Effects of medullary area I cooling on respiratory response to muscle stimulation

The respiratory response to afferent input from both peripheral and central chemoreceptors can be decreased progressively by bilateral graded cooling of the intermediate, or I(s), areas of the ventral medulla (Millhorn et al., 1982). The present study was undertaken to determine if the phrenic nerve response to calf muscle afferent test stimulation is also affected by graded cooling of the I(s) areas. Anesthetized, paralyzed cats whose vagi and carotid sinus nerves had been cut were studied. Unlike the response to chemoreceptor input, the response to calf muscle input was not affected by moderate (40 degrees C to 20 degrees C) cooling of the I(s) areas. Thus, we conclude that input from both chemoreceptors travels to the respiratory controller via a common pathway that includes the I(s) areas, whereas input from peripheral muscle reaches the controller via an independent pathway that does not include the I(s) areas.