Hnn:23:pbl10 Eyes And Vision

Front 1

eye diagram

Back 1

Front 2

what are the three layers of the eye?

Back 2

fibrous tunic

vascular tunic

retina

Front 3

describe the fibrous tunic

Back 3

fibrous tunic

superficial layer consisting of the cornea (anterior) and sclera (posterior), the junction of these is called the scleral venous sinus, into which aqueous humor drains

cornea

is a transparent epithelium covering the iris

curved - helps focus light on the retina

receives oxygen from outside air

sclera

white of the eye, a layer of dense connective tissue

site of attachment for extrinsic eye muscles

Front 4

describe the vascular tunic

Back 4

vascular tunic

middle layer composed of choroid, ciliary body and iris

choroid

posterior portion

highly vascularised - supply the retina

contains melanocytes producing melanin which absorbs stray light, keeping images on the retina sharp and clear

ciliary body

extends from ora serrata (jagged anterior margin of the retina) to just posterior to the scleral venous sinus

consists of ciliary processes and ciliary muscle

ciliary processes

secrete aqueous humor from capillaries

extend zonular fibres that attach to lens

ciliary muscle

circular band of smooth muscle that contracts and relaxes to alter the shape of the lens for near/far vision

lens is transparent and avascular, behind the pupil and the iris, within the cavity of the eyeball

Front 5

describe the iris

Back 5

* suspended between the cornea and the lens, attached to ciliary processes
* contains melanocytes (giving eye colour) and smooth muscle to regulate the amount of light entering the eye (parasympathetic (CNIII) constrict the pupil, sympathetic dilate the pupil)

Front 6

diagram and histological section of the retina

Back 6

Front 7

describe the layers of the retina

Back 7

pigmented layer (outer)

sheet of melanin-containing epithelial cells - helps to absorb stray light

between choroid and the neural part

neural layer (inner) containing, from outer to inner…

photoreceptors - rods (black and white) and cones (colour - blue, green and red cones)

outer synaptic layer

bipolar cell layer - modifies signals from photoreceptors, contains bipolar cells, amacrine cells and horizontal cells

inner synaptic layer

ganglion cell layer - axons of ganglion cells extend posteriorly to the optic disc (‘blind spot’ - site where optic nerve, central retinal artery and central retinal vein exit the eyeball) to exit the eyeball as the optic nerve

Front 8

cavities and chambers f the eye

Back 8

the lens divides the interior of the eyeball into anterior cavity and vitreous chamber

anterior cavity

divided into anterior chamber (between cornea and iris) and posterior chamber (between iris and zonular fibres/lens)

filled with aqueous humor - transparent fluid that nourishes cornea and lens which is constantly supplied from ciliary processes of the ciliary body into the posterior chamber, and drains from the anterior chamber via the scleral sinus

vitreous chamber

between lens and the retina

contains vitreous body - jelly-like substance that holds the retina flush against the choroid giving the retina an even surface, with some phagocytes to keep it clear

Front 9

picture and description of rods and cones

Back 9

each rod and cone is divided into an outer segment, inner segment (with nuclear region) and a synaptic zone

saccules and discs in the outer segment contain photosensitive compounds that react to light to initiate APs in the the visual pathways

rods predominate outside the fovea

images of objects are focused on the retina, the light rays striking the retina generate APs in the rods and cones, these are conducted to the cerebral cortex where they produce the sensation of vision

Front 10

explain how light is refracted by the eye

explain long and short sightedness too

Back 10

* light is refracted mostly at the anterior surface of the cornea, but also at the anterior and posterior surfaces of the lens
* the retinal image is inverted

short-sightedness

eyeball is too long and light focuses in front of the retina

biconcave lens causes light rays to diverge before reaching the eye

long-sightedness

eyeball is too short and light focuses behind the retina

biconvex lens adds to the refractive power of the eye

Front 11

explain accommodation

Back 11

* the process by which the curvature of the lens is increased by contraction of the ciliary muscles in order to focus on close objects (less than 6m away)
* this, along with convergence of visual fields and pupillary constriction is the near response - allowing focused images of near objects

Front 12

basic explanation of electrical response to light in the eye

Back 12

* when light is absorbed by the photosensitive compounds (made up of the protein opsin and retinene1) in the rods and cones their structure changes, triggering a series of events that initiate neural activity
* rods, cones and horizontal cells are hyper-polarising, bi-polar cells are either hyper- or hypo-polarising and amacrine cells are hypo-polarising
* rods are much more sensitive, they respond to levels of illumination below the threshold for cones
* cones are sensitive to high levels of illumination when rod responses are maximum and cannot change

Front 13

explain the ionic basis of photoreceptor potentials

Back 13

* Na+ channels in the outer segments of the rods and cones in the dark are held open by cGMP
* NaK pumps in the inner segment maintain ionic equilibrium
* release of synaptic transmitter is steady in the dark
* light leads to conversion of cGMP to 5’-GMP, and some of the channels close, produces hyper-polarisation of the synaptic terminal of the photoreceptor
* this hyper-polarisation reduces the release of synaptic transmitter - generating a signal in the bipolar cells that ultimately leads to APs in the ganglion cells, which are transmitted to the brain

Front 14

explain photosensitive compounds

Back 14

made up of the protein opsin and retinene1

rods

the photosensitive pigment in the rods is called rhodopsin (contains retinene1 and an opsin called scotopsin), a G-protein linked receptor

light causes a conformational change in the rhodopsin, starting a G-protein linked pathway leads to the conversion of cGMP to 5’-GMP (causing some Na+channels to close)

cones

3 different kinds of cones with a different structured opsin for each

light causes a conformational change in the receptor, starting a G-protein linked pathway leads to the conversion of cGMP to 5’-GMP (causing some Na+channels to close)

melanopsin

a small number of photoreceptors contain melanopsin instead of rhodopsin or cone pigments

the axons of this neurons lead to the suprachiasmatic nuclei and the part of the lateral geniculate nucleus that controls pupillary response to light

Front 15

flowchart from light to neural response

Back 15

Front 16

describe how the image changes throughout the different neural pathways of the eye

Back 16

* in a sense the processing of visual images in the retina involves three images
* the first is formed by action of the photoreceptors, this is altered by the horizontal cells to a second image on the bipolar cells
* which is altered by the amacrine cells to a third image in the ganglion
* it is essentially this third image that reaches the occipital cortex

Front 17

psychological and social implication of blindness

Back 17

economic

* unable to work - loss of employment and income
* requires increased care - more likely to need residential care
* health risks - falls and fractures
* domestic - cooking, eating, dressing, etc., etc.
* shopping
* finance - bills, bank statements
* navigation - safety
* hearing impairment
* non-verbal communication
* social interactions
* TV, film and media
* isolation
* anxiety
* depression