FINE-STRUCTURAL ORGANIZATION OF SPINOTHALAMIC AND TRIGEMINOTHALAMIC LAMINA-I TERMINATIONS IN THE NUCLEUS SUBMEDIUS OF THE CAT

We examined lamina I trigemino- and spinothalamic tract (TSTT) termina ls labeled with Phaseolus vulgaris leucoagglutinin in the nucleus subm edius (Sm), a nociceptive relay in the cat's thalamus. Volume-rendered (three-dimensional) reconstructions of ten lamina I TSTT terminals id entified with light and electron microscopy were built from serial ult rathin sections by computer, which enabled the overall structures of t he terminal complexes to be characterized in detail. Two fundamentally different terminations were observed: compact clusters of numerous bo utons, which predominate in the dense focus of a lamina I terminal fie ld in the Sm, and boutons-of-passage, which are present throughout the terminal field and predominate in its periphery. Reconstructions of c luster terminations reveal that all boutons of each cluster make synap tic contact with protrusions and branch points on a single dendrite an d involve presynaptic dendrites (PSDs) in triadic arrangements, provid ing a basis for the secure relay of sensory information. In contrast, reconstructions show that boutons-of-passage are generally characteriz ed by simple contacts with PSDs, indicating an ascending inhibitory la mina I influence. These different synaptic arrangements are consistent with physiological evidence indicating that the morphologically disti nct nociceptive-specific and thermoreceptive-(cold) specific lamina I TSTT neurons terminate differently within the Sm. Thus, a suitable str uctural substrate exists in the cat's Sm for the inhibitory effect of cold on nociception, a behavioral and physiological phenomenon of fund amental significance. We conclude that the Sm is more than a simple re lay for nociception, and that it may be an integrative comparator of a scending modality-selective information that arrives from neurons in l amina I. (C) 1996 Wiley-Liss, Inc.