1. A total of 59 medullary raphespinal neurons antidromically activated from the T2-T5 segments were tested for responses to coronary artery occlusion, epicardial application of bradykinin, and mechanical probing of the epicardium in cats anesthetized with alpha-chloralose. With the exception of five neurons, only those neurons that were responsive to electrical stimulation of the left stellate ganglion were tested for responses to these stimuli. Neurons in this study are a subset of those in the companion report. 2. Six neurons (12%) responded to epicardial bradykinin. Five cells were excited, and one was inhibited. Nine neurons (18%) had cardiac receptive fields, and one neuron (2%) had a field confined to the pericardium. 3. Neurons were tested for responses to separate occlusions of the left anterior descending (LAD) and circumflex (CX) coronary arteries. Overall, 13/34 (38%) of tested raphespinal neurons were responsive to coronary artery occlusion. Responses to coronary artery occlusion consisted of two major patterns. One pattern consisted of either an increase or decrease in neuronal firing rate after the heart became ischemic; this pattern was termed an ischemic (IS) response. Ten neurons (29%) exhibited an IS response to occlusion of the LAD and/or CX coronary arteries. Because four neurons exhibited IS responses to occlusion of each artery, there were a total of 14 IS responses. Of these, 10 were inhibitory, and 4 were excitatory. The second pattern of response consisted of a rapidly adapting excitation or inhibition at the onset or release of occlusion, but cell activity was unchanged during cardiac ischemia; this pattern was termed an onset (ON) response. Three neurons (9%) exhibited ON responses; two were excited, and one was inhibited. No neurons demonstrated both ON and IS responses during occlusion. 4. Twenty-one neurons were tested for responses to occlusion of each artery. Seventeen neurons (81%) exhibited similar patterns of responses to occlusion of each artery; that is, they either showed the same pattern of response to occlusion of each artery or they were unresponsive to either occlusion. For the 5 of these 17 neurons that were responsive, direction of change in neuronal activity (excitation or inhibition) was the same for occlusion of each artery. 5. All raphespinal neurons tested for responses to epicardial bradykinin and coronary artery occlusion were responsive to electrical stimulation of the left stellate ganglion. In addition, 16/28 (57%) of neurons tested for responses to occlusion were responsive to electrical stimulation of the right cervical vagus nerve. vagal input appeared to be more important in predicting whether a neuron would be excited or inhibited by cardiac ischemia than did sympathetic afferent input. However, vagal input was not required for an IS response to occur. Sympathetic afferents seemed to mediate ON responses and neuronal responses to epicardial bradykinin and epicardial probing. 6. The primary response to coronary artery occlusion was inhibition, but the main response to bradykinin and probing the epicardium was excitation. Taken together, the results of this study and the companion report indicate that the influence of cardiac vagal afferents on raphespinal neurons depends on the state of cell activation, whereas cardiac sympathetic afferents are chiefly excitatory.
