Chapter 16
I.
INTRODUCTION
Receptors for the special senses - smell,
taste, vision, hearing, and equilibrium - are housed in complex
sensory organs.
II. OLFACTION: SENSE OF SMELL
A.
Both
smell and taste are chemical senses.
B.
Anatomy
of olfactory receptors
1.
The
receptors for olfaction, which are bipolar neurons, are in the nasal
epithelium in the superior portion of the nasal cavity (Figure 16.1).
2.
They
are first-order neurons of the olfactory pathway.
3.
Supporting
cells are epithelial cells of the mucous membrane lining the nose.
4.
Basal
stem cells produce new olfactory receptors.
5.
Olfactory
glands produce mucous which odorant molecules dissolve in.
C.
Physiology
of Olfaction
1.
Genetic
evidence suggests there are hundreds of primary scents.
2.
In
olfactory reception, a generator potential develops and triggers one or more
nerve impulses.
D.
Adaptation
to odors occurs quickly, and the threshold of smell is low: only a few
molecules of certain substances need be present in air to be smelled.
E.
Olfactory
receptors convey nerve impulses to olfactory nerves, olfactory bulbs, olfactory
tracts, and the cerebral cortex and limbic system.
III. GUSTATORY: SENSE OF TASTE
A.
Taste
is a chemical sense.
1.
To
be detected, molecules must be dissolved.
2.
Taste
stimuli classes include sour, sweet, bitter, salty, and umami.
3.
Other
“tastes” are a combination of the four taste sensations plus olfaction.
B.
Anatomy
of Taste Buds and Papillae
1.
The
receptors for gustation, the gustatory receptor
cells, are located in taste buds (Figure 16.2).
2.
Taste
buds consist of supporting cells, gustatory receptor cells, and basal cells
(Figure 16.2c). Basal cells produce supporting cells, which then develop into
gustatory receptor cells.
3.
Taste
buds are found in papillae (Figure 16.2a, b).
a.
The
papillae include circumvallate, fungiform, foliate,
and filiform papillae.
b.
They
appear as elevations on the tongue.
c.
You
should know the types of papillae and their functions as discussed in class.
C.
Physiology
of Gustation
1.
When
a tastant is dissolved in saliva it can make contact
with the plasma membrane of gustatory receptor cells.
2.
Receptor
potentials developed in gustatory hairs cause the release of neurotransmitter
that gives rise to nerve impulses.
3.
Individual
gustatory receptors in certain regions of the tongue are more sensitive than
others to the primary taste sensations (Figure 16.2a).
4.
Figure
16.3 shows the responses of three groups of taste neurons to sweet, salty, and
sour chemicals.
D.
Taste
Thresholds and Adaptation
1.
Taste
thresholds vary for each of the primary tastes with the threshold for bitter
being the lowest, then sour, and finally salty and sweet.
2.
Adaptation
to taste occurs quickly.
E.
Gustatory
receptor cells convey nerve impulses to cranial nerve VII, IX, and X, which
carry the impulses to the medulla, the thalamus, and the parietal lobe of the
cerebral cortex (Figure 14.15).
IV. VISION
A.
Introduction
1.
More
than half the sensory receptors in the human body are located in the eyes.
2.
A
large part of the cerebral cortex is devoted to processing visual information.
B.
Accessory
Structures of the Eyes
1.
Eyelids
a.
The
eyelids shade the eyes during sleep, protect the eyes from excessive
light and foreign objects, and spread lubricating secretions over the eyeballs
(Figure 16.4).
b.
From
superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous
tissue, fibers of the orbicularis
oculi muscle, a tarsal plate, tarsal glands, and
conjunctiva (Figure 16.5a).
1)
The
tarsal plate gives form and support to the eyelids.
2)
The
tarsal glands secrete a fluid to keep the eye lids from adhering to each
other.
3)
The
conjunctiva is a thin mucous membrane that lines the inner aspect of the
eyelids and is reflected onto the anterior surface of the eyeball.
2.
Eyelashes and eyebrows
help protect the eyeballs from foreign objects, perspiration, and the direct
rays of the sun.
3.
The
lacrimal apparatus consists of
structures that produce and drain tears as discussed in class (Figure 16.5b).
4.
The
six extrinsic eye muscles move the eyeballs laterally, medially,
superiorly, and inferiorly (Exhibit 11.2, Figures 16.5a and 16.6).
C.
Anatomy
of the Eyeball
1.
The
eye is constructed of three layers (Figure 16.6).
a.
The
fibrous tunic is the outer coat of the eyeball. It can be divided into
two regions: the posterior sclera and the anterior cornea.
At the junction of the sclera and cornea is an opening known as the scleral venous sinus or
1)
The
sclera, the “white” of the eye, is a white coat of dense fibrous tissue
that covers all the eyeball, except the most anterior
portion, the iris; the sclera gives shape to the eyeball and protects its inner
parts. Its posterior surface is pierced by the optic nerve.
2)
The
cornea is a nonvascular, transparent, fibrous coat through which the
iris can be seen; the cornea acts in refraction of light.
b.
The
vascular tunic is the middle layer of the eyeball and is composed of
three portions: choroid, ciliary body, and iris (Figure 16.6).
1)
The
choroid absorbs light rays so that they are
not reflected and scattered within the eyeball; it also provides nutrients to
the posterior surface of the retina.
2)
The
ciliary body consists of the ciliary processes and ciliary
muscle.
a)
The
ciliary processes consist of
protrusions or folds on the internal surface of the ciliary
body where epithelial lining cells secrete aqueous humor.
b)
The
ciliary muscle is a smooth muscle that
alters the shape of the lens for near or far vision.
3)
The
iris is the colored portion seen through the cornea and consists of circular
iris and radial iris smooth muscle fibers (cells) arranged to
form a doughnut-shaped structure.
a)
The
black hole in the center of the iris is the pupil, the area through
which light enters the eyeball.
b)
A
principal function of the iris is to regulate the amount of light entering the
posterior cavity of the eyeball (figure 16.7)
c.
The
third and inner coat of the eye, the nervous tunic, lines the posterior
three-quarters of the eyeball and is the beginning of the visual pathway
(Figure 16.6).
1. The nervous tunic consists of the
retina and the pigment
epithelium which aids the choroid
in absorbing stray light rays.
The
retina contains three zones of neurons that are named in the order
in which they conduct nerve impulses: photoreceptor neurons, bipolar neurons, and ganglion neurons
(Figure 16.9).
(1) The photoreceptor neurons are called
rods or cones because of the differing shapes of their outer segments.
(2) Rods are specialized for black-and-white
vision in dim light; they also allow us to discriminate between different
shades of dark and light and permit us to see shapes and movement.
(3) Cones are specialized for color vision and
sharpness of vision (high visual acuity) in bright light; cones are most
densely concentrated in the central fovea, a small depression in the center of
the macula lutea.
(a) The macula lutea
is in the exact center of the posterior portion of the retina, corresponding to
the visual axis of the eye.
(b) The fovea is the area of
sharpest vision because of the high concentration of cones.
(c) Rods are absent from the fovea and
macula and increase in density toward the periphery of the retina.
2.
The
eyeball contains the nonvascular lens, just behind the pupil and iris. The lens fine tunes the focusing of light
rays for clear vision.
3.
The
interior of the eyeball is a large space divided into two cavities by the lens:
the anterior cavity and the vitreous chamber (posterior cavity)
(Figure 16.10).
a.
The
anterior cavity is subdivided into the anterior chamber (which lies
behind the cornea and in front of the iris) and the posterior chamber
(which lies behind the iris and in front of the suspensory
ligaments and lens).
1)
The
anterior cavity is filled with a watery fluid called the aqueous humor
that is continually secreted by the ciliary processes
behind the iris.
2)
The
aqueous humor flows forward from the posterior chamber through the pupil into
the anterior chamber and drains into the scleral
venous sinus (
a)
The
pressure in the eye, called intraocular pressure, is produced
mainly by the aqueous humor. The intraocular pressure, along with the vitreous
body, maintains the shape of the eyeball and keeps the retina smoothly applied
to the choroid so the retina will form clear images.
b)
Excessive
intraocular pressure, called glaucoma, results in degeneration of the
retina and blindness.
b.
The
second, and larger, cavity of the eyeball is the vitreous chamber (posterior
cavity). It lies between the lens and the retina and contains a gel called
the vitreous body. It is formed during embryonic life and is not
replaced thereafter. It holds the retina flush against the back of the eyeball
and helps in maintaining intraocular pressure.
4.
Table
16.1 summarizes the structures associated with the eyeball.
D.
Image
Formation
1.
Image
formation on the retina involves refraction of light rays by the cornea
and lens, accommodation of the lens, and constriction of the
pupil.
a.
The
bending of light rays at the interface of two different media is called
refraction; the anterior and posterior surfaces of the cornea and of the lens
refract entering light rays so they come into exact focus on the retina (Figure
16.11a).
1)
Images
are focused upside-down (inverted) on the retina and also undergo mirror
reversal (Figure 16.11b,c); these inverted images are
rearranged by the brain to produce perception of images in their actual
orientation.
2)
The
lens fine tunes image focus and changes the focus for near or distant objects.
b.
Accommodation
and Near Point of Vision
1)
Accommodation
is an increase in the curvature of the lens, initiated by ciliary
muscle contraction decreasing tension on the suspensory
ligaments, which allows the lens to focus on near objects (figure 16.11c). To
focus on far objects, the ciliary muscle relaxes,
which increases tension on the suspensory ligaments
and the lens flattens.
2)
The
near point of vision is the minimum distance from the eye that an object
can be clearly focused with maximum effort.
3)
With
aging the lens loses elasticity and its ability to accommodate resulting in a
condition known as presbyopia
c.
Refraction
Abnormalities
1)
Myopia is
nearsightedness (Figure 16.12).
2)
Hyperopia is farsightedness (Figure 16.12).
3)
Astigmatism is
a refraction abnormality due to an irregular curvature of either the cornea or
lens.
d.
Constriction
of the pupil means narrowing the diameter of the hole through which light
enters the eye; this occurs simultaneously with accommodation of the lens and functions
to prevent light rays from entering the eye through the periphery of the lens.
2.
In
convergence, the eyeballs move medially so they are both directed toward
an object being viewed; the coordinated action of the
extrinsic eye muscles bring about convergence.
E.
Physiology
of Vision
1.
The
first step in vision transduction is the absorption of light by photopigments (visual pigments) in rods
and cones (photoreceptors) (Figure 16.13).
a.
Photopigments are colored proteins that undergo structural changes
upon light absorption.
b.
The
single type of photopigment in rods is called rhodopsin. A cone contains one of three different
kinds of photopigments so there are three types of
cones.
1)
All
photopigments involved in vision contain a
glycoprotein called opsin and a derivative of
vitamin A called retinal.
2)
Retinal is the
light absorbing part of all visual photopigments.
3)
There
are four different opsins, one for each cone photopigment and another for rhodopsin.
c.
Figure
16.14 shows how photopigments are activated and
restored. Remember, light inhibits the release of an inhibitory
neurotransmitter from the rods. Please refer to your class notes for more
details.
2.
Bleaching
and regeneration of the photopigments accounts for
much but not all of the sensitivity change during light and dark
adaptation. Please refer to your class notes for more details.
3.
Once
receptor potentials develop in rods and cones, they release neurotransmitters
that ultimately induce action potentials in the bipolar cells.
F.
Visual
Pathway
1.
Impulses
from ganglion cells are conveyed through the retina to the optic nerve, the
optic chiasma, the optic tract, the thalamus, and the
occipital lobes of the cortex (Figure 16.16). The optic nerve exits the eye at
the optic disc (blind spot).
V.
HEARING AND EQUILIBRIUM
A.
The
ear consists of three anatomical subdivisions.
1.
The
external (outer) ear collects sound waves and passes them
inwards; it consists of the auricle (pinna),
external auditory canal (meatus), and tympanic
membrane (eardrum) (Figure 16.17)
a.
Ceruminous glands in the external auditory canal secrete cerumen
(earwax) to help prevent dust and foreign objects from entering the ear.
b.
Excess
cerumen may become impacted, causing temporary
partial hearing loss before it is removed.
2.
The
middle ear (tympanic cavity) is a small, air-filled cavity in the
temporal bone that is lined by epithelium. It contains the auditory (Eustachian)
tube, auditory ossicles (middle ear
bones, the malleus, incus,
and stapes), the oval window, and the round window (Figure
16.18). Two muscles involved in
protecting the ear by limiting the movement of the auditory ossicles
are the tensor tympani and the stapedius muscles.
3.
The
internal (inner) ear is also called the labyrinth
because of its complicated series of canals (Figure 16.19). Structurally it consists of two main
divisions: an outer bony labyrinth that encloses an inner membranous
labyrinth.
a.
The
bony labyrinth is a series of cavities in the petrous
portion of the temporal bone.
1)
It
can be divided into three areas named on the basis of shape: the semicircular
canals and vestibule, both of which contain receptors for
equilibrium, and the cochlea, which contains receptors for hearing.
2)
The
bony labyrinth is lined with periosteum and contains
a fluid called perilymph. This fluid,
chemically similar to cerebrospinal fluid, surrounds the membranous labyrinth.
b.
The
membranous labyrinth is a series of sacs and tubes lying inside and
having the same general form as the bony labyrinth.
1)
The
membranous labyrinth is lined with epithelium.
2)
It
contains a fluid called endolymph, chemically
similar to intracellular fluid.
c.
The
vestibule constitutes the oval central portion of the bony labyrinth.
The membranous labyrinth in the vestibule consists of two sacs called the utricle
and saccule.
d.
Projecting
upward and posteriorly from the vestibule are the
three bony semicircular canals. Each is arranged at approximately right
angles to the other two.
1)
The
anterior and posterior semicircular canals are oriented vertically; the lateral
semicircular canal is oriented horizontally.
2)
One
end of each canal enlarges into a swelling called the ampulla.
3)
The
portions of the membranous labyrinth that lie inside the semicircular canals
are called the semicircular ducts (membranous semicircular canals).
e.
The
vestibular branch of the vestibulocochlear
nerve consists of ampullary, utricular, and saccular
nerves.
f.
Anterior
to the vestibule is the cochlea, which consists of a bony spiral canal
that makes almost three turns around a central bony core
called the modiolus (Figure 16.20a).
1)
Cross
sections through the cochlea show that it is divided into three channels by
partitions that together have the shape of the letter Y (Figure 16.20 a-c).
a)
The
channel above the bony partition is the scala vestibuli, which ends at the oval window.
b)
The
channel below is the scala tympani,
which ends at the round window. The scala vestibuli and scala tympani both
contain perilymph and are completely separated
except at an opening at the apex of the cochlea called the helicotrema.
c)
The
third channel (between the wings of the Y) is the cochlear duct (scala media). The vestibular membrane
separates the cochlear duct from the scala vestibuli, and the basilar membrane separates the cochlear
duct from the scala tympani.
2)
Resting
on the basilar membrane is the spiral organ (organ of Corti), the organ of hearing (Figure 16.20, c,d).
3)
Projecting
over and in contact with the hair cells of the spiral organ is the tectorial membrane, a delicate and flexible
gelatinous membrane.
B.
Sound
waves result from the alternate compression and decompression of air molecules.
1.
The
sounds heard most acutely by human ears are from sources that vibrate at
frequencies between 1000 and 4000 Hertz (Hz; cycles per minute).
2.
The
frequency of a sound vibration is its pitch (measured in Hertz, Hz); the greater
the intensity (size) of the vibration, the louder the sound (as measured in
decibels, dB).
3.
Exposure
to loud sounds can damage hair cells of the cochlea and possibly lead to
deafness.
C.
Physiology
of Hearing
1.
The
events involved in hearing are seen in Figure 16.21.
a.
The
auricle directs sound waves into the external auditory canal.
b.
Sound
waves strike the tympanic membrane, causing it to vibrate back and forth.
c.
The
vibration conducts from the tympanic membrane through the ossicles
(through the malleus to the incus
and then to the stapes).
d.
The
stapes moves back and forth, pushing the membrane of the oval window in and
out.
e.
The
movement of the oval window sets up fluid pressure waves in the perilymph of the cochlea (scala vestibuli).
f.
Pressure
waves in the scala vestibuli
are transmitted to the scala tympani and eventually
to the round window, causing it to bulge outward into the middle ear.
g.
As
the pressure waves deform the walls of the scala vestibuli and scala tympani, they
push the vestibular membrane back and forth and increase and decrease the
pressure of the endolymph inside the cochlear duct.
h.
The
pressure fluctuations of the endolymph move the
basilar membrane slightly, moving the hair cells of the spiral organ against
the tectorial membrane; the bending of the hairs
produces receptor potentials that lead to the generation of nerve impulses in
cochlear nerve fibers.
i.
Pressure
changes in the scala tympani cause the round window
to bulge outward into the middle ear.
2.
Differences
in pitch are related to differences in the width and stiffness of the basilar
membrane and sound waves of various frequencies that cause specific regions of
the basilar membrane to vibrate more intensely than others.
a.
High-frequency
or high-pitched sounds cause the basilar membrane to vibrate near the base of
the cochlea.
b.
Low-frequency
or low-pitched sounds cause the basilar membrane to vibrate near the apex of
the cochlea.
3.
Differences
in loudness are related to the numbers of hair cells stimulated.
Hair cells convert a mechanical force
(stimulus) into an electrical signal (receptor potential); hair cells release
neurotransmitter, which initiates nerve impulses.
4.
The
cochlea can produce sounds called otoacoustic
emissions. They are caused by vibrations of the outer hair cells that occur
in response to sound waves and to signals from motor neurons.
D.
Auditory
Pathway
1.
Nerve
impulses from the cochlear branch of the vestibulocochlear
nerve (Figure 14.15) pass to the cochlear nuclei in the medulla. Here, most
impulses cross to the opposite side and then travel to the midbrain, to the
thalamus, and finally to the auditory area of the temporal lobe of the cerebral
cortex.
E.
Physiology
of Equilibrium
1.
There
are two kinds of equilibrium.
a.
Static equilibrium refers to the maintenance of the position of the body (mainly the head)
relative to the force of gravity.
b.
Dynamic equilibrium is the maintenance of body position (mainly the head) in response to
sudden movements, such as rotation, acceleration, and deceleration.
2.
Otolithic Organs: Saccule and Utricle
a.
The
maculae of the utricle and saccule
are the sense organs of static equilibrium; they also sense linear dynamic
equilibrium (Figure 16.22).
b.
The
maculae consist of hair cells, which are sensory receptors, and
supporting cells.
c.
An
otolithic membrane containing otoliths
(crystals of calcium carbonate) sits on top of the hair cells. It slides over the hair cells during linear
movements. The hair cells also provide information about the position of the
head in space. See the p. 553 – 554 for more detailed information.
3. Membranous Semicircular Ducts
a.
The
three semicircular ducts, along with the saccule and
utricle maintain equilibrium (Figure 16.23).
b.
The
cristae in the ampullae
of the semicircular ducts are the primary sense organs of rotational dynamic
equilibrium. Each crista
contains a group of hair cells and supporting cells covered by a gelatinous cupula.
See p. 555 for more detailed information.
3.
Equilibrium
Pathways
a.
Most
vestibular branch fibers of the vestibulocochlear
nerve enter the brain stem and terminate in the medulla; the remaining fibers
enter the cerebellum.
b.
Various
pathways between the vestibular nuclei, cerebellum, and cerebrum enable the
cerebellum to play a key role in maintaining static and dynamic equilibrium.
F.
Table
16.2 summarizes the structures related to hearing and equilibrium.