Module 7 - The Eye Flashcards
Why are the eyes and ears considered special senses?
there are specialized and complex organs devoted to them
they are sensory - they convey nervous information about the world or about the body to the brain
involve both afferent and efferent nerve pathways
both involve the autonomic (involuntary-motor) nervous system
The eye also includes somatic (voluntary-motor) nerve pathways to control eye-movement
What is the conjunctiva?
specialized epithelium covering the inner surfaces of the eyelids and the outer white surface of the eye (not the cornea)
What is the lacrimal apparatus?
tears: salt, mucus, antibodies, lysozyme (antibacterial)
constant flow of tears keeps conjunctiva clean: removes debris, reduces friction, prevents infection, provides O2 and nutrients
What are the 3 layers of the hollow, fluid filled anterior and posterior cavities, eyeball?
sclera, choroid and retina (neural)
What is the posterior segment (cavity)?
contains mostly vitreous humor, a jelly-like substance: holds the retina against the choroid and prevents the eyeball from collapsing
what is the sclera?
“white” of the eye
- FIBROUS layer
- protects and shapes the eyeball
- provides anchoring for extrinsic eye muscles
- continuous with the cornea
What is the choroid?
VASCULAR layer
- provides nutrition to all layers
- brown pigment reduces internal reflection and scattered light
- continuous with the ciliary body
What is the retina?
NEURAL layer
- contains light sensitive photoreceptors
- stops just before the cornea at the front
What is the macula lutea and fovea centralis?
Macula lutea: highest concentration of photoreceptors
Fovea centralis is the central area of macula lutea
- when you look directly at an object, the image falls on this area
- this area is the area of sharpest vision
What are the structures at the anterior pole of the eye?
ciliary body: thick ring of muscle that controls lens shape
iris: muscles that control pupil size
ciliary processes: contain capillaries that secrete fluid - aqueous humor - into posterior chamber
Scleral venous sinus (drains aqueous humor)
ora serrata: junction between retina and ciliary body
suspensory ligaments: holds the lens in place
How does circulation of aqueous humor work?
- aqueous humor is formed by filtration from the capillaries in the ciliary processes
- aqueous humor flows from the posterior chamber through the pupil into the anterior chamber, some also flows through the vitreous humor
- aqueous humor is reabsorbed into the venous blood by the scleral venous sinus via the trabecular meshwork (not shown) that provides slight resistance to the outflow
What is glaucoma?
damage of the optic nerve due to inadequate drainage of aqueous humour causing increased intraocular pressure
- excessive intraocular pressure
- if untreated, results in retinal degeneration and may cause blindness
- initially manifests as tunnel vision
What are the 4 steps to image formation in the retina?
- refraction of light rays
- accommodation of the lens
- constriction of the pupils
- convergence of the eyes
What is refraction of light rays? Where does it happen in the eye?
- bending of light rays at the junction of two transparent substances with different densities
- sites of refraction in the eye: cornea(75%), lens(25%)
- refractive capability of the cornea is fixed, lens can change (accommodate)
How does accommodation of the lens work?
Close vision:
ciliary muscle contracts
suspensory ligaments relax
lens bulges
Distant vision:
ciliary muscle relaxes
suspensory ligaments tense
lens flattens
Far point of vision:
Distance beyond which no change in lens shape is needed to focus
-lens is maximally flat NORMAL: 20 ft
Near point of vision:
Distance under which focusing is not possible
-lens is maximally bulged NORMAL: 4 inches
