The special senses a. Visual sensation Accessory structures of the eye



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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).
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).




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

Describe the following:
Photoreceptors -- The outermost zone of neurons, lying next

to the pigmented epithelium, is the layer of photo-receptor 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. outnumber cones 20:1

2. most numerous at periphery of retina (ora

serrata) and decrease toward the center (fovea centralis)

3. 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.


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 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. refraction of light rays

2. accommodation of the lens



3. 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.
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 suspen-sory 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 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.
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 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.
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.
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.

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.

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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