The goal of this report is to gain a better, and hopefully complete, understanding of the Lateral Geniculate Nucleus.
The optic nerve projects onto a structure called the Lateral Geniculate Nucleus. The LGN is structured such that it has a left and a right hemisphere, within which there are six distinct layers of the LGN.
Neurons of the LGN
The receptive fields of adjacent neurons overlap since neighbouring regions of the retina make connections with neighbouring geniculate cells. This overlap means that the receptive field of the LGN is even more scrutinous when it comes to diffuse light than is the ganglion receptive field. Therefore, whereas the ganglion receptive field does somewhat respond to diffuse light, the LGN receptive field is very poor at responding to diffuse light. Although the primary driving input of the LGN is the retinal activation, this only accounts for 30% of total input; the majority of the input comes in the form of feedback information from the primary visual cortex (V1). Also, the input from the optic nerve is separated into 3 types, M, P and K, based on the size of the cells, and they project to M, P and K cells within the LGN. The difference in size has a direct bearing on the propogation delay, with the larger M cells arriving first, and the smaller K cells arriving last
Historically the layers are numbered from the bottom up, and layers 1 and 2 have the larger, and much faster cells. These M cells work much faster, however, they do not process as much information. The P cells on the other hand, which are found in layers 3 to 6, are much smaller and slower, but come with the added advantage that they can do complex calculations such as colour detection. Between each of the M and P layers lies a zone of very small cells, the K cells. Layers 2, 3 and 5 receive their inputs from the ipsilateral eye (w.r.t the left or right hemisphere) whereas layers 1, 4 and 6 receive their inputs from the contralateral eye (w.r.t. the left or the right hemisphere). There are roughly 1 million cells in the LGN, however, the optic nerve fibres each connect to multiple LGN cells, as opposed to a simple one to one mapping. Not only are the cells topographically ordered (neighbouring photoreceptors project to neighbouring cells) but they are retinotopically registered along the different levels. Therefore, if a sample rod is taken from layer 1 to layer 6, then (for each eye) those appropriate cells will have the same receptive fields.
Feedback from V1
Feedback connections from V1 form about 70% of the connections in the LGN, with axon size playing no important role. Connections were never made to both P and M layers from the same cell, and generally axons which connect to P or M layers also connect to a K layer. Also there was never the case that one cortical cell would innervate two K layers. It seems like the K layer is some sort of buffer.
Ichida, Jennifer M (2002). "Organization of the Feedback Pathway from Striate Cortex (V1) to the Lateral Geniculate Nucleus (LGN) in the Owl Monkey". Comparative Neurology 454: 272–283.
Callaway, Edward M (2005). "Structure and Function of Parallel Pathways in the Primate Early Visual System". Journal of Physiology. 566.1: 13–19.