Eye

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Describe how the lens of the eye focuses the image onto the retina.

  • two lenses (a fixed and a flexible) focus light

On what two factors does one's ability to focus an image clearly depend?

  1. shape of lens (affected by ciliary muscles)
  2. the length of the eye (from lens to retina)

What are two functions of the iris?

  1. look purty.
  2. constricts to make pupil smaller
  3. can also sharpen focus

List the five cell types in the retina and draw a diagram of their anatomical connections arrangement

  1. Sensitive receptors (i.e., rods and cones)
  2. Ganglion cells - only output from the eye
  3. Bipolar - connect sensitive receptors to ganglion cells
  4. Horizontal - converge signals from several receptors
    • this determines how many receptors each ganglion cell "sees"
  5. Amacrine - same convergance as horizontal cells, but for peripheral receptors (namely rods)

Describe how light hyperpolarizes rods/cones and how a molecular cascade amplifies this process

  • Light hyperpolarizes rods and cones, while dark depolarizes them.
  • These occur by amplification via a molecular cascade:
    • Cascade's purpose is the control of cGMP concentration
    • In dark, high cGMP concentration keeps Na+ channels open, causing depolarization
    • In light, cGMP concentration drops, Na+ channels close, and hyperpolarization occurs (inside is more negative)
  • one photon converts a rhodopsin molecule
  • this activates 100 molecules of G-protein transducin
  • These activate cGMP phosphodiesterases, which break down 100s of cGMPs
  • this closes down thousands of Na+ gates
  • leads to hyperpolarization of cell


Define the receptive field of a sensory neuron and the receptive field of a ganglion cell in particular

  • Receptive field of a sensory neuron - area over which a sensory neuron is altered by stimuli
  • Receptive field of a ganglion cell - area over which a ganglion cell is altered by light stimuli

Use this definition to show which light stimulus would best activate ganglion cells

  • what the hell does this objective even mean?

Explain the function of these receptive fields

  • Depends on the receptive field
  • Generally, the function is to "compress" data so that it can all fit through the optic nerve
  • The on-center/off-surround and vice versa are used to elaborate edges 'n' shit.

Explain why it is difficult to read by moonlight

  • Cones, which provide high acuity, are not very sensitive to light
  • Rods, which are very sensitive to light, do not provide high acuity
    • Since more than one rod converges on each ganglion cell
    • therefore information is integrated over 3deg instead of 0.3 deg in cones
  • Hence, trying to read at night sucks ass.

List the anatomical and functional differences between ganglion cells in the fovea and ganglion cells in the peripheral retina

Anatomical

  • Ganglion cells in the periphery connect to more rods than cones.
  • Ganglion cells in the periphery connect to more receptors per ganglion than in the fovea (3deg vs. 0.3deg)

Functional

  • Ganglion cells in the periphery represent lower acuity
  • Ganglion cells in the periphery basically represent black and white vision, whereas they represent colour vision in the fovea

Show how a two-color cone system is better at distinguishing an object from its background than a single color cone system

  • The problem with a single colour cone system is that it has a normal curve-shaped response to the visible light spectrum
  • This means that two colours would produce the same response in a single colour receptor
  • With two- or three-colour cone systems, each of the different kinds of receptors produces a slightly different response, causing greater colour differentiation

How many gradations of color can the human eye distinguish? Explain how a three-cone system achieves this

  • 200 hues x 20 saturations x 500 brightness levels = 2 million gradiations of colour