The thalamus (dorsal) is divided by the internal medullary lamina (IML) into 4 anatomical groups: 1) an anterior nuclear group enclosed by the anterior bifurcation of IML, 2)a mediodorsal group medial to the IML, 3)a lateral group (lateral to IML), and 4) some intralaminar nuclei within the lamina. The largest group, the lateral group, is then subdivided into a ventral tier (VA,VL,VP, MGN, LGN) and a dorsal tier above it (LD,LP, and Pulvinar). The LGN and MGN together are sometimes called the “metathalamus”; however, if LGN and MGN are instead considered the posterior-most members of the ventral tier then all of the primary sensory and motor relay nuclei are conveniently located in the ventral tier (of the lateral group). Finally, the shell-like thalamic reticular nucleus (TRN) is delineated by the external medullary lamina is its own special group/division of the thalamus.
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