- light absorption - epithelial transport - active potassium transport - vitamin A metabolism (visual cycle) - photoreceptor membrane recycling (phagocystosis) - hormone production + secretion

RETINA Flashcards by louisa reavy

WHAT IS THE VASCULAR TUNIC OF EYE?

CHOROID

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WHEN ENTERING A DARK ROOM ON A SUNNY DAY, TAKES SOME TIME TO BE ABLE TO SEE, WHY?

TAKES TIME FOR RETINA TO REBUILD ITS RHODOPSIN

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INTRAOCULAR PRESSURE PRIMARILY DUE TO WHAT?

AQUEOUS HUMOUR

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AS NERVE FIBRES EXTEND FROM RETINA TO BRAIN…

FROM NASAL HALF OF EACH RETINA CROSS TO REACH THE OPPOSITE VISUAL CORTEX

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WHICH CRANIAL NERVE CARRIES VISUAL IMPULSES TO BRAIN?

II (OPTIC)

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CONSTRICTION OF PUPIL BROUGHT ABOUT BY CONTRACTION OF?

CIRCULAR MUSCLE OF IRIS

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PHOTOPIGMENT IN RODS + CONES IS EMBEDDED IN THE?

MEMBRANE FOLDS OF THE OUTER SEGMENT OF THESE CELLS

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IN DAYLIGHT, RODS CONTRIBUTE LITTLE TO VISION BECAUSE?

THE RHODOPSIN IS BLEACHED AS FAST AS IT IS REGENERATED

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WHEN LIGHT HITS THE RODS..

NA+ CHANNELS CLOSE + LESS INHIBITORY NEUROTRANSMITTER IS RELEASED

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TRANSDUCTION

light -> electrical potential

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ENCODING

electrical potentials -> action potential sequences for retinal output

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TEN LAYERS OF RETINA

inner limiting membrane 
nerve fibre layer 
ganglion cell layer
inner plexiform layer
inner nuclear layer
outer plexiform layer
outer nuclear layer 
outer limiting membrane
photoreceptor layer
retinal pigmented epithelium

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PARAFOVEA

rods + cones

inner + outer nuclear layers

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PERIFOVEA

all retinal layers

thickest part

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FOVEA

depression with inner retinal layers pushed aside

foveola - only long, slender cone outer segments

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FUNCTIONS OF RPE

light absorption
epithelial transport
active potassium transport
vitamin A metabolism (visual cycle)
photoreceptor membrane recycling (phagocystosis)
hormone production + secretion

RETINAL

small molecule made by modifying vitamin A

photons of light are absorbed by a transformation of retinal in photoreceptor membranes

retinal transformed in the rods is restored to its active state by enzymes in RPE

RODS

110-130 million/ retina

long, slim cells

separate membrane discs

slow to respond

very sensitive

CONES

5-7 million/ retina

wider cells

outer segment membrane continuous with plasma membrane

respond quickly

RHODOPSIN

only in rods

a G-protein-coupled receptor

PHOTOTRANSDUCTION IN DARKNESS

rod photoreceptors release the neurotransmitter GLUTAMATE

this inhibits bipolar cells from transmitting signals to ganglion cells which provide output from retina to brain

PHOTOTRANSDUCTION IN LIGHT

rod photoreceptors are bleached + hyperpolarise

glutamate production stops + a sign reversal occurs in some bipolar cells that are excited into sending signals to ganglion cells which provide output from retina to brain

light is the ligand that triggers activation of the enzyme

PHOTOTRANSDUCTION IN RODS

1 - activation of rhodopsin (R->R*)
2 - activation of transducin (G protein)
3 - activation of phosphodiesterase
4 - deactivation of chemical messenger, cyclic guanosine monophosphate (cGMP) closes cell

OUTER PLEXIFORM LAYER

code neural signals for contrast (centre vs surround)

establish separate channels for detecting lighter objects + darker objects (ON vs OFF channels)

WHAT IS NUCLEAR LAYERS MADE UP OF?

horizontal amacrine muller bipolar cells

WHAT IS PLEXIFORM LAYER MADE UP OF?

synapses

RECEPTIVE FIELD

area of visual field that causes a response from a visual neuron (i.e. not necessarily the location of the cell)

WHERE DO RODS + CONES SYNAPSE WITH INTEGRATOR NEURONS?

OUTER PLEXIFORM LAYER

HORIZONTAL CELLS

communicate in outer plexiform layer connect rods + cones with bipolar cells

WHAT IS THE SIMPLEST + FASTEST PATHWAY FOR TRANSMISSION OF A SIGNAL?

CONE -> BIPOLAR CELL -> GANGLION CELL

VISUAL PATHWAY

OPTIC NERVE (II) - > OPTIC CHIASM - > OPTIC TRACT - > LATERAL GENICULATE NUCLEUS OF THALAMUS - > OPTIC RADIATION - > PRIMARY VISUAL AREA OF VISUAL CORTEX IN OCCIPITAL LOBE

MULLER CELL FUNCTIONS

1. support metabolism to fuel aerobic production in nerve cells 2. mops up neural waste products e.g. CO2 + NH3 3. protects neurons from exposure to excess neurotransmitters e.g. glutamate (uptake + recycling mechanisms) 4. control homeostasis + protect neurons by maintaining ionic environment 5. regulation of K+ distribution across retinal vitreous border

RPE - LIGHT ABSORPTION

melanin pigment granules absorb stray light reduces light scatter from behind retina + between photoreceptors (apical processes) improves resolution

RPE - EPITHELIAL TRANSPORT

active import from CC: - glucose, omega-3 fats, vit A diffusion from CC: - water + oxygen active export to CC: - waste products e.g. lactic acid diffusion to CC: - water, co2 + chloride

RPE - ACTIVE POTASSIUM TRANSPORT

glial cell functions ion levels must be stable for photoreceptors to function ions rapidly exchanged during light-dependent activity rapid ion compensation mechanisms in outer retina are by RPE cells + by muller cells in inner retina

RPE - VITAMIN A METABOLISM

visual cycle ‘retinal’ is a small molecule made by modifying vitamin A photons of light absorbed by a transformation of retinal in photoreceptor membranes retinal transformed in rods is restored to its active state by enzymes in RPE

RPE - PHOTORECEPTOR MEMBRANE RECYCLING

phagocytosis photoreceptor membranes produces continuously + shed outer segments: rod outer segments shed in morning (1-2 hours after dawn) cone outer segments shed at night (3-5 hours after dark)

RPE - HORMONE PRODUCTION + SECRETION

RPE produces + secretes a variety of factors + signalling molecules for regulation of metabolism, reactions to pathology/ injury etc VEGF (vascular endothelial growth factor) - involved in making new blood vessels: angiogenic factor PEDF (pigmented epithelium derived factor) - does opposite: antiangiogenic + neurotrophic protein of eye