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Abstract
Neurons of several nuclei within the medial pontomedullar reticular formation are
active during mastication, but their relationship with other elements of the pattern
generating circuits have never been clearly defined. In this paper, we have studied
the connection of this area with the trigeminal motor nucleus and with pools of last-order
interneurons of the lateral brainstem. Retrograde tracing techniques were used in
combination with immunohistochemistry to define populations of glutamatergic and GABAergic
neurons. Injections of tracer into the Vth motor nucleus marked neurons in several
trigeminal nuclei including the ipsilateral mesencephalic nucleus, the contralateral
Vth motor nucleus, the dorsal cap of the main sensory nucleus and the rostral divisions
of the spinal nucleus bilaterally. Many last-order interneurons formed a bilateral
lateral band running caudally from Regio h (the zone surrounding the Vth motor nucleus),
through the parvocellular reticular formation and Vth spinal caudal nucleus. Injections
of tracer into Regio h, an area rich in last-order interneurons, marked, in addition
to the areas listed above, a large number of neurons in the medial reticular formation
bilaterally. The major difference between injection sites was that most neurons projecting
to the Vth motor nucleus were located laterally, whereas most of those projecting
to Regio h were found medially. Both populations contained glutamatergic and GABAergic
neurons intermingled. Our results indicate that neurons of the medial reticular formation
that are active during mastication influence Vth motoneurons output via relays in
Regio h and other adjacent nuclei.