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THE SPECIAL SENSES
A.
VISUAL SENSATION
1.
ACCESSORY STRUCTURES OF THE EYE
Identify the various features of the surface of the eye
Name the accessory structures of the eye.
The accessory structures of the eye include the eyelids, eyelashes,
eyebrows, lacrimal apparatus, and the extrinsic eye muscles.
What is the lacrimal apparatus?
The lacrimal apparatus is a group of structures that produces and
drains tears.
Describe the following:
Lacrimal glands -- The lacrimal glands, which produce tears, each
about the size of an almond, lie between the skin and bone
of the upper lateral eye. Each gland empties 6 - 12
excretory lacrimal ducts onto the surface of the conjunctiva
of the upper lateral eyelid.
Lacrimal puncta -- The tears are swept medially across the eye by
blinking of the eyelids to enter two small openings at the
medial eye called the lacrimal puncta.
Pathway of flow -- From the puncta, tears flow through the lacrimal
canals into the nasolacrimal duct, which empties into the
nasal cavity.
Tears -- Tears contain water, salts, and a bactericidal enzyme
called lysozyme. These components function to clean,
lubricate, and moisten the surface of the eyeball.
2.
ANATOMY OF THE EYEBALL
Describe the gross anatomy of the eyeball.
The adult eyeball is 1" in diameter, with 1/6 of its surface exposed
to the outside. The rest is recessed within the bony orbit and
packed in adipose tissue. The wall of the eye is divided into three
layers: from the outside in, the fibrous tunic, vascular tunic, and
nervous tunic (retina).
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a.
FIBROUS TUNIC
Describe the following:
Fibrous tunic -- The fibrous tunic is the outermost portion of
the eyeball and consists of the anterior cornea and
the posterior sclera.
Cornea -- The cornea is a nonvascular, transparent layer
that covers the iris. Because it is curved, it helps to
focus light onto the retina. It is covered exteriorly by a
layer of simple epithelium that becomes continuous
with the conjunctiva lining the eyelids.
Sclera -- The sclera, continuous with the dura mater, is a
coat of dense connective tissue that covers all of the
eyeball except the cornea, giving it shape and rigidity
and protecting its inner parts. The posterior surface
of the sclera is pierced by the optic nerve as it passes
through the optic foramen of the orbit.
Canals of Schlemm -- Along the circumferential junction of
sclera with cornea are the canals of Schlemm, venous
sinuses that drain aqueous humor from the anterior
chamber. This drainage is vital to the maintenance of
intraocular pressure. A build-up in intraocular
pressure leads to glaucoma.
b.
VASCULAR TUNIC
Describe the following:
Vascular tunic -- The vascular tunic is the middle layer of the
eyeball and consists of the choroid, ciliary body, and
iris.
Choroid -- The choroid is highly vascular and lines most of
the inner surface of the sclera. It provides nutrients to
the retina and contains melanocytes that produce
melanin.
Ciliary body -- The anterior extension of the choroid is the
ciliary body, a structure that contains the ciliary
muscle, smooth muscle that is used in the process of
accommodation (focusing the lens for near vision).
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Ciliary process -- On the surface of the ciliary body are folds
called the ciliary processes. They secrete aqueous
humor into the posterior chamber of the eye.
Iris -- The anterior-most extensions of the eyeball is shaped
like a flattened doughnut. It is suspended between
the cornea and the lens by its attachments at its outer
margins to the ciliary body.
Iris muscles -- In addition to its pigments, the iris consists of
the sphincter pupillae and the dilator pupillae smooth
muscles that alter the shape of the pupil, the hole in
the center of the iris through which light passes to
enter the eye.
Size of pupil -- Parasympathetic innervation stimulates the
sphincter pupillae to contract, causing the pupil to
constrict and therefore the amount of light entering
the eye to decrease.
Sympathetic innervation stimulates the dilator pupillae
to contract, causing the pupil to dilate and therefore
the amount of light that enters the eye to increase.
c.
RETINA (NERVOUS TUNIC)
Describe the following:
Nervous layer – The third and innermost coat of the eyeball
is the retina or nervous tunic. It lines the posterior ¾
of the eyeball and is the beginning of the visual
pathway of neurons.
The retina consists of an outer pigmented epithelium
and an inner nervous layer formed by neurons.
Pigmented layer – The pigmented layer contains
melanocytes that produce melanin that functions to
absorb stray light rays and prevent reflection and
scattering of light within the eye.
Nervous layer – The nervous layer contains three zones of
neurons:
1.
photoreceptors
2.
bipolar neurons
3.
ganglion cell neurons
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Describe the following:
Photoreceptors -- The outermost zone of neurons, lying next
to the pigmented epithelium, is the layer of photoreceptor cells, the rods and cones. Photoreceptors
respond to light energy by decreasing their secretions
of the inhibitory neurotransmitter glutamate and
therefore initiate the visual pathway.
Rod cells –
1.
2.
3.
outnumber cones 20:1
most numerous at periphery of retina (ora
serrata) and decrease toward the center (fovea
centralis)
responsible for peripheral, black-and-white,
and low-light vision
Cone cells –
1.
most numerous at the center of the visual axis
(fovea centralis) and decrease towards the
periphery
2.
stimulated by bright light only
3.
function in color vision and visual acuity
Bipolar cells -- The middle zone of neurons is the bipolar cell
layer. Bipolar neurons, once their inhibition by
glutamate is removed, initiate action potentials to start
the visual pathway. Also in the middle zone are
amacrine and horizontal cell neurons that modify the
signals.
Ganglion cells -- The third zone, and innermost portion of the
retina, is the ganglion cell layer, consisting of ganglion
cells that receive action potentials from the bipolar
neurons.
Optic disc -- The axons of ganglion cells converge on the
optic disc and exit the eyeball as the optic nerve.
Their point of exit is called the “blind spot” since no
photoreceptors can be found there.
d.
LENS
Describe the following concerning the lens of the eye:
Location -- Just behind the pupil and iris, within the cavity of
the eye, is the nonvascular biconvex lens.
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Crystallins -- The lens consists of proteins called crystallins.
They are arranged like an onion, with all of the
crystallin fibers lying in parallel, to make the lens
perfectly transparent.
Suspensory ligaments -- The lens is enclosed in a clear
connective tissue capsule and held in place by
suspensory ligaments that are attached to the capsule
and the ciliary body.
Function -- The lens, by changing its biconvexity, is used to
focus light rays on the fovea centralis of the retina as
the distance between the object and the retina
changes.
e.
INTERIOR OF THE EYEBALL
Describe the following:
Anterior cavity --The interior of the eyeball is a large space
divided by the lens into two cavities: the anterior and
posterior cavities.
The anterior cavity, the space anterior to the lens, is
further subdivided into two chambers, the anterior and
posterior chambers.
Chambers -- The posterior chamber lies behind the iris and
in front of the suspensory ligaments and lens.
The anterior chamber lies behind the cornea and in
front of the iris.
Aqueous humor -- The anterior cavity (both chambers) is
filled with a watery fluid called the aqueous humor
that is continually secreted by the ciliary processes
behind the iris
Fluid flow -- The fluid flows from the posterior chamber
forward between the iris and lens, through the pupil,
and into the anterior chamber. Aqueous humor is
then drained from the anterior chamber into canals of
Schlemm at the border between the cornea and
sclera, and thus into the blood.
Fluid pressure -- The fluid pressure exerted by aqueous
humor is called the intraocular pressure. This
pressure helps to maintain the shape of the eyeball
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and to keep the retina smoothed out on the surface of
the choroid for reception of clear images. Excessive
intraocular pressure leads to glaucoma.
Describe the following:
Posterior cavity -- The second and larger cavity of the
eyeball is the posterior (vitreous) cavity. The
posterior cavity lies between the lens and the retina
and contains jellylike fluid called the vitreous humor
(body).
Vitreous humor -- Vitreous humor contributes to intraocular
pressure, helps prevent the eyeball from collapsing,
and holds the retina flush against the choroid. Unlike
the aqueous humor, the vitreous humor is not
continually produced; once formed it is not replaced.
3.
IMAGE FORMATION
Formation of images on the retina involves three processes. Name them.
1.
2.
3.
refraction of light rays
accommodation of the lens
constriction of the pupil
What are the intrinsic muscles of the eye?
Smooth muscle fibers of the ciliary body and the iris (sphincter
pupillae and the dilator pupillae)
a.
REFRACTION OF LIGHT RAYS
What is refraction?
When light rays traveling through a transparent medium,
such as air, pass into a second medium with a different
density, such as water, they bend at the junction between
the two. This process is known as refraction and is essential
to proper vision.
Where is light refracted in the eye?
As light enters the eye, it is refracted by the anterior, then
posterior surfaces of the cornea (75% of total refraction).
Then, both sides of the lens further refract the light so that
they come into exact focus on the cone cells of the fovea
centralis.
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b.
ACCOMMODATION AND NEAR POINT OF VISION
Describe the process of accommodation and how the lens changes
its focal length depending upon the distance an object is from the
eye.
The lens of the eye is biconvex, causing the light rays
passing through it to be refracted towards each other so that
they eventually intersect.
The point of intersection is the point of fine focus and is on
the fovea centralis in the normal eye.
By changing the shape of the lens, more or less biconvex,
the point of intersection changes. In this way we focus on
objects at different distances from our eye so that the point
of focus is always on the fovea centralis.
The focusing power of the lens increases as its curvature
becomes greater. Near objects = more convex, far objects =
less convex.
When the lens is focusing on a near object, the lens become
more convex. This increase in its curvature for near vision is
called accommodation.
When viewing distant objects, the ciliary muscle is relaxed
and dropped away from the lens. As a result, the suspensory ligaments attached to the lens are pulled taut and the
lens is pulled to a less convex shape.
When focus shifts to a near object, the ciliary muscle
contracts, the suspensory ligaments are relaxed, and the
lens resumes its more natural more convex shape.
c.
CONSTRICTION OF THE PUPIL
Why does the pupil constrict reflexively as a part of the
accommodation reflex? What purpose is served?
Part of the accommodation reflex is a constriction of the
pupil, a narrowing of pupil diameter so that less light enters
the eye. This prevents light rays from the periphery of the
visual field from entering the eye. As a result, peripheral
light would not be focused on the retina. If this happened
during near vision the result would be blurred vision. Also as
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a result, peripheral vision is greatly diminished during near
vision.
4.
CONVERGENCE
What is single binocular vision?
Both eyes focus on one set of objects, a characteristic known as
single binocular vision. It allows us to see one image with two
eyes.
What are corresponding points of the retina?
Corresponding points are photoreceptors in the two eyes which are
complementary to one another and which send visual information to
the same place in the cerebral cortex.
Describe the process of convergence of the eyes?
As the distance between an object and our eyes decreases, our
eyes must rotate medially to maintain focus on corresponding
points. This medial rotation is called convergence.
5.
PHYSIOLOGY OF VISION
a.
PHOTORECEPTORS AND PHOTOPIGMENTS
b.
RECEPTOR POTENTIAL AND NEUROTRANSMITTER
RELEASE
Describe photoreceptors and the mechanism by which they inhibit
bipolar cells when they are at rest and excite bipolar neurons when
they are stimulated.
An image focused on the retina stimulates photoreceptors,
which transduce the light stimuli into receptor potentials then
pass the information to bipolar neurons.
Rod and cone cells are named for the appearance of their
outer segment, the distal end of the cells next to the
pigmented epithelium.
The outer segments are filled with specific visual pigments
that absorb light rays as they pass through the cells.
Under dark conditions, Na+ channels are open in the
photoreceptor membrane, causing continual release of the
inhibitory neurotransmitter glutamate onto the bipolar
neurons.
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Glutamate, in turn, causes hyperpolarization of the bipolar
neurons, thus inhibiting them.
When light rays strike the visual pigments, they undergo
structural changes that cause the Na+ channels to close.
As a result, the photoreceptors no longer release glutamate
and the bipolar neurons are therefore no longer inhibited;
instead, they become stimulated.
Excitation of the bipolar neurons in this way result in the
formation of an action potential and the inhibition of the
visual pathway.
6.
VISUAL PATHWAY
Where does the visual pathway begin?
The visual pathway begins with stimulation of the bipolar neurons
by photoreceptor cells.
Describe the relationship between rods and bipolar cells?
Depending on location in the retina, 6 – 600 rod cells converge on
one bipolar neuron. They also converge on the association neurons
(amacrine and horizontal cells). As a result, information from many
rod cell leads to a summative effect and vision that is not sharp.
Describe the relationship between cones and bipolar cells.
Cone cells have a 1:1 relationship with bipolar cells, resulting in
sharpness of vision.
Describe the visual pathway.
From (1) bipolar neurons, action potentials are passed to the (2)
ganglion cells of the retina. Axons of ganglion cells are collected
together on the surface of the eyeball at the optic disk as the (3)
optic nerve (II). The optic nerve travels on the inferior surface of
the brain to a structure known as the (4) optic chiasma, a crossing
point of the optic nerves. All axons of ganglion cells originating on
the lateral half of a retina remain ipsilateral, passing through the
optic chiasma without decussating. Exiting from the optic chiasma
are the (5) optic tracts, the right tract carries information from the
right lateral and left medial eye and the left tract carrying
information from the left lateral and right medial eye. The optic
tracts pass into the thalamus, and synapse there. From the
thalamic nuclei come axons collected together as the (6) optic
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radiations, which pass to the (7) primary visual areas in the occipital
lobes of the cerebral cortex.
B.
AUDITORY SENSATIONS
1.
EXTERNAL (OUTER) EAR
Describe the external ear.
The external (outer) ear collects sound waves and passes them
inward towards the middle ear. It consists of the auricle (pinna), the
external auditory meatus (canal), and the tympanic membrane.
What is the tympanic membrane?
The tympanic membrane is a thin, semi-transparent fibrous
membrane that separates the external ear from the middle ear.
What are ceruminous glands?
Near the external opening of the meatus, the skin is rich with hairs
and ceruminous glands that secrete cerumen, a waxy antimicrobial
substance that provides nonspecific resistance to disease
organisms that could enter the body through the ear canal.
2.
MIDDLE EAR
Describe the middle ear.
The middle ear (tympanic cavity) is a small, air-filled, mucous
membrane-lined cavity carved from the petrous portion of the
temporal bone. It is separated from the external ear by the
tympanic membrane and from the internal ear by a thin bony
partition that contains two membrane-covered “windows”.
What are mastoid air cells?
The posterior wall of the middle ear communicates with the mastoid
air cells of the mastoid process of the temporal bone.
What is the auditory tube?
The anterior wall of the middle ear communicates with the
nasopharynx through the auditory (Eustachian) tube. It functions to
equalize the air pressures between the atmosphere and the middle
ear cavity, ensuring the free movement of the tympanic membrane
as it vibrates.
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Describe the auditory ossicles and their articulations. Describe the
locations of the oval and round windows. What are the tensor tympani
and stapedius muscles?
Three tiny bones known as the auditory ossicles extend across the
middle ear and are attached to it by ligaments.
The malleus (hammer) is attached to the internal surface of the
tympanic membrane laterally and to the incus (anvil), via a synovial
joint, medially.
The incus, in turn, articulates with the stapes (stirrups), the
footplate of which fits into a membrane covered opening in the
partition between the middle and the internal ears called the oval
window.
Just below the oval window is a second membrane covered
opening in the bony partition called the round window.
Inserted into the ossicles are two muscles, the stapedius and
tensor tympani, which serve to increase the tension of the ossicles
against each other and the tympanic membrane.
These muscles reflexively contract as a result of loud noises, thus
dampening the sound before it is passes on to the internal ear.
Listening to loud music for a long time causing tetany in these
muscles, so you feel like you can’t hear after the music stops
3.
INTERNAL (INNER) EAR
In general, describe the internal ear. What is the bony labyrinth and what
are its components?
The internal (inner) ear is also called the labyrinth (maze) because
of its complicated series of canals and ducts.
Structurally, the internal ear consists of two main divisions:
1.
an outer bony labyrinth
2.
An inner membranous labyrinth
The bony labyrinth is a series of three cavities carved from the
petrous portion of the temporal bone:
1.
three semicircular canals
2.
the vestibule
3.
the cochlea
The three semicircular canals are placed at right angles to each
other in the frontal, horizontal, and sagittal planes.
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The bony labyrinth is lined with periosteum and is filled with a fluid
known as perilymph.
Describe the membranous labyrinth. What are its parts?
Perilymph surrounds the membranous labyrinth within the bony
labyrinth.
The membranous labyrinth is a membrane bound series of tubes
and sacs filled with a fluid called the endolymph.
Within the bony semicircular canals are the membranous
semicircular ducts, which communicate with the vestibule.
The bony vestibule contains the membranous vestibule, which is
subdivided into two smaller sacs, the saccule and the utricle.
Anterior to the vestibule, lying within the bony cochlea, is the
membranous cochlear duct.
Describe the cochlea. What are its parts?
Cross sections of the cochlea show that it is subdivided into three
channels in the shape of the letter “Y”. The wings of the “Y” are
formed by the membranous labyrinth.
Above the bony partition forming the stem of the “Y”, called the
modiolus, is the scala vestibuli, filled with perilymph.
Below the modiolus is the scala tympani, which is also filled with
perilymph.
Between the wings of the “Y”, within the membranous labyrinth, is
the scala media or cochlear duct, filled with endolymph.
The vestibular membrane separates the cochlear duct from the
scala vestibuli and the basilar membrane separates the cochlear
duct from the scala tympani.
Resting on the basilar membrane is the spiral organ of Corti, the
organ of hearing. It is a sheet of epithelium consisting of support
cells and about 16,000 hair cells, the receptors for hearing.
Projecting over and in contact with the stereocilia of the hair cells is
the tectorial membrane, a delicate and flexible membrane
suspended within the endolymph.
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4.
PHYSIOLOGY OF HEARING
In a step-wise fashion, describe the events of hearing.
The auricle directs sound waves into the external auditory canal so
that they can strike the tympanic membrane.
The tympanic membrane vibrates, causing the malleus, incus, and
then the stapes to vibrate.
The footplate of the stapes moves like a piston in the oval window,
forming pressure waves in the perilymph of the scala vestibuli.
This, in turn, causes deformation of the vestibular membrane of the
cochlear duct, forming pressure waves in the endolymph of the
scala media.
These pressure fluctuations cause the basilar membrane to move
slightly, moving the stereocilia of the hair cells against the tectorial
membrane.
Deformation of the hairs initiates action potentials that pass into the
cochlear portion of the cranial nerve VIII and thus pass to the
primary auditory areas of the cerebrum.
Highest frequencies move the basilar membrane at the base of the
cochlea and lowest frequency sounds move the basilar membrane
at its apex.
5.
PHYSIOLOGY OF EQUILIBRIUM
What is static equilibrium?
Static equilibrium refers to the maintenance of body position
(mainly the head) relative to the force of gravity. It is the perception
of orientation of the head when the body is stationary.
Describe the receptors for this sense?
Hair cell receptors for this are found in the utricle and saccule within
the vestibule of the bony labyrinth. Overlying the hair cells are
heavy calcium carbonate structures called otoliths into which the
tips of the stereocilia are embedded.
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How does this system work?
When the head moves, the otoliths remain motionless (due to
inertia), while the hair cells are moved beneath them. Movement of
a hair in a given direction initiates an action potential that is carried
by the vestibular portion of cranial nerve VIII to the brain, where it is
interpreted.
What is dynamic equilibrium?
Dynamic equilibrium refers to the maintenance of body position
(mainly the head) in response to motions such as rotation,
acceleration, and deceleration.
Describe the receptors for this sense.
Hair cell receptors for this are located at the origins of the
semicircular ducts in areas called the ampullae. Overlying the hair
cells are masses of gelatinous material, each called cupula.
How does this system work?
When the head moves, the endolymph moves the hair cells against
the cupula, which remains in place due to inertia. Deformation of
the hair cells in a particular direction initiates an action potential that
is passed via the vestibular portion of cranial nerve VIII into the
brain.
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