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== 独立的温度感受器检测暖和冷 ==
<b style=color:#0ae>Separate thermoreceptors detect warmth and cold</b>


Neurons are sensitive to changes in temperature, as are all of life’s chemical reactions. Neuronal temperature sensitivity has two consequences: first, neurons can measure temperature; but second, to work properly, most neural circuits need to be kept at a relatively stable temperature. Neurons of the mammalian CNS are especially vulnerable to temperature changes. Whereas skin tissue temperatures can range from 20°C to 40°C without harm or discomfort, brain temperature must be near 37°C to avoid serious dysfunction. The body has complex systems to control brain (i.e., body core) temperature tightly (see pp. 1198–1201). Even though all neurons are sensitive to temperature, not all neurons are thermoreceptors.

神经元对温度的变化很敏感，生命中的所有化学反应也是如此。神经元对温度的敏感有两个后果：首先，神经元可以测量温度;但其次，要正常工作，大多数神经回路需要保持在相对稳定的温度下。哺乳动物 CNS 的神经元特别容易受到温度变化的影响。虽然皮肤组织温度可以在 20°C 到 40°C 之间而不会造成伤害或不适，但大脑温度必须接近 37°C 以避免严重功能障碍。身体有复杂的系统来严格控制大脑（即身体核心）的温度（见第 1198-1201 页）。尽管所有神经元都对温度敏感，但并非所有神经元都是温度感受器。


Because of specific membrane mechanisms, some neurons are extremely sensitive to temperature and seem to be adapted to the job of sensing it. Although many temperature-sensitive neurons are present in the skin, they are also clustered in the hypothalamus and the spinal cord (see pp. 1198–1199). The hypothalamic temperature sensors, like their cutaneous counterparts, are important for regulation of the physiological responses that maintain stable body temperature.

由于特定的膜机制，一些神经元对温度极为敏感，并且似乎适应了感知温度的工作。尽管皮肤中存在许多对温度敏感的神经元，但它们也聚集在下丘脑和脊髓中（见第 1198-1199 页）。下丘脑温度传感器与它们的皮肤传感器一样，对于调节维持稳定体温的生理反应很重要。


Perceptions of temperature apparently reflect warmth and cold receptors located in the skin. Thermoreceptors, like mechanoreceptors, are not spread uniformly across the skin. When you map the skin’s sensitivity to temperature with a small cold or warm probe, you find spots ~1 mm across that are especially sensitive to either warmth or cold, but not to both. In addition, some areas of skin in between are relatively insensitive. The spatial dissociation of the hot and cold maps shows that they are separate submodalities, with separate receptors to encode each. Recordings from single sensory fibers have confirmed this conclusion. The responses of both warmth and cold thermoreceptors adapt during long stimuli, as many sensory receptors commonly do. Most cutaneous thermoreceptors are probably free nerve endings, without obvious specialization. Their axons are small, either unmyelinated C fibers or the smallest-diameter myelinated Aδ fibers (see Table 12-1).

对温度的感知显然反映了位于皮肤中的温暖和寒冷感受器。温度感受器与机械感受器一样，不会均匀分布在皮肤上。当您使用小型冷探头或暖探头绘制皮肤对温度的敏感性图时，您会发现 ~1 毫米宽的点对暖或冷特别敏感，但对两者都不敏感。此外，介于两者之间的某些皮肤区域相对不敏感。热图和冷图的空间解离表明它们是独立的子模态，每个子模态都有单独的受体来编码。来自单个感觉纤维的记录证实了这一结论。温暖和寒冷的热感受器的反应都会在长时间刺激期间进行调整，就像许多感觉感受器通常所做的那样。大多数皮肤热感受器可能是游离神经末梢，没有明显的特化。它们的轴突很小，要么是无髓 C 纤维，要么是最小直径的有髓 Aδ 纤维（见表 12-1）。


We can perceive changes in our average skin temperature of as little as 0.01°C. Within the skin are separate types of thermoreceptors that are sensitive to a range of relatively hot or cold temperatures. Figure 15-28A shows how the steady discharge rate of both types of receptors varies with temperature. Warmth receptors begin firing above ~30°C and increase their firing rate until 44°C to 46°C, beyond which the rate falls off steeply and a sensation of pain begins, presumably mediated by nociceptive endings (see the next section). Cold receptors have a much broader temperature response. They are relatively quiet at skin temperatures of ~40°C, but their steady discharge rate increases as the temperature falls to 24°C to 28°C. Further decreases in temperature cause the steady discharge rate of the cold receptors to decrease until the temperature falls to ~10°C. Below that temperature, firing ceases and cold becomes an effective local anesthetic.

我们可以感知到低至 0.01°C 的平均体表温度变化。 皮肤内有不同类型的热感受器，它们对一系列相对较热或较冷的温度敏感。图 15-28A 显示了两种受体的稳态放电率如何随温度变化。温暖感受器在 ~30°C 以上开始放电，并增加其放电速率，直到 44°C 至 46°C，超过此温度后，速率急剧下降并开始疼痛感，可能是由伤害性结束介导的（见下一节）。冷受体具有更广泛的温度响应。它们在 ~40°C 的皮肤温度下相对安静，但随着温度降至 24°C 至 28°C，其稳定的放电速率会增加。 温度进一步降低会导致冷接收器的稳定放电速率降低，直到温度降至 ~10°C。 低于该温度时，燃烧停止，寒冷成为一种有效的局部麻醉剂。


In addition to the tonic response just described (i.e., the steady discharge rate), cold receptors also have a phasic response that enables them to report changes in temperature. As shown in Figure 15-28B, when the temperature suddenly shifts from 20.5°C to 15.2°C (both points are to the left of the peak in Fig. 15-28A), the firing rate transiently increases (i.e., the phasic response). However, the new steady-state level is lower, as suggested by the left pair of points in Figure 15-28A. When the temperature suddenly shifts from 35°C to 31.5°C (both points are to the right of the peak in Fig. 15-28A), the firing rate transiently increases, and the new steady-state level is higher, as suggested by the right pair of points in Figure 15-28A.

除了刚才描述的强直反应（即稳定的放电率）外，冷受体还具有阶段性反应，使它们能够报告温度的变化。如图 15-28B 所示，当温度突然从 20.5°C 变为 15.2°C 时（两个点都在图 15-28A 中峰值的左侧），放电速率瞬时增加（即相位响应）。然而，新的稳态水平较低，如图 15-28A 中左边的一对点所示。当温度突然从 35°C 变为 31.5°C 时（图 15-28A 中两个点都在峰值的右侧），放电速率瞬时增加，新的稳态水平更高，如图 15-28A 中右边的一对点所示。

[[文件:Ch15-28.png]]


The transduction of relatively warm temperatures is carried out by several types of TRPV channels (specifically TRPV1 to TRPV4—see Table 6-2, family No. 5) expressed in thermoreceptors. TRPV1 is a vanilloid receptor—it is activated by the vanilloid class of compounds that includes capsaicin, the pungent ingredient that gives spicy foods their burning quality. Aptly enough, chili peppers taste “hot” because they activate some of the same ion channels that heat itself activates! TRPV1 and TRPV2 channels have painfully high temperature thresholds (~43°C and ~50°C, respectively) and thus help mediate the noxious aspects of thermoreception (see p. 387). Other TRPV channels (TRPV3 and TRPV4) are activated at more moderate temperatures and presumably provide our sensations of warmth. Yet another TRP channel, TRPM8, mediates sensations of moderate cold. TRPM8 channels begin to open at temperatures below ~27°C and are maximally activated at 8°C. In a remarkable analogy to the hot-sensitive TRPV1 channel (the capsaicin receptor), the cool-sensitive TRPM8 channel is a menthol receptor. Menthol evokes sensations of cold because it activates the same ion channel that is opened by cold temperatures.

相对较暖温度的转导由热感受器中表达的几种类型的 TRPV 通道（特别是 TRPV1 到 TRPV4 — 参见表 6-2，家族 5）进行。TRPV1 是一种香草素受体——它被香草素类化合物激活，其中包括辣椒素，辣椒素是一种刺激性成分，使辛辣食物具有燃烧感。恰如其分地，辣椒尝起来“辣”，因为它们激活了一些与加热本身相同的离子通道！TRPV1 和 TRPV2 通道具有非常高的温度阈值（分别为 ~43°C 和 ~50°C），因此有助于介导热接收的有害方面（参见第 387 页）。其他 TRPV 通道（TRPV3 和 TRPV4）在更温和的温度下被激活，并可能为我们提供温暖的感觉。另一个 TRP 通道 TRPM8 介导中度寒冷的感觉。TRPM8 通道在低于 ~27°C 的温度下开始打开，并在 8°C 时最大激活。 与热敏感的 TRPV1 通道（辣椒素受体）有一个显著的类比，冷敏感的 TRPM8 通道是一种薄荷醇受体。薄荷醇会引起寒冷的感觉，因为它会激活与低温相同的离子通道。

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