Terminal arbors of individual, physiologically identified geniculocortical axons in the tree shrew's striate cortex

We studied the terminal patterns of single, physiologically identified geniculocortical axons in the striate cortex of the tree shrew by using intracellular recording and labeling methods. Axons were classified by their response to the onset (ON-center) or offset (OFF-center) of a light stimulus presented to the ipsilateral or contralateral eye. Then, we attempted to penetrate each axon for labeling with horseradish peroxidase. We recovered 23 axons and studied 16 of these in detail. Light microscopic reconstructions of these axons revealed several distinct terminal patterns within cortical layer IV. ON-center axons had terminal arbors that ended mainly in the upper part of layer IV (IVa), while OFF-center axons ended in the lower part of layer IV (IVb). Within layers IVa and IVb, axons driven by the ipsilateral eye and those driven by the contralateral eye had overlapping distributions. However, their terminal arbors differed in size, in shape, and in the number of boutons. Compared with contralateral eye arbors, ipsilateral eye axons were on average three times larger in lateral extent (925 microns vs. 325 microns), spread over four times the surface area (0.13 mm2 vs. 0.03 mm2), and supported one and one-half times as many terminal boutons (1,647 vs. 1,086). The ipsilateral eye axons had more boutons at the edges of layer IV (i.e., the upper part of layer IVa and the lower part of layer IVb), while those from the contralateral eye axons were more evenly distributed. These results show that each functional class of geniculocortical fiber has a different laminar and areal arrangement of boutons and we consider the significance of these differences for visual cortical function.