All of the following factors increases the level of respiratory neuron activity in the medulla, EXCEPT:

A rise in the PCO2 or H+ concentration of aerial blood or a drop in its Po2 increases the level of respiratory neuron activity in the medulla, and changes in the opposite direction have a slight inhibitory effect. The effects of variations in blood chemistry on ventilation are mediated respiratory chemoreceptors--the carotid and aoic bodies and collections of cells in the medulla. They initiate impulses that stimulate the respiratory center. Stimuli Affecting the Respiratory Center: Chemical control: CO2 ( CSF and brain interstitial fluid H+ concentration) O2& H+ ( carotid and aoic bodies) Nonchemical control: Vagal afferents from receptors in the airways and lungs Afferents from the pons, hypothalamus, and limbic system Afferents from proprioceptors Afferents from baroreceptors: aerial, atrial, ventricular, pulmonary Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 36. Regulation of Respiration. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.