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

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Abstract
The morphology of 11 physiologically identified, deep, spinothalmic tract (STT) cells in the 7th lumbar segment of the cat were studied after being intracellularly injected with horseradish peroxidase (HRP). Four of these cells, prepared for combined light microscopy and EM, did not appear to be as well filled with HRP as the other cells prepared solely for light microscope. In these 4 cells the axons were not stained and significantly fewer distal dendrites were stained. The axons of the other 7 cells projected medially, crossed the midline in the ventral white commissure and ascended in the contralateral ventral funiculus. No axon collaterals were found. The dendrites of 8 cells could be divided into 3 groups. The 1st group projected laterally across lamina VII, usually passing through the ventrolateral portion of lamina VI before entering the lateral funiculus. The 2nd group projected ventrally, ventromedially and ventrolaterally through the ventral parts of laminae VII and VIII and into the ipsilateral ventral funiculus. The 3rd 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. Of the remaining 3 cells, 1 was located more deeply than any other cell, 1 was probably incompletely filled, and 1 was located more laterally. The first 2 cells had dendritic trees which bore strong similarities to the other 8 cells described above. In all, 12.3-37.5% of dendritic tips were found in the white matter of the ventral or lateral funiculi. 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 gray-white border. These data are discussed in relation to the excitatory and inhibitory responses of STT cells to somatic stimuli and the axonal projections of physiologically identified primary afferents and spinal interneurons.