Copyright 2007 by Saunders/Elsevier. All rights reserved. Chapter 22: Special Senses Color Textbook...

Copyright 2007 by Saunders/Elsevier. All rights reserved.

Chapter 22:

Special Senses

Color Textbook of Histology, 3rd ed.

Gartner & Hiatt Copyright 2007 by Saunders/Elsevier. All rights reserved.


Specialized Peripheral Receptors

The dendritic endings of certain sensory receptors, located in various regions of the body, including muscles, tendons, skin, fascia, and joint capsules, are specialized to receive particular stimuli.

Merkel’s disks are slightly more complex mechanoreceptors that perceive discriminatory touch. They are located mostly in nonhairy skin and regions of the body more sensitive to touch.

Meissner’s corpuscles are specialized for tactile discrimination. They are located in the dermal papillae of the glabrous portion of the fingers and palms of the hands.

Pacinian corpusclesare located in the dermis and hypodermis in the digits of the hands and in the breasts as well as in connective tissue of the joints and the mesentery. They perceive pressure, touch, and vibration.

Naked nerve endings penetrate the epidermis. They are nociceptors that respond to pain and thermoreceptors that respond to temperature differences.

Ruffini’s endings (corpuscles) are located in the dermis of the skin, nail beds, and joint capsules. The connective tissue capsule surrounding each of these receptors is anchored at each end, increasing their sensibility to stretching and pressure in the skin and in the joint capsules.

For more information see the Specialized Peripheral Receptors section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–1 Various mechanoreceptors. A, Merkel’s disk. B, Meissner’s corpuscle. C, Pacinian corpuscle. D, Peritricial (naked) nerve endings. E, Ruffini’s corpuscle. F, Krause’s end bulb. G, Muscle spindle. H, Golgi tendon organ.


Specialized Peripheral Receptors (cont.)

Krause’s end bulbs are spherical, encapsulated nerve endings located in the papillary region of the dermis. They were thought to be receptors sensitive to cold, but present evidence does not support this concept. Their function is unknown.

Both muscle spindles and Golgi tendon organs are encapsulated mechanoreceptors involved in proprioception.

Muscle spindles provide feedback concerning the changes in muscle length as well as the rate of alteration in the length of the muscle.

Golgi tendon organs monitor the tension as well as the rate at which the tension is being produced during movement. Information from these two sensory structures is processed mostly at the unconscious levels within the spinal cord; however, the information also reaches the cerebellum and even the cerebral cortex, so that the individual may sense muscle position.

For more information see the Specialized Peripheral Receptors section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–1 Various mechanoreceptors. A, Merkel’s disk. B, Meissner’s corpuscle. C, Pacinian corpuscle. D, Peritricial (naked) nerve endings. E, Ruffini’s corpuscle. F, Krause’s end bulb. G, Muscle spindle. H, Golgi tendon organ.


Eye

Eyes are the photosensory organs of the body. Light passes through the cornea, lens, and several refractory structures of the orb; light is then focused by the lens on the light-sensitive portion of the neural tunic of the eye, the retina, which contains the photosensitive rods and cones. Through a series of several layers of nerve cells and supporting cells, the visual information is transmitted by the optic nerve to the brain for processing.

The bulb of the eye is composed of three tunics, the fibrous, vascular, and neural tunics.

The external fibrous tunic of the eye, the tunica fibrosa, is divided into the sclera and the cornea. The white, opaque sclera covers the posterior five sixths of the orb, whereas the colorless, transparent cornea covers the anterior one sixth of the orb.

The lens of the eye is a flexible, biconvex, transparent disk composed of epithelial cells and their secretory products.

The retina, the third and innermost tunic of the eye, is its neural portion, which contains the photoreceptor cells, known as rods and cones.

For more information see the Eye section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–4 Anatomy of the eye (orb).


Retina

The retina is formed of an outer pigmented layer and a neural portion, called the retina proper. The cells composing the retina constitute a highly differentiated extension of the brain.

The portion of the retina that functions in photoreception lines the inner surface of the choroid layer from the optic disk to the ora serrata and is composed of 10 distinct layers: pigment epithelium; layer of rods and cones; outer limiting membrane; outer nuclear layer; outer plexiform layer; inner nuclear layer; inner plexiform layer; ganglion cell layer; optic nerve fiber layer; and the inner limiting membrane.

The optic disk, located on the posterior wall of the orb, is the exit site of the optic nerve. Because it contains no photoreceptor cells, it is insensitive to light and is therefore called the “blind spot” of the retina. Approximately 2.5 mm lateral to the optic disk is a yellow-pigmented zone in the retinal wall called the macula lutea. Located in the center of this spot is an oval depression, the fovea centralis, where visual acuity is greatest. The fovea contains only tightly packed cones. As distance from the fovea increases, the number of cones decreases and the number of rods increases.

The optical portion of the retina houses two distinct types of photoreceptor cells called rods and cones. Both rods and cones are polarized cells whose apical portions, known as the outer segments, are specialized dendrites. The outer segments of both rods and cones are surrounded by pigmented epithelial cells.

For more information see the Retina section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–8 Cellular layers of the retina. The space observed between the pigmented layer and the remainder of the retina is an artifact of development and does not exist in the adult except during detachment of the retina.


Rods and Cones

Rods are specialized receptors for dim light; cones are specialized receptors for bright light reception. Cones are further adapted for color vision, whereas rods perceive only light. Rods and cones are unevenly distributed in the retina, in that cones are highly concentrated in the fovea; thus, this is the area of the retina where high-acuity vision occurs.

Rods are composed of an outer segment, an inner segment, a nuclear region, and a synaptic region.

The outer segment of the rod, presents several hundred flattened membranous lamellae oriented perpendicular to its long axis, forming a disk. Each disk is composed of two membranes separated from each other by an 8-nm space. The membranes contain rhodopsin (visual purple), a light-sensitive pigment. Because the outer segment is longer in rods than in cones, rods contain more rhodopsin, respond more slowly than cones, and have the capacity to collectively summate the reception.

Although the mode of function of the cones is similar to that of rods, cones are activated in bright light and produce greater visual acuity compared with rods. There are three types of cones, each containing a different variety of the photopigment iodopsin. Each variety of iodopsin has a maximum sensitivity to one of three colors of the spectrum—red, green, and blue—and the difference resides in the opsins rather than in the 11-cis retinal.

For more information see the Rods and Cones section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–9 Morphology of a rod and a cone. BB, basal body; C, connecting stalk; Ce, centriole; IS, inner segment; M, mitochondria; NR, nuclear region; OS, outer segment; SR, synaptic region; SV, synaptic vesicles. (Modified from Lentz TL: Cell Fine Structure: An Atlas of Drawings of Whole-Cell Structure. Philadelphia, WB Saunders, 1971.)


The External and Middle EarThe ear, the organ of hearing as well as the organ of equilibrium or balance, is divisible into three parts: (1) the external ear, (2) the middle ear (tympanic cavity), and (3) the inner ear.

Sound waves received by the external ear are translated into mechanical vibrations by the tympanic membrane. These vibrations then are amplified by the bony ossicles in the middle ear (tympanic cavity) and transferred to the fluid medium of the inner ear at the oval window. The inner ear, a perilymph-filled bony labyrinth in which is suspended a membranous labyrinth, regulates hearing (the cochlear portion) and maintains balance (the vestibular portion). The middle ear, or tympanic cavity, is an air-filled space located in the petrous portion of the temporal bone. This space communicates, via the auditory tube (eustachian tube), with the pharynx. The bony ossicles are housed in this space, spanning the distance between the tympanic membrane and the membrane at the oval window.

Located within the medial wall of the tympanic cavity are the oval window and the round window, which connect the middle ear cavity to the inner ear. The malleus, incus, and stapes are articulated in series by synovial joints. The malleus is attached to the tympanic membrane, with the incus interposed between it and the stapes, which in turn is attached to the oval window. Two small skeletal muscles, the tensor tympani and the stapedius, aid movements of the tympanic membrane and the bony ossicles. Vibrations of the tympanic membrane set the ossicles into motion, and the oscillations are magnified to vibrate the membrane of the oval window, thus setting the fluid medium of the cochlear division of the inner ear into motion.

For more information see the External and Middle Ear sections in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–12 Anatomy of the ear.


Inner Ear

The inner ear is composed of the bony labyrinth, an irregular, hollowed-out cavity located within the petrous portion of the temporal bone, and the membranous labyrinth, which is suspended within the bony labyrinth.

The bony labyrinth is separated from the membranous labyrinth by the perilymphatic space. This space is filled with a clear fluid called the perilymph, within which the membranous labyrinth is suspended. The central region of the bony labyrinth is known as the vestibule.

The three semicircular canals (superior, posterior, and lateral) are oriented at 90 degrees to one another. One end of each canal is enlarged; this expanded region is called the ampulla. Suspended within the canals are the semicircular ducts, which are regionally named continuations of the membranous labyrinth.

The vestibule is the central portion of the bony labyrinth located between the anteriorly placed cochlea and the posteriorly placed semicircular canals. Its lateral wall contains the oval window (fenestra vestibuli), covered by a membrane to which the footplate of the stapes is attached, and the round window (fenestra cochleae), covered only by a membrane. The vestibule also houses specialized regions of the membranous labyrinth (the utricle and the saccule).

The cochlea arises as a hollow bony spiral that turns upon itself, like a snail shell, two and one-half times around a central bony column, the modiolus. The modiolus projects into the spiraled cochlea with a shelf of bone called the osseous spiral lamina, through which traverse blood vessels and the spiral ganglion.

The membraneous labyrinth is filled with endolymph and possesses the following specialized areas: the saccule and utricle, the semicircular ducts, and the cochlear duct.

For more information see the Inner Ear section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–13 Cochlea of the inner ear. A, Anatomy of bony labyrinth. B, Anatomy of the membranous labyrinth. C, Sensory labyrinth.


Utricle

The saccule and utricle, sac-like structures lying in the vestibule. Specialized regions of the saccule and utricle act as receptors for sensing orientation of the head relative to gravity and acceleration, respectively. These receptors are called the macula of the saccule and the macula of the utricle.

The maculae are composed of two types of neuroepithelial cells, called type I and type II hair cells, as well as of supporting cells that sit on a basal lamina.

Innervation of the hair cells is derived from the vestibular portion of the vestibulocochlear nerve. The rounded bases of the type I hair cells are almost entirely surrounded by a cup-shaped afferent nerve fiber. Type II hair cells exhibit many afferent fibers synapsing on the basal area of the cell. Structures resembling synaptic ribbons are present near the bases of type I and type II hair cells. The synaptic ribbons of the type II hair cells probably function in synapses with efferent nerves, which are thought to be responsible for increasing the efficiency of synaptic release.

For more information see the Utricle section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–14 Hair cells and supporting cells in the macula of the utricle.


Organ of Corti

The cochlear duct (scala media) and its organ of Corti are responsible for the mechanism of hearing.

The roof of the cochlear duct is the vestibular membrane, whereas its floor is the basilar membrane. The perilymph-filled compartment lying above the vestibular membrane is called the scala vestibuli, whereas the perilymph-filled compartment lying below the basilar membrane is the scala tympani. These two compartments communicate at the helicotrema, near the apex of the cochlea.

The basilar membrane, extending from the spiral lamina at the modiolus to the lateral wall, supports the organ of Corti and is composed of two zones: the zona arcuata and the zona pectinata. The zona arcuata is thinner, lies more medial, and supports the organ of Corti.

Interdental cells located within the body of the spiral limbus secrete the tectorial membrane, a proteoglycan-rich gelatinous mass containing numerous fine keratin-like filaments, that overlies the organ of Corti. Stereocilia of specialized receptor hair cells of the organ of Corti are embedded in the tectorial membrane.

For more information see the Organ of Corti section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–17 Organ of Corti.


Organ of Corti (cont.)

The organ of Corti, the specialized receptor organ for hearing, lies on the basilar membrane and is composed of neuroepithelial hair cells and several types of supporting cells. Although the supporting cells of the organ of Corti have different characteristics, they all originate on the basilar membrane and contain bundles of microtubules and microfilaments, and their apical surfaces are all interconnected at the free surface of the organ of Corti. Supporting cells include pillar cells, phalangeal cells, border cells, and cells of Hensen.

Neuroepithelial hair cells are specialized for transducing impulses for the organ of hearing. Depending on their locations, these cells are called inner hair cells and outer hair cells.

Because the organ of Corti is firmly attached to the basilar membrane, a rocking motion within the basilar membrane is translated into a shearing motion on the stereocilia of the hair cells embedded in the overlying rigid tectorial membrane. When the shearing force produces a deflection of the stereocilia toward the taller stereocilia, the cell becomes depolarized, thus generating an impulse that is transmitted via the afferent nerve fibers.

How differences in sound frequency or pitch are distinguished is not understood. It has long been thought that the basilar membrane, which becomes longer with each turn of the cochlea, vibrates at different frequencies relative to its width. Therefore, low-frequency sounds would be detected near the apex of the cochlea, whereas high-frequency sounds would be detected near the base of the cochlea. Evidence suggests that outer hair cells contain the necessary machinery to react rapidly to efferent input, causing them to vary the length of their stereocilia and consequently altering the shear force between the tectorial membrane and the basilar membrane, thus “tuning” the basilar membrane. This action then alters the response of the sound-detecting inner hair cells, affecting their reaction to different frequencies.

For more information see the Organ of Corti section in Chapter 22 of Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

Figure 22–17 Organ of Corti.

Copyright 2007 by Saunders/Elsevier. All rights reserved. Chapter 22: Special Senses Color Textbook...

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