Spinal cord collaterals from axons of type II slowly adapting units in the cat.

1. The morphology of single axons, and their collaterals, of Type II slowly adapting mechanoreceptors situated at the claw bases was studied. Intra-axonal injections of horseradish peroxidase were made into the axons near their entrance to the lumbosacral spinal cord of anaesthetized cats. The morphology was revealed by subsequent histochemistry. 2. Nine Type II axons were stained. All but one bifurcated into ascending and descending branches upon entering the cord. Eighty-nine collaterals arose from the axons at a mean spacing of about 570 micrometers. 3. The collaterals formed plate-like arborizations usually about 500-600 micrometers wide in the transverse plane but only 100-300 micrometers thick in the longitudinal axis of the cord. The terminal arborizations were in laminae III-VI. 4. Synaptic boutons in laminae III and IV were more numerous than in laminae V and VI. Boutons en passant were common in laminae III and IV and arranged in series of three to six, whereas in deeper laminae only two or three boutons formed a series de passage. 5. The morphology of the slowly adapting Type II collateral is discussed. 6. Some general principles of the organization of cutaneous afferent fibres in the lumbosacral cord are presented.