Stephenson Richard
Department of Zoology, University of Toronto, Toronto, Ontario, Canada M5S 3G5.
J Exp Biol. 2005 May;208(Pt 10):1971-91. doi: 10.1242/jeb.01583.
Despite being obligate air breathers, many species of marine mammal are capable of spending most of their lives submerged in water. How they do this has been a subject of intense interest to physiologists for over a century, yet we still do not have a detailed understanding of the physiological mechanisms underlying this behaviour. What are the proximate mechanisms that trigger the 'decisions' to submerge and return to the surface? The present study proposes a model intended to address this question, based on fundamental concepts of cardiorespiratory control. Two basic hypotheses are examined by computer simulation, using a mathematical model of the mammalian cardiorespiratory control system with parameter values for an adult Weddell seal: (1) that the control of diving can be considered to be a respiratory control problem, and (2) that dives are initiated and maintained by disfacilitation of respiratory drive, not inhibition. Computer simulations confirmed the plausibility of these hypotheses. Simulated diving behaviour and physiological responses (ventilation, cardiac output, blood and tissue gas tensions) were consistent with published data from freely diving Weddell seals. Dives up to the estimated aerobic dive limit (ADL, 18-25 min) could be simulated without the need for active inhibition of breathing in this model. This theoretical analysis suggests that the most important physiological adjustments occur during the surface interval phase of the dive cycle and include hyperventilation accompanied by high cardiac output, appropriate regulation of cerebral blood flow and central chemoreceptor threshold shifts. During dives, cardiac output, distribution of peripheral blood flow, splenic contraction and peripheral chemoreflex drives were found to modulate physiological and behavioural responses, but were not essential for simulated dives to occur. The main conclusion from this study is that the central chemoreceptor may be an important mechanism involved in the regulation of diving behaviour, implying that CO2, not O2, is the key regulatory variable in this model. This model includes and extends the ADL concept and suggests an explicit mechanism by which the respiratory control system may play a central role in the regulation of diving behaviour. It is likely that respiratory mechanisms are an important component of a hierarchical behavioural control system and further studies are required to test the qualitative and quantitative validity of the model.
尽管海洋哺乳动物是专性需氧呼吸者，但许多种类的海洋哺乳动物能够在水中度过大部分生命时光。一个多世纪以来，它们是如何做到这一点的一直是生理学家们极为感兴趣的课题，然而我们仍然没有对这种行为背后的生理机制有详细的了解。触发潜入水中和返回水面“决策”的近端机制是什么？本研究基于心肺控制的基本概念提出了一个旨在解决这个问题的模型。通过计算机模拟，使用具有成年威德尔海豹参数值的哺乳动物心肺控制系统数学模型，检验了两个基本假设：（1）潜水控制可被视为一个呼吸控制问题；（2）潜水是由呼吸驱动的去易化而非抑制启动和维持的。计算机模拟证实了这些假设的合理性。模拟的潜水行为和生理反应（通气、心输出量、血液和组织气体张力）与自由潜水的威德尔海豹的已发表数据一致。在该模型中，无需主动抑制呼吸就可以模拟长达估计有氧潜水极限（ADL，18 - 25分钟）的潜水。这一理论分析表明，最重要的生理调节发生在潜水周期的水面间隔期，包括伴有高心输出量的过度通气、脑血流量的适当调节以及中枢化学感受器阈值的变化。在潜水过程中，发现心输出量、外周血流分布、脾脏收缩和外周化学反射驱动可调节生理和行为反应，但对于模拟潜水的发生并非必不可少。这项研究的主要结论是，中枢化学感受器可能是参与潜水行为调节的重要机制，这意味着在这个模型中，二氧化碳而非氧气是关键的调节变量。该模型纳入并扩展了有氧潜水极限的概念，并提出了一种明确的机制，通过该机制呼吸控制系统可能在潜水行为调节中发挥核心作用。呼吸机制很可能是分层行为控制系统的一个重要组成部分，但需要进一步研究来检验该模型的定性和定量有效性。
