Eye anatomy and vision Flashcards
Identify three layers of the eye and how the eye is divided up
- 3 layers: outer fibrous tunic (maintains shape), middle vascular tunic (blood supply), inner neural layer (nerve cells that convert light into action potentials)
- Interior of the globe divided into anterior and posterior segments
a) anterior (front) = everything infront of iris. Further divided into anterior chamber = infront of iris, posterior chamber = between lense and iris
b) posterior = behind lense - internal fluid support by aqueous (anterior) and viterous (posterior) humours
Purpose of the eye
- organ of sight
2. turn incoming light into action potentials
What does the optic nerve do
- Take action potentials to the visual cortex of the brain (occipital lobe)
Which cranial nerve is the optic?
SECOND
Where does the eye develop from and were deos it “sit”
- develops from the optic cup of the embryonic diencephalon
2. sits in the bony orbit of the skull which varies between species
Where does light have to pass through
- Cornea at very front
- through anterior segment
- pupil (hole in iris)
- lense
- viterous humour (jelly like)
What are the functions of the first adnexa of the eye?
- eyelid
- protect eye
- Produce the lipid component of the tear film
- spread and move tears over eye surface
- remove foreign material
- close to exclude light
What is the space between the eyelids called, corners of eye called, movement controlled
- Palperbral Fissue
- corners = canthi = medial and lateral ligaments which hold the lids at these points
- movement controlled by cranial nerve 7, facial nerve
What is the conjunctiva and what is its role
- the mucous membrane that is inside eyelids (pink stuff)
- covers everything, highly vascular, lot of lymphoid tissue, lot of gobles cells produce mucin
- folds back on itself (formix)
What are the different names for the conjunctiva?
- palperbral conjunctiva - on eyelid
- Bulbar conjunctiva - onto eyeball
- Nicititating conjunctiva - covers thirs eyeball
3rd eyelid
- nicitating membrane
- sits in ventral medial aspect of orbit
closes when eyeball retracts passively - supported by cartilage
- lacromal (tear) gland at base produces some aqueous component of tear film
What are the three components of tears and why
three components
- lipid = eyelids tarsal gland
- mucin = goblet cells of conjuctiva
- aqueous 1/3 from gland at base of 3rd eyelid, 2/3 from lacrimal gland in dorsal lateral part of orbit
Structures of the outer fibrous tunic
- made up of 2 things
- Sclera = white = covrs whole eyeball except for cornea
- Cornea = transparent at front of eye = transmit and refract light
Avascular
What are the layers of the cornea
- Anterior epithelium - squamous outermost and columnar inner. Many pain receptors
- Stroma = collagen fibrils which form lamellae. If cloudy = disruption to lamellae arrangement
- descement’s membrane lies causal to stroma, produced by endothelium
- ENdothelium = innermost. Produced Descement’s membrane and actively pumps water out of cornea = maintains clarity of stroma
Structures of vascular tunic
- sits beterrn fibrous tunic and neural layer.
- termed uvea
- Anterior uvea = iris and ciliary body
- Posterior uvea = choroid
Iris
- sphincter with a constrictor (around pupil) and dilator muscle
- arises from cilary body
What are the names of constriction and dilation of iris. what kind of control is this under
- Constricting (miosis)
- dilating (mydriasis)
- autonomic control
What are the functions of the ciliary body?
- Production and drainage of aqueous
humour - Anchors the lens via zonular fibres
- Changes the size and shape of the lens
through contraction and relaxation of the ciliary body muscles
What is the name and function of the posterior uvea
- Choroid
- lies between sclera and retina. highly vascularised and pigmented.
- provides blood supply to deep layers of retina
- tapetum Lucidum structure within = increases night vision in animals = greenish reflection =
Lens
- job: reflect light by changing shape
- Sits behind iris, attached to ciliary process by zonular fobres
- transparent to allow it to transmit light
- shorter, fatter lens = close vision
- longer thinner = distance
What is accommodation
How contraction or relaxation of the lens changes the depth of focus
what is the retina
- the neural layer that converts light into action potentials
- innermsot layer of the wall of the posterior segment
- 4 main cell types: outside = retinal pigment epithelium, photoreceptor cells (rods and cones), bipoalr ganglion, inside = multipolar ganglion
What are the structures visible in the posterior segment of the eye via an ophthalmoscope known as
- Ocular fundus
- includes optic disc, retina, retinal pigment epithelium, choroid including tapetum, sometmes sclera
3.
species differences
- position of orbit within skull: predators more rostrally, prey laterally
- Cats very little sclera, horses lot
- Horses = lacrimal caruncle in medial canthus of eye
- Rabbits one lower lacrimal punctum: other = dorsal and ventral
- ungulates = horizontal oval shaped cornea = wider horizontal field of vision vs round cornea of carnivores
- ungulates = oval pupil, cats upright slit shaped, pigs and dogs rounded. depends on where constrictor muscle lies
- Fundus- dog optic disc vaguely triangular, white fuzzy outline (myelination of ganglion cell axons). Horse and guinea pigs = tiny blood vessels, rabbits= white band across retina
- where aqueous humour produced from and drained
- produced = ciliary process, drains Iridocorneal angle
Which layer of cornea is composed of collagen lamellae?
- stroma
Describe the role of rods
i. Black and white images
ii. High sensitivity to light
iii. Rhodopsin pigment
Describe the role of cones
i. Colour vision
ii. Low sensitivity
Describe role of Bipolar neuron
i. First neuron in the pathway
ii. Connected by photoreceptors
Describe role of ganglion cells
i. Second neuron in pathway
ii. Axons comprise the optic nerve
What is the visual (neural) pathway from the retina to cortex?
Photoreceptor –> bipolar neurone –> ganglion –> brain
Early sensory deprivation
i. Affective blindness – lack of full sensory experience as a neonate leaves animals unable to experience it in alter life
ii. Eyelids closed from birth, if opened = blind no matter what
iii. If only see certain things e.g. horizontal likes = can’t process vertical
Where do neurons travel
i. Most synapse in the LATERAL GENICULATE NUCLEUS.
ii. SOME travel to the hypothalamus = involved in day/night cycle regulation
iii. Some travel to the superior colliculus = involved in visual reflexes
b. What is visual decussation and sterio/binocular vision?
i. Visual decussation = crossing from one side of the brain to the other = widely spaced eyes = broad field of view
ii. Stereo or binocular vision = particular point is visualised by BOTH eyes = see in 3D/ depth
c. What are the names of the different visual fields of vision and what do carnivores vs herbivores possess?
i. Monocular lateral field of vision
ii. Binocular area
iii. Carnivore eyes = frontal position = horizontal field of vision which means restricted monocular lateral field of vision and large central binocular area. Dog and cat = good depth perception, half visual
iv. Herbivore eyes = lateral position = wide monocular lateral field of vision and narrow binocular area = very poor stereoscopic vision. Horse and rabbit = huge visual, poor depth
a. Why do birds have better vision than us?
i. Birds have 10X density of photoreceptors than us
b. Why can’t cats be vegetarian with NO supplements. What supplement needed
i. Need Vitamin A supplement
ii. Cats require preformed vit A because they lack enzyme dioxygenase necessary to cleave Beta Carotene therefore can’t create the biologically active vit A. Dogs however can synthesise vit A from carotenoids
a. What muscle ??? orchestrates the pupillary dilation reflex? What should happen in this reflex? What are you testing?
i. Superior colliculus
ii. When shine torch in one eye BOTH should constrict
iii. Testing whether the animal is blind or the afferent cranial nerve is lesioned
b. If the motor nerve is damaged in one eye what will happen when shine torch in it?
i. When shine torch only one eye constricts each time
c. If blind in one eye what happens when shine torch?
i. Shine torch in both eyes. If one doesn’t constrict BUT when shine light in the other eye (non blind) BOTH constrict = motor working fine, vision is damaged
