Visual system

Optic Tract 

LGN

- The vast majority (~80%) of retinal ganglion neurons terminate in the LGN, which is a dorsal thalamic structure.
- BUT 80% of the excitatory input to the LGN comes not from retina but V1 and other cortical structures -- truly, it is the case that the eyes do not see what the mind does not know
- LGN also receives input from brainstem nuclei involved in attentiveness, which may be another way that attention modifies not only how well we see but what we see



Histologically, the LGN is a 6-layered structure

- Neurons from ipsilateral eye terminate on layers, 2, 3, and 5

- Neurons from contralateral eye terminate on layers, 1, 4, 6

- Layers 1 and 2 contain larger nuclei than those of layers 3-6 and are called magnocellular and parvocellular, and they contain input form the magnocellular (bigger receptive field, fast processing, low-contrast stimuli -- important for motion) and parvocellular (smaller receptive field, important for color, detail, and shape) retinal ganglion neurons, respectively. 

- the non-staining layers in between are koniocellular (greek for dust) - unknown function 

 LGN on MRI

now why does this GBM patient have homonymous hemianopsia? it looks like the T2 signal is invading into the distal optic tract, possibly LGN, and all of the bundles of right optic radiation. Remember: when its GBM, the T2 is not edema, its tumor.

Other places where retinal nuclei project:

- superior colliculus - important for coordinating eye movements - fixation (staring at one point, adjustments only to compensate for head motion), smooth pursuit (following one object smoothly over time), saccades (rapid movement in one direction) and vergence (eyes moving independently to maintain binocular/focused motion). Outputs go, as one would expect, to the nuclei of CN 3, 4, and 6. In higher primates, most of input to superior colliculus is not retina but visual cortex and frontal eye fields. 

- suprachiasmatic nucleus in hypothalamus - important for circadian rhythms and day/night cycles 

- pretectum - pupillary light reflex 

LGN to Visual cortex 

- There are two or three tracts depending on who you asked, and they can be named upper/lower, anterior/middle/posterior, ventral/dorsal etc. But the point is there is a bundle of fibers that carry upper visual quadrant information (meyer's loop) through temporal lobe and that terminates in the lower bank of the calcarine sulcus (lingual gyrus) and a bundle of fibers that carry lower visual quadrant information through the parietal lobe that terminates in the upper bank of the calcarine sulcus (the cuneus)
- The anatomy of meyer's loop is highly variable -- in some people, it will project all the way to the temporal pole. The traditional teaching that anterior/inferior temporal lobe is safe to resect is not always 100% true.

V1 lines the calcarine sulcus and extends into occipital pole

From V1, there is the dorsal stream "where" stream into parietal lobe (identifying motion, and relative positions in space of objets) and the ventral stream "what" stream into temporal lobe (object and person recognition). More on that next time 

sources

http://journal.frontiersin.org/article/10.3389/fnana.2010.00015/full

http://thebrain.mcgill.ca/flash/i/i_02/i_02_cr/i_02_cr_vis/i_02_cr_vis.html

http://iovs.arvojournals.org/article.aspx?articleid=2480802

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3236807/pdf/nihms319970.pdf