The morphology of group Ib afferent fibre collaterals in the spinal cord of the cat.

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Abstract
Horseradish peroxidase (HRP) was injected into single Ib muscle afferent fibers in anesthetized cats. Subsequently, histochemistry allowed the morphology of the axons and their collaterals in the lumbosacral spinal cord to be determined. Ib axons (11) were stained, 7 from lateral gastrocnemius-soleus, 1 from medial gastrocnemius and 3 from muscles innervated by the posterior tibial nerve. Of the axons 10 were traced into the dorsal roots and all but 1 (from the posterior tibial nerve) bifurcated upon entering the cord. Between 5.1 and 9.9 mm of each axon was stained and the fibers gave off 84 collaterals at intervals of 100-2300 .mu.m, at an average spacing of about 900 .mu.m. The spacing between collaterals on the (finer) descending axon branches was generally less than the intervals between collaterals on ascending branches. All Ib collaterals had a characteristic morphology. The collaterals coursed cranially on a direct path through the dorsal horn to lamina IV or V before branching. They arborized widely in the intermediate region, mainly in lamina IV and in the dorsal part of lamina VII. Occasionally, less extensive arborizations were seen more dorsally in lamina IV and V. The rostro-caudal extent of individual collateral arborizations was limited to 200-400 .mu.m and there was no overlap between adjacent collaterals. Each terminal arborization gave rise to 56-384 boutons, mainly of the en passant type. The results are discussed in relation to previous anatomical and electrophysiological studies.