NEUROBIOLOGY B5

• On-Centre Bipolar Cells

  • Increases in glutamate close Na+ channels and opens K+ channels via metabotropic GluRs
  • In response to light, they depolarise
  • In response to darkness, they hyperpolarise
• Off-Centre Bipolar Cells

  • Increases in glutamate opens Na+ channels and closes K+ channels via ionotropic GluRs
  • In response to light, they hyperpolarise
  • In response to dark, they depolarise
• Centre-Surround Antagonism (Lateral Inhibition)

  • Horizontal cells receive photoreceptors input and have gap junctions between themselves
  • They have graded responses and inhibit 'centre' photoreceptors via a lateral feedback mechanism
  • Release inhibitory neurotransmitter GABA onto a 'centre' bipolar cell (feed-forward inhibition)
  • Due to lateral inhibition in the retina, we are poor at judging the absolute brightness of objects, rather we encode the contrast between objects and surround
• Amacrine Cells

  • Interneurons that receive input from one or more bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
  • Graded amacrine cells release inhibitory neurotransmitter, resulting in suppression (hyperpolarisation) of postsynaptic ganglion cells
• Retinal Ganglion Cells

  • Spiking RGC's transmit both image forming and non-image forming information to several regions of the brain (e.g. circadian rhythms, pupillary light reflex)
  • RGCs are 'ON' and 'OFF' centre type (polarity)
  • Two 'classes' (M and P), but over 20 types of RGC (polarity, RF size, adaptation)
  • M and P cells project to the magnocellular and parvocellular layers of the LGN, also Koniocellular (blue-yellow)
  • Both neuron types exhibit adaptation, however responses from M-type ganglion cells are more transient
• M-Type Ganglion Cells

  • Aka parasol cells
  • 10% of total, larger receptive fields
  • Encode achromatic contrast (not colour)
  • Faster responses and faster adaptation
  • Transient responses for sensitivity to movement
• P-Type Ganglion Cells

  • Aka midget cells
  • 70% of ganglion cell type
  • Sustained responses
  • Selective for opponent colour, red-green
  • Encode detail and form
• Centre-Surround Interactions (Centre Only)

  • ON cells fire when the centre is illuminated.
  • OFF cells fire when the light is off or removed.
• Centre-Surround Interactions (Surround Only)

  • ON cells show decreased firing when the surround is illuminated due to the inhibitory effect.
  • OFF cells show increased firing when the surround is illuminated.
• Centre-Surround Interactions (Centre and Surround Uniform)

  Both ON and OFF cells have little to no response because the uniform illumination cancels out due to the centre-surround antagonism
• Centre-Surround Interactions (Centre and Surround Opposite)

  • ON cells fire strongly when the center is illuminated, and the surround is dark.
  • OFF cells fire strongly when the center is dark, and the surround is illuminated.
• Optic Nerve and Chiasm

  • RGC axons become myelinated past the optic disk and form the optic nerve
  • About 90% of the ganglion cell axons go to the lateral geniculate nucleus (LGN) in the thalamus
  • Partial cross-over of the optic nerves from each eye align the visual fields
• Lateral Geniculate Nucleus (LGN)

  • Nucleus in the lateral-dorsal thalamus subserving visual perception - projects to the visual cortex
  • 6 layers with alternating input from the ipsilateral and contralateral visual fields
  • Therefore both eyes use retinotopic mapping
• The pathways maintained by the LGN are magnocellular, parvocellular and koniocellular pathways
• Magnocellular Pathway

  Input from M ganglion cells, larger receptive field, involving in motion vision
• Parvocellular Pathway


  Input from P ganglion cells, involved in form and colour
• Koniocellular Pathway

  Input from bistratified ganglion cells, large receptive field, likely involved in colour vision
• Potential Functions of the LGN

  • Bring visual field's from both eyes into register to make it easier for the cortex to combine
  • Gating of visual information flow
  • Feedback regulation --> spatial attention and arousal, saccadic eye movements, our brains predict what we will see to guide vision