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

1. The enzyme horseradish peroxidase (HRP) was injected into single Ia muscle afferent fibres in anaesthetized cats. Subsequent histochemistry allowed the morphology of the axons and their collaterals in the lumbosacral spinal cord to be determined. 2. Fifteen Ia axons were stained, four from medial gastrocnemius, four from lateral gastrocnemius-soleus and seven from muscles innervated by the posterior tibial nerve. All thirteen axons that could be traced into the dorsal roots bifurcated upon entering the cord. Between 4 and 11 mm of axons were stained and they gave off eighty seven collaterals over distances between 3 and 9 mm. Collaterals were given off at intervals of 100-2600 micron at an average spacing of about 1000 micron. 3. All Ia collaterals had a characteristic morphology. After leaving the parent axon they ran ventrally to lamina VI and then ventrolaterally to the motor nuclei. The collaterals coursed cranially from their point of origin to the motor nuclei so that their lamina VI terminations were about 100-300 micro caudal to their terminations in motor nuclei. Terminal arborizations were limited to three sites; lamina VI (the intermediate region), lamina VII (the Ia inhibitory interneurone region) and lamina IX (the motor nuclei). The three sets of terminals had characteristic arborizations and bouton arrangements. 4. The results are discussed in relation to previous anatomical studies. In particular the present results suggest that a single Ia collateral makes contact with many more motoneurones than has previously been suggested in fact with fifty to sixty rather than with about ten.