Transcript Document

Chapter 22:
Special Senses
Color Textbook of Histology, 3rd ed.
Gartner & Hiatt
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Eye
Figure 22–4 Anatomy of the eye (orb).
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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 yellowpigmented 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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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.)
Copyright 2007 by Saunders/Elsevier. All rights reserved.
The External and Middle Ear
The 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 perilymphfilled 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.
Figure 22–12 Anatomy of the ear.
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
Inner Ear
The inner ear is composed of the bony labyrinth, an irregular, hollowedout 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.
Figure 22–13 Cochlea of the inner ear. A, Anatomy of bony
labyrinth. B, Anatomy of the membranous labyrinth. C, Sensory
labyrinth.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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.
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.
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.
Figure 22–17 Organ of Corti.
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.
Copyright 2007 by Saunders/Elsevier. All rights reserved.