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

Abstract

The enzyme horseradish peroxidase (HRP) was injected into single Ia muscle afferent fibers in anesthetized cats. Subsequent histochemistry allowed the morphology of the axons and their collaterals in the lumbosacral spinal cord to be determined. Fifteen Ia axons were stained, 4 from medial gastrocnemius, 4 from lateral gastrocnemius-soleus and 7 from muscles innervated by the posterior tibial nerve. All 13 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 87 collaterals over distances between 3-9 mm. Collaterals were given off at intervals of 100-2600 .mu.m at an average spacing of about 1000 .mu.m. 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 .mu.m caudal to their terminations in motor nuclei. Terminal arborizations were limited to 3 sites; lamina VI (the intermediate region), lamina VII (the Ia inhibitory interneuron region) and lamina IX (the motor nuclei). The 3 sets of terminals had characteristic arborizations and bouton arrangements. The results were discussed in relation to previous anatomical studies. A single Ia collateral apparently makes contact with many more motoneurons than previously suggested, in fact with 50-60 rather than with about 10.