Eye

• changing pupil diameter (by use of the circular and radial muscles) enables the eye to control the amount of light hitting the retina.
• the circular muscles contract to constrict the pupil
• the radial muscles contract to dilate the pupil
• the radial and circular muscles in an eye work antagonistically (when one set of muscles contracts, the other relaxes)
• events in bright light;

  bright light (stimulus) > light receptors in the eyes > sensory neurone > CNS > motor neurone > circular muscles in iris
• events in low light;

  low light (stimulus) > light receptors in eyes > sensory neurone > CNS > motor neurone > radial muscles in iris
• in low light, radial muscles contract and cause the iris to dilate which maximises the amount of light entering the eye - improving vision
• Focusing light in the eye:

  Light enters through the pupil and is focused on the fovea of the retina. Iris muscles control light entry. The lens focuses light; its shape is adjusted by the ciliary muscles via suspensory ligaments to focus on objects at different distances.
• the fovea contains many light receptors / photoreceptors
• the retina contains two types of photoreceptors; rod cells, cone cells
• Features of Rod cells:

  • located in the outer retina
  • sensitive to light intensity - detects bright light
  • images generated from rod cells are black and white
• Features of Cone cells:

  • mostly found grouped together in the fovea
  • sensitive to different wavelengths of visible light so detect colour
  • cone cells can be red-sensitive, green-sensitive, blue-sensitive
  • the number of red, green, and blue-sensitive cones stimulated will determine the colour seen
• action potentials generated in the photoreceptor are transmitted to the brain via the optic nerve
• rod cells contain a light-sensitive pigment called rhodopsin
• light-sensitive pigments inside the photoreceptors are bleached when light falls on them - for example, when light hits rhodopsin, it breaks apart into retinal and opsin
• the bleaching of light-sensitive pigments causes a chemical change in the photoreceptor - triggering a nerve impulse. This travels along a bipolar neurone to the optic nerve, which sends signals to the brain.
• rather than initiating an action potential when depolarised, rod cells initiate action potentials in neighbouring bipolar neurones when they are hyperpolarised
• Rod cells action in the dark:

  1. sodium ions are actively pumped out of rod cells, generating a concentration gradient
  2. sodium ions diffuse back down the concentration gradient into the rod cell via sodium channels (at this stage the cell is slightly depolarised -40mV)
  3. inhibitory neurotransmitters are released from the rod cell and diffuse to a bipolar neurone and causes hyperpolarisation (prevents an action potential in the bipolar neurone)
• Rod cells action in the light:

  1. Rhodopsin breaks down into retinal and opsin when its bleached
  2. opsin activates a series of membrane-bound reactions which causes the channels in the membrane of the rod cell to close - prevents the diffusion of Na+ back into the rod cell.
  3. Sodium ions continue to be pumped out by active transport.
  4. The membrane becomes hyperpolarised (-90mV).
  5. No neurotransmitter released at inhibitory synapse. Sodium ion channels in bipolar cell open and the membrane becomes depolarised, generating an action potential which travels to the ganglion neurone in the optic nerve.