CHAPTER 17 SPECIAL
SENSES
General senses such as touch and pain were discussed in Chapter
16. The special senses include: smell, taste, vision, hearing, and equilibrium . Receptors are highly 6specialized and located
only in certain areas of the body.
I. OLFACTORY
SENSE (SMELL)
This is a complicated sense that we do not fully understand (we
are just realizing how complicated it really is). It is a chemical
sense--odorant molecules interact with smell receptors, which are located in a
specific area of the nasal epithelium lining the nose (high in the nasal
cavity). Humans have 10 million to 100 million receptors concentrated in about
1 square inch.
Olfactory epithelium (the area of the nasal lining containing
the receptors) contains 3 kinds of cells:
1. Olfactory receptors-bipolar neurons with a knob-shaped
dendrite. Cilia called olfactory hairs protrude from the dendrite and these
hairs are the parts that actually come in contact with and have binding sites
for specific odorant molecules. These cells usually live only about 1 month and
then are replaced.
2. Supporting cells--columnar epithelium of the mucous membrane
lining the nose.
3. Basal stem cells--these lie between the supporting cells.
These are stem cells that are capable of producing new olfactory cells, which
is an exception to the rule of “no new neurons.”
Within the CT deep to the olfactory epithelium are olfactory
(Bowman's) glands, which produce mucus that moistens the epithelium and
provides a medium in which gases dissolve.
PHYSIOLOGY OF SMELL
We now know that there are certain scents that are primary
smell sensations and interact with a particular type of receptor on the plasma
membrane of olfactory receptors specific to that primary sensation. Other olfactory
receptors would react only to their own primary sensation. If the thing being
smelled is a mixture of several primary sensations (most things will be), then
a combination of receptors would react. We are uncertain just exactly how many
different primary smell sensations and therefore how many different types of
specific olfactory receptors exist, but it is probably several hundred or more.
Adaptation occurs rapidly in this sense and the
threshold is very low.
For a substance to be smelled, these steps must occur:
1. Substance must enter a
gaseous state
2. Gaseous particles enter nostrils
3 Particles dissolve
in nasal mucus
4. Molecules attach to
specific receptors in the plasma membrane of their own specific type of
olfactory receptor and this attachment causes depolarization and generation of
a nerve impulse. We now believe that this occurs without the odorant molecule
ever actually entering the receptor cell.
OLFACTORY PATHWAY
Olfactory hairs
¯
Dendrite of olfactory neuron
¯
Olfactory neuron
¯
Axon of olfactory neuron
¯
Axons unite passing through foramina of cribriform plate and form:
¯
Olfactory nerves (I)
¯
Olfactory bulbs--located on ventral aspect of frontal lobes of cortex
¯
Olfactory tracts-deeper
into brain
Limbic
system Thalamus
¯
Frontal lobes
Emotional responses to smell. Temporal lobes of
cortex of cortex
May
evoke strong feelings and memories
(Lateral olfactory area)
and memories
¯ ¯
Conscious perception and
identification
II. GUSTATORY SENSATIONS (TASTE)
This is also a chemical sense--substance being tasted comes in
contact with gustatory (taste) receptors.
These are located in taste buds, which are mostly on the tongue but also
on the soft palate, larynx and pharynx. Each taste bud is an oval body with 3
kinds of epithelial cells:
1. Supporting cells-form a capsule with taste receptors inside
(protection)
2. About 50 gustatory receptor cells (which are not neurons,
but epithelial cells)--a single process called a gustatory hair projects from
the receptor cell through an opening
called a taste pore and comes in contact with the substance being tasted (which
is dissolved in saliva).
3. Basal cells--epithelial cells which produce new gustatory
receptors.
Taste buds are found mostly in elevations on the tongue called
papillae (these are the bumps you can see and feel, the taste buds themselves
are microscopic). There are 4 types of papillae:
1. Vallate (circumvallate)--large, rounded
and form a V on the back of the tongue.
2.
Fungiform--mushroom-shaped and found on the tip and sides of the tongue
3.
Foliate—in trenches on lateral margins of tongue—their taste buds degenerate in
childhood
4.
Filiform--thread-like and cover the anterior surface of the tongue—no taste
buds
Circumvallate and fungiform contain taste buds
PHYSIOLOGY OF GUSTATION
1. Substance (tastant) enters mouth and dissolves in saliva.
2. Dissolved substance touches gustatory hair.
3. Neurotransmitter
contained in gustatory receptor cells is released in response and
generates a nerve impulse in associated sensory neurons.
There are 5 primary taste
sensations:
Sour Sweet Umami (meaty taste)
Salty Bitter
Taste receptors respond to these primary taste sensations in
several different ways, but the end result is always release of
neurotransmitter. Each type of taste receptor reacts most strongly with a
certain one of the primary sensations, but most receptors respond to some
extent to others also. Certain areas of the tongue react most strongly with
each of these.
All tastes other than the 5 primary sensations are combinations
of these 5 plus olfactory sensations. What we call taste is really a little
taste and a lot of smell. Gaseous particles from substances in the mouth pass
up the nasopharynx to reach the olfactory receptors.
Adaptation occurs rapidly, and the threshold varies, with
bitter tastes having the lowest.
GUSTATORY PATHWAY
Gustatory
receptors
¯
Sensory
neurons
¯
Facial VII-anterior 2/3 of tongue
1 of 3 cranial nerves
Glossopharyngeal IX-posterior 1/3
Vagus X-pharynx
¯
Medulla
¯
Thalamus
¯
Primary gustatory area in parietal lobe
of cortex
III. VISUAL SENSATIONS
Ophthalmology is study of the eye.
A. Accessory structures of the eye
1. Eyelids
2. Eyebrows
3. Eyelashes
4. Lacrimal (tear) apparatus
5. Extrinsic eye muscles
1. Eyelids (palpebrae)—protection .
Each eyelid consists of epidermis, dermis, subQ tissue, muscle fibers
(orbicularis oculi), tarsal plate, tarsal glands, conjunctiva.
Tarsal plate--thick fold of CT that gives form and support to
the eyelid. Each tarsal plate contains a row of modified sebaceous glands
called tarsal (Meibomian glands) which produce an oily secretion that prevents
eyelids from sticking together.
The conjunctiva is a thin mucous membrane. Palpebral
conjunctiva lines the inside of the eyelids; bulbar conjunctiva passes from the
eyelids to the anterior surface of the eyeball (conjunctival sac). Irritation
of the conjunctiva causes dilation of its blood vessels-bloodshot eyes.
2. Eyebrows--protection
3. Eyelashes--also protection. Sebaceous glands called sebaceous
ciliary glands or
glands of Zeis are located at the base of the hair follicles of the
lashes and pour a
lubricating fluid into the follicles. Infection of these is a sty.
4. Lacrimal apparatus--structures that produce and drain tears.
Lacrimal glands--1 for each eye located above the lateral
canthus or commissure (canthus = area where upper and lower lids meet,
"corner"). Size and shape of an almond and secretes tears, which
travel through 6 - 12 excretory lacrimal ducts and empty onto the upper
conjunctiva. The tears then travel across the anterior surface of the eye and
drain as shown:
Lacrimal fluid (tears)--watery solution containing salts, mucus,
and an enzyme, lysozyme, which kills bacteria. Tears clean, lubricate and
moisten the eyeball. Blinking helps spread tears across the surface of the eye
and about 1 ml per day is normally produced.
Irritating substances cause increased tear production. This may
result in tears spilling out of the eye, as may also occur if the nasolacrimal
duct is blocked (swelling from a cold, etc.) and in crying.
5. Extrinsic eye muscles--these skeletal muscles attach to the
outer surface of the eyeball and point the eyes in the proper direction. They
are innervated by cranial nerves III, IV, VI.
4 rectus muscles: superior, medial, lateral,
inferior
2
oblique muscles--inferior & superior
B. Eyeball
Hollow sphere, measures about 1" in diameter. Its walls
consist of 3 layers called coats or tunics.
1. Fibrous
tunic--this outermost layer consists of 2 parts:
a. Sclera--the
white of the eye. It is dense CT that gives shape to the eyeball and protects
its inner parts. An opening in the sclera at the back allows the exit of the
optic nerve.
b. Cornea--this
is a thin, transparent, avascular, curved covering of the anterior surface of
the eyeball (1/6). We must have a clear window to allow light rays to enter.
The outer surface of the cornea is an epithelial layer that is continuous with
the bulbar conjunctiva. It contains a large number of nerve endings and is very
sensitive.
The area where cornea and sclera meet is the limbus.
2. Vascular layer
(uvea)--middle layer
a. Choroid--thin dark tissue layer that
lines the posterior 3/4 of the eyeball. It is highly vascular and nourishes the
retina. Dark color is from melanin and allows it to absorb light rays after
they once strike the retina.
b. Ciliary body--as the choroid approaches the anterior
potion of the eyeball, it becomes the ciliary body, which consists of:
1) Ciliary
processes--folds of the ciliary body with capillaries that secrete aqueous
humor
2) Ciliary
muscle--circular band of smooth muscle to which the lens is attached. It alters
the shape of the lens to adjust for near and far vision.
c. Iris--colored portion of the eye
visible through the transparent cornea. It is
shaped like a
flat doughnut and suspended between the cornea and the lens. A central
opening appears black and is called the pupil. Muscles of the iris
adjust the
size of the pupil according to the amount of light -present. The
iris contains 2 layers of muscle fibers:
1) Circular (constrictor) muscles--contract and decrease the diameter of the pupil
2) Radial
(dilator) muscles--contract and increase the diameter of the pupil
3. Nervous tunic (retina)--lines the
posterior 3/4 of eyeball and this is the area where
vision actually begins. The edge of the retina ends in a jagged
anterior margin called the ora
serrata, which is located where the choroid changes to form the
ciliary body. The retina
consists of a pigmented epithelial portion next to the choroid and the
neural (visual) portion.
The neural portion contains
3 layers of neurons:
1)
Photoreceptor neurons
2)
Bipolar neurons
3) Ganglion neurons
2 additional types of neurons, horizontal cells and amacrine
cells, are also found in the retina. Synapses occur between the various neurons
and a good bit of processing of visual information is done in the retina
(before it is sent to the brain). The axons of the ganglion neurons form the
optic nerves.
Photoreceptor neurons are of 2 types, rods and cones. Each
retina contains 6 million cones and 120 million rods. Rods give black &
white vision in dim light, as well as detecting shape and movement. Cones
function only in brighter light and give color vision and sharpest vision.
The exact center of the retina is an area known as the macula
lutea. In the center of it is a small depression, the central fovea. This
contains cones only (no rods) and is the area of absolute sharpest vision. Rods
appear and increase in number towards the periphery of the retina as cones thin
out.
An area in the retina slightly off center (medial) called the
optic disc or blind spot is where the optic nerve leaves the eyeball. In this
small area no vision occurs.
LENS
The interior of the eyeball is divided into 2 cavities
(anterior and posterior) by the lens. The lens is located just behind the pupil
and iris. It is made of layers of special proteins called crystallins, arranged
like the layers of an onion. A normal lens has no blood supply and is
transparent. It is enclosed in a CT capsule and held in place by suspensory
ligaments (zonular fibers) attached to the ciliary body. Its function is focusing
light rays for clear vision.
If the lens becomes cloudy (due to changes in lens
proteins) this is a cataract. Surgery involves removal and replacement of the
lens with an artificial one.
INTERIOR OF THE EYEBALL
1. Anterior cavity--behind the cornea and anterior to the
lens--relatively small area
a. Anterior
chamber---behind cornea and anterior to iris
b. Posterior
chamber---behind iris and in front of lens
The anterior cavity contains a fluid called aqueous humor,
which is continuously secreted by the ciliary processes and flows throughout
the anterior cavity. It drains out through a
venous sinus, the
1)
Nourishes the lens and cornea
2) Contributes
to intraocular pressure, which maintains the shape of the eyeball and keeps the
retina applied to the choroid. Variations in the amount of aqueous humor
present can cause the pressure to vary. If this pressure rises, this is
glaucoma and can lead to blindness.
2.
Posterior cavity (vitreous chamber)—lies between the lens and retina (most of
the area inside the eyeball). This cavity is filled with the vitreous body, a
jellylike substance which forms during embryonic development and is not
replaced. Functions of the vitreous:
1)
Contributes to intraocular pressure—this is constant and doesn't vary
2)
Presses retina firmly against the choroids, which maintains the blood supply
and gives a smooth "screen" for the image to form on
The hyaloid canal is a narrow channel that runs
through the vitreous from the optic disc to the lens. It was occupied by an
artery during fetal development.
MAJOR
EYE PARTS & FUNCTIONS
|
|
REFRACTS AND FOCUSES LIGHT RAYS |
|
IRIS |
REGULATES ENTRANCE OF LIGHT; COLORED PART OF
EYE |
|
PUPIL |
ADMITS LIGHT |
|
CHOROID |
ABSORBS STRAY LIGHT, NOURISHES RETINA |
|
SCLERA |
PROTECTS EYEBALL |
|
CORNEA |
REFRACTS LIGHT RAYS; COVERS IRIS & PUPIL |
|
AQUEOUS HUMOR |
WATER FLUID; FILLS FRONT OF EYEBALL |
|
CILIARY BODY |
HOLDS |
|
RETINA |
CONTAINS RECEPTORS FOR SIGHT |
|
RODS |
BLACK & WHITE VISION IN DIM LIGHT |
|
CONES |
COLOR VISION, SHARP VISION IN BRIGHT LIGHT |
|
OPTIC NERVE |
TRANSMITS IMPULSES TO BRAIN |
|
OPTIC DISC |
EXIT OF OPTIC NERVE, NO VISION |
|
MACULA
LUTEA AND FOVEA CENTRALIS |
AREA OF SHARPEST VISION (ALL CONES) |
|
VITREOUS BODY |
THICK
GELATINOUS MATERIAL THAT FILLS BACK OF EYEBALL |
IMAGE FORMATION
Everything
we see must either give off light (like a light bulb) or reflect light rays.
(No
light rays, no vision). The
cornea and lens focus these light rays on the retina to form a tiny
image of the object. Image formation
involves these basic processes:
1. Refraction (bending) of light rays
2. Accommodation of the lens
3. Constriction of the pupil
1. Refraction of light rays—when light rays pass from one transparent substance to another
transparent
substance of different density, their speed changes and the rays are
bent(refracted). In the eye, light rays are bent as they move
through:
1 ) Cornea
2) Aqueous humor
3) Lens
4) Vitreous body
The eye is designed so that the end result of all this bending
(refraction) is the exact focusing of the light rays on the retina--if this
does not occur, vision will be blurred. From a distant object only the nearly
parallel rays will enter the eye and less refraction is required. From a near
object light rays from many angles will enter and more refraction is
needed. About 75% of refraction occurs
at the cornea. The lens fine-tunes the focus as it changes shape for near and
far vision. The fluids of the eye do only a little refraction.
As light rays are focused on the retina, a tiny image of the object is formed on the retina. The image is upside down (inverted) and right and left are reversed. We perceive the image right-side-up) and right-side-around because early in life the brain learns to turn the visual image.
2. Accommodation of the
lens--the lens must change shape for
focusing rays from
near or far objects (cornea and fluids remain constant, so the
lens is the adjustable part). Clear focus for distant objects requires that the
lens become thinner and less curved. For near objects, it needs to become
thicker, shorter and more curved (see Fig. 17-11 P. 589). A condition called presbyopia occurs when the
lens loses its elasticity with age and cannot accommodate for near vision. It
also may not stretch as well for far vision.
The near point of vision is the minimum distance at which an
object can be clearly focused, using maximum accommodation. This will be about
4 inches in a young person with good vision.
A normal eye that focuses clearly is called an emmetropic eye. Glasses mostly are worn to correct errors of refraction (Fig. 16-12 P. 541).
l) Myopia (near-sightedness)--eyeball longer from
front to back or lens unusually thick. The best efforts of the refraction
system still result in all light rays except those coming from near the eye
being focused in front of the retina.
2) Hypermetropia (far-sightedness)--eyeball shorter
from front to back or lens unusually thin. All light rays except those coming from
a distance are focused behind the retina.
3) Astigmatism--irregular curvature of
cornea or lens
3. Constriction of the
pupil--this adjusts the diameter of the
pupil and regulates the amount of light entering the eye. In bright light, the
pupil constricts and light rays are prevented from passing through the
periphery of the lens, where they cannot be properly refracted. In dim light
the pupil dilates to allow all possible light rays to enter. This tends to blur
vision, but vision is not really sharp with rods anyway.
Refraction by the lens and constriction of the pupil both are
controlled by the intrinsic eye muscles, which are smooth muscle fibers of the
ciliary muscle and iris.
Another important process in human vision is called convergence. In humans both eyes focus on one set of objects--binocular vision. As we look straight ahead at a distant object, this type of vision occurs without special adjustment, but as we move the object closer both eyes must rotate medially so both eyeballs are directed at the object being viewed. This is done by the extrinsic eye muscles.
To achieve what we often refer to as depth perception, we use
two factors related to vision.
1. We judge whether
the eyes are pointed straight ahead or inward to allow both retinas to have the
object focused in the same place on both retinas
2. How curved is the
lens to give us a clear focus on the object.
Putting these two things together, we are able to judge how far
or near the object is.
PHYSIOLOGY OF VISION
An image focused on the retina stimulates the photoreceptor
neurons, which generate a nerve impulse that is passed along the visual
pathway. The photoreceptor neurons are called rods and cones, which are named
according to shape—see Fig. 17.12 P. 590
The light rays forming the image are absorbed by special
pigments of the rods & cones called photopigments. Photopigments are integral proteins in the cell
membranes of the outer segments of the photoreceptor neurons. Photoreceptor
neurons have 2 main parts:
1. Outer segment, which sort of corresponds
to the dendrite since this is where the action begins—outer segments of cones
have their plasma membranes in pleats; outer segments of cones have the pleats pinched off to form a stack of discs.
2. Inner
segment—rest of neuron (cell body and axon)
All phototpigment molecules contain 2 parts, a glycoprotein called
an opsin and a Vitamin A derivative, retinal. When no light is striking a
dendrite, the 2 parts of its photopigment molecules are chemically joined. The
energy in light rays striking a photopigment molecule causes the 2 parts to
break apart and this results in generation of a nerve
impulse.
Four different types of opsin molecules are found in normal
human eyes. Rhodopsin is the opsin of all rods. There are 3 different types of
cones, each
containing one of the following opsins, which responds to light rays of a
certain wave length (color):
Erythrolabe--red light
Chlorolabe--green light
Cyanolabe--blue
light
If any kind of cone is lacking or deficient in number, we do
not see that color.
Eyes
adjust in several ways to the amount of light present: