Morphology and organization of axon collaterals from afferent fibres of slowly adapting type I units in cat spinal cord.

1. The morphology of the collaterals of single axons innervating Type I slowly adapting receptors was studied by using the intra-axonal injection of the enzyme horseradish peroxidase in anaesthetized cats. The axons were impaled near their dorsal root entrance zone in the lumbosacral cord. The morphology was revealed by subsequent histochemistry. 2. Thirteen Type I axons were stained, nine with receptors in the hairy skin and four with receptors in the glabrous foot pad skin. Twelve axons could be traced back into their dorsal roots and 11 of these divided into rostral and caudal branches shortly after entering the spinal cord. 3. One hundred and twelve collaterals were given off the thirteen axons and all well filled collaterals had a similar morphology. In the dorsal horn they gave rise to wide elliptical areas of terminal arborization (in transverse sections of cord) that were limited to laminae III, IV and the dorsal part of lamina V. The terminal arborizations of collaterals from the same axon were in line in the saggittal plane, but only rarely did the terminal arborizations of adjacent collaterals overlap; usually there was a gap between adjacent terminal arborizations. 4. Synaptic boutons of Type I units from hairy skin were mainly of the "en passant" variety whereas those of Type I units from glabrous skin were generally "boutons terminaux" with very few boutons "de passage". 5. The morphology of axon collaterals of Type I units is compared with that of hair follicle units.