Responses of sympathetic postganglionic neurons to peripheral nerve stimulation in pigeon (Columba livia).

Reflex changes in heart rate and arterial blood pressure can be elicited in pigeons with high cervical transection by stimulation of brachial or lumbosacral peripheral and spinal nerves. This extends the phenomenon of spinally mediated, somatosympathetic reflexes to another vertebrate class. In a preliminary attempt to explore the spinal circuitry mediating these reflexes, the responses of single sympathetic postganglionic neurons were studied during spinal and peripheral nerve stimulation. With stimulation and recording at the same spinal segment, calculation of the central delay suggests the segmental reflex circuitry may be relatively simple, possibly trisynaptic. As the distance between stimulating and recording sites increases, postganglionic neuronal responsiveness decreases and becomes more variable. However, there is clear evidence that lumbosacral afferents can activate postganglionic neurons at brachial levels, indicating an effective propriospinal circuitry for somatosympathetic reflexes. Experiments on birds with intact spinal cords demonstrate that these spino-spinal pathways are also functional in the intact animal. While the segmental reflex is not different in the intact bird, the propriospinal pathways do behave somewhat differently, possible suggesting tonic central control.