Lec 18 Flashcards
What three accessory structures to the eye are important? What do they do?
- The eyebrow shades the eye and protects it from perspiration
- The eyelids (palpebrae) hold the tarsal glands (oily protective secretion) and lacrimal caruncle (sebaceous/sweat glands).
- The bulbar conjunctiva is a lining of the eyelids that covers the white of the eye and produces a lubricating mucus.
What separates the eyelids?
The palpebral fissure.
What contains sweat glands and sebaceous glands around the eye?
The lacrimal caruncle.
What is special about eyelash follicles?
They are highly innervated and stimulate the blinking reflex.
Where do tears drain?
Into the lacrimal sac
Where do tears go from the lacrimal sac?
Through the nasolacrimal duct into the inferior meatus of the nasal cavity.
What produces tears?
Lacrimal glands
What structure do tears pass through to go to the eye?
Excretory ducts of the lacrimal glands
Which direction do tears flow across the eyeball?
Inferiorly and medially
Where do tears go after crossing the eye?
Into the lacrimal canaliculi at openings called the lacrimal puncta
In which directions do the rectus eye muscles move the eye?
Superior = elevates
Inferior = depresses
Medial = Medially
Lateral = Laterally
In which directions do the oblique eye muscles move the eye?
Superior = depresses eye + lateral
Inferior = elevates eye + lateral
What are the three layers of the eye?
- Fibrous layer
- Vascular layer
- Inner (retina) layer
What structure divides the eye into posterior and anterior sections?
The lens
What are the two parts of the fibrous layer of the eyeball?
The sclera and cornea
What is the sclera?
fibrous tissue that surrounds the eyeball, it is an anchoring site for muscles and is seen as the white of the eye
What is the sclera continuous with at the site of the optic nerve?
The dura mater
Where is the cornea?
The anterior 1/6 of the fibrous layer
What is the role of the cornea?
It allows light entry and is important in refraction
What are the stratified squamous cells that protect and renew the cornea called?
External epithelial sheet
What is the corneal endothelium?
It is stratified squamous cells, it holds sodium pumps to maintain corneal clarity
How can we describe the vascularity of the cornea?
It has lot of nerve endings and can repair itself, but is not vascular so it has no access to the immune system.
What are the three parts of the vascular layer of the eyeball?
- The choroid
- The Ciliary body
- Iris
Where is the choroid located and what are its two functions?
It is deep to the sclera and is vascularized (brings nutrients to the eye), and contains melanin to minimize scatter
Where is the ciliary body located and what does it do?
It is located in the anterior 1/6 of the vascular layer and holds ciliary muscles that influence the shape of the lens
What structure gives the eye color and makes the pupil?
The iris
What pigment in differing amounts gives different eye colours?
Brown pigment
What kinds of muscle constricts and dilates the pupil?
Smooth circular muscles of the PNS constrict the iris, and smooth straight muscles of the SNS dilate the pupil.
How many photoreceptors are on the retina?
Millions
What are the two layers of the retina?
The pigmented layer and the neural layer (vision)
What is the role of the pigmented layer?
It absorbs light, cells can be phagocytic, and store vitamin A
What composes the neural layer?
photoreceptors, bipolar cells, and ganglion cells
What is the blind spot of the retina called?
The optic disc
What are the differences in function of rods and cones?
Rods are for low light and peripheral vision (more numerous but more blurry), cones are for bright light and color
What is the area of dense cones for visual acuity called?
The macula lutea
What is the central pit of the macula called? What happens here?
It is called the fovea and all other cells are pushed away for photoreceptors
What is the dispersion pattern of rods and cones?
All cones in the fovea, mostly cones in the macula lutea, moving farther out means more rods
What fluid in the eye supplies nutrients/oxygen to the lens and cornea and takes away waste?
Aqueous humor
What does the aqueous humor come out of? Where does it go to be reabsorbed?
It filters out of the ciliary capillaries and is reabsorbed into the venous blood by the scleral venous sinuses.
What is vitreous humor? What is its function?
It is a lifetime lasting embryonic fluid that is found in the posterior of the eye and transmits light, holds the two retinal layers together, and creates intraocular pressure.
How does the lens flip light signals?
Upside down and also left right flipped
Why does light have color?
Some wavelengths are reflected
What is the wavelength of visible light to humans?
400-700nm
What is the six step pathway of light in the eye?
It goes through the cornea, aqueous humor, lens, vitreous humor, neural layer of the retina, and then the photoreceptors
Where is light refracted in the eye?
- Cornea
- First side of lens
- Second side of lens
What structure of the eye adjusts to see things at different lengths away?
The lens
What kind of vision are our eyes adapted to?
distant vision
How far is an object usually when we don’t need to flex our ciliary muscles to see it clearnly?
6m
What is another word for an eye at rest creating a crisp image?
An emmetropic eyes
With distant vision, what is the behavior of the lens?
It is relaxed.
With close vision, what is the behavior of the lens? What happens to the light hitting the eye?
It bulges as the light hitting the eye diverges
What are three events of near vision?
- The lens bulges (accommodates) (more can be done for children, less for adults).
- Pupils constrict to keep the most divergent light waves out
- Convergence of eyeballs to keep the fovea on the object -medial rectus
What is presbyopia?
It is the lens not being able to accommodate at older age, reading glasses are needed.
What is myopia?
Nearsightedness, the eyeball is long and the lens cannot stretch enough and faraway images converge before the retina.
What is hyperopia?
farsightedness, the eyeball is too short and the lens isn’t able to bulge enough and close images converge “behind” the retina.
Where are the receptive regions of photoreceptors located?
In the pigmented retina layer.
What connects the inner and outer segments of photoreceptors?
The cilium
What is the structure of the outer segment of a photoreceptor?
It contains visual pigments called rhodopsins embedded in disc membranes.
What is a rhodopsin?
It is a visual pigment that changes shape as it absorbs light.
How do rods and cones deal with damage?
The top disc is renewed every day and old ones are detached at the other end.
Do rods or cones have higher sensitivity?
Rods do, they can see in lower light
How can we compare the acuity of rods and cones?
Rods have a lower acuity, many rods converge onto a ganglion where cones have a higher acuity where one cone is attached to one ganglion cell in the fovea.
What is the order of the 4 types of cones in reference to what wavelengths they are responsive to? (longest wavelength to shortest)
Red Cones (560)
Green Cones (530)
Rods (500)
Blue Cones (420)
When do we see white?
When all the different color rhodopsins are stimulated equally.
How are rhodopsins formed?
retinal combined with different kinds of opsins (in the dark)
How do we best see dimly lit objects?
If they are in peripheral vision or moving.
What happens when light is absorbed by rhodopson?
Pigment bleaching happens where retinal releases from opsin.
How do we convert trans retinol back into pigment?
Pigment regeneration takes ATP to turn trans retinol back into cis retinol
What are the 5 steps of phototransduction? (in the discs of the cells)
- The visual pigment activates when the retinal changes shape.
- The visual pigment activates transducin.
- Transducin activates phosphodiesterase (PDE)
- PDE causes cGMP to convert into GMP
- As there is less cGMP levels, the channels close leading to hyperpolarization.
How can we describe the relationship between light levels and polarization of the photoreceptors?
They hyperpolarize when exposed to light, which is a signal. They depolarize when exposed to dark.
What is the function of the bipolar cells in signal transmission of photo-information?
The presence of light inhibits the release of ISPS in the bipolar cell, which depolarizes the bipolar cell, releasing NTs from the bipolar cell which activate ganglion cells.
When going from dark to light, what happens?
rhodopsin in the rods bleaches, and cones take 5-10min to reach max acuity
When going from light to dark, what happens?
rhodopsin starts to accumulate in the rods, it takes 20-30m
Where do most optic tract fibers end up?
The lateral geniculate body of the thalamus
Where do optic tract fibers end up that don’t end in the thalamus?
The superior colliculi for reflexes, and the pretectal nuclei for pupillary reflexes.
Where do optic radiations travel?
From the thalamus to the primary visual cortex
Which fibers of the optic nerve desiccate at the optic chiasm? Why?
The medial ones because they carry the information from the lateral half of the opposite field of view.
Why are there 4 optic tracts?
Each one carries one half of the field of view of the eyes visual field.
How do we perceive depth?
The primary visual cortex blends together the angles from the different eyes.
