Lecture 22 (last lecture)

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Cell and Systems Biology
Melanie Woodin

CSB332H1S L22; Apr. 04, 2012  “OFF” centre cells ↓ firing of Sensory Systems APs when central area of RF illuminated & ↑ when light What is significance of bipolar cell RFs? turned off  D & H bipolars not only responding to light  Light shone onto surround of  Begin to analyze info about light patterns “OFF” centre RF excitation of  Tell the CNS about small spots of light surrounded by dark (& vice neuron versa)  Essentially responding to contrasting patterns of light & dark over small patch of retina “On” and “Off” Ganglion Cells  APs evoked in “on” ganglion cells by small spots of light shone onto center of field surrounded by darkness  APs evoked in “off” ganglion cells by small dark spots surrounded by light Receptive Fields Sizes of Receptive Fields  Neighbouring RFs collect info from similar but not identical areas of retina  Small spot of light (0.1 mm)  activate RFs of many ganglions  Throughout visual system: Neurons processing related info clustered together  Size of ganglion RF depends on location in retina o Ganglions in centre have much smaller RFs than those in periphery o Smallest in fovea st  RFs of ganglions 1 characterized by Stephen Kuffler in 1950s Ganglion Cell Types  Eyes of anesthetized cat focused on screen w various patterns of light generated by computer 2 main grps; project to dif regions of LGN:  Electrode records responses from single cell in visual pathway 1. P – parvocellular  RF for ganglion determined by correlating area of screen o Smaller o Small RFs illuminated w neuron’s firing o Respond to colour & fine detail 2. M - magnocellular Ganglion Cell Receptive Fields  Ganglions normally fire APs even at rest o Large  RF mapped by correlating light shone on retina w increases or o Large RFs o Respond well to movement decreases in AP firing of ganglion  2 main grps of ganglions: “ON” & “OFF” centre fields What Info do ganglion cells convey?  “ON” centre cells respond best to spot of light shone onto central pat Photorecs ~ light meters of RF; illumination of surrounding area ↓ firing of APs  Illumination of entire RF elicits weak discharges because centre & o Ganglions ignore much of this info from photorecs o Ganglions don’t convey info about absolute levels of illumination surround cancel each other out  Ganglion cells measure difs in their RFs by comparing degrees of illumination
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