Morphology of single intracellularly stained axons terminating in area 3b of macaque monkeys.

We have studied the morphology of single thalamocortical axons innervating area 3b of postcentral somatosensory cortex in macaque monkeys. We recorded from axons in the white matter below the representation of the hand in postcentral cortex in two monkeys (Macaca fascicularis) by using micropipettes filled with horseradish peroxidase (HRP). When an axon was recorded, we delineated its receptive field and determined its modality, and if cutaneous, whether it was slowly or rapidly adapting (SA or RA). We then impaled the axon and injected it with HRP. We recorded and successfully injected many more RA than SA axons, possibly because of differences in their true proportions. The RA axonal arbors varied in mediolateral extent from 350 to 800 microns with a mean of 600 microns. One of the RA axons gave rise to four separate arbors spanning 2.5-3.0 mm of cortex. The single SA axon we recovered was 370 microns in width. We suggest that the individual terminal zones underlie the columnar parcellation of the somatosensory cortex. The presence of arbors spanning several such columns suggests that all regions within the arbor may not be equally effective in driving cortical cells under normal conditions, and such arbors may provide the substrate for a cortical response to alterations in the pattern of input.