How can light hit one part of a ganglion cell but not another part?

[sazr]

In regards to human vision and specifically the retina and ganglion cells.

Each receptive field is arranged into a central disk, the "center", and a concentric ring, the "surround", each region responding oppositely to light. For example, light in the centre might increase the firing of a particular ganglion cell, whereas light in the surround would decrease the firing of that cell.

I find it hard to comprehend that somehow the light can only hit one part of the retinal ganglion cells (either the centre or the surround) and not the other? Doesn't light go everywhere? Is the lens focusing light so accurately that light is hitting certain parts of a tiny cell (the centre) but not others (the outside/surround)?

Can anyone provide more insight as to how it works that light can hit one tiny part of a tiny cell but not another part of that tiny cell?

 

[BillTre]

Light comes in photons. Therefore a particle of light (a photon) can interact with only small cellular bits.
At low light levels, in a dark adapted retina, a single photo can interact with a photo-receptor.
Light usually interact with the photo-receptors not the ganglion cells, however in some species, some ganglion cells can react to light.

 

[Ygggdrasil]

Retinal ganglion cells do not respond directly to light. Rather, they indirectly receive input from photoreceptor cells (rods and cones) and are part of the downstream neural circuit that helps interpret visual information before it gets sent to the brain via the optic nerve. These cells are wired to other types of neurons such that some sets of photoreceptor cells activate excitatory connections that increase their firing while other sets of photoreceptor cells activate inhibitory connections that decrease their firing rate.

For more information see: https://en.wikipedia.org/wiki/Retinal_ganglion_cell

 

[BillTre]

Retinal ganglion cells do not respond directly to light.

Most retinal ganglion cells...

There are photosensitive retinal ganglion cells, even in humans.
They are rare, respond slowly to light, and apparently primarily involved in circadian rhythms (day-night cycles) rather than image forming.

Some think that photoreceptor cell types evolved from the equivalent of photosensitive ganglion cell types (the projection or output cells from the eye).

Sister and homologous cell types. Arendt (231) defined 'sister' and 'homologous' cell types as follows. Within an organism, 'sister cell types' are those that have evolved from one common precursor, by diversification of cell type. And across the evolutionary tree, 'homologous cell types' are those that have evolved from the same type of precursor cell in the last common ancestor of the groups being compared. Using these definitions he argued that, within any vertebrate species, the horizontal cells, amacrine cells, and ganglion cells are 'sister' cell types, having descended from a single ancestral cell type. And across phyla, he argued that rhabdomeric photoreceptors and vertebrate retinal ganglion cells are 'homologous' cell types, having descended from a microvillar photoreceptor in the last common (bilaterian) ancestor.

from here, an extensive review of evolutionary changes in the visual system.

 

[atyy]

The center and surround are not due to light hitting different parts of a ganglion cell.

Light hits photoreceptors (cones and rods), which are the cells in the retina that convert light into neural signals. The center and surround of a ganglion cell are due to connections that the it receives indirectly from different photoreceptors. The surround is due to indirect connections from photoreceptors to a ganglion cell via horizontal cells and bipolar cells. Note that a photoreceptor that contributes to the center of a particular ganglion cell contributes to the surround of other ganglion cells.
https://en.wikipedia.org/wiki/Retina_horizontal_cell
https://en.wikipedia.org/wiki/Retina_bipolar_cell

It is possible to focus light to hit a single photoreceptor.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2735134/Nat Neurosci. 2009 Aug;12(8):967-9. doi: 10.1038/nn.2352. Epub 2009 Jun 28.
Sincich LC, Zhang Y, Tiruveedhula P, Horton JC, Roorda A.