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The tidal volume and corresponding efferent phrenic activity of spontaneously occurring and provoked "augmented" breaths, AB, and the subsequent post-augmented breaths were studied in cats anesthetized with pentobarbitone during hypercapnia and hypoxia. The augmentation phase (phase II) begins at, or close to, the crest of what appears as a "normal" inspiration (phase I). The amplitude and duration of phase II remained fairly constant whereas the amplitude and the duration of phase I changed with chemical drive just as in control breaths. The smaller amplitude and shorter duration of post-augmented breaths as compared to control breaths seems to be due to both a lower-than-normal inspiratory "off-switch" threshold following the AB and an increased rate of rise of inspiratory activity. With increasing hypercapnia and hypoxia both the time interval between AB and the refractory period following an AB during which a new AB cannot be provoked were reduced. Following bilateral vagotomy AB was temporarily abolished but reappeared after 1-2 h. The relatively low rates of occurrence after vagotomy still showed the same type of dependence on chemical stimuli. The refractory period was not abolished although usually decreased by gallamine paralysis or high thoracic spinalization.

SUMMARY The function of the anal sphincters has been studied by obtaining continuous recordings of the pressure in the anal canal and the electromyographic activity in the striated sphincter muscles during expansion of the ampulla recti by means of an air balloon. Ten healthy subjects were examined before and after the striated muscles had been entirely paralysed by bilateral pudendal block, making it possible to record the activity from the internal sphincter alone.The results show that the internal sphincter contributes about 85 % of the pressure in the anal canal at rest but only about 40 % after a sudden substantial distension of the rectum. During constant substantial rectal distension, the internal sphincter accounts for about 65 % of the anal pressure. It is concluded that the internal sphincter in the adult is chiefly responsible for anal continence at rest. In the event of sudden substantial distension of the rectum, continence is maintained by the striated sphincter muscles, whereas both sphincter systems probably have an important function during constant distension of the rectum.

Recent advances in several laboratories concerning the respiration-related medullary neurons, their locations, projections, interconnections, morphological and physiological properties, and patterns of inhibitory postsynaptic potentials, excitatory postsynaptic potentials, and discharge rate, on the one hand, and the "systems behavior," on the other, have provided the basis for new hypothesis concerning the neural mechanisms underlying the central pattern generator (CPG) for breathing and its different parts. The onset of the "ramp"-like increase in inspiratory activity is due to an abrupt release of inhibition and a subsequent progressively increasing synaptic excitation of inspiratory premotor neurons. The integration of the excitatory "drive" inputs underlying the ramp inspiratory activity seems to depend on structures in the ventrorostral medulla, including nucleus paragigantocellularis. The termination of this activity by the off-switch mechanisms is actuated when a critical threshold is attained by the excitatory inputs of 1) a slowly increasing inspiration-related activity and 2) the afferent input from the pulmonary stretch receptors. The nature of the former activity is discussed. During the expiratory phase, an inhibitory activity suppresses inspiration-facilitating inputs with a slowly decaying power that controls the expiratory duration. The postinspiration activity, which brakes the rate of exhalation during the first part of the expiratory phase, depends on mechanisms separate from those responsible for the inspiratory ramp activity. The respiratory CPG seems to be organized with considerable amount of redundancy, or "degeneracity."

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