LECTURE 12 EYE
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major structures included in the eye
includes the eyeball, optic nerve, and a number of associated accessory structures including the eyelids, extra-ocular muscles, and the components of the lacrimal apparatus
- a bony structure where they eye is housed
- houses the eyeball, lacrimal gland, and extrinsic muscles of the eye
- protects the eye on all aspects except the anterior side
- anterior opening of the orbit
- bordered by the orbital margin which is subjectively subdivided into supraorbital, infraorbital, medial and lateral borders
seven bones which contribute to the formation of the orbit
- major portions of the superior, inferior, and posterior walls formed by the frontal, zygomatic, maxillary, and sphenoid bones
- medial wall is formed primarily by small portions of the lacrimal, ethmoid and palatine bones
fossa for the lacrimal sac
a small depression within the orbit
4 major openings present at posterior aspect of orbit
- aditus orbitalis
- optic canal of sphenoid bone - a passageway in sphenoid bone that transmits the optic nerve
- superior orbital fissue - a cleft btwn the greater and lesser wings of sphenoid bone, transmits cranial nerves 3,4,6, part of 5 as well as the ophthalmic veins
- inferior orbital fissure - a cleft btwn the greater wing of sphenoid bone and maxilla; transmits maxillary branch of trigeminal nerve and the infraorbital vessels
- a spheroid organ
- structurally, has three named layers: fibrous, vascular and internal tunics (coats)
anterior pole of eyeball
an unmarked reference point at the center of eyeballs anterior aspect
posterior pole of eyeball
the posterior aspect of the anterior pole
axis of the eyeball
an imaginary line connecting the anterior pole and the posterior pole
equator (of eyeball)
a plane perpendicular to the axis
includes the sclera and cornea
- forms the "white" of the eye
- composed of dense, irregular, fibrous connective issue
- less than 1.0mm think over most of it's extent
- the transparent portion on the anterior aspect of the fibrous tunic
- bulges anteriorly w/ a smaller radius of curvature than the sclera
- junction btwn cornea and sclera called limbus of the cornea
limbus of the cornea
the junction btwn the cornea and sclera
cornea's 5 named layers
anterior epithelium, anterior limiting layer, substantia propria, posterior limiting layer, and posterior epithelium
also called uveal tract, has three major components: choroid, ciliary body, and iris
- component of vascular tunic
- forms the posterior part of the vascular tunic and lines most of the sclera
- has four identifiable layers: suprachoroid, vascular choroidocapillary, and basal layers
forms a thickened ring-like structure slightly posterior to the unction of the cornea and sclera
suspend the lens, attach to the margin of the ciliary body, and the ciliary body contains the ciliary muscle
a SMOOTH MUSCLE that controls the thickness of the lens
- immediately anterior to the ciliary body
- a circular, diaphragm-like structure with a central opening, (the pupil)
- size is controlled by two sets of smooth muscles
- the "colored" portion of the eye and it's pigmentation varies among individuals
"of the eyeball" includes the retina, lens, and the chambers (internal spaces)
- part of the internal tunic
- portion of the eyeball that lines the choroid, ciliary body, and posterior aspect of the iris
- the ciliary and iridial parts of the retina have no photoreceptor cells and are histologically termed the blind part of the retina
ora serrata of the retina
an irregular margin which separates the blind part of the retina from the optical part of the retina
optical part of the retina
- consists of 10 named layers...
- Only 2 Shively wants me to know: #9 Layer of Rods and Cones (neuroepithelial layer) and #10 Pigmented layer
- optical part of the retina layer #9 (layer of rods and cones
- the outer, light-sensetive portions of the visual cells.
- Rods allow ONLY black and white vision and function only in LOW LIGHT LEVELS
- Cones are responsible for color vision (including black and white) and require high light intensity
Layer #10 of optical part of retina
- Pigmented layer: a single layer of low cuboidal cells adjacent to the choroid that contain a pigment called FUSCIN.
- the pigmented layer of the retina is the most superficial retinal layer and is the last layer encountered by the incoming ray of light
- a photoreceptor located in the optic part of the retina that generates impulses under dim light conditions resulting in black and white (only) visual images
- give fuzzy images
- the term "rod" technically refers only to the superficial rod-shaped end of the cell where the visual pigment is located
- a photoreceptor located in the optic part of the retina that generates impulses interpreted in all color, including black and white
- require more light than rods to function, responsible for all vision at medium/high light levels
- 3 kinds: some respond to red, some respond to blue, and some respond to green light. NO YELLOW
the lack or dysfunction of one or more of the three types of cones (respond to either red, green, or blue)
what forms the optic nerve which courses to the brain?
the axons of ganglionic cells
disc of the optic nerve
- area where axons of ganglionic cells (which form the optic nerve) leave the retina, which is devoid of visual cells.
- functionally a "blind spot"
- *the vessels from the choroid which supply retina enter through optic disc
- 'of the retina' ; a small area (about 3 x 5 mm) located below and lateral to the optic disc near the posterior pole of the eyeball.
- central portion is fovea centralis
- central portion of macula
- provides the best visual acuity because it is composed almost entirely of cone cells and also because of the inner layers of the retina are absent allowing more direct access of light to the photoreceptors
- a transparent biconvex structure which refracts light and focuses inverted and reversed images onto the retina
- somewhat elastic with natural tendency to become thicker (front to back)
- DECREASED tension allows lens to become THICKER, and vise versa
- has a capsule, an anterior epithelial layer, and is composed primarily of lens fibers which are united by an amorphous ground substance
- what the lens is primarily composed of
- formed by epithelial cells which lose their nuclei and become transparent
- outer ones are firmer and for the lens cortex
- inner fibers are softer and form the lens nucleus
- group of zonular fibers which suspend the lens (at it's equator) from the inner edge of the ciliary body
- these anchor to about 70 tiny projections from the ciliary body termed ciliary processes
deepest layer of the retina
would be layer #1, since "deep" refers to the most medial, or toward the middle of a structure
- opacity (clouding) of the lens
- can be caused by # of different factors
chambers of the eye
anterior chamber, posterior chamber, and vitreous chamber
- anterior chamber is the space btwn cornea and iris, continuous through the pupillary aperture w/ the posterior chamber, which is located btwn the iris and the lens.
- These chambers are filled with aqueous humor
- a clear, watery fluid continually secreted by specialized cells on POSTERIOR ASPECT OF IRIS
- keeps anterior parts of eyeball from collapsing
- reabsorbed from anterior chamber by flowing back into venous system through a number of openings termed scleral venous sinuses
scleral venous sinuses
- "canals of Schlemm"
- venous system which reabsorbs aqueous humor from anterior chamber
- located at periphery of anterior chamber near iridocorneal angle
- interferfence w/ the reabsorption of aqueous humor causes increased intra-ocular pressure due to it's continual production; clinically called glaucoma, can lead to blindness if not corrected
- largest chamber, located btwn the lens and the retina; filled with amorphous gelatinous material called vitreous humor
- encapsulated by a transparent vitreous membrane to form the vitreous body
- the vitreous chamber which is filled with vitreous humor and encapsulated by vitreous membrane
- keeps the posterior portion of the eyeball from collapsing
- is not continuously produced and cannot be replaced if traumatically lost
the visual field projected onto both retinae
the visual field that is perceived by only one eye or the other
define landmarks: anterior and posterior poles (of the eyeball); axis of eye, and equator of eyeball
- the center of it's anterior aspect is an unmarked reference point termed the anterior pole
- the corresponding point of the posterior aspect is the posterior pole
- an imaginary line connecting these poles is the axis of the eyeball
- A plane perpendicular to the axis halfway btwn the two poles intercepts the surface of the eyeball at it's equator
- the process by which the lens changes anterior/posterior thickness in order to properly focus the images of nearby vs. distant objects of the retina
- to focus on nearby objects (up close), lens need to be THICK
- to focus on objects further away (20+ ft), lens needs to be thin
process of lens thickening
- increased thickness occurs by contraction of ciliary muscle in ciliary body, which is innervated by postganglionic parasympathetic fibers which is carried by the oculomotor nerve
- ciliary muscle consists of bundles of smooth m. fibers which form a functional sphincter m. When they contract, they decrease overall diameter of ring-like ciliary body. This reduces tension on zonular fibers, allows natural elasticity of lens to "ball up" and thicken along axis
process of lens thinning
- the ciliary muscle must relax. When it does, diameter of ring formed by ciliary body increases. This increase due to tension of conn. tissues initially stretched by original ciliary muslce contraction.
- When these tissues assume their original position (due to elastic recoil) they cause increased tension on the zonular fibers which pulls lens back into thinner configuration
if accommodation mechanism fails to properly focus images onto the retina...
myopia or hyperopia results
(nearsighted) - image is focused anterior to retina; occurs cause lens gets too thick or eyeball abnormally elongated in anterior/posterior direction. CONCAVE (diverging) lenses for correction. Common aging change related to elongation of eyeball (from re-absorption of the fat located behind it
(farsighted) - focus images behind the retina. Occurs because lens fails to thicken enough during accommodation or in cases where the dydball is too short along it's anterior/posterior diameter. CONVEX (converging) lenses for correction
reduced ability to accommodate due to loss of lens flexibility
what determines the amount of light that strikes the retina
pupillary sphincter muscle
- when it contracts, it makes the pupil smaller
- is a smooth muscle innervated by POSTGANGLIONIC PARASYMPATHETIC FIBERS from branches of oculomotor nerve
- embedded within pupillary margin of iris
pupillary dilator muscle
- causes enlargement of pupil by contracting (SYMPATHETIC), also embedded in tissue of iris
- consists of radially oriented fascicles of smooth muscle originating to the ciliary border of the iris and inserting at the pupillary margin
skeletal muscles associated with the eye include...
4 rectus muscles, 2 oblique muscles, and the levator palpebrae superioris m.
- superior, inferior, medial and lateral; originate via a common anular tendon
- individual contration rotates (tilts) eyeball in the direction indicated by their name
superior oblique muscle
- smallest of eye muscles
- originates near posterior aspect of orbit, courses anteriorly and passes through the trochlea of the superior oblique muscle
- inserts on superolateral aspect of eyeball. It's contraction tilts the visual axis inferiorly and laterally
inferior oblique muscle
- originates from medial aspect of orbit, inserts on lateral aspect of eyeball deep to lateral rectus m.
- contraction moves the visual axis superiorly and laterally
levator palpebrae superioris m.
- does not attach to eyeball. originates from sphenoid bone, inserts into tissue of superior eyelid.
- opens eye by elevating upper eyelid.
- closure of eye effected by contraction of orbicularis oculi m.
muscles of eye
- innervated by 3rd, 4th, and 6th cranial nerves; orbicularis oculi m. innervated by facial nerve (CN 7)
- use memory aid LR6SO4OR7Rest3
- Where LR stands for lateral rectus m., SO stands for superior oblique m., OR stands for orbicularis oculi m., Rest represents other eye muscles
the superior and inferior eyelids
medial/lateral palpebral commissures
the actual attachments of the lids to each other (corners)
- eyelashes; embedded in anterior edges of each eyelid
- have glands associated with follicles (ciliary glands and sebaceous glands) and tarsal glands
"glands of Moll" - sudoriferous (sweat glands)
"glands of Zeis" - sebum (oily/fatty secretion) glands associated with follicles
- "meibomian" - are sebaceous glands that discharge near the posterior edge of each eyelid.
- these oily secretions help to prevent lacrimal secretion from spilling over the palpebral margins
- a modified mucous membrane which covers the posterior aspect of each eyelid
- reflects onto sclera at sharp angle called the conjuctival fornix to form the bulbar conjunctiva
on front of cornea, this conjunctiva forms the outer two of 5 named corneal layers: anterior corneal epithelium and anterior limiting layer
- the potential space btwn the palpebral and bulbar conjunctiva
- the presence of conjunctiva and it's reflection at the fornix prevents a contact lens from getting completely"lost" by wedging itself around behind the eyeball
All eye muscles originate from the posterior aspect of orbit EXCEPT:
- inferior oblique m.
- contractions of various muscles rotates visual axis in direction implied by muscle name
- contraction of the oblique muscles tilts the visual axis slightly
includes the lacrimal gland, lacrimal punctae, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct
- a flattened exocrine gland roughly dime-sized in diameter.
- locatted in conn. tissue of superolateral aspect of orbit, continually secretes water fluid know as lacrimal secretion "tears"
- fluid enters superoplateral aspect of conjunctival fornix, flows over surface of eyeball, distributed by opening and closing eyelids.
superior/inferior lacrimal puncta
as tears are distributed across eyeball, enters sup/inf lactimal puncta on medial margins of eyelids and drains into the lacrimal sac by following through the sup/inf lacrimal canaliculi
- occupies a small depression on medial spect of orbit termed fossa for the lacrimal sac
- sac drains into inferial nasal meatus of nasal cavity through the nasolacrimal duct.
- Lacrimation is normally evaporated from nasal cavity; irritation of eye from trauma and some emotional states can overload lacrimal drainage system and nose "runs"
what major parts of the eyeball develop from neuroectoderm
- each eye begins developing in 4th wk of gestation as outgrowth of n.ectoderm (from prosencephalon)
- this protrusion is called an optic vesicle and remains attached to the prosencephalon by a narrow optic stalk (which forms the optic canal)
what major parts of eye formed from surface ectoderm
forms the cornea, conjunctiva, and lacrimal gland
- a condition in which the visual axes do not converge on the same object.
- termed convergent strabismus in cross-eyed individuals and divergent strabismus ("lazy eye") if affected eye is rotated laterally
- surgical correction often required, involves lengthening or shortening of eye muscles
- caused by separation of retina from surrounding tissues
- can be caused by trauma, neoplasia, or degenerative changes and is sometimes correctable
- an irregular curvature in the cornea or lens which results in distortion of the visual image
- # of types depending on locus of irregular curvature and type of distortion
- often treated with corrective lens
- a localized infection involving the follicle of an eyelash
- painful but not serious
a reduced or lack of visual perception related to developmental anomalies, trauma, or diseases of the eyeballs, optic nerves, or occipital lobes of cerebral hemisphers
complete absence of light perception
- quantitative loss of visual acuity defined by statute
- in US, person is legally blind if neither eye has better than 20/20 vision (after correction) and visual field is only 20 degress of less
refers to abnormally poor vision in dim light conditions
temporary partial blindness related to exposure to exceptionally bright light, often caused by the glare of the sun upon snow
color blindness (3 types)
- due to congenital lack of or nonfunction of one or more of three types of cones in retina
- individuals with normal color vision have all three types of cones and are called trichromatesthose with only two of the cone types are called dichromates
- those with only one functional type of cones are called monochromats and perceive only black, white and shades of gray
- dichromats who are missing red cones
- X-linked traits affecting about 1% of males, rarely females
- dichromates who has dysfunctional green cones
- X-linked trait
dichromasy involving blue cones, very rare
- inflammation of the conjunctiva and may result from traumatic irritation ("something in the eye"), sunburn, or infectious organism
- if caused by microbial infection, termed "pinkeye"
- inflammation of cornea
- may be caused by severe conjunctivitis and may also occur as secondary complication of systemic disease
surgical procedure used to change focal length of eyeball as treatment for visual deficits
- drooping of an upper eyelid (usually caused by dysfunction of oculomotor nerve)
- damage to facial nerve results in inability to close ipsilateral eye due to loss of innervation
- leads to drying of cornea, irritation, and excessive lacrimation
also called keratitis sicca; drying of the cornea
why are rod and cone cells oriented (pointed) away from the incoming light that enters the eye?
- light apparently passes through the layer of rods and cones, strikes the pigmented layer, and then is reflected back into the photoreceptors.
- it's not clear if the receptors get "double" stimulated by this process as some have suggested
Explain how myopia can result from either an eyeball that is too "long" or a lens that remains too thick
- In myopia, the lens brings light rays to focal pint in front of the retina (rather than right on retina) If lens were not as thick, the focal point of light rays would be further posterior (toward the retina)
- Likewise, if the eyeball were "shorter" overall from front to back, the retina and focal point of the light rays could intersect.
- Elongagion of eyeball is normal aging phenomenon due to loss of fat in posterior aspect of orbit. That is why so many older people are myopic
Why are periodic eye exams recommended even for individual with no visual impairment or known eye problems?
increased intra-ocular pressure (glaucoma) can cause serous eye damage before the individual is aware of the problem. Periodic eye exams include a check for this problem
Explain why a crying individual usually has a runny nose
- tears from each eye normally enter lacrimal punctae, drain through the lacrimal canaliculi to the lacrimal sac, and then flow inferiorly through the nasolacrimal duct into inferior nasal meatus where they are evaporated by air movements of respiration.
- during emotional stress, the ANS increases the secretion rate of lacrimal glands. Excess lacrimation exceeds the evaporation rate and the nose runs
why are images in one's peripheral vision not as sharp as those in the central visual field (at normal light levels)?
The central visual field is projected onto the fovea centralis of the retina-an area entirely formed of cones. Peripheral vision images are projected on adjacent parts of the retina where there is a mix of rods and cones. At normal light levels cones are functional but rods are not. Thus, lowered peripheral vision acuity is primarily due to reduced number of functional photorectptors in peripheral visual field. Even at lower light levels (where some rods are functional) the visual acuity is reduced cause they do not produce images as sharp as cones
What keeps contact lenses in place and what prevents them from slipping all the way around behind the eyeball when they are displaced?
Contact lenses tent to stay centered on corneas cause their redii of curvature match. Part of eyeball covered by sclera has larger radius (ex. it flatter). Contact lenses connot get "behind" the eyeball cause the bulbar conjunctiva on it's surface is continuous wth the palpebral conjunctiva lining the syslids. A displaced contact lens is stopped by conjunctival fornix where bulbar conjunctiva meets (and is continuous) with palpebral conjunctiva
Which two anatomic features of the fovea centralis contribute to the visual clarity of the images projected thereon?
The fovea centralis is entirely cones which produce greater visual acuity than rods, and there are fewer retinal layers in the foveal region that light must pass through before activating the cones
functions of aqueous and vitreous humor
- the humors keep the eyeball inflated to it's proper shape.
- The aqueous humor probably serves as nutrient source for some of the living cells of the posterior and anterior chambers
Explain the phrase "so mad he couldn't see straight"
Anger causes a release of epinephrine from adrenal medulla. This , in turn, results in a generalized sypathomimetic response including pupillary dilation. The pupil is actually dilated too much and allows too much light to strike the retina. Over dirves the cones and reduces visual acuity
In addition to the photoreceptors, how many other layers of cells are found in the 10 layers of the retina?
Two. There are three layers of cells (total) in the retina. Their cell bodies make up the retinal layers termed ganglionic layer, internal nuclear layer, and external nuclear layer
Is the retina limited to vitreous chamber
No. The back side of iris is covered by retina too. This iridial part of the retina, as well as the retina covering the back side of the ciliary body, has no photoreceptors
What is the physical relationship of the "blind spot" to the fovea centralis?
The "blind spot" is the disc of the optic nerve. It is located slightly above and medial to the fovea centralis
In theiry, which eye muscles could be eliminated without seriously impairing eye movements?
One could logically produce all necessary eye movements with the rectus muscles alone. The other two oblique muscles seem superfluous
How would unilateral dysfunction of an abducens nerve affect the eyes?
Unilateral dysfunction of an abducens nerve will paralyze the ipsilateral lateral rectus m. Tone in the medial rectus m. of that eye will draw the visual axis medially. The individual would suffer convergent strabismus (will be cross-eyed)
What special purpose do sebaceous glands located at the margins of the eyelids serve?
The oily secretion of these glands coats the palpebral margins (edges of the lids) and prevents tears from spilling over them (normally)
What three fluids must be transversed by light with reaches the retina?
Lacrimal secretion, aqueous humor, and vitreous humor
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