Intercostal‐to‐Phrenic Reflexes in the Spinal Cat

The diaphragm, which previously has been shown to contain relatively few proprioceptors and to lack autogenetic facilitation, was found to be under reflex control from intercostal afferents. Electrical stimulation of the intercostal nerves of the lowest thoracic segments elicited a polysynaptic reflex excitation of phrenic motoneurones followed by a depression of ‘spontaneous’ phrenic motor activity. No phrenic responses were elicited from the upper thoracic segments. Increased phrenic activity was also elicited by mechanical stimulation of the trunk wall; the receptive area of these effects was similarly limited to the region of the insertion of the diphragm. Alike reflex effects were obtained from both the internal and the external intercostal nerves. In the internal nerve both group Ib and II afferents were found to contribute to the reflex whereas in the external nerve it was mainly the secondary muscle spindle endings which gave rise to the response. In contrast to the intercostal‐to‐intercostal reflexes cutaneous afferents proved to be without effect in the intercostal‐to‐phrenic reflex.