Spinal projections from the lower brain stem in the cat as demonstrated by the horseradish peroxidase technique. I. Origins of the reticulospinal tracts and their funicular trajectories.

Using a retrograde tracer technique with horseradish peroxidase (HRP) attempts were made to determine the origins of reticulospinal tracts and their funicular trajectories. Reticulospinal tracts originating from the mesencephalic reticular formation (RF) were composed of: (1) descending projections arising from the cluster of cells located just lateral to the periaqueductal gray that course in the anterior funiculus (AF) and ventral part of the lateral funiculus (LF) with ipsilateral predominance; and (2) projections from the cluster of cells located dorsal to the brachium conjunctivum that course in the ipsilateral LF. Origins of the pontine reticulospinal tracts arising from the n. reticularis pontis oralis (Poo) have been divided qnto three parts: (1) medial one-third; (2) middle; and (3) ventrolateral. The axons from the medial part descend ipsilaterally via the medial part of the AF, while the axons from the ventrolateral part of the Poo give rise to diffuse descending projections in the AF and LF. The middle part of the Poo has been further subdivided into: (1) dorsal part that gives rise to spinal projections ipsilaterally in the ventrolateral funiculus (VLF); and (2) ventral, particularly its upper part, whose axons descend bilaterally via the DLF. Origins of reticulospinal tracts from the n. reticularis pontis caudalis (Poc) could be divided into three parts: (1) medial; (2) dorsolateral; and (3) ventrolateral. The medial part of the Poc is a source of axons via the medial part of the ipsilateral AF, while the ventrolateral part of the nucleus is a source of axons via the contralateral LF. The spinal projections from the dorsolateral part of the Poc appears to course diffusely in the AF and LF, but with DLF predominance. The n. reticularis gigantocellularis (Gc) was found to be a main medullary source of the spinal projections in the ipsilateral AF, while n. reticularis magnocellularis (Mc) is the major source of the fibers coursing ipsilaterally in the VLF. The most medial part of the Mc descends ipsilaterally via the medial part of the AF, while the ventrolateral part of the nucleus together with the n. reticularis lateralis of Meesen and Olszewski descends ipsilaterally via the DLF. It has also been found that the axons from the n. reticularis paramedianus pass via both the AF and LF with ipsilateral predominance, while the n. reticularis dorsalis and ventralis course via the LF with ipsilateral predominance.