The morphology of physiologically identified deep spinothalamic tract cells in the lumbar spinal cord of the cat.

1. The morphology of eleven physiologically identified, deep, spinothalmic tract (s.t.t.) cells in the seventh lumbar segment of the cat were studied after being intracellularly injected with horseradish peroxidase (HRP).2. Four of these cells, prepared for combined light and electron microscopy, did not appear to be as well filled with HRP as the other cells which were prepared solely for the light microscope. In these four cells the axons were not stained and significantly fewer distal dendrites were stained.3. The axons of the other seven cells projected medially, crossed the mid line in the ventral white commissure and ascended in the contralateral ventral funiculus. No axon collaterals were found.4. The dendrites of eight cells could be divided into three groups. The first group projected laterally across lamina VII, usually passing through the ventrolateral portion of lamina VI before entering the lateral funiculus. The second group projected ventrally, ventromedially and ventrolaterally through the ventral parts of laminae VII and VIII and into the ipsilateral ventral funiculus. The third group projected towards the central canal, the ventral border of the dorsal columns and the dorsal parts of the ipsilateral ventral funiculus. This group usually branched profusely and projected into lamina X.5. Of the remaining three cells, one was located more deeply than any other cell, one was probably incompletely filled, and one was located more laterally. The first two cells had dendritic trees which bore strong similarities to the other eight cells described above.6. In all, 12.3-37.5% of dendritic tips were found in the white matter of the ventral or lateral funiculi.7. Dendrites never entered the dorsal columns but they could often be traced to points very close to the dorsal columns before they turned abruptly and ran parallel to the grey-white border.8. These data are discussed in relation to the excitatory and inhibitory responses of s.t.t. cells to somatic stimuli and the axonal projections of physiologically identified primary afferents and spinal interneurones.