Caudal medullary pathways to lumbosacral motoneuronal cell groups in the cat: Evidence for direct projections possibly representing the final common pathway for lordosis

Abstract

The nucleus retroambiguus (NRA) projects to distinct brainstem and cervical and thoracic cord motoneuronal cell groups. The present paper describes NRA projections to distinct motoneuronal cell groups in the lumbar enlargement. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify the retrogradely labeled neurons in the caudal medullary lateral tegmentum. These injections were combined with spinal hemisections to distinguish between neurons having ipsi-or contralaterally descending axons. The NRA-lumbosacral fibers descend almost exclusively contralaterally, but neurons in areas surrounding the NRA project mainly ipsilaterally. In an anterograde tracing study, injections of WGA-HRP or tritiated leucine were made in the region of the NRA to determine the NRA targets in the lumbosarcral cord. Hemisections in C2 made it possible to distinguish between NRA projections and projections from neurons in the adjoining lateral tegmentum. The results show delicate NRA projections to distinct lumbosacral motoneuronal cell groups innervating specific hindlimb muscles (iliopsoas, adductors, and hamstrings) as well as axial muscles (medial longissimus and proximal tail muscles). The projection is bilateral, with a contralateral predominance. Ipsilaterally terminating fibers are derived from NRA neurons whose axons cross the midline at the level of the obex, descend through the contralateral spinal white matter, and recross at the level of termination. A conceptual description is presented in which the periaqueductal gray-NRA-lumbosacral projections form the final common pathway for lordosis in the cat.

The Nucleus Retroambiguus Control of Respiration

“…NRA neurons at this level have axonal projections to the motoneurons in the thoracic, lumbar, and upper sacral spinal cord and are thought to mediate the excitatory expiratory drive to the external intercostal and abdominal muscles. However, NRA cells at this level have also been shown to project to the hindlimb (Vanderhorst and Holstege, 1995) and pelvic floor muscles (Holstege and Kuypers, 1982;Holstege and Tan, 1987;Van der Horst and Holstege, 1998;Boers et al, 2006). Ac- cording to Miller et al (1995), the NRA also controls pudendal motoneurons innervating the pelvic floor.…”

“…Outside the caudal brainstem, the NRA receives strong projections from the lateral, ventrolateral, and to a lesser extent the dorsomedial part of the caudal midbrain periaqueductal gray (PAG) (Holstege, 1989), but not from any other suprapontine brain structure (Holstege, 1991). The NRA projects directly to the laryngeal and pharyngeal motoneurons in the nucleus ambiguus in the lateral medulla (Holstege, 1989;Boers et al, 2002) and to the motoneurons of the diaphragm, external and internal intercostal, abdominal (Merril, 1970(Merril, , 1974Holstege and Kuypers, 1982), pelvic floor, upper leg, and hip muscles (Holstege and Tan, 1987;Vanderhorst and Holstege, 1995) in the spinal cord. The NRA is also called caudal component of the ventral respiratory group (VRG) (Long and Duffin, 1986) because its rostral part contains two types of inspiration-related neurons while its caudal part contains expiration-related neurons (Merril, 1970(Merril, , 1974Long and Duffin, 1986).…”

Projections from bed nuclei of the stria terminalis, magnocellular nucleus: Implications for cerebral hemisphere regulation of micturition, defecation, and penile erection

“…The caudal central gray appears to be important for integrating autonomic and somatomotor responses in a variety of motivated behaviors involving ingestion, mating, and defense. In particular, the caudal central gray sends massive projections to the medullary reticular formation, which in turn projects to distinct lumbosacral somatomotor neurons innervating specific hindlimb and axial muscles (Vanderhorst and Holstege, 1995). For example, these pathways have been proposed as a final common pathway for lordosis in the cat (Vanderhorst and Holstege, 1995;Holstege, 1998).…”

“…In particular, the caudal central gray sends massive projections to the medullary reticular formation, which in turn projects to distinct lumbosacral somatomotor neurons innervating specific hindlimb and axial muscles (Vanderhorst and Holstege, 1995). For example, these pathways have been proposed as a final common pathway for lordosis in the cat (Vanderhorst and Holstege, 1995;Holstege, 1998). Obviously, the same muscles might be used to prepare appropriate body posture for micturition and defecation.…”

Descending projections from the nucleus retroambiguus to the iliopsoas motoneuronal cell groups in the female golden hamster: Possible role in reproductive behavior

“…Main targets of the descending projections are the phrenic nucleus, which contains motoneurons innervating the diaphragm; the motoneuronal cell groups of the cutaneus trunci [cervical segment 8 (C8) through thoracic segment 1 (T1)]; and the intercostal and abdominal muscles at the levels T1 through lumbar segment 3 (L3; Holstege and Kuypers, 1982;Holstege, 1989;Holstege and Blok, 1989;Zhang et al, 1992). Furthermore, NRA fibers project to a distinct set of motoneuronal cell groups in the lumbosacral cord (VanderHorst and Holstege, 1995, 1996, 1997aVanderHorst et al, 1997; see also Holstege and Tan, 1987). VanderHorst and Holstege have put forward the concept that these latter projections subserve mating behavior, because the complete set of lumbosacral motoneuronal cell groups that receive NRA afferents plays a role in the particular movements necessary for mating and not in other specific motor behaviors, such as stepping or jumping.…”