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
How does constriction of the pupil work?
decreased light intensity and distance vision:
the pupillary dilator (radial) muscles extend radially away from the edge of the pupil. Contraction of these muscles enlarges the pupil.
increased light intensity and close vision:
the pupillary constrictor muscles form a series of concentric circles around the pupil. When these sphincter muscles contract, the diameter of the pupil decreases
What controls convergence of the eyes?
extrinsic eye muscles
MEDIAL AND LATERAL RECTUS control horizontal movements
SUPERIOR AND INFERIOR RECTUS control vertical movements
OBLIQUE MUSCLES control the rotation of the eyeball
What are the 3 main types of retinal cells?
- Photoreceptors
- rods - monochrome
- cones -color - bipolar cells
- relay information to ganglion cells - ganglion cells
- converge into the optic nerve –> brain
What are the roles of the photoreceptors?
rods
role: dim light and peripheral vision
distribution: density is highest in the peripheral retina and decreases towards the fovea (where it is nil)
visual pigment: rhodopsin
cones
role: high-acuity color vision
distribution: density is highest in the fovea and decreases towards the peripheral retina
visual pigment: cone opsins (blue, green and red)
What are the 3 types of cones?
there are 3 types of cones,each responds to one of the primary colours
red light
green light
blue light
the range of wavelength sensitivities for the three types of cones , each of which contains a different form of cone opsin
How to turn light into electrical impulses? Visual pigment
- visual pigment is a compound embedded into membrane discs or folds inside the photoreceptors
- made of opsin, a protein, and retinal, derived from vitamin A
- when light strikes a visual pigment, retinal changes shape. this sets off a cascade of enzyme driven reactions inside the rod or cone
- the result is graded potential changes inside the photoreceptor
- this electrical information is relayed via synapses to bipolar cells, then onto ganglion cells where action potentials are generated and sent onto the brain
What is the path to the brain and the processing of right and left visual fields?
the visual field is the entire area that a person is able to see
both hemispheres of the brain contain a visual cortex; the visual cortex in the left hemisphere receives signals from the right visual field and the visual cortex in the right hemisphere receives signals from the left visual field. This is due to nerves crossing over at the optic chiasma
