Physiology 3120 Lecture Notes - Amacrine Cell, Ganglion Cell, Axon Terminal

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Published on 26 Nov 2011
Department
Professor
Human Physiology
Wednesday, November 4, 2009
“Neuro VI”
The Visual System
Cell types in the retina
Rods/cones: receptors
Bipolar cell
Ganglion cell
Horizontal cell
Acts on axon terminal of rods/cones
Amacrine cells
Functional characteristics of the rod & cone systems
Rods
Night vision
Peripheral (few in fovea)
Sensitive to faint light (single photon)
Due to higher photopigment & convergence
Cones
Day vision/bright light/colour vision
Concentrated in the fovea
Most sensitive part of the eye
Must constantly move the eye to focus objects of interest on the fovea
Greater acuity (less convergence)
Flow diagram of the steps in the retina by which light is transduced to action potentials
Dark
Sodium channels open, so inward sodium current
Depolarization of rods & cones
Release of inhibitory NT
Hyperpolarization of on-centre bipolar cells
Light
Activated pigment closes sodium channels
Hyperpolarization of rods & cones proportional to amount of light
Decrease in inhibitory transmitter
Depolarization of on-centre bipolar cells, leading to depolarization & action potentials in
ganglion cells (the first cells in the pathway that generate action potentials)
Bipolar cells are very short, so current spreads electrotonically
Receptive fields of retinal ganglion cells
Circular
Can be one of 2 types
ON-centre; OFF-surround
OFF-centre; ON-surround
ON = increase in APs; OFF = decrease in APs
OFF surround mediated by inhibitory (“sign-changing”) interneurons around the ON centre (see
neuro 26)
Major feature of information processing by the retina
Responds to contrast (neuro 29)
Retinal ganglion cells don’t respond to light per se, but contrast in light shone on the
retina
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Document Summary

The visual system: cell types in the retina. Amacrine cells: functional characteristics of the rod & cone systems. Sensitive to faint light (single photon: due to higher photopigment & convergence. Concentrated in the fovea: most sensitive part of the eye, must constantly move the eye to focus objects of interest on the fovea. Greater acuity (less convergence: flow diagram of the steps in the retina by which light is transduced to action potentials. Sodium channels open, so inward sodium current. Hyperpolarization of rods & cones proportional to amount of light. On = increase in aps; off = decrease in aps. Off surround mediated by inhibitory ( sign-changing ) interneurons around the on centre (see neuro 26: major feature of information processing by the retina. Retinal ganglion cells don"t respond to light per se, but contrast in light shone on the retina. First step of processing in visual system.

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