查看Ch14 The Autonomic Nervous System的源代码

=== CNS 控制中心监督本能反馈回路并协调前馈响应以满足预期需求 ===
<b style=color:#0ae>CNS control centers oversee visceral feedback loops and orchestrate a feed-forward response to meet anticipated needs</b>

The ANS maintains physiological parameters within an optimal range by means of feedback loops made up of sensors, afferent fibers, central autonomic control centers (discussed in the preceding section), and effector systems. These feedback loops achieve homeostasis by monitoring input from visceral receptors and adjusting the output of both the sympathetic and parasympathetic divisions to specific organs so that they maintain activity at a set-point determined by involuntary CNS control centers. As we have already noted, the sympathetic and parasympathetic divisions usually act in opposite ways to make these adjustments. Blood pressure control is an example of a visceral feedback loop in which the CNS monitors current blood pressure through afferents from baroreceptors, compares it with an internally determined set-point, and appropriately adjusts output to the heart, blood vessels, adrenal gland, and other targets. An increase in blood pressure (see pp. 537–539) causes a reflex decrease in sympathetic output to the heart and an increase in parasympathetic output.

ANS 通过由传感器、传入纤维、中枢自主神经控制中心（在上一节中讨论）和效应系统组成的反馈回路将生理参数保持在最佳范围内。这些反馈回路通过监测来自内脏受体的输入并调整交感神经和副交感神经对特定器官的输出来实现体内平衡，以便它们将活动维持在由非自愿 CNS 控制中心确定的设定点。正如我们已经指出的，交感神经和副交感神经的划分通常以相反的方式进行这些调整。血压控制是本能反馈回路的一个例子，其中 CNS 通过压力感受器的传入神经监测当前血压，将其与内部确定的设定点进行比较，并适当调整到心脏、血管、肾上腺和其他目标的输出。血压升高（见第 537-539 页）导致心脏交感神经输出的反射性减少和副交感神经输出增加。


Instead of merely responding through feedback loops, the ANS also anticipates the future needs of the individual. For example, when a person begins to exercise, sympathetic output increases before the increase in metabolic need to prevent an exercise debt from occurring (see p. 1214). Because of this anticipatory response, alveolar ventilation rises to such an extent that blood levels of CO2 (a byproduct of exercise) actually drop at the onset of exercise. This response is the opposite of what would be expected if the ANS worked purely through feedback loops, in which case an obligatory increase in CO2 levels would have preceded the increase in respiratory output (see pp. 716–717). Similarly, a trained athlete’s heart rate begins to increase several seconds before the starting gun fires to signal the beginning of a 100-m dash. This anticipation of future activity, or feedforward stimulation prior to (and during) exercise, is a key component of the regulation of homeostasis during stress because it prevents large changes in physiological parameters that could be detrimental to optimal function. This type of response probably resulted in an evolutionary advantage that permitted the body to respond rapidly and more efficiently to a threat of danger. A system relying solely on feedback could produce a response that is delayed or out of phase with respect to the stimulus. The central neuronal pathways responsible for this anticipatory or feed-forward response are not known.

ANS 不仅通过反馈循环做出响应，还预测个人的未来需求。例如，当一个人开始锻炼时，交感神经输出在防止运动债务发生的新陈代谢需求增加之前增加（见第 1214 页）。由于这种预期反应，肺泡通气量上升到如此程度，以至于血液中的 CO2（运动的副产品）水平实际上在运动开始时下降。这种反应与 ANS 纯粹通过反馈回路工作时的预期相反，在这种情况下，CO2 水平的强制性增加会在呼吸输出增加之前（见第 716-717 页）。同样，训练有素的运动员的心率在发令枪响前几秒钟开始增加，以发出 100 米冲刺的开始。这种对未来活动的预期，或在运动前（和运动中）的前馈刺激，是压力期间调节体内平衡的关键组成部分，因为它可以防止可能对最佳功能有害的生理参数发生巨大变化。这种类型的反应可能导致一种进化优势，使身体能够快速、更有效地对危险的威胁做出反应。仅依赖反馈的系统可能会产生与刺激有关的延迟或异相响应。负责这种预期或前馈反应的中枢神经元通路尚不清楚。

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