Functional Heterogeneity Among Neurons in the Nucleus Retroambiguus With Lumbosacral Projections in Female Cats

1 Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, United Kingdom; 2 Department of Anatomy and Embryology, Faculty of Medicine, University of Groningen, Groningen, The Netherlands; and 3 University of Nottingham School of Nursing, Queen's Medical Centre Nottingham, United Kingdom Submitted 11 April 2005; accepted in final form 10 June 2005 Nucleus retroambiguus (NRA), in the caudal medulla, projects to all spinal levels. One physiological role is abdominal pressure control, evidenced by projections to intercostal and abdominal motoneurons from expiratory bulbospinal neurons (EBSNs) within NRA. The roles of NRA projections to the lumbosacral cord are less certain, although those to limb motoneurons may relate to mating behavior and those to Onuf's nucleus (ON) to maintaining continence. To clarify this we physiologically characterized NRA projections to the lumbosacral cord. Extracellular recordings were made in NRA under anesthesia and paralysis in estrus cats. Administered CO 2 gave a strong respiratory drive. Antidromic unit responses were recorded to stimulation of the contralateral ventrolateral funiculus of L 6 , L 7 , or sacral segments and to microstimulation in the region of semimembranosus motor nucleus or ON. All units were found at sites showing expiratory discharges. Units that showed collisions between antidromic and spontaneous spikes (all in late expiration) were identified as EBSNs. These were common from the ventrolateral funiculus (VLF) of L 6 (42.5%) or L 7 (32.9%), but rare from the sacral VLF or the motor nuclei. Antidromic latencies revealed a subthreshold respiratory drive in some non-EBSNs. This group had lower conduction velocities than the EBSNs. The remainder, with a negligible respiratory drive, had even lower conduction velocities. A new population of NRA neurons has thus been defined. They are not active even with a strong respiratory drive, but may provide most of the synaptic input from NRA to lower lumbar and sacral segments and could subserve functions related to mating behavior. Address for correspondence: P. A. Kirkwood, Sobell Dept. of Motor Neuroscience and Movement Disorders, Inst. of Neurology, Univ. College London, Queen Square, London WC1N 3BG, UK (E-mail: pkirkwoo{at}ion.ucl.ac.uk )