Morphological and electrophysiological identification of gigantocellular tegmental field neurons with descending projections in the cat: II. Bulb.

There are three different descending projections from the bulbar gigantocellular tegmental field (BFTG) in the cat, as defined by intracellular recording and intracellular horseradish peroxidase (HRP) techniques. The first pathway arises from neurons which send axons to the contralateral medial longitudinal fasciculus (cMLF neurons); cMLF neurons show excitatory postsynaptic potentials (EPSPs) after stimulation of the ipsilateral pontine gigantocellular tegmental field (PFTG). Most cMLF neurons have large ellipsoid-polygonal somata (mean, 56.8 micron), thick axons (average diameter, 3.09 microns), mostly non-spiny dendrites and dendritic fields flattened in the anteroposterior direction. No cMLF neurons with axon collaterals in the BFTG are present in the data of this study. The second pathway arises from neurons which send axons to the ipsilateral MLF (iMLF neurons); iMLF neurons show EPSPs after stimulation of the ipsilateral PFTG. Most iMLF neurons have large ellipsoid-polygonal somata (mean, 60.2 microns), thick axons (average diameter, 3.00 microns), mostly non-spiny dendrites and dendritic fields that are only slightly flattened in the anteroposterior direction. As with cMLF neurons, no iMLF neurons with axon collaterals in the BFTG are present in the data of this study. The third pathway arises from neurons that send axons directly into the ipsilateral caudal bulbar reticular formation (iBRF neurons). Most iBRF neurons have smaller ellipsoid-polygonal somata (mean, 38.6 microns), thinner axons (average diameter, 1.84 microns), mostly nonspiny dendrites, and dendritic fields that are flattened in the anteroposterior direction. In contrast to cMLF and iMLF neurons, axon collaterals are present in 73% of iBRF neurons. About half of iBRF neurons have bifurcated axon collaterals with both anterior and posterior projections, and in these neurons antidromic spike potentials are elicited by stimulation of the ipsilateral PFTG.