Flashcards in The visual system Deck (35):
What are the 3 coats of the eye?
-Outer = cornea, sclera
-Middle (Uvea) = iris, ciliary body, choroid
-Inner = retina
Describe the sclera, what is its function
-Tough outer collagen layer
Describe the cornea, what is its function
-Transmission of light
-Important for refraction
-must be transparent, smooth spherical surface
-allows attachment of extra-ocular muscles
-trigeminal nerve (400 X that of skin)
-epithelium must last a lifetime
-endothelium continuously replaced by Limbal stem cells
Describe the iris, what is its function
-Contains iris colour
-pupil reflexes, light/dark, near
Describe the ciliary body, what is its function
Production of aqueous humour
Nutrients for cornea and lens
Maintains intraocular pressure (15 mmHg)
Controls accommodation (12 dioptre)
Adjustment of the lens in the eye so that clear images of objects at different distances are formed on the retina
What does parasym/sym innervation to the iris do?
Sphincter muscle: parasympathetic innervation = lets less light in
Dilator muscle: sympathetic innervation = more light is allowed in
Describe the choroid body, what is its function?
Nutrition of outer retina (photoreceptors)
Absorbs stray photons
Describe the retina, what is its function?
-Complex layered structure, 10 layers
-light sensitive at the back of the eye
-contains the 'specialised cells' that are responsible for convertinf light energy into cell membrane potential for neurotransmission
1. Retinal pigment epithelium
3. Ganglion cell
4. Nerve fibre
-colour vision R, B, G
-night time vision
-more photopigment discs than cones
-slower reaction than cones
-contains Rhodopsin (opsin and retinal)
-mostly peripheral vision
What happens when photoreceptors are struck by a photon?
Photoreceptors contain rhodopsin (rods) or iodopsin (cones), this is the photosensitive pigments that have the cis isomer of retinal bound
-When struck by a photon, the cis retinal is converted into the trans form and activation of chemical signalling cascades involving cGMP, resulting in hyperpolarisation of the cell
Describe the aqueous humour what is its function?
-Mainly composed of water and electrolytes
-Circulates around lens, through pupil, exits through trabecular meshwork
-Maintains intraocular pressure needed to inflate the eye and lens
-provides nutrients for the lens and central cornea as they do not have their own blood supply
Describe the Crystalline lens
-Transparent biconvex structure
- functions - maintain its own clarity. to refract light. to provide accommodation.
Describe the eyelids
Meibomium glands (lipid)
Posterior mucous membrane
Firm tarsal plate
Orbicularis oculi (closure)
Levator palpabrae superioris (LPS) and sympathetic muscle (opening)
1.Anterior lipid (meibomium glands) - oil layer, line upper and lower eyelids
2. Middle aqueous (lacrimal glands) water layer, under orbital rim bone below eyebrow
3. Posterior mucous (microscopic goblet cells in the conjunctiva)
-Nutrition to cornea
Describe the blood supply to the eye
1. Internal carotid artery
=Ophthalmic artery, gives off central retinal artery (most important as supplies internal surface of the retina, occlusion=blindness), branches into superior and inferior, temperal and nasal branch retinal arteries
Ciliary arteries (anterior and posterior)
Ethmoid and eyelid
2. External carotid
angular artery supplies medial lid and orbit
Describe venous drainage of the eye
Venous drainage of the eyeball is carried out by the superior and inferior ophthalmic veins. These drain into the cavernous sinus, a dural venous sinus in close proximity to the eye.
Describe the path of a photon in the eye
Tear film (transmission)
Cornea (transmission and refraction)
Lens (transmission and refraction)
Inner retina (transmission)
Outer retina (transduction)
Why is it important that the extra-ocular muscles move the eyes in synchrony
The extra-ocular muscles move the eyes in synchrony to maintain binocular single vision and allow depth perception (stereopsis)
Which two layers refract light?
What is the fovea centralis?
-Depression in the centre of the retina containing a high concentration of only cones
Describe the extra ocular muscles of the eye
medial rectus - adduction
lateral rectus - abduction CN6
superior rectus - eleveation, intorsion, adduction
inferior rectus - depression, intorsion, adduction
superior oblique - intorsion, depression, abduction CN4
inferior oblique - extorsion, elevation, abduction
(down and out) if CN3 damaged
Describe innervation of the extra ocular muscles
-lateral rectus CN6
-superior oblique CN4
-the rest CN3
(down and out) if CN3 damaged
What is the optic chiasm?
-mechanism for the fusing of two images to prevent double vision
What are the dorsal and ventral streams?
DORSAL - 'where' pathway
puts objects in spatial location
from primary visual cortex in occipital lobe to the parietal lobe (spatial
VENTRAL - 'what' pathway
object and visual identification and recognition
- has strong connections to the medial temporal lobe (which stores long-term memories), the limbic system (which controls emotions), and the dorsal stream (which deals with object locations and motion).
-enters via primary visual cortex
Where are the face-selective regions?
-Fusiform face area
-Occipital face area
-Superior temporal sulcus
What does the 'vestibular' movement of the eye do?
Vestibular - holds images of the seen world steady on the retina during brief head rotations of translations
What does the 'visual fixation' movement of the eye do?
Visual fixation holds images of the seen world steady on the retina during sustained head rotation
What does the 'smooth pursuit' movement of the eye do?
Smooth pursuit holds the image of a small moving object steady on the fovea
What does the 'nystagmus - quick phases' movement of the eye do?
-This resets the eyes during prolonged rotation and directs the gaze towards the oncoming visual scene
(nystagmus is also a pathological condition)
What does the 'saccades' movement of the eye do?
Saccades - brings images of objects of interest onto the fovea
What does the 'Vergence' movement of the eye do?
Vergence - moves the eyes in opposite directions so that the images of a single object are placed or held simultaneously on fovea of each eye
What is hemianopia?
Blindness is over half field of vision
eg left homonymous hemianopsia- when L side of each eye is damaged to vision is on right side in each eye
What is a heteronymous hemianopsia?
A Heteronymous hemianopsia is the loss of half of the visual field on different sides in both eyes. It is separated into two categories:
Binasal hemianopsia - The loss of the fields surrounding the nose;
Bitemporal hemianopsia - The loss of the fields closest to the temples.