S1 to S2 hind- and forelimb projections in the agouti somatosensory cortex: Axon fragments morphological analysis

The integration of cutaneous, proprioceptive, and motor information in area S2 seems to be essential for manual object recognition and motor control. Part of the inputs to S2 comes from area S1. However no detailed investigations of the morphology of this projection are available. In the present study we describe and quantify the morphology of axon fragments of S1 to S2 ipsilateral projections in the agouti somatosensory cortex. Two groups of projecting axon arbors in S2 were individually reconstructed in three dimensions using Neurolucida, after a single electrophysiological guided BDA injection in either the forelimb (n = 4) or the hindlimb (n = 4). Electrophysiological mapping was performed 15 days after injections, allowing the localization of S2. Cluster analysis of 40 fragments after hindlimb and 40 after forelimb distinguished two clusters of terminals designated as type I and type II. On average, Type I fragments had greater surface areas and segment lengths than type II fragments, whereas type II fragments had higher number of terminal boutons, number of segments and branching points/mm than type I fragments. Type I corresponded to 58% of the axons projecting from the hindlimb representation in S1 whereas 63% of the sample originating from the forelimb representation in S1 corresponded to type II axons. The results suggest possible parallel processing by two stereotyped classes of axon terminals in the S1 to S2 projections that may represent at least part of the circuitry groundwork associated with distinct somatomotor skills of these limbs in agoutis.

Research highlights▶ Cortical S1 to S2 axon arbors were individually reconstructed. ▶ Cluster analysis of the morphologies distinguished two types of axon terminals. Type II arbors may recruit more neurons and potentially more synapses than type I. ▶ We suggest possible parallel processing by two stereotyped classes of axon terminals. A higher proportion of Type II fragment was found within the forelimb representation. ▶ Bi-lateral sensorimotor behavior in the agouti is restricted to the forelimbs. ▶ Axon morphological differences may contribute to these distinct somatomotor skills.