Distribution patterns of individual medial lemniscal axons in the ventrobasal complex of the monkey thalamus.

Twenty-seven medial lemniscal axons were traced to their terminations in the thalamic ventrobasal complex of monkeys, following injection of horseradish peroxidase into the lemniscus at midbrain levels. Most axons had terminal ramifications at one horizontal level in the ventrobasal complex. All terminations were focal, many were anteroposteriorly elongated, though none were sufficiently long to occupy more than one-third to one-half of the anteroposterior extent of the VPLc nucleus. All terminal ramifications were compressed sagittally into a slab 200-300 micrometers wide. There was modest overlap in the terminal territories of adjacent labelled axons and, as judged light microscopically, only a small amount of convergence onto the vicinities of single cells. Axons terminating selectively in the anterodorsal shell or central core of VPLc could be identified. In the anterodorsal shell (of neurons responding to stimulation of deep tissues) axons tended to terminate either in the part projecting to area 3a or in that projecting to areas 3a and 2 of the cortex. In the central core (of neurons responding to cutaneous stimulation and projecting to areas 3b and 1) larger ramifications were present in its center and smaller ramifications in its ventral part. A few axons had terminal ramifications at two dorsoventral levels in the cutaneous core; others had terminations in both the dorsal deep shell and the cutaneous core. In both cases, one ramification was always smaller than the other. The termination of lemniscal axons at defined foci instead of along the dorsoventral extent of the representational lamellae of the ventrobasal complex has implications for the nature of the body representation in the nuclei. Their tendency to be focal implies that not all members of an anteroposteriorly elongated rod of place-and-modality-specific ventrobasal cells that project their axon to a single column in the somatic sensory cortex receive afferent inputs from the same lemniscal fibers.