8.1.6 Detection of Light

• 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