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arXiv 0905.3690q-bio.NCq-bio.CBq-bio.SC
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热力学约束对神经维度、放电频率、脑温度及脑尺寸的影响

Thermodynamic constraints on neural dimensions, firing rates, brain temperature and size

Jan Karbowski

AI总结:

本文基于热平衡热力学,分析Na$^{+}$/K$^{+}$泵耗散与脑温度对频率的双相依赖,推导纤维直径下限,证明哺乳动物大脑处于热力学安全区间,并给出估算平均放电频率的解析公式。

摘要:

已有研究表明,大脑代谢活动产生的热量可能约束哺乳动物大脑的进化、结构和功能。本文研究了由Na$^{+}$/K$^{+}$-ATP酶、脑血流和热传导主导的热平衡热力学对大脑布线和相应脑温度变化所施加的物理限制。研究发现,即使中等放电频率也会导致显著的细胞内Na$^{+}$积累,而泵出这些离子所伴随的ATP消耗率随频率非线性增长。令人惊讶的是,Na$^{+}$/K$^{+}$泵耗散功率对频率呈双相依赖,这可能导致脑温度对频率也呈双相依赖。钠泵总功率和脑温度在纤维直径极小时均发散,表明过细的纤维不利于热平衡。对于极小的大脑,血流在大脑深处不足以作为有效的冷却机制。纤维直径的理论下限(高于此下限时脑温度处于工作区间)强烈依赖频率,但由于突触抑制而有限。在正常神经生理条件下,该下限比经验纤维直径的平均值至少小一个数量级,表明哺乳动物大脑的神经解剖学处于热力学安全区间。所提出的解析公式可用于估计哺乳动物的平均放电频率,并将其变化与脑温度变化相关联,这具有重要的实际应用价值。总体而言,较大大脑的活动比较小大脑的活动更慢。

英文摘要:

There have been suggestions that heat caused by cerebral metabolic activity may constrain mammalian brain evolution, architecture, and function. This article investigates physical limits on brain wiring and corresponding changes in brain temperature that are imposed by thermodynamics of heat balance determined mainly by Na$^{+}$/K$^{+}$-ATPase, cerebral blood flow, and heat conduction. It is found that even moderate firing rates cause significant intracellular Na$^{+}$ build-up, and the ATP consumption rate associated with pumping out these ions grows nonlinearly with frequency. Surprisingly, the power dissipated by the Na$^{+}$/K$^{+}$ pump depends biphasically on frequency, which can lead to the biphasic dependence of brain temperature on frequency as well. Both the total power of sodium pumps and brain temperature diverge for very small fiber diameters, indicating that too thin fibers are not beneficial for thermal balance. For very small brains blood flow is not a sufficient cooling mechanism deep in the brain. The theoretical lower bound on fiber diameter above which brain temperature is in the operational regime is strongly frequency dependent but finite due to synaptic depression. For normal neurophysiological conditions this bound is at least an order of magnitude smaller than average values of empirical fiber diameters, suggesting that neuroanatomy of the mammalian brains operates in the thermodynamically safe regime. Analytical formulas presented can be used to estimate average firing rates in mammals, and relate their changes to changes in brain temperature, which can have important practical applications. In general, activity in larger brains is found to be slower than in smaller brains.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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