Effects of thoracic dorsal rhizotomies on the respiratory pattern in anesthetized cats

Experiments were conducted to determine if thoracic wall proprioceptor afferents are involved in the modulation of respiratory activity during eupnea. The effects of elimination of thoracic wall afferents (thoracic dorsal rhizotomies (TDR) on tidal volume (VT), frequency (f), inspiratory time (ti) and expiratory time (te) were studied in vagotomized cats anesthetized with diallylbarbituric acid (Dial). Dorsal rhizotomies 1-12 resulted primarily in a decreased VT and ti, and an increased f. Further experiments were performed to determine if these changes in respiratory pattern could be correlated with known reflexes from the middle and lower intercostal muscles, or lungs, via thoracic dorsal roots. Afferents from these sources were eliminated by TDR 5-9, 10-13, and 1-4. TDR 1-4 had no significant effect on the respiratory pattern. TDR 5-9 and TDR 10-13 produced changes similar in direction to TDR 1-12. The results indicate that: a) afferents 1-4 from the upper intercostal muscles and lungs (sympathetic afferents) do not contribute significantly to the control of the spontaneous respiratory rhythm, and b) afferents via the middle thoracic roots, 5-9, and the lower thoracic roots, 10-13, contribute significantly to the rhythm. The results do not completely correlate with known intercostal reflexes, but it is suggested that elimination of intercostal muscle proprioceptor afferents is responsible for the observed effects of thoracic dorsal rhizotomies.