeds 25 mm hg, damage will occur and may cause blindness; this condition is called glaucoma.

Figure 6-55.— Eye Structure Horizontal section through the eyeball. The eye is viewed from above.

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & physiology (6th ed.). St. Louis: Elsevier Health Sciences.

The posterior cavity of the eye is larger than the anterior cavity, occupying the entire space posterior to the lens to include suspensory ligaments and ciliary body. The posterior cavity contains a substance, with the consistency similar to soft gelatin, called vitreous humor. Vitreous humor and aqueous humor help maintain sufficient pressure inside the eye to prevent the eyeball from collapsing (Figs. 6-55 and 6-56).

The eyeball is composed of three layers; sclera, choroid, and retina (Fig. 6-56).

OUTER LAYER.—The outer layer of the eye is the sclera. It is the tough, fibrous, protective portion of the globe, called the white of the eye. The anterior outer layer of the sclera is transparent and called the cornea, or the window of the eye. It permits light to enter the globe. The exposed sclera is covered with a mucous membrane, the conjunctiva, which is a continuation of the inner lining of the eyelids. The lacrimal gland produces tears that constantly wash the front part of the eye and the conjunctiva. Excess secretions flow toward the inner angle of the eye (canthus) and drain down ducts into the nose.

MIDDLE LAYER.—The middle layer of the eye is the choroid. It is a highly vascular, pigmented tissue that provides nourishment to the inner structures. Continuous with the choroid is the ciliary body. The ciliary body is formed by a thickening of the choroid and fits like a collar into the area between the retina and iris. Attached to the ciliary body are the suspensory ligaments, which blend with the elastic capsule of the lens and holds it in place (Fig. 6-56).

    Iris.—The iris is continuous with the ciliary body. It is a circular, pigmented muscular structure that gives color to the eye. The iris separates the anterior cavity into anterior and posterior chambers. The opening in the iris is called the pupil. The amount of light entering the pupil is regulated through the constriction of radial and circular muscles in the iris. When strong light is flashed into the eye, the circular muscle fibers of the iris contract, reducing the size of the pupil decreasing the amount of light. If the light is dim, the pupil dilates to allow as much of the light in as possible. The size and reaction of the pupils of the eyes are an important diagnostic tool.

    Lens.—The lens is a transparent, biconvex (having two convex surfaces) structure suspended directly behind the iris. The optic globe posterior to the lens is filled with a jellylike substance called vitreous humor to maintain the shape of the eyeball by maintaining intraocular pressure. The lens separates the eye into anterior and posterior cavities.

Figure 6-56.—Lens, cornea, iris, and ciliary body. Note the suspensory ligaments that attach the lens to the ciliary body.

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & physiology (6th ed.). St. Louis: Elsevier Health Sciences.

INNER LAYER.—The inner layer of the eye is the retina (Fig. 6-57). It contains layers of nerve cells, rods, and cones, which are the receptors of the sense of vision. The retina is continuous with the optic nerve, entering the back of the globe carrying visual impulses received by the rods and cones to the brain. The area where the optic nerve enters the eyeball contains no rods and cones and is called the optic disc (blind spot).

Figure 6-57.—Opthalmoscope View of the Eye

    Rods.— Rods respond to low intensities of light and are responsible for night vision. They are located in all areas of the retina, except in the small depression called the fovea centralis, where light entering the eye is focused, and has the clearest vision. If a person looks slightly to the side (where most of the cones are at) it will be clearer at night.

    Cones.—Cones require higher light intensities for stimulation and are most densely concentrated in the fovea centralis. The cones are responsible for color vision and vision in very bright light.

Vision Process

The vision process begins with rays of light from an object passing through the cornea. The image is then received by the lens, by way of the iris. Leaving the lens, the image falls on the rods and cones in the retina. The image is then sent by the optic nerve to the brain for interpretation (Fig. 6-58). Note the image received by the retina is upside down, but the brain turns it right-side up.

Figure 6-58.—The Vision Process

REFRACTION.—Deflection or bending of light rays results when light passes through substances of varying densities in the eye (cornea, aqueous humor, lens, and vitreous humor). The deflection of light in the eye is refraction.

ACCOMMODATION. —Accommodation is the process by which the lens increases or decreases its curvature to refract light rays into focus on the fovea centralis.

CONVERGENCE.—The movement of the globes toward the midline causes a viewed object to come into focus on corresponding points of the two retinas. This process produces clear, three-dimensional vision.

HEARING

The ear is the primary organ of hearing and the sense organ for balance. Its major parts are illustrated in Figure 6-59. The ear is divided into three parts: the external, middle, and inner ear.

External Ear

The external (outer) ear is composed of two parts, the auricle and the external auditory canal (see Fig. 6-15). The auricle, or pinna, is a cartilaginous structure located on each side of the head.

The auricle collects sound waves from the environments that are conducted by the external auditory canal (about 3cm long) to the eardrum. The lining of the external auditory canal contains glands that secrete a wax-like substance called cerumen. Cerumen aids in protecting the eardrum against foreign bodies and microorganisms.

The tympanic membrane, or eardrum, is an oval sheet of fibrous epithelial tissue that stretches across the inner end of the external auditory canal (Fig. 6-59). The eardrum separates the outer and middle ear. Sound waves cause the eardrum to vibrate, and this vibration transfers the sounds from the external environment to the auditory ossicles.

Figure 6-59.— Effect of sound waves on cochlear structures. A, Sound waves strike the tympanic membrane and cause it to vibrate. This causes the membrane of the oval window to vibrate, which causes the perilymph in the bony labyrinth of the cochlea and the endolymph in the membranous labyrinth of the cochlea, or cochlear duct, to move. This movement of endolymph causes the basilar (spiral) membrane to vibrate, which in turn stimulates hair cells on the organ of Corti (spiral organ) to transmit nerve impulses along the cranial nerve. Eventually, nerve impulses reach the auditory cortex and are interpreted as sound. B, High-frequency (high-pitch) waves stimulate hair cells nearer the stapes (oval window) and low-frequency (low-pitch) waves stimulate hair cells nearer the distal end of the cochlea. The location of peak stimulation of the hair cells allows the brain to interpret the pitch of the sound. (B: Adapted from Guyton A, Hall J: Textbook of medical physiology, ed 11, Philadelphia, 2006, Saunders.)

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

Middle Ear

The middle ear is a cavity in the temporal bone, lined with epithelium. It contains three auditory ossicles the malleus (hammer), the incus (anvil), and the stapes (stirrup) which transmit vibrations from the tympanic membrane to the fluid in the inner ear (Fig. 659). The malleus is attached to the inner surface of the eardrum and connects with the incus, which in turn connects with the stapes. The base of the stapes is attached to the oval window, the membrane-covered opening of the inner ear. These tiny bones, which span the middle ear, are suspended from bony walls by ligaments. This arrangement provides the mechanical means for transmitting sound vibrations to the inner ear.

The eustachian tube, or auditory tube, connects the middle ear with the nasopharynx. It is lined with a mucous membrane and is about 36 mm long. Its function is to equalize internal and external air pressure. For example, while riding an elevator in a tall building, a person may experience a feeling of pressure in the ear. This condition is usually relieved by swallowing, which opens the eustachian tube and allows the pressurized air to escape and equalize with the area of lower pressure. Divers who ascend too fast to allow pressure to adjust may experience rupture of their eardrums. The eustachian tube can also provide a pathway for infection of the middle ear.

Inner Ear

The inner ear is filled with a fluid called endolymph. Sound vibrations cause the stapes to move against the oval window create internal ripples that run through the endolymph. These pressurized ripples move to the cochlea, a small snail-shaped structure where the cochlear duct (the only part of the inner ear concerned with hearing) is located housing the organ of Corti, the hearing organ (Fig. 6-60).

The cells protruding from the organ of Corti are stimulated by the ripples to convert these mechanical vibrations into nerve impulses, and these impulses are relayed through the vestibulocochlear (8th cranial) nerve to the auditory area of the cortex in the temporal lobe of the brain. There they are interpreted as the sounds a person hears

The vestibule constitutes the central section of the bony labyrinth. The bony labyrinth opens to the oval window as well as the three semicircular canals which are situated at right angles to each other (Fig. 6-60). Movement of the endolymph within the canals, caused by general body movements, stimulates nerve endings, which report these changes in body position to the brain, which in turn uses the information to maintain equilibrium.

The sense of organs located in the utricle and saccule function in static equilibrium, a function needed to sense the position of the head relative to gravity or sense acceleration or deceleration of the body. “The sense organs associated with semicircular ducts function in dynamic equilibrium – a function needed to maintain balance when the head or body itself is rotated or suddenly moved.”

The round window is another membranecovered opening of the inner ear. It is the opening for the auditory tube.

Figure 6-60.—The inner ear. A, The bony labyrinth (bone colored) is the hard outer wall of the entire inner ear and includes semicircular canals, vestibule, and cochlea. Within the bony labyrinth is the membranous labyrinth (purple), which is surrounded by perilymph and filled with endolymph. Each ampulla in the vestibule contains a crista ampullaris that detects changes in head position and sends sensory impulses through the vestibular nerve to the brain. B, The inset shows a section of the membranous cochlea. Hair cells in the organ of Corti (spiral organ) detect sound and send the information through the cochlear nerve. The vestibular and cochlear nerves join to form the eighth cranial nerve.

Image reprinted  from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

TOUCH

Until the beginning of the last century, touch (feeling) was treated as a single sense. Thus, warmth or coldness, pressure, and pain, were thought to be part of a single sense of touch or feeling. It was discovered that different types of nerve ending receptors are widely and unevenly distributed in the skin and mucous membranes. For example, the skin of the back possesses relatively few touch and pressure receptors while the fingertips have many. The skin of the face has relatively few cold receptors, and mucous membranes have few heat receptors. The cornea of the eye is sensitive to pain, and when pain sensation is abolished by a local anesthetic, a sensation of touch can be experienced.

Receptors are considered to be sensory organs. They provide the body with the general senses of touch, temperature, and pain. In addition, these receptors initiate reactions or reflexes in the body to maintain homeostasis. For example, receptors in the skin perceive cold, resulting in goose bumps. This reaction is the body's attempt to maintain internal warmth.

Receptors are classified according to location, structure, and types of stimuli activating them. Classified according to location, the three types of receptors are as follows: superficial receptors (exteroceptors), deep receptors (proprioceptors), and internal receptors (visceroceptors). See Table 6-7 for receptor locations and the senses resulting from the stimulation of these receptors.


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David L. Heiserman, Editor

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Revised: June 06, 2015