Nonuniformity of the descending pathways activating the sympathetic preganglionic neurons of the superior thoracic segment of the spinal cord

In anesthetized, immobilized and artificially ventilated cats, excitatory influences from the ventrolateral part of medulla are mediated to sympathetic preganglionic neurons of T2 segment via two types of descending pathways with conduction velocity 12.3 +/- 3.2 m/sec and about 6 m/sec. The descending excitatory influences on sympathetic neurons of T4 and more caudal segments are mediated via pathways of the latter type only. According to characteristics of the white rami responses elicited by stimulation of different points of the bulbar ventrolateral surface, both types of descending fibers are the axons of neurons located in caudal part of zone M and in zone S. Moreover, the T2 sympathetic neurons are activated by descending pathways with conduction velocity about 5.7 +/- 0.6 m/sec originating in dorsocaudal part of the medulla. The high speed descending pathways to T2 were shown to take part in transmission of pupillo-dilatory impulses. On the basis of comparison of the white rami responses elicited by stimulation of different points of the bulbar ventrolateral surface with different impulse intensity, the neuronal organization of sympathoexcitatory structures of this region of medulla is suggested.