Lateral tegmental field neurons of cat medulla: a source of basal activity of ventrolateral medullospinal sympathoexcitatory neurons.

Barman SM, Gebber GL.
J Neurophysiol. 1987 May;57(5):1410-24.

   The RVLM does not exist all by itself, no matter how much I focus my attention on it, that is not going to change. This goal of this paper was to look at another region involved in control of SNA, the lateral tegmental field (LTF) of the cat medulla, and how it interacted with the RVLM. Both the RVLM and the LTF  contain neurons with activity correlated with cardiac SNA, slow or stop their rate of firing with increases in blood pressure, and glutamate or electrical stimulation of both areas increases SNA. The hypothesis was that the LTF drives activity in the RVLM, which then drives SNA.
   They located neurons in the LTF, were able to antidromically activated them from the ipsilateral or contralateral RVLM (conduction velocity ~0.5m/s), and check them for barosensitivity and cardiac related rhythmic activity. They also found a few neurons that had 2 different conduction velocities when the stimulating electrode was placed at different depths of the RVLM, suggesting multiple axon branches within the RVLM region from the same LTF neuron that either took a short path and a long path, or had very different properties between collaterals. The long/slow path was always more ventral than the short/fast path, with the ventral sites being located outside the RVLM.
   Then things got more complicated!  They found that if they recorded spinally projecting RVLM neurons and stimulated in the LTF, they could find cases where some RVLM neurons could be antidromically activated, some neurons could be synaptically activated (variable latency action potentials that could not follow at high frequencies), and some neurons showed both characteristics, suggesting reciprocal communication between the LTF and the RVLM. They also found a number of RVLM neurons which could be antidromically activated by stimulating either in the T2 IML or the LTF, which indicates the presence of collateralizing axons with both ascending and descending projections.
   There was a lot more to this paper that I did not cover in this blog entry, including where the axons of RVLM neurons likely branched, latencies of efferent signals from the different regions, discusson on potentially sympathoinhibitory cells in the LTF, and more. There's a lot of info in this paper and it's worth a couple of reads to try to get it all. -DH