BMC L9 Flashcards
What are photons ?
Particles of light
Travel in wave like fashion
What is visible light ?
Small part of electromagnetic spectrum
VISIBLE to humans
What does the lacrimal apparatus consist of ?
Lacrimal gland
Lacrimal ducts (excretory + nasolacrimal)
State the function of the lacrimal gland
Secretion of tears
State the composition of tears
Dilute saline solution
(MAL: mucus, antibodies, lysozymes)
State the function of lacrimal ducts
Carry tears from caruncle to nasal cavity via lacrimal sac
Name the 6 extrinsic eye muscles
Superior rectus
Inferioir rectus
Lateral rectus
Medial rectus
Superior oblique
Inferior oblique
Where do the rectus muscles orignate in the eye
Common tendinous ring
Where do the oblique muscles originate in the eye
bony orbit
State the general function of extrinsic eye muscles
Movevement of eye
Maintain shape of eye
State the function of the lateral rectus
Moves eye laterally
State the function of the medial rectus
Moves eye medially
State the function of the superior rectus
Elevates eye, moves eye medially
State the function of the inferior rectus
Depresses eye, moves eye medially
State the function of the superior oblique
Depresses eye, moves eye laterally
State the function of the inferior oblique
Elevates eye, moves eye laterally
Describe what happens during distant vision
Sympathetic activation
This relaxes cillary muscles
Tightens cillary zonule
Flattens lens
Describe what happens during close vision
Parasympathetic activation
Contracts cillary muscle
Loosens cillary zonule
Lens bulges
How can we recognise if the eye
Describe the myopic eye
Myopic - nearsighted
Eyeball too long
Focal point in front of retina
How is the myopic eye corrected ?
Concave lens, moves focal point further back
Describe the hyperopic eye
Farsighted
Eyeball too short
Focal point behind retina
How is the hyperopic eye corrected
Convex lens
Moves focal point further forward
Describe the emmetropic eye
Norml sighted
Focal point on retina
State the function of the cornea
Bending of light / refraction
Foucses light on retina
State the function of lens
Adds fine control
State the 3 layers of the wall of the eyeball
Fibrous layer
Vascular layer
Sensory layer
What are the two segments of the internal eyeball cavity?
Anterior (aqueous humor)
Posterior (vitrous humor)
What does the fibrous layer consist of?
Dense, avascular connective tissue
Sclera + cornea
State the position + function of the sclera
Posterioir, opaque (white, not transparent) region
Protects + maintains shape of eyeball
Anchors 6 extrinsic eye muscles
State the position + function of the cornea
Anterior, transparent region
Bends / refracts light as it enters eye
Contains pain receptors, involved in tearing relfex, blink reflex
State the functions of the posterior segment
Vitreous humor
Transmits light
Supports posterior region of lens
Intraocular pressure
State the functions of the anterior segment
Aqueous humor, blood plasma like, filtered continuosly by capillaries in cillary processes
Supplies oxygen + nutrients to
-lens
-retina
-cornea
removes o2 + waste
drained by scleral venous sinus - canal of shclemm at scleral - corneal junction
State the two chambers of the anterior segment
Anterior chamber: between cornea + iris
Posterior chamber: between iris + lens
What is the cause of glaucoma and state the effects
Blockage of drainage of aqueous humor
Leads to compression of retina + optic nerve
Describe the 2 types of glaucoma
Open angle glaucoma - slow exit / drainage of aqeous humor
Closed angle glaucoma - iris completely blocks exit / drainage of aqueous humor
State what the sensory layer of the eyeball wall consists of
This is the retina - double membrane, 2 layers
Pigmented layer
Nueral layer
State the functions of the pigmented layer
Vit A storage
Absorbs light, prevents it from scattering
State the functions of the neural layer
Photoreceptor cells - transduce light energy
Bipolar cells - 1st order neurons, transmit impulses from photoreceptors to ganglion cells
Amacrine cells - interneurones, transmit impulses to ganglion cells
Horizontal cells - lateral interconnecting cells, integrate signals from multiple photoreceptors, for light adjustment
Ganglion cells - 2nd order neurones. Transmit impulses to brain via optic tracts
Describe the blood supply of the retina
2 SOURCES OF BLOOD SUPPLY
Inner layers - central artery and vein of retina
Outer layers (photoreceptors) - choroid
State the two types of photoreceptors
Rods
Cones
Describe the structure of photorecptors
Outer segment - of rodes + cones, embedded in pigmented layer of retina. contains Photopigements - molecules change shape when absorb light
Inner segement - join synapse with cell bodies of horizontal / bipolar cells
Compare rod cells tro cone cells
Rod cells are found more commonly at the peripheral region of the retina, away from the macula lutea. Cones are found at the macula luta
Rods have high sensitivity. Sensitive to dim light. Cones have low sensitivity. Sensitive to bright light
Rods produce a grey image. Cones produce vivid coloured image as they can have 1 of 3 pigments
In rods, pathways converge, therefore, producing “fuzzy” images. In cones, pathways do not converge, producing high resolution images
State the function of the Macula Lutea
This is the fovea
It is a yellow spot. It filters some of the blue light to prevent chromatic abberation
Focussing region
State the 3 different types of cone cells
Red cones (10%) L CONES
Blue cones (45%) S CONES
Green cones (45%) M CONES
Why is red-green colour blindness common in males?
The genes which code for the photopsin proteins (red + green forms of opsin pigments) found on X chromosomes
Males only have one X chromosome. Therefore, only 1 copy of these genes
Therefore, if any defect to these genes, men will have red-green colour blindness - dichromatic vision
Describe how visual pigments are formed
Visual pigments are formed from one of four proteins (opsin) and retinal protein.
State the two isomers of retinal protein
11-cis retinal
All-trans retinal
What is retinal protein?
Light absorbing molecules formed from vitamin A
State the visual pigment of rods and describe its formation
Rhodopsin
Opsin + 11-cis retinal
Explain the excitation of rods
Rods are activated in dim light.
In dim light, regeneration of 11-cis retinal from all-trans retinal occurs. All-trans retinal (formed from Vit a) is converted into 11-cis retinal slowly by enzymes. This requires ATP. (11-cis retinal combines with opsin forming rhodopsin)
Rhodopsin is broken down / bleached in bright light. Bright light stimulates a series of reactions leading to 11-cis retinal to be converted into all-trans retinal. Therefore, this leads to opsin separating from retinal. We end with all-trans retinal and opsin.
Explain signal transmission in the retina
- Light hyperpolarises photoreceptor cells
This stops them from releasing inhibitory neurotransmitter - glutamate - Bipolar cells no longer inhibited. DEPOLARISE AND They release excitatory neurotransmitter to ganglion cells
- Ganglion cells transmit APs via optic nerve to brain
Explain light adaptation
Moving from darkness to bright light
Instantaneous breakdown of rhodopsin - glare - pupils constrict
rapid change in retinal sensitivity - rod function ceases, cones rapidly adapt
visual acuity improves after 5 min
Explain dark adaptation
Moving from darkness to bright light
Accumulation of rhodopsin
Rapid change in retinal sensitivity, cone function ceases, rods rapidly adapt
Pupils dilate
