eye and retina Flashcards
what does vision allow us to do
-Detect prey (in modern world = source food)
-Detect predators (in modern world = danger)
-Detect mates
-Communicate
-More than a third of the human neocortex is involved in analysing the visual world
why is light important in vision
-Electromagnetic radiation that is visible
-Light has:
=a wavelength – distance between peaks or troughs
=a frequency – number of waves per second
=an amplitude – difference between wave peak and trough (intensity of light)
how do light rays interact
-Electromagnetic light travels in straight lines, known as rays, until it interacts with atoms and molecules
Light rays interact in three ways:
=relefction
=absorption- e.g. photoreceptors and pigmented epithelium of retina
=refraction-bending due to difference in speed of light through different media
e.g. used by cornea to form images on the retina
why can Eye position can vary with species
-Monocular vs binocular vision
-What can you see with one eye?
-Field of view
-Depth perception
-Predator vs prey
what are the components of the eye
-Pupil – lets light inside the eye
-Iris – contains muscles which control the amount of light entering the eye
-Cornea – glassy, transparent covering of the pupil and iris that refracts light
-Sclera – continuous with cornea, forms the tough, protective wall of the eyeball to give it its shape
-Extraocular muscles – move the eyeball, controlled by oculomotor nerve (cranial nerve III)
-Optic nerve (cranial nerve II) – carries axons from retina to brain
What does the eye look like using an opthalmoscope
-Optic disk (blind spot)– origin of blood vessels and optic nerve, cannot sense light
-Macula (blood vessels get thinner as you reach this) – region of retina for central vision, devoid of large blood vessels to improve vision quality
-Fovea – retina is thinnest here and is the area of highest visual acuity
what does the retina contain
-The retina contains the sensory receptor cells and afferent neurons
whats the lens suspended by
-The lens is suspended by zonal fibres (suspensory ligaments) which are attached to the ciliary muscle, enabling stretching of the lens
whats image formation
-Light rays must be focussed onto the retina (ideally the fovea)
-Refraction occurs at the:
cornea, ~80%
lens, remaining 20%
-Degree of refraction is determined by:
=Difference in refractive indices between the two media
=The angle at which light hits the interface between these two media
what happens with refraction by the cornea
-Light arrives at the cornea through air but the cornea is mainly water
-Light travels more slowly through water than air due to higher density = refraction occurs
-Distance from refractive surface to convergence of parallel light rays = focal distance
whats Accommodation by the lens
-how the eye changes
-Distant objects
=almost parallel light rays
=cornea provides sufficient refraction to focus them on the retina
=cillary muscles relaxed, suspensory muscles are taut
-Closer objects (<7m away)
=light rays are not parallel
=requires additional refraction to focus them on the retina
=provided by the fattening of the lens
=cillary muscles fatter an push into circle
-Rounding of the lens increases the refractive power to focus closer objects on the fovea
what are the problems with focussing
-Eye is emmetropic when lens is flat and we are focussing a distant object
-Farsightedness- can see far away but not up close
=eye is too short
=near objects are focussed behind the retina
=not enough refraction
-Reading glasses
=make light rays more parallel
=requires less refraction
=convex lens which increases refraction and makes light rays parallel
-Near (short) sightedness - things are focused before the retina
=eye is too long
=distant objects are focussed before retina
=too much refraction
-Glasses
=make light rays diverge
=requires more refraction
=concave lens so it splits the light rays and ,over them further apart
whats Laminar organisation of the retina
-Light focussed on the retina must now be converted into neural activity
-Light must pass through ganglion cells and bipolar cells before it reaches the photoreceptors
-Light that passes all the way through the retina is absorbed by the pigmented epithelium
what are cells of the retina like
-Ganglion cells – output from retina
-Amacrine cells – modulate information transfer between GCs and BCs
-Bipolar cells – connect photoreceptors to ganglion cells
-Horizontal cells – modulate information transfer between photoreceptors and BCs
-Photoreceptors – sensory transducers, both rods and cones
what are photoreceptors like
-Membranous disks contain light-sensitive photopigments that absorb light
-Duplicity theory: can’t have high sensitivity and high resolution in single receptor
-Separate systems for monochrome and colour
what are rod photoreceptors
-Greater number of disks
-Higher photopigment concentration
-1000 times more sensitive to light than cones
-Enable vision in low light (scotopic) conditions i.e. at night time
-Low visual acuity/resolution
what are cone photoreceptors
-Fewer disks
-Used during daylight (photopic) conditions
-Enable colour vision
-High visual acuity/resolution
-Lower sensitivity
why does retinal structure vary with regions
-The fovea contains most of the 5 million cones and no rods
-Central retina = low convergence, low sensitivity, high resolution
-Peripheral retina = high convergence, high sensitivity, low resolution
