EFFECTS OF SENSORY NERVE STIMULATION ON COSTAL AND ABDOMINAL BREATHING IN THE ANAÆSTHETISED DOG

Attention is called to the significance of localised differences and localised changes in respiratory movements in the chest and abdomen in different animals and under varying conditions. Inasmuch as costal or abdominal breathing may shift from one to the other it is concluded that all dogs are provided with mechanisms permitting either type or any combination of types of breathing. The question is considered whether such localised differences in breathing are inherent in a compact respiratory centre in the brain, or whether the control of breathing is more diffuse and emanating from out the entire central grey axis stem. In this connection the influence of reflex action on the control of breathing is studied by electrical stimulation of sensory nerves. To determine whether the level of entrance in the central grey axis stem of sensory nerve impulses exerts a localised effect on breathing, such nerves as the saphenous, radial cutaneous, infra orbital and phrenic were stimulated faradically. A comparable response of accelerated breathing, decreased costal expiratory circumference and a tendency towards increased abdominal circumference was obtained with all. The effects of graded sensory stimulation of the mixed cervical vagus and of the pulmonary branches of the same differed considerably. The occurrence of acceleration during graded stimulation of the cervical vagus was attributed to the abundance of extra pulmonary nervefibres of excitatory function. Faradic stimulation of the central end of the vagus nerve, which will produce pure inhibition when applied during expiration, may increase the depth of one introductory inspiration if applied during the phase of inspiration. The accessibility of the centre to excitatory and inhibitory influences, therefore, varies with the phase of the respiratory cycle. Rhythmic breathing may be produced by rhythmic faradic sensory stimulation, which confirms the findings of others. Such rhythmic breathing produced by stimulation of the cervical vagus, pulmonary branches of the vagus and the saphenous nerve indicates that the respiratory centre may become accessible to rhythmic impulses from many sources agreeing with the synchronisation of breathing and rhythmic muscular movements. Rhythmic stimulation of the vagus nerve may also produce complete respiratory inhibition. Faradic stimulation of the saphenous nerve, or chemical excitation of the carotid gland with sodium sulphide or cyanide removes the inhibition and forces the animal to breathe with each vagal stimulation. When rhythmic stimulation of the vagus nerve produces rhythmic breathing, the action is, most frequently, locally selective. Costal breathing is partly or completely inhibited, and abdominal breathing is usually augmented. Under these conditions cyanide or sulphide lead to powerfully augmented costal respiration and, less markedly, augmented abdominal respirations. Faradic stimulation of the saphenous nerve and intravenous injection of sulphide or cyanide produce much greater and more rapid ventilation with the vagus nerves intact than during vagal block, clearly indicating the great importance of the vagus nerves in the control of breathing. These findings show the interdependence of vagal and other afferent nerve impulses, and support the general principle that breathing may be fundamentally a resultant of numerous and various afferent nerve impulses.