pupil hole light entry

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pupil - hole light entry
Anatomy of the human eyeball
sclera - protective white outer surface
iris - outer pigmented layer inner layer of
blood vessels
cornea - very sensitive to touch
ciliary muscles - control shape of lens
aqueous humor - fluid in anterior chamber
continually replenished
vitreous humor - fluid in posterior chamber not
replenished floaters form
Coren et al. (2004)
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Anatomy of the human eyeball
choroid - blood vessels nourishes retina
retina - detects light initiates neural messages
macula - yellow pigmented spot
fovea - best vision
optic disk - axons leave eye
optic nerve - axons of retinal neurons
Coren et al. (2004)
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INSTRUCTIONAL OBJECTIVES

• By the end of the class, you will be able to
• 1. List the layers of the retina and the type of
  cell found in each layer.
• 2. Describe the distribution of rods and cones
  across the retina and explain why we have a
  blindspot.
• 3. Describe photoactivation and transduction in
  the photoreceptors.
• 4. List the 3 types of bipolar cell and describe
  how their graded potentials change with light
  levels.
• 5. Define receptive field, and discuss how
  receptive-field-centre size across the retina
  affects acuity and sensitivity.
• List 3 types of retinal ganglion cell and
  describe their luminance (ON/OFF), spatial and
  temporal responses.

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Layers of the retina
side view of retina
light capture transduction in photoreceptors
pigment epithelium keeps photopigment from
bleaching
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Layers of the retina
Fig 2.7
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Photoreceptors and transduction
the fovea

• most neurons pushed away light passes through
  thinner layer
• only cones in centre of fovea densely packed
• more rods, fewer cones away from fovea less
  dense

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Photoreceptors and transduction
distribution of rods and cones across the retina
Fig 2.9
no rods or cones in the region of the optic disk
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Photoreceptors and transduction
finding your blindspot
look at the square with your left eye and move
the page closer until the top circle disappears
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Photoreceptors and transduction
stores photopigments
makes photopigments
nucleus (outer nuclear layer)
axon (outer plexiform layer)
Fig 2.8
several synaptic terminals
one synaptic terminal
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Photoreceptors and transduction
wavelength sensitivity
S-, M- and L-cones near fovea
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Photoreceptors and transduction
red due to rod photopigment rhodopsin
light turned on no bleaching yet
(frog retina)
photoactivation bleaching of photopigment
colour fades as photopigment bleaches with more
time in the light
Goldstein (2007)
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Photoreceptors and transduction
photoactivation
rhodopsin molecule
shape in dark
Goldstein (2007)
shape after light absorption
transduction process by which light is
transformed into electrical energy
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Photoreceptors and transduction
transduction
photopigment in outer segment
light absorption reduces the flow of sodium
bipolar cell
no signal while glutamate is released in the dark
Goldstein
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Photoreceptors and transduction
photoreceptor hyperpolarizes --calcium
reduced--glutamate reduced--bipolar cell
stimulated
cyclicGMP keeps sodium flowing
deactivation of cGMP blocks sodium
Coren et al. (2004)
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Synapses and neural transmission
in the dark
light absorption
photoreceptor hyperpolarizes
resting membrane potential -40 mV
less glutamate membrane channels open
glutamate binds to bipolar membrane receptors
ion channels close
bipolar cell depolarizes
graded potential slow change in membrane
potential
occurs in photoreceptors, bipolar cells,
horizontal cells
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Synapses and neural transmission

• Diffuse bipolar cells
• depolarize with increase in photon catch by
  photoreceptors increase in rate of
  neurotransmitter release
• connected to rods or to cones in peripheral
  retina
• up to 50 photoreceptors per bipolar cell
• ON midget bipolar cells
• depolarize with increase in photon catch by
  photoreceptors increase in rate of
  neurotransmitter release
• connected to cones in fovea 1 cone per bipolar
  cell
• OFF midget bipolar cells
• depolarize with decrease in photon catch by
  photoreceptors increase in rate of
  neurotransmitter release
• connected to cones in fovea 1 cone per bipolar
  cell

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Synapses and neural transmission
bipolar cell depolarizes
glutamate released into synaptic cleft ion
channels open ganglion cell depolarizes
depolarization of bipolar cell happens with more
light in ON pathway and with less light in OFF
pathway
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Synapses and neural transmission
source Coren et al. (2004)
action potential (spike) rapid depolarization
occurs in amacrine cells and retinal ganglion
cells
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Retinal ganglion cells (anatomy)
parasol
midget
bistratified
Wolfe et al. (2006)
thick axons 10 also called M cells
thin axons 80 also called P cells
intermediate axons lt10 also called K cells
photosensitive retinal ganglion cells are
involved in circadian rhythms and pupil reflexes,
but not vision
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Retinal ganglion cells (wiring)
receptive field the part of the retina or the
part of the visual field to which a neuron
responds
ganglion cell rf size determined by
photoreceptors connected to that cell (through
bipolar cell)
large receptive field
small receptive fields
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Retinal ganglion cells (wiring)
rods
cones
convergence of rods onto ganglion cells yields
high sensitivity
lack of convergence in cones yields high acuity
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Retinal ganglion cells (spatial responses)
Coren et al. (2004)
centre surround give opposite response (spatial
opponency)
centre size determined by photoreceptor
connections through bipolar cells
surround size determined by photoreceptor
connections through horizontal cells
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Retinal ganglion cells (spatial responses)
midget ganglion cells have smaller receptive
fields than parasol and bistratified ganglion
cells
www.sinauer.com/wolfe2e/chap2/ganglionF.htm
Fig 2.13
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Retinal ganglion cells (temporal responses)
midget ganglion cells
sustained response lasts entire time light is on
or off in receptive field
transient response brief response at onset
and/or offset of light in receptive field
parasol ganglion cells
Mather (2006)
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Summary

• Light on
• ON bipolars
• depolarize neurotransmitter increase
• ON-centre ganglion cells fire (depolarize)
• OFF bipolars
• hyperpolarize neurotransmitter decrease

• Light off
• ON bipolars
• hyperpolarize neurotransmitter decrease
• OFF bipolars
• depolarize neurotransmitter increase
• OFF-centre ganglion cells fire (depolarize)