Secher Neils H, Seifert Thomas, Van Lieshout Johannes J
Department of Anesthesia, The Copenhagen Muscle Research Center, Rigshospitalet, Denmark.
J Appl Physiol (1985). 2008 Jan;104(1):306-14. doi: 10.1152/japplphysiol.00853.2007. Epub 2007 Oct 25.
During exercise: the Kety-Schmidt-determined cerebral blood flow (CBF) does not change because the jugular vein is collapsed in the upright position. In contrast, when CBF is evaluated by (133)Xe clearance, by flow in the internal carotid artery, or by flow velocity in basal cerebral arteries, a approximately 25% increase is detected with a parallel increase in metabolism. During activation, an increase in cerebral O(2) supply is required because there is no capillary recruitment within the brain and increased metabolism becomes dependent on an enhanced gradient for oxygen diffusion. During maximal whole body exercise, however, cerebral oxygenation decreases because of eventual arterial desaturation and marked hyperventilation-related hypocapnia of consequence for CBF. Reduced cerebral oxygenation affects recruitment of motor units, and supplemental O(2) enhances cerebral oxygenation and work capacity without effects on muscle oxygenation. Also, the work of breathing and the increasing temperature of the brain during exercise are of importance for the development of so-called central fatigue. During prolonged exercise, the perceived exertion is related to accumulation of ammonia in the brain, and data support the theory that glycogen depletion in astrocytes limits the ability of the brain to accelerate its metabolism during activation. The release of interleukin-6 from the brain when exercise is prolonged may represent a signaling pathway in matching the metabolic response of the brain. Preliminary data suggest a coupling between the circulatory and metabolic perturbations in the brain during strenuous exercise and the ability of the brain to access slow-twitch muscle fiber populations.
通过凯蒂-施密特法测定的脑血流量（CBF）不变，因为在直立位时颈静脉塌陷。相比之下，当通过（133）Xe清除率、颈内动脉血流或脑基底动脉血流速度评估CBF时，会检测到约25%的增加，同时代谢也平行增加。在激活过程中，由于脑内没有毛细血管募集，且代谢增加依赖于氧扩散梯度的增强，因此需要增加脑氧供应。然而，在最大强度全身运动期间，由于最终的动脉血氧饱和度降低以及与过度通气相关的显著低碳酸血症对CBF产生影响，脑氧合会降低。脑氧合降低会影响运动单位的募集，补充氧气可增强脑氧合和工作能力，而对肌肉氧合无影响。此外，运动期间的呼吸功和脑温升高对所谓的中枢性疲劳的发展也很重要。在长时间运动期间，主观用力感觉与脑内氨的积累有关，数据支持星形胶质细胞内糖原耗竭限制脑在激活过程中加速其代谢能力的理论。长时间运动时脑内白细胞介素-6的释放可能代表了一种使脑代谢反应相匹配的信号通路。初步数据表明，剧烈运动期间脑内循环和代谢紊乱之间存在耦合，以及脑获取慢肌纤维群的能力。
