Physiology of the retina Flashcards
What does the lacrimal apparatus consist of?
Lacrimal glands and associated ducts
Function of the lacrimal glands
Secrete tears which provide an optically smooth refracting surface
Also contain antibodies and lysozyme to prevent bacterial growth
What stimulates lacrimal gland secretion?
parasympathetic stimulation via facial nerve
How do tears enter and exit the eye?
Where do they drain?
Enter via the excretory duct
Exit via the lacrimal punctum
Drain into the nasolacrimal duct
What is contained in the outer layer of the eye?
Cornea at the front, sclera at the back. Fuses with the dura around the optic nerve
What is contained in the middle layer of the eye?
Iris, ciliary body and choroid
What is the inner layer of the eye
Retina
What type of humour is the cornea filled with?
Aqueous humour (protein free filtrate of blood)
What type of humour is the posterior chamber of the eye filled with?
Vitreous humour (watery gel)
Function of vitreous humour
Holds shape of eye and maintains focussing accuracy
What is the main refractive surface of the eye?
Cornea
Is the cornea vasculated and innervated?
Nearly avascular, richly supplied by nerve fibres
Function of tears related to the cornea?
maintain oxygen exchange and water content
tears prevent scattering and improve optical quality
How do the exterior and interior of the sclera differ?
Exterior is smooth and white
Interior is brown and grooved
Rough diameter of average pupil?
3-7mm
What is the conjuctiva
layer of stratified columnar epithelium, goblet cells and capillaries that covers the sclera
Function of goblet cells in the conjuctiva?
Produces mucus that mixes with tears
Where is the blind spot?
The point where visual axons leave the eye to form the optic nerve (no photoreceptors here)
Where does aqueous humour drain?
Canal of Schlemm
How is intraocular pressure formed?
Difference between formation and drainage of aqeuous humour
Name the the 2 main sub groups of glaucoma?
Open angle glaucoma
Primary angle closure glaucoma
What is open angle glaucoma?
Slowly progressive condition, trabecular meshwork gradually becomes blocked
Normal angle between cornea and iris
What is primary angle closure glaucoma?
Angle between cornea and iris is reduced
Flow of fluid cannot pass through canal of Schlemm
Causes rapid increase
Describe the surgery that can be used to treat glaucoma
Incision mae at cornea-sclera junction
What is the fovea?
The point where the cones are most closely packed
What type of cells are present in the fovea?
Cones
How many types of cone cells are there?
3
Function of rod cells?
Very sensitive to light- active in the dark. no colour differentiation, suppressed in daylight
Which part of the rods and cones are photoreceptive?
the outer segment
What is contained in the inner segment of the rods and cones
Cell body and synaptic terminal
Describe the structure of the outer segment of rod cells
Consists of disks of membrane containing the photopigment ‘rhodopsin’
What protein is contained in cone cells?
Cone opsins
What is the advantage of the stacking arrangment of rod cells?
The light passes through all the segments in sequence, maximising the chance of a photon interacting with molecule of pigment
How can cones afford to have a shorter outer segment
There enough light in the daylight to guarentee a photon with interact with a photopigment
Why does the eye require such high amounts of oxygen?
Because photoreceptors are one of the most metabolically active cells in the body
How does the cells of the eye remain depolarised?
Constant inward leak of sodium keeps cell depolarised and toonically release glutamate from synaptic ending
What is the effect of light on eye cells?
Hyperpolarises the cell and stops the tonic glutamate release
What is the effect of light absorption of the rhodopsin?
Changes the shape
This acts via G protein coupled receptor to reduce level of cyclic GMP in rod
This closes sodium channel and allows cell to stop releasing glutamate
Light is an inhibitory stimulus
What is the role of bipolar cells?
Converts this continuous release of glutamate into varying depolarisation. This is transmitted to ganglion cells which project axons into the optic tract
Three main neurons in the path from photoreceptor to optic nerve?
Photoreceptor
Bipolar cells
Ganglion cells
What is the blood supply of the inner retina?
Central retina artery
What is the blood supply of the photoreceptors?
Supplied by choroid (network of capillaries suppled by ciliary arteries)
