The Eye

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Iris and pupil size

The iris consists of antagonistic muscles arranged in two layers:

  • the circular muscle
  • the radial muscle

In BRIGHT light:

  • Circular muscles contract / radial muscles relax
  • The iris widens and pupil constricts
  • Less light enters the eye
  • Prevents over-stimulation of light-sensitive cells

In DIM light:

  • Radial muscles contract / circular muscles relax
  • Iris constricts and pupil widens
  • More light enters the eye
  • Ensures stimulation of light sensitive cells in low light
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Lens and accommodation of light

Light travels in straight lines. The eye has the ability to adjust the refraction of light to focus on near or far objects, by adjusting the thickness of the lens.

Light rays from a DISTANT object are nearly parallel - require less refraction of light.

  • Ciliary muscles relax - increase tension in the suspensory ligaments
  • Pulling the lens into a thinner shape - less converging

Light rays from a NEAR object are diverging - require more refraction of light.

  • Ciliary muscles contract - release tension on eyeball
  • Lens adopts a fatter (thicker) shape - more converging
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Structure of rods and cones

The retine contains two cells which are receptors to light: rods, which are sensitive to dim light; and cones, which only respond in bright light but are able to discriminate fine detail and distinguish different colours.

Cones concentrated at fovea (centre of retina), while rods are mainly found around periphery of retina. The rods and cones synapse with bipolar neurones which in turn synapse with ganglion cells. The axons in the ganglion cells group together to make up the optic nerve.

Many rods synapse with each bipolar neurone and many bipolar cells connect with each neurone of the optic nerve. This is known as retinal convergence. Allows the generator potentials from individual rods to combine together (summation) and reach the threshold required to produce an AP.

Each cone cell generally synapses with a single bipolar neurone and a single ganglion cell. This allows high visual acuity - ability to produce highly precise colour vision of high resolution.

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  • The membranes of rod cells contain the pigment rhodopsin (protein opsin + retinal).
  • When light strikes rhodopsin, it breaks down into its components. This changes the membrane potential of rod cell and creates a generator potential.
  • If threshold level is reached, causes adjacent bipolar neurone to become depolarised and conduct an action potential
  • Rods are sensitive, because rhodopsin absorbs light readily and is easily broken down, and because retinal convergence enables many rods to be added together  - summation
  • However, there is decreased visual acuity and rods lack the ability to discriminate detail.

Dark adaptation

  • In bright light, all rhosopsin is broken down (bleached) and it takes time to be re-synthesised.
  • Explains why our vision in the dark is poor initially and gradually improves
  • Eyes have changed from being light-adapted to being dark-adapted
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  • Contain the pigment iodopsin, which is less readily broken down and will only produce a generator potential in bright light.
  • Cones provide high visual acuity and colour vision
  • Iodopsin exists in three different forms, each sensitive to different colours and each found in different types of cone
  • Iodopsin sensitive to colours blue, green and red - the trichromatic theory of colour vision
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Binocular vision

Some prey species, such as rabbit, have their eyes positioned on opposite sides of their heads so as to give the widest possible field of view - facilitates detection of potential predators.

Binocular vision - the use of two eyes positioned on the front of the head to create a single image, allowing more accurate judgement of distance.

Stereoscopic vision - the ability to form 3D images

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