Afferent Fibers of Cardiovascular and Pulmonary Origins and Their Intrabulbar Connections (A Review)

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Abstract
The respiratory reaction to an afferent stimulation of the cervical vagus depends on stimulus strength and frequency. At threshold, fibers of the A-group are activated. With low frequencies of stimulation respiration will be activated, with higher frequencies inhibited. At about double stimulus strength fibers of the B-group are activated, which leads at lower frequencies of stimulation to a strong inspiratory reaction. The respiratory effect of vagal stimulation show great differences of species. Predominantly inhibitory in cats, the vagal afferents activate respiration in rabbits and guinea-pigs. In the latter animal the dynamogenic action of vagal reflexes is indispensable for the maintenance of adequate breathing rhythms. Stimulation of the superior laryngeal nerve always leads to respiratory inhibition. The results of afferent stimulation of the phrenic nerve are variable. Electrically induced activity of the buffer has little effect on respiration; if any influence occurs, it is inhibitory. Respiratory depth and frequency increase under chemoreceptor influence. The blood-pressure reaction to stimulation of the cervical vagus depends on the admixture of aortic depressor and chemoreceptor fibers. At threshold stimulation the activated A-fibers are depressor fibers, as well as the less excitable fibers in the B-group. Activation of C-fibers leads mainly to pressor reactions. The aortic depressor and tne baroreceptor branch to the carotid sinus contain two sets of fibers, both depressor. A slight fall in systemic arterial pressure follows the stimulation of A- and B-fibers. To obtain a maximal depressor effect stimulus strength has to be increased to intensities where C-fibers become activated. Chemoreceptor fibers show high thresholds to stimulation and are all pressor. In some cases a selective stimulation of these fibers in the Hering''s nerve is possible when shocks of longer duration and low stimulation frequencies are used. Stimulation of Hering''s nerve in man is followed by identical respiratory and vasomotor reactions to those in animals.