[Rhombencephalon and vasomotor reflexes to impulses of somatic A-fibers].

Cats decerebrated so that the medulla oblongata remained connected with a greater or lesser part of the pons Varolli were immobilized with flaxedyl or succinylcholine; arterial pressure (AP) responses to impulses (at the frequency of 10 s-1) of different subgroups of the tibial nerve A-afferents were studied. Impulses of the Abeta + Adelta1-afferents and of the most excitable part of Adelta2-afferents lowered AP in all the animals. Addition of all the Adelta2-afferent impulses enhanced this AP fall in 79% of the cats and only 21% of them did it evoke a rise of AP. Inclusion of Adelta2-fiber impulses into the afferent inflow either did not alter the average AP decrease or evoked a weak, and only sometimes, a more pronounced AP rise. It was found crucial to keep intact the structures of the ponto-medullary junction, as well as the most rostral part of the medulla for sharp decrease of the pressor reflexes or for the appearance of the depressor ones, instead. Consequently, in this part of the hind brain there are structures exerting a tonic depressive influence on the exciting effect of the somatic A-afferent impulses on the vasoconstrictor neurons, thus unmasking the existence of the inhibitory component of their action on these neurons. In unanesthetized cats this action of the hind brain structures is tonically inhibited by the midbrain.