Termination areas of corticobulbar and corticospinal fibres in the rat.

Solution of the cobaltic lysine complex compound injected into the bulbar pyramid was taken up by pyramidal tract (PT) fibres and intraaxonally transported as far rostral as the internal capsule, and as far caudal as segment C4 of the spinal cord. With sulfide-precipitation of the cobalt and silver-intensification of the CoS, terminal branches of corticobulbar and corticospinal fibres could be shown in the following structures: In the pons the medial, ventral and the medial part of the lateral pontine nuclei are supplied with collaterals of PT fibres. The medial pontine nucleus receives contralateral PT fibres as well. The nucleus raphe magnus receives PT fibre collaterals from both sides. In the reticular formation contralateral fibres terminate in n. reticularis pontis caudalis, n. gigantocellularis, n. paramedianus, n. reticularis ventralis and dorsalis. Ipsilateral fibers terminate in the n. gigantocellularis and n. reticularis ventralis. The inferior olive receives both ipsilateral and contralateral PT fibres. All parts of the central gray substance are supplied by ipsi-and contralateral fibres which emerge from the PT at the level of the pyramidal decussation. The caudal nucleus of the spinal nucleus of the trigeminus receives a strong bundle of crossing PT fibres which terminate in the subnucleus magnocellularis. The dorsal column nuclei are virtually outlined by the numerous terminals of PT fibres. An oval area at ventrolateral aspect of the cuneate nucleus is innervated by PT fibres. The neurones of this area send their axons to the dorsal white column. In the spinal cord PT fibre terminals delineate four areas of termination. The first is the internal basilar nucleus which is the most richly supplied. The second lies lateral and ventral to this nucleus including segments of laminae V, VI, and VII. The third area extends into the ventral horn including a small zone of the ventral part of lamina VII. Thin PT fibre collaterals may establish here direct contacts with motoneurone dendrites. The fourth area includes laminae III and IV and receives individual terminal formations of thicker PT fibres. Findings which corroborate and which extend previous results are discussed. A few notes are made on the functional significance of some of these fibre connections.