Photoreception

• Rod cells are numerous and evenly spread across the retina
• Rod cells are absent from the foveal region
• Rods operate at low light intensities
• Rods contain the light sensitive pigment rhodopsin
• Rhodopsin absorbs light and is bleached to form retinal and opsin (protein)
• Rods provide low visual acuity due to retinal convergence
• Rhodopsin reforms as light intensity falls
• Cone cells are less numerous than rod cells in the retina
• Cone cells are concentrated in the fovea
• Cones operate at high light intensities
• Cones contain light sensitive pigments called iodopsins
• There are three types of cone cell with different iodopsins sensitive to red green and blue lights giving trichromatic vision
• Cones provide colour vision when the relative stimulation of different types of cone cell is interpreted by the brain
• Cones provide high visual acuity as in the fovea a single cone is connected to a single optic nerve fibre via a relay neurone
• Upon sudden light, all rhodopsin is bleached instantly
• There is a delay in the reformation of rhodopsin when lights turn off
• The impulses from the optic nerve are interpreted by the occipital lobe of the brain
• The lens gets less elastic over time
• There are no receptor cells in the blind spot
• Laminations in rod and cone cells are covered in photosensitive pigment and increase surface area
• The tapetum in the choroid can be pigmented (reflective/black) and affects light reflection
• retinal is a derivative of vitamin A
• Variation in the stimulation of different cones results in trichromatic vision
• The photoreceptor cells of the retina are arranged with the photosensitive part of the cell towards the outer region of the retina. Light must therefore pass through a thin layer of nerve tissue to impact on the photosensitive region
• At low light intensities each rod releases small amount of neurotransmitter. It accumulated until there is enough to trigger an nerve impulse in the relay neurone. This is spatial summation
• Visual acuity is a measure of the capacity of the brain to resolve stimuli that are separated by space
• The cone density is the main factor that influence visual acuity
• Spatial summation results in a decrease in acuity because stimulation of several receptors only generates a single impulse
• The axons of the sensory neurones connected to the cones and rods form the optic nerve
• Types of vision

  A) peripheral

  B) central

  
• Our visual resolution is lowest in the peripheral vision, because this area is mostly made of rod cells
• Multiple rod cells connect to a single bipolar cell (retinal convergence). An action potential is triggered in this cell before being sent to the brain. The brain has no way of determining which rod cell the signal came from, since the bipolar cell response is the same, lowering the resolution since the exact location of light cannot be determined
• Each cone cell is connected to a single bipolar cell. This means the exact location of light can be determined, so there is a higher visual acuity
• To create a generator potential, the rhodopsin must be broken down by light energy. If enough pigment has broken down for the threshold to be met in the bipolar cell, an action potential is triggered
• The threshold can be reached in low light intensities because multiple rod cells are connected to a single bipolar cell
• The only difference between cone cells is the type of iodopsin pigment they contain
• Iodopsin is only broken down if there is a high light intensity, so action potentials can only be generated with enough light - why we can't see colour in the dark
• The fovea receives the highest light intensity, so this is where most cone cells are found