• The eye is a?sense organ?containing receptors sensitive to light intensity and wavelength
  • Receptors?are specialised cells that can generate an electrical impulse in a sensory neurone when stimulated by a particular stimulus e.g. light receptors are stimulated when light falls on them
• Light enters the eye through the?pupil?and is focused onto a region of the?retina?called the?fovea
  • The amount of light that enters the eye is controlled by the muscles of the?iris
  • Light is focused using the?lens, the shape of which is controlled by?ciliary?muscles?attached to the lens by?suspensory ligaments
    • The muscles change the shape of the lens to allow it to focus light reflected from objects at different distances from the eye
  • The fovea contains many?light receptors,?or?photoreceptors
• The retina contains two types of photoreceptors
  • Rod cells
    • Primarily located around the?outer retina
    • Sensitive to?light intensity?so can detect the presence and brightness of light
    • Images generated using information from only rod cells is?black and white
  • Cone cells
    • Mostly found grouped together in the?fovea
    • Sensitive to?different wavelengths?of visible light and so detect colour
      • Cone cells can be?red-sensitive, green-sensitive, or?blue-sensitive
      • The number of red-, green-, and blue-sensitive cone cells stimulated will?determine the colours seen
    • Images generated using information from cone cells will be in?colour
• Action potentials generated in the photoreceptor are transmitted to the brain via the?optic nerve
  • The optic nerve leaves the back of the eye from a region known as the?blind spot
    • The blind spot contains no photoreceptors

The eye focuses light on the retina, which contains many light receptors

Photoreceptors generate nerve impulses

• Photoreceptors in the eye generate action potentials when?stimulated?by?bright enough light?(rods), or by?light of a particular wavelength?(cones)
• Light-sensitive pigments?inside the photoreceptors are?bleached?when light falls on them e.g.
  • Rod cells contain a light-sensitive pigment called?rhodopsin
  • When light hits rhodopsin it?breaks apart?into constituent parts?retinal?and?opsin
  • The breaking apart of rhodopsin is known as?bleaching
• The bleaching of light-sensitive pigments causes a?chemical change?in the photoreceptor that results in the generation of a?nerve impulse
• Nerve impulses travel along a?bipolar neurone?to the optic nerve, which carries information to the brain

Information passes from rod and cone cells to the optic nerve via bipolar neurones. Note that you do not need to know about ganglion cells here

The action of rod cells

• The way in which rod cells pass information to the optic nerve is a bit back-to-front in comparison to the action of other nerve cells; rather than initiating an action potential when they are?depolarised, rod cells initiate action potentials in neighbouring bipolar neurones when they are?hyperpolarised
• In the?dark?the following occurs inside rod cells
  • Sodium ions?are actively pumped out of rod cells, generating a?concentration gradient
    • Sodium ions (Na+) are?positively charged?ions, also known as?cations
  • Sodium ions?diffuse?back down this concentration gradient into the rod cell via?sodium channels
    • Sodium channels are also known as?cation channels?because they allow the movement of positively charged ions
  • At this stage there is little difference in charge between the outside and inside of the rod cell, and the cell is said to be?depolarised
    • In reality the inside of the rod cell is?slightly negative?in comparison to the outside
  • The depolarised rod cell?releases neurotransmitters?which diffuse across a synapse to a?bipolar neurone
  • Rather than initiating an action potential in the bipolar neurone?this neurotransmitter inhibits the generation of an action potential, preventing a nerve impulse from being sent to the optic nerve
    • This neurotransmitter is said to be an?inhibitory neurotransmitter
• In the?light?the following occurs inside rod cells
  • Light?bleaches rhodopsin, causing it to break apart into retinal and opsin
  • The bleaching of rhodopsin causes the sodium ion channels in the cell surface membrane of the rod cell to?close,?preventing sodium ions from diffusing?back into the rod cell
    • The active transport of sodium ions out of the cell is still taking place, so sodium ions are removed from the cell but not able to return
  • The lack of positively charged ions entering the rod cell causes its interior to become?more negative?until it reaches a?hyperpolarised state
    • A membrane that is hyperpolarised has a?more negative?potential difference across it than the resting -70 mV
  • The hyperpolarised rod cell?stops releasing an inhibitory neurotransmitter, so the generation of an action potential in the neighbouring bipolar neurone is?no longer inhibited
  • An?action potential?is generated in the bipolar neurone attached to the rod cell and?an impulse is sent to the optic nerve

Rod cell membranes are depolarised in the dark and hyperpolarised in the light