On the physiological action of carbon dioxide on cortex and hypothalamus.

Abstract Inhalation of 10 per cent CO 2 reduces the responsiveness of specific sensory projection areas to optic and acoustic stimuli while it increases the reactivity of the hypothalamic cortical system. The activation of the latter is evident from the increased asynchrony of potentials in all parts of the cerebral cortex which is associated with similar changes in hypothalamic potentials. In sensitive preparations and/or under the influence of slightly higher concentrations of carbon dioxide neuronal recruitment may occur resulting in a marked increase in the amplitude of the potentials. This effect of carbon dioxide on the hypothalamic-cortical system is similar to that produced by proprioceptive or nociceptive stimuli which evoke arousal reactions in lightly anesthetized animals. It is shown that the action of proprioceptive and nociceptive stimuli is greatly magnified in hypercapnia. The excitability of the cortex is thought to depend, other conditions being equal, on the hypothalamic-cortical system which influences the cerebral cortex as a whole and afferent impulses whose action is confined to a specific cortical projection area (optic stimulus → visual cortex). A functional separation of these two systems can be achieved by the action of carbon dioxide. The effect of high concentrations of carbon dioxide (30 to 40 per cent) administered in oxygen or air is fundamentally different from that of "low" carbon dioxide. The chief actions of "high" carbon dioxide are: It reduces or eliminates the small, frequent background potentials while the grouped, Dial potentials are relatively unchanged. If high carbon dioxide is administered for longer periods cortical activity may temporarily disappear. Thereafer, Dial potentials reappear and may occur more frequently than prior to the administration of carbon dioxide. Topically induced cortical strychnine spikes undergo a slight reduction in amplitude and a marked diminution in frequency under the influence of high carbon dioxide concentrations. Potentials recorded from the posterior hypothalamic area show similar changes. The reduction in amplitude and frequency of the potentials suggests neuronal decruitment in cortex and hypothalamus under the influence of anesthetic carbon dioxide concentrations. The potential changes described in the preceding paragraphs occur in carbon dioxide-air as well as in carbon dioxide-oxygen mixtures. They are reversible on readmission of air. Occasionally, rebound phenomena are observed under these conditions. Optic and acoustic stimuli still elicit typical responses in their specific cortical projection areas even when the resting potentials of these or other parts of the cortex have vanished. The reduction in amplitude of the acoustic potentials is less than that of the visual cortical responses. While specific cortical projection areas remain excitable in high carbon dioxide, the generalized response of the whole cerebral cortex to proprio- and nociceptive stimuli is abolished. Since hypothalamic excitation is likewise missing this result is interpreted as indicating that "high" carbon dioxide eliminates the reactivity of the hypothalamic-cortical system while the responsiveness of specific cortical projection areas is retained. It is suggested: ( a ) that the selective functional elimination of the hypothalamic-cortical system, which is activated in arousal, is characteristic for the state of loss of awareness and narcosis; ( b ) that the background potentials of the cortex depend primarily on hypothalamic-cortical discharges; ( c ) that functional elimination of the activating hypothalamic-cortical system releases the inhibitory system regulated by intralaminar thalamic nuclei and leads to more frequent Dial potentials in the cortex.