A study of fine and coarse retino‐fugal axons terminating in the geniculate C laminae and in the medial interlaminar nucleus of the mink

When retino‐geniculate axons of mink are labelled with 3 H proline, the laminar structure of the dorsal lateral geniculate nucleus is seen to resemble that previously described in the cat. Layers A, C and C2 receive a crossed input and layers A1 and C1 receive an uncrossed input. The parvocellular layer C2 is relatively thicker in the mink than it is in the cat, and studies of retino‐geniculate fiber degeneration show that it receives fine caliber retinal afferents, whereas layers A and C receive mainly coarse afferents. The medial interlaminar nucleus receives two quite distinct crossed retino‐geniculate inputs. There is a coarse fibered component, which is continuous with lamina C, and a fine fibered component, continuous ventrolaterally with lamina C2 and extending rostro‐dorsally as a narrow streak along the margin of the nucleus lateralis posterior. The uncrossed afferents to lamina A1 are as coarse as the crossed axons to layers A and C. The uncrossed afferents to lamina C1 are somewhat finer, but are not as fine as the afferents to lamina C2. There are not many uncrossed fibers to the medial interlaminar nucleus and the distinction between coarse and fine components is not shown clearly. However, it is possible to recognize fine axons extending towards the margin of the nucleus lateralis posterior. We interpret these results as showing that there are two functionally distinct inputs to the medial interlaminar nucleus, and that the rest of the lateral geniculate nucleus may be divided into two distinct systems: A magnocellular, receiving coarse axons, and a parvocellular, receiving fine axons. The magnocellular system has one “ipsilateral” layer (A1) flanked by two “contralateral” layers (A and the dorsal part of C), while the parvocellular system similarly consists of a single “ipsilateral” layer (C1) flanked by two “contralateral” layers (C2 and the ventral part of C).