Morphology of physiologically characterized medial lemniscal axons terminating in cat ventral posterior thalamic nucleus

Medial lemniscal axons were identified by extra- and intracellular recording in the thalamic ventral posterior lateral nucleus (VPL) of cats and injected intracellularly with horseradish peroxidase (HRP). Axons were characterized in terms of their latencies of response of stimulation of the medial lemniscus in the medulla, their receptive fields, and the temporal patterns of their discharge in response to stimulation of the receptive field with natural, hand-held stimuli. One-hundred sixty-six axons were placed in five operational groups: hair transient (Ht) (n = 41); hair sustained (Hs) (n = 45); pressure transient (Pt) (n = 14); pressure sustained (Ps) (n = 27), and deep or joint (Jt) (n = 39). There was a tendency for Jt axons to have their terminations in anterodorsal parts of VPL and for those in the four cutaneous categories to have theirs in more central parts of the nucleus. Nineteen injected axons with receptive fields mainly on the distal forelimb were subjected to detailed morphological analysis in terms of extent of terminal field and number of boutons. All axons ended in localized terminal fields that were more extensive anteroposteriorly than in the other dimensions. All showed an overall similarity and similar ranges of variation. There was a tendency, however, for Jt axons to have the least extensive terminations with fewest boutons. Ps axons had the most extensive terminations and largest number of boutons; Hs axons had small terminations and few boutons but Ht axons had small-to-medium arborizations with many boutons; no Pt axons were sufficiently well stained to enable comparisons of them with the others. There were no marked differences in axon diameter or conduction velocity among the five types. Boutons identified light microscopically tended to be clustered in linear chains along proximal dendrites of relay neurons and electron microscopy revealed that they were terminals making synaptic contacts on relay cell dendrites and on presynaptic dendrites of interneurons. These results reveal more similarities than differences among lemniscal axon terminations in VPL. Further studies of a quantitative nature on stimulus-response coupling and on the geographic distribution of lemniscal synapses on relay neurons will be required to reveal how lemniscal input is translated into relay cell output in VPL.