Eyeball: Hollow sphere
- 3 coats
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- Fibrous outer coat
- Sclera: tough, dense fibrous coat that covers 5/6 of eyeball
- Cornea: continuous with sclera, transparent
- Vascular middle coat—three components are continuous
- Choroids: highly vascularzied, most richly vascularized structure in body
- Ciliary body: donut shaped, surrounds lens
- Ciliary zonule of Zinn: radial fibers from ciliary body to lens
- Suspends lens
- Works with zonular fibers—radial (suspensory ligament of lens)
- See more about function in accommodation of lens section below
- Ciliary zonule of Zinn: radial fibers from ciliary body to lens
- Ciliary processes projects from chillier body: produces aqueous humor
- Humor flows from posterior to anterior chamber thru pupil
- Nourishes and bathes cornea and lens (avascular)
- Liquid flows between iriscornea angle→venous channels (sinus venosus sclera, canal of schlemm)
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- Channels make circle around perimeter of cornea
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- Fluid is replenished every 90 minutes
- Iris: surrounds pupil (hole=aperture of camera)
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- Controls amount of light entering by changing shape and size of pupil
- Contraction controlled by inferior division of oculomotor (parasymp)
- Goes thru ciliary ganglion
- Dependent on circular, concentric fibers
- Dilation: sympathetic control (long ciliary nerves)
- Dependent on radial fibers
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- Fibrous outer coat
- Neural inner coat: retina—posterior 5/6
- Retina has two different layers: developed separately
- Pigmented layer: fused with choroids
- Neural layer: picks up light rays
- Axons of ganglion cells travel towards optic disk
- Connect to bipolar cells→rods and cones
- Detached retina: separation of two layers
- Happens to prizefighters
- Continuous with optic nerve at optic disk/papilla:
- In the middle: blind spot with no photoreceptors
- With the optic nerve, find vessels (arteries are branches of ophthalmic artery)
- Travels within dura
- Central artery of retina pierces dura, arachnoid, pia to enter substance of optic nerve
- Macula lutea (lateral to optic disk): yellow spot
- Pit: Fovea centralis—only has cones
- Area of acute vision
- Pit: Fovea centralis—only has cones
- Retina has two different layers: developed separately
- Vitreous humor: holds retina in place (primary function)
- Vitreous body: 2/3-3/4 of back of eye
- Transmits light to retina
- Pushes against lens
- 99% H2O (not replenished) + collagen fibers (replenished)
- Separates and dries out over life
- Retina can peel away→spots in vision
- Lens, suspensory ligament (zonular fibers)
- Lens is normally clear
- Can get cloudy as you age (cataract)
- Treatment: replace lens by sucking out old eye with needle
- Fibers changes shape of lens by contraction (more info below in accommodation of lens section)
- Lens is normally clear
- Common lesions
- Papilldema: swelling of optic disk due to increase in CSF pressure
- Disruption of arachnoid granulations
- Closes off ophthalmic veins→doesn’t allow blood to drain out→veins swell up (detectable)
- Conjunctivitis: inflammation of conjunctiva
- Glaucoma: elevated pressure of aqueous humor in anterior chamber
- Block of iriscornea angle
- Cornea bulges
- Lens pushed backwards, pushes on vitreous body→retinal vessels obstructed→death of retina→blindness
- Papilldema: swelling of optic disk due to increase in CSF pressure
- How image is made in the retina?
- Image formed by refraction (bending) of light rays
- Rays converge on retina
- Parameters of refraction
- Refractive index: related to density
- Change in density across interface affects speed of light and angle of refraction
- Refractive index: related to density
- Angle of incidence of light rays
- Affected by curvature of interface
- Image formed by refraction (bending) of light rays
- 3 surfaces where refraction occurs
- Cornea: air/cornea interface
- Cornea: curvature is constant (therefore index is constant)
- Can use laser surgery to shave off cornea to change curvature to change refractive index
- Most refraction occurs here because biggest difference in indices
- Lens (anterior): aqueous/lens interface
- Lens changes shape: different indices
- Lens (posterior): lens/vitreous interface
- Parameters of total refractive power
- Refraction at all 3 surfaces
- Distance between cornea and lens
- Refractive index of aqueous humor
- Not additive
- Cornea: curvature is constant (therefore index is constant)
- Cornea: air/cornea interface
Accommodation
- Lens: gel substance in capsule
- More rounded on back
- Changes in lens curvature allows rays to focus on retina
- Contraction of ciliary muscles under parasymp ctrl (CN III)
- Circular ciliary muscles contract→zonular fibers are relaxed→lens capsule are relaxed→lens deform toward spherical shape: bend light more, see closer objects (parasymp)
- Circular ciliary muscles relax→zonular fibers are taut→lens capsule stretches→lens is flattened (symp)
- Lens elasticity and accommodation decreases with age: presbyopia
- Become near sighted and far sighted
- Need gradual lenses
Iris
- Pupil contracts (sphincter pupillae) to limit light entrance to central of lens
- Increases depth of field for near objects
- Contraction: parasympathetic (CN III)
- Dilation: sympathetics
Eye lids:
- Tarsal plate (superior / inferior tarsus):
- The core of the eyelid
- Held in place to Lateral and medial side of the orbit by lateral / medial palpebral ligaments.
- Anywhere have an opening, the Orbital Septum fills in the space
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- Attaches eyelids to rib of orbit
- Is made of fascia of tarsal plates
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- Is continuous with periorbita
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- Some muscles attach directly to upper / lower edge of tarsal plates
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- Superiorly: Superior tarsal muscle (Miller’s muscle)
- Inferiorly: inferior tarsal muscle
- These are smooth muscle structures
- These raise upper eye lid / lower the lower eyelid.
- Increase the size of the palpebral fissure (open the eyelids)
- Levator Palpebrae Superioris
- On superior aspect of the eye, passing deep to frontal bone
- Tendons of attachment pass out to the skin of the eyelid
- Interdigitates with circular muscle in upper eye lid, called Ovicularis Oculi (palpebral part)
- Elevates the upper eyelid
- Some fibers come off inferior aspect and attach to the superior tarsal plate
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- This defines the superior tarsal muscle
- superior tarsal muscle originates from levator palpebrae superioris and inserts in the superior tarsal plate
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- Tarsal plate: Contains tarsal glands
- Secretion prevents lids from adhering to eachother
- Is a sebaceous gland (modified meiobian gland)
- Orbital septum: allows levator palpebrae fibers to pass through, elsewhere it acts as a barrier
Conjunctiva:
- Serous membrane that lines the inner aspect of the eye and part of the eye itself
- Provides thin film of moisture that allows lids to slide over eye w/o irritation
- Has 2 parts
- Palpebral portion: lines the inner aspect of the eyelids
- Bulbar portion: on the eyeball from the fornices to the corneoscleral junction.
- Stops at cornea
- At center of eye
- Covers 1/6 of the eye
- Not covered by the conjunctiva
- Remainder white portion of eye (non-cornea surface) is the sclera
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- Where the two parts of the conjunctiva come together:
- Superior / inferior fornices
- This is where palpebral conjuntiva reflects onto eye to become bulbar conjunctiva
- Innervation: All from CN V (trigeminal)
- Palpebral portion: upper lid from V1, lower lid from V2
- Bulbar portion: from V1 (ophthalmic)
Fat: inside the orbital around the optic nerve is either intraconal fat or extraconal fat
Orbital Fascia:
- Muscles of the eye have fascia
- Deep fascia thickens as approach the eye
- Wraps onto the eye and wraps back onto optic nerve
- Tenon’s capsule
- This is the fascial sheath derived from extraoccular muscles
- Thin fascia that envelops eye
- Extends from optic nerve to corneoscleral junction
- Perforated by tendons of extraoccular muscles (EOMs)
- Continuous with deep fascia of EOMs
- Recent studies:
- Show that the deep fascia of the EOMs not only extends onto the eye, but it also attaches to the walls of the orbit
- As a result, the pull (or functional origin) of these muscles is as much on the orbital walls as at the muscle origins
- This is important for surgical procedures to correct amblyopia (lazy eye)
- Thick portion of fascia extends to medial / lateral walls
- Called Medial / Lateral Check Ligaments
- These limit inward / outward motion of the eye
- They are strong expansions of the fascial sheaths of the horizontal recti that attach the bony orbit.
- Not the same as palpebral ligaments
- Suspensory ligament (lockwood) of the eye:
- Blended fascial sheaths of the inferior oblique and inferior rectus muscles
- Is a sling-like ligament for inferior aspect of eye
- Are continuous with Tenon’s capsule and thus attach to the check ligaments
- This creates a continuous fascial hammock below the eye
Lacrimal Apparatus
- The lacrimal gland is in the lateral / anterior / superior aspect of the orbit (laterally behind the upper eyelid just inside the orbit)
- Flow of tears from lacrimal gland to nasal cavity: Pathway
- Excretory ducts of lacrimal gland (about 12) in the superior conjunctival fornixà
- Wash over bulbar conjunctiva / corneaà
- Gather in the lacrimal lakeà
- As you blink, flows into openings within the medial edges of upper / lower lid called lacrimal puncta (as they touch lacrimal lake)→
- Fluid enters lacrimal canaliculi→ empty into lacrimal sac → leads to canal that contains the nasolacrimal duct→
- Leads to the inferior nasal cavity- into the inferior nasal concha, which is continuous with the inferior nasal meatus which finally receives tears.
- Innervation of the lacrimal gland (3 types)
- Parasympathetic:
- CN VII (Facial Nerve) contains preganglionic parasympathetic fibers that leave facial nerve as the Greater Petrosal Nerve
- Greater Petrosal nerve passes through pterygoid canal→ leads to pterygopalantine ganglion (a parasympathetic ganglion in floor of orbit)
- In this ganglion, preganglionic fibers in the greater petrosal nerve synapse and form post ganglionic parasympathetic fibers→ rejoin V2 (maxillary nerve) for short time→ jump onto zygomatic nerve
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- From zygomatic nerve→ travel on the lacrimal nerve→ to the lacrimal gland.
- These are secretomotor for the gland- cause secretion of tears.
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- Sympathetic innervation:
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- Preganglionic cells from upper thoracic spinal cord → use chain
- Post ganglionic cells are in superior cervical ganglion→ jump onto internal carotid artery to form sympathetic plexus→ leave carotid plexus on the deep petrosal nerve→ pass right through the pterygopalantine ganglion (no synapse)à take similar path as parasympathetic and end up on the vasculature of the gland.
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- Sensory fibers
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- From capsule of gland (not parenchyma)
- Lacrimal nerve is a branch of V1à goes through the gland
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- Carries sensory info from lateral upper eyelid and conjunctiva (bulbar and upper palpebral)
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- Lacrimal nerve as it approaches the lacrimal gland is joined by sympathetic and parasympthatic fibers
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Extrinsic Muscles of the Eye (EOMs) (7)
- Levator palpebrae superioris – raises the upper eyelid
- Vertical Rectus muscles:
- Superior rectus
- Inferior rectus
- Horizontal Rectus muscles:
- Medial rectus
- Lateral rectus
- Oblique muscles: fibers run obliquely to attach eye itself
- Superior oblique–
- Fibers approach the trochlea
- Tendon passes through the trochlea (acts as a pulley), turns sharply and attaches onto eye
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- Inferior oblique
- Innervation reminder: LR6 SO4 AO3
- LR6: lateral rectus by CN VI
- SO4: Superior Oblique by CN IV
- AO3: All Others by CN III
- All muscles penetrate the fascial capsule to reach the eye
- Origins:
- All rectus muscles (4) take origin from the Common tendinous ring (Anulus of Zinn / Anulus fibrosis)
- Superior Oblique: from the sphenoid bone (posterior roof of orbit)
- Inferior Oblique: takes origin from the maxilla (anterior floor of orbit)
- Insertions:
- All into the Sclera of the eye
- Spiral of Tillaux: each of the four rectus muscles inserts into sclera at increasing distances from the edge of the cornea, forming a spiral.
- Medial rectus is closest to cornea
- Superior rectus is farthest from cornea
- Actions of the EOMs
- General rule: with the exception of the horizontal rectus muscles (medial / lateral rectus) the actions of the other 4 muscles depends on which way the eye is directed
- Adductors: look in
- Primary: Medial rectus- innervated by CN III Oculomotor
- Inferior / superior rectus also contribute
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- Abductors: look out
- Primary: Lateral rectus: -innervated by CN VI Abducens
- Inferior / superior obliques also contribute
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- Elevators: look up
- When looking straight ahead: use
- Inferior oblique
- Superior rectus
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- Depressors: look downward
- When looking straight ahead: use
- Superior oblique
- Inferior rectus
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- Cross-Pairs rule:
- You need a pair of EOMs to look either straight up or straight down
- Rule: change to opposite name to get pair
- Look up: superior rectus works with inferior oblique
- Look down: inferior rectus works with superior oblique
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- Rotators: eyes rotate slightly to adjust for slight head tilting
- Is not a voluntary action
- Eye rotation helps to maintain a visual horizon when head is tilted
- When neck (head) is flexed laterally to one side, the eyes rotate in opposite direction several degrees.
EOM Testing:
- Some EOMs have multiple and complex actions on the eye
- Their actions depend on direction of gaze
- Superior rectus
- CN III
- Look laterally 23 degrees and upward
- Inferior rectus
- CN III
- Look laterally 23 degrees and downward
- Lateral rectus
- CN VI
- Look laterally
- Medial rectus
- CN III
- Look medially
- Inferior oblique
- CN III
- Look medially (as far as possible) and upward
- Superior oblique
- CN IV
- Look medially (as far as possible) and downward
EOM innervation:
- Cranial Nerve III comes through superior orbital fissure within common tendinous ring and divides
- Superior branch: supplies
- Levator palpebrae superioris
- Superior rectus
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- Inferior branch: supplies
- Medial rectus
- Inferior oblique
- Inferior rectus
- Trochlear nerve CN IV
- Goes through superior orbital fissure
- Passes into superior oblique muscle far back into orbital
- Abducens nerve CN VI – innervates lateral rectus
Visual problems
- Emmetropia: image focuses on retina—20/20 vision
- Myopia (near sighted): image focuses in front of retina
- Eyeball is too long
- Correct by diverging light with concave/diverging lens
- Reduce power of cornea
- Hyperopia (far sighted): image focuses behind retina
- Eyeball is shortened
- Correct by converging light with convex/converging lens
- Increase power of cornea
- Astigmatism
- Irregular curvature of lens or cornea→poor focus
- Strabismus:
- Non-parallel visual axes