Effect of intramedullary procaine injection on tracheal tone and phrenic neurogram.

To map the superficial locations which are involved in the control of respiration and tracheal smooth muscle tone in ventrolateral medulla, we examined the effects of local anesthesia on phrenic activity and tracheal tone in twelve anesthetized, paralyzed, and artificially ventilated dogs. 0.5 microliter of 5% procaine was injected 0.3 to 0.5 mm below the surface unilaterally to the ventral superficial layer (from the rostral part of the trapezoid body to the caudal hypoglossal rootlets and lateral from the pyramids to 5.5 mm from the midline), which included rostral, intermediate and caudal areas, and the area lateral to the hypoglossal rootlets. The peak amplitude of the integrated phrenic neurogram was decreased by procaine injection to the intermediate area and the area lateral to the hypoglossal rootlets. Tracheal tone decreased only by procaine injection to the intermediate area. In the intermediate area, some injections decreased either phrenic output alone or tracheal tone alone. These results suggest that the two ventral medullary areas, i.e. the intermediate and caudolateral parts, contain neural structures which are involved in the shaping of phrenic output, but only the intermediate area is involved in the regulation of tracheal tone. It is also suggested that, in the intermediate area, the structures responsible for the maintenance of respiration and tracheal tone are, at least in part, separable.