Physiology Flashcards
What cells connect rods and cones to retinal ganglion cells?
Bipolar cells
What is myopia?
Short-sightedness
Images focussed in front of retina instead of on retina
Concave lens corrects
What is hyperopia?
Long-sightedness
Images focussed behind retina
Convex lens corrects
Describe rods
Colour vision at low light levels
One form of opsin
Do not mediate colour vision, low spatial acuity
Describe cones
Responsible for colour vision
3 types depending on opsin type responding to red, green and blue respectively
Colour blindness
Usually genetic, carried on X chromosome so more men affected than women
Most commonly affects red/green colours and is thought to be due to dysfunctional cone cells
Presbyopia
Form of hyperopia (long-sightedness) caused by loss of elasticity of lens in eye
Typically middle and old age
Astigmatism
occurs when cornea is slightly curved rather than completely round
refractive error where the light is focussed in many spots not just one area of retina
Either glasses or toric contact lenses
Describe the optic disc
Entry point of optic nerve
Blind spot
Describe the fovea
Area where photoreceptor cells are smaller and more densely packed than other areas of retina
Do off-centre retinal ganglion cells fire in response to light object on dark background?
No, dark object on light background
What is a chromophore?
Molecule that absorbs light at specific wavelength; colour
Describe how vision occurs
Outer segments of rods and cones contain region of membrane bound discs which contain proteins (opsin) bound to chromophore 11-cis-retinal
When visible light hits this it goes from 11-cis to all-trans retinal
New form does not fit as well so series of conformational changes occurs during which phosphodiesterase hydrolyses cGMP
cGMP is required to open Na channels, so these now close causing hyperpolarisation and sending an impulse to the brain
What is rhodopsin?
Opsin bound to 11-cis-retinal
Describe the radial muscle of the iris
Radial muscle of iris aka dilator pupillae
Causes dilation of the pupil in response to sympathetic nerve activity
