Multisegmental spinal sympathetic reflexes originating from the heart

Activation of cardiac sympathetic afferent nerves can initiate excitatory cardiocardiac reflexes through pathways that are exclusively spinal. In addition, stimulation of the same nerves also causes lower thoracic and lumbar sympathetic excitation, but the contribution of spinal pathways to these reflexes is unknown. Therefore experiments were performed to compare cardiac, splenic, and renal sympathetic responses to cardiac sympathetic afferent stimulation before and after cervical spinal cord transection in anesthetized, vagotomized, sinoaortic-denervated cats. Electrical stimulation of afferent cardiac sympathetic nerves produced excitatory responses in cardiac and renal nerves before transection but only cardiac nerve responses after transection. In contrast, afferent stimulation by epicardially applied bradykinin excited cardiac, renal, and splenic nerves before and after cord transection. Splenic nerve responses were greater than renal nerve responses in intact and spinal cats. These results demonstrate that spinal reflexes initiated by activation of cardiac sympathetic afferent nerves are not limited to cardiocardiac pathways. The similarity of patterns of responses in intact and spinal cats suggests that spinal pathways contribute significantly to the reflex excitation observed in intact animals.