Eye

• What is the function of the retina?


  contains photoreceptors that detect light intensity and colour
• what is the function of the pupil?


  hole that allows light to enter the eye
• What is the function of the optic nerve?


  group of neurones that carry the impulses between the eye and the brain
• what is the function of the lens?


  focuses light rays on the retina
• What is the function of the iris?


  Controls how much light enters the pupil
• what is the eye?


  sense organ that contains receptors sensitive to light intensity and colour
• circular muscles in the eye contract and radial muscles relax
• photoreceptors

  -light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments

  -light bleaches the pigment which causes a chemical change and alters the Na+ permeability

  -generator potential is created and if it reaches threshold, an impulse is sent to the bipolar cell
• photoreceptor synapses with bipolar cell which synapses with ganglion cell which converges to the optic nerve as the axon extends to it
• light rays must pass through sensory neurone and bipolar cell before reaching the receptor cell
• -each rod and cone is divided into an outer segment, an inner segment, a nuclear region and a synaptic zone

  -highly specialised eukaryotic cells
• Rods

  -high visual sensitivity

  -several rods connected to a single neurone

  -spatial summation allows threshold to be overcome
• Rods

  -multiple rods synapse with a single bipolar cell/neurone

  -brain is unable to determine exactly where on the retina the rays of light struck

  -converging: brain receives a single action potential for a number of activated rods so low visual acuity

  -spatial summation:combined generator potentials combine to overcome threshold

  -high sensitibity
• Cones

  -high visual acuity

  -each cone is connected to a single bipolar cell/neurone

  -cone sends separate sets of impulses to the brain
• Cones

  -single cone cell synapses with a single bipolar cell

  -each action potential is generated by a specific cone, brain is able to determine exactly where on the retina the ray of light struck

  -high visual acuity

  -no spatial summations

  -no convergence of small generator potentials as there is no opportunity for an additive effect so low sensitivity
• Rods


  -photosensitive pigment is rhodopsin

  -combination of retinal (vitamin A derivative) and opsin (protein) which when split causes bleaching

  -very sensitive to light and so are used in low light intensity

  -opsin binds to the membrane and changes Na+ permeability in the membrane (depolarises) and evokes a generator potential

  -rhodopsin has to be re synthesised which requires ATP

  -monochromatic vision
• Cones


  -photosesitive pigment is iodopsin, contains retinal (vitamin A derivative) and photopsin (protein)

  -bright light splits and causes bleaching

  -3 different cones found in the human eye each have a different photosensitive pigment

  -different colours are seen due to the stimulation of more than one cone, combined effect allow humans to observe all colours on visible spectrum

  -less sensitive to light intensity

  -sensitive to wavelength (colour) of light
• Cone pigments

  -red-sensitive

  -blue-sensitive

  -green-sensitive

  (TRICHROMATIC THEORY OF COLOUR)
• Gene to make photosensitive pigments are on the non-homologous x chromosome and so mutation/abnormalities are sex-linked

  -non functional or missing

  e.g colour blindness is more common in males
• what is visual sensitivity?


  the amount of light required to stimulate a receptor
• what is visual acuity?


  clarity and ability to distinguish 2 separate points
• Fovea


  -only contains cone cells (max number of cones)