Submitted on April 11, 2005
Accepted on June 10, 2005
FUNCTIONAL HETEROGENEITY AMONG NEURONS IN THE NUCLEUS RETROAMBIGUUS WITH LUMBOSACRAL PROJECTIONS IN FEMALE CATS
J. Boers1, T. W. Ford2, G. Holstege3, and P. A. Kirkwood4*
1 Department of Anatomy and Embryology, Faculty of Medicine, University of Groningen, Groningen, The Netherlands; Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, United Kingdom
2 School of Nursing, University of Nottingham, Nottingham, United Kingdom
3 Department of Anatomy and Embryology, Faculty of Medicine, University of Groningen, Groningen, The Netherlands
4 Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, United Kingdom
* To whom correspondence should be addressed. E-mail: pkirkwoo{at}ion.ucl.ac.uk.
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, though those to limb motoneurons may relate to mating behavior and those to Onuf's nucleus (ON) to maintaining continence. To clarify this we have physiologically characterized NRA projections to the lumbosacral cord.
Extracellular recordings were made in NRA under anesthesia and paralysis in estrus cats. Administered CO2 gave a strong respiratory drive. Antidromic unit responses were recorded to stimulation of the contralateral ventrolateral funiculus of L6, L7 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 VLF of L6 (42.5%) or L7 (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.
