查看Ch14 The Autonomic Nervous System的源代码

=== 大多数节后交感神经元将去甲肾上腺素释放到内脏靶标上 ===
<b style=color:#0ae>Most postganglionic sympathetic neurons release norepinephrine onto visceral targets</b>

Most postganglionic sympathetic neurons release norepinephrine (see Fig. 15-8), which acts on target cells through adrenergic receptors. The sympathetic innervation of sweat glands is an exception to this rule. N14-4 Sweat glands are innervated by sympathetic neurons that release ACh and act via muscarinic receptors (see p. 571). The adrenergic receptors are all GPCRs and are highly homologous to the muscarinic receptors (see p. 341). Two major types of adrenergic receptors are recognized, α and β, each of which exists in multiple subtypes (e.g., α1, α2, β1, β2, and β3). In addition, there are heterogeneous α1 and α2 receptors, with three cloned subtypes of each. Table 14-2 lists the signaling pathways that are generally linked to these receptors. For example, β1 receptors in the heart activate the Gs heterotrimeric G protein and stimulate adenylyl cyclase, which antagonizes the effects of muscarinic receptors.

大多数节后交感神经元释放去甲肾上腺素（见图 15-8），它通过肾上腺素能受体作用于靶细胞。汗腺的交感神经支配是这个规则的一个例外[N14-4]。汗腺由交感神经元支配，交感神经元释放 ACh 并通过毒蕈碱受体发挥作用（见第 571 页）。肾上腺素能受体都是 GPCR，与毒蕈碱受体高度同源（见第 341 页）。肾上腺素能受体有两种主要类型，α 和 β，每种受体都存在于多个亚型中（例如，α1、α2、β1、β2 和 β3）。此外，还有异质性 α1 和 α2 受体，每种受体都有三个克隆亚型。表 14-2 列出了通常与这些受体相关的信号转导通路。例如，心脏中的 β1 受体激活 Gs 异源三聚体 G 蛋白并刺激腺苷酸环化酶，从而拮抗毒蕈碱受体的作用。


Adrenergic receptor subtypes have a tissue-specific distribution. α1 receptors predominate on blood vessels, α2 on presynaptic terminals, β1 in the heart, β2 in high concentration in the bronchial muscle of the lungs, and β3 in fat cells. This distribution has permitted the development of many clinically useful agents that are selective for different subtypes and tissues. For example, α1 agonists are effective as nasal decongestants, and α2 antagonists have been used to treat impotence. β1 agonists increase cardiac output in congestive heart failure, whereas β1 antagonists are useful antihypertensive agents. β2 agonists are used as bronchodilators in patients with asthma and chronic lung disease.

肾上腺素能受体亚型具有组织特异性分布。α1 受体在血管中占主导地位，α2 在突触前末梢，β1 在心脏中，β2 在肺支气管肌中高浓度，β3 在脂肪细胞中。这种分布允许开发许多临床上有用的药物，这些药物对不同的亚型和组织具有选择性。例如，α1 激动剂作为鼻减充血剂有效，α2 拮抗剂已用于治疗阳痿。β1 受体激动剂可增加充血性心力衰竭患者的心输出量，而 β1 受体拮抗剂是有用的降压药。β2 激动剂用作哮喘和慢性肺病患者的支气管扩张剂。


The adrenal medulla (see pp. 1030–1034) is a special adaptation of the sympathetic division, homologous to a postganglionic sympathetic neuron (see Fig. 14-8). It is innervated by preganglionic sympathetic neurons, and the postsynaptic target cells, which are called chromaffin cells, have nicotinic ACh receptors. However, rather than possessing axons that release norepinephrine onto a specific target organ, the chromaffin cells reside near blood vessels and release epinephrine into the bloodstream. This neuroendocrine component of sympathetic output enhances the ability of the sympathetic division to broadcast its output throughout the body. Norepinephrine and epinephrine both activate all five subtypes of adrenergic receptor, but with different affinities (see Table 14-2). In general, the α receptors have a greater affinity for norepinephrine, whereas the β receptors have a greater affinity for epinephrine.

肾上腺髓质（见第 1030-1034 页）是交感神经的特殊适应，与节后交感神经神经元同源（见图 14-8）。它由节前交感神经元支配，突触后靶细胞（称为嗜铬细胞）具有烟碱 ACh 受体。然而，嗜铬细胞并不具有将去甲肾上腺素释放到特定靶器官上的轴突，而是位于血管附近并将肾上腺素释放到血液中。交感神经输出的这种神经内分泌成分增强了交感神经部门将其输出广播到全身的能力。去甲肾上腺素和肾上腺素都激活肾上腺素能受体的所有五种亚型，但亲和力不同（见表 14-2）。一般来说，α 受体对去甲肾上腺素具有更大的亲和力，而 β 受体对肾上腺素的亲和力更大。

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