Effects of Noxious Peripheral Nerve Stimulation on the Activity of Respiratory Neurons in the Ventrolateral Medulla of Cats

Kim, Chang Jin; Kim, Sang Jung; Goo, Yong Sook; Kim, Jun; Choi, Kil Soo

Abstract

Noxious peripheral nerve stimulation causes changes of arterial blood pressure by way of somatosympathetic reflex, and may play an important role in determining the periodicity of respiratory neurons at the same time. However, little has been known about how noxious stimulation does mediate respiratory control. In the present study, the respiratory neurons of the ventrolateral medulla were identified and characterized, and the responses of these neurons to noxious stimulation were observed, to investigate the role of noxious stimulation in determining respiratory rhythm. Cats were anesthetized with alpha-chloralose and mechanically ventilated after muscle relaxation. Occipital craniectomy was performed and the floor of the fourth ventricle was exposed. Single cell activities of the ventrolateral medulla were recorded using carbon-filament microelectrodes. Neurons related with respiratory rhythm were identified and the responses to vagal and sciatic nerve stimulation were observed. The following results were obtained. 1) A total of 46 neurons related with mechanical ventilatory cycle was identified, including 13 expiratory augmenting neurons, 11 inspiratory decrementing neurons, and 9 unclassified neurons. 2) The activity of inspiratory augmenting neurons was increased by A-delta intensity stimulation of the vagal and sciatic nerves, and inhibited by C-intensity stimulation of the sciatic nerve. 3) The inspiratory decrementing neurons did not show a significant response to vagal nerve stimulation, but the cell activity was inhibited by A-delta and C-intensity stimulation of the sciatic nerve. 4) The expiratory neuron did not show a significant response to peripheral nerve stimulation. 5) 27 neurons had slower rhythmic activity than mechanical ventilation. Sciatic nerve stimulation increased frequency of rhythmic discharges in these cells. 6) Most of the neurons with slower rhythmic activity during spontaneous respiration followed faster rhythm of mechanical ventilation after muscle relaxation. From these results, it is concluded that the activities of the respiratory neurons, especially inspiratory ones, in ventrolateral medulla are under influence of noxious peripheral stimulation, and they are dependent on the state of the experimental animal.