Functional significance of thoracic vagal branches in the chicken.

One minute electrical stimulation was used to excite the right and left cervical vagi as well as specific points on, or branches of, the left thoracic vagus. Respiratory, heart rate and blood pressure responses were observed with the nerves intact and cut. Stimulation of either intact cervical vagus produced apnea, bradycardia and blood pressure depression. Stimulation of the cut ends after nerve section demonstrated that the heart rate and blood pressure effects were efferent and the respiratory change was afferent. No responses were observed due to stimulation of the vagus caudal to the lungs. Stimulation of cardiac branches reduced heart rate and blood pressure but did not produce significant respiratory effects. Middle and anterior pulmonary branches were found to contribute only to respiratory changes through afferent nerves. Sudden, sustained reduction of CO2 in the airways produced immediate, sustained apnea. The data suggest that CO2 sensitive thoracic receptors important in regulation of respiration are confined primarily to the lungs and that these receptors play no direct role in cardiovascular function.