Coordination in Living Organisms | 生物体内的协调

📚 Coordination in Living Organisms | 生物体内的协调

In multicellular organisms, the survival and proper functioning of every cell depend on the ability to detect changes in the internal and external environment and to respond appropriately. Coordination is the process by which different parts of the body work together to bring about a unified and efficient response to a stimulus. This is achieved primarily through two major communication systems: the nervous system, which uses electrical impulses for rapid, short-lived responses, and the endocrine system, which relies on chemical messengers called hormones for slower, longer-lasting effects. Plants, lacking a nervous system, coordinate their responses through plant hormones (plant growth regulators) that control growth and development.

在多细胞生物中，每个细胞的生存和正常功能都依赖于检测内、外环境变化并作出恰当反应的能力。协调是指身体不同部分共同作用，对刺激产生统一且高效的反应过程。主要依靠两大通信系统实现：一是神经系统，利用电冲动产生快速、短效的反应；二是内分泌系统，借助称为激素的化学信使产生较慢但持久的效果。植物没有神经系统，则通过植物激素（植物生长调节物质）来控制生长和发育，实现协调。

1. Introduction to Coordination | 协调概述

Coordination involves three essential components: receptors that detect a stimulus, coordination centres that process the information, and effectors that carry out the response. In animals, the nervous system provides point-to-point rapid coordination using neurones and synapses, while the endocrine system broadcasts signals more slowly via the bloodstream to reach target cells throughout the body. Together they maintain homeostasis and enable complex behaviours. Plant coordination is primarily based on changes in growth rates, mediated by auxins and other plant growth substances.

协调包含三个基本组成部分：探测刺激的感受器、处理信息的协调中枢，以及执行反应的效应器。在动物中，神经系统通过神经元和突触实现点对点的快速协调，而内分泌系统则通过血液将信号较慢地播散至全身的靶细胞。二者共同维持稳态并实现复杂行为。植物的协调主要基于生长速率的变化，由生长素和其他植物生长物质介导。

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2. The Nervous System: Neurones | 神经系统：神经元

Neurones are specialised cells designed to transmit electrical impulses rapidly. A typical motor neurone consists of a cell body containing the nucleus, numerous short dendrites that receive signals, and a long axon that carries impulses away to effectors such as muscles or glands. Many axons are surrounded by a myelin sheath formed by Schwann cells, which insulates the axon and greatly increases the speed of impulse conduction through saltatory conduction. Between adjacent Schwann cells are small uninsulated gaps called nodes of Ranvier, where depolarisation can occur.

神经元是专门用于快速传递电冲动的特化细胞。一个典型的运动神经元包括含有细胞核的胞体、多条接收信号的短树突，以及一根将冲动传向肌肉或腺体等效应器的长轴突。许多轴突被由施万细胞形成的髓鞘包裹，髓鞘起绝缘作用，并通过跳跃传导大大加快冲动传导速度。相邻施万细胞之间的小段裸露区域称为郎飞结，是发生去极化的部位。

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3. Resting and Action Potentials | 静息电位与动作电位

When a neurone is not transmitting an impulse, its membrane is polarised with a resting potential of approximately -70 mV. This is maintained by the sodium–potassium pump, which actively transports 3 Na⁺ out and 2 K⁺ in using ATP, and by the differential permeability of the membrane, which is more permeable to K⁺ than to Na⁺. When a stimulus causes the membrane potential to reach the threshold (around -55 mV), voltage-gated Na⁺ channels open, Na⁺ rushes in, and depolarisation occurs. The potential rises to about +30 mV, after which Na⁺ channels inactivate and voltage-gated K⁺ channels open, allowing K⁺ to flow out and causing repolarisation. The potential may briefly overshoot, producing hyperpolarisation, before the resting state is restored.

当神经元不传递冲动时，其细胞膜处于极化状态，静息电位约 -70 mV。这通过钠-钾泵消耗 ATP 主动转运出 3 个 Na⁺ 并运入 2 个 K⁺，以及膜对 K⁺ 的通透性大于 Na⁺ 来维持。当刺激使膜电位达到阈电位（约 -55 mV）时，电压门控 Na⁺ 通道开放，Na⁺ 大量内流，发生去极化。膜电位上升至约 +30 mV，随后 Na⁺ 通道失活，电压门控 K⁺ 通道开放，K⁺ 外流引起复极化。电位可能短暂反超，形成超极化，最终恢复静息状态。

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4. Synaptic Transmission | 突触传递

Synapses are the junctions between neurones or between a neurone and an effector. When an action potential arrives at the presynaptic terminal, it triggers voltage-gated Ca²⁺ channels to open, allowing Ca²⁺ ions to enter. This influx causes neurotransmitter-containing vesicles to fuse with the presynaptic membrane and release the neurotransmitter (e.g., acetylcholine) into the synaptic cleft by exocytosis. The neurotransmitter diffuses across and binds to specific receptors on the postsynaptic membrane, leading to the opening of ligand-gated ion channels and generation of a postsynaptic potential. Excitatory neurotransmitters cause depolarisation (EPSP), while inhibitory ones cause hyperpolarisation (IPSP). The neurotransmitter is then rapidly removed by reuptake into the presynaptic neurone or by enzymatic breakdown to prevent continuous stimulation.

突触是神经元之间或神经元与效应器之间的连接点。当动作电位到达突触前末梢时，电压门控 Ca²⁺ 通道开放，Ca²⁺ 内流。这一内流促使含有神经递质的囊泡与突触前膜融合，以胞吐方式将神经递质（如乙酰胆碱）释放到突触间隙。神经递质扩散通过间隙并与突触后膜上的特异性受体结合，导致配体门控离子通道开放，产生突触后电位。兴奋性递质引起去极化（EPSP），抑制性递质则引起超极化（IPSP）。随后神经递质被迅速重摄取至突触前神经元或被酶降解，以防止持续刺激。

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5. The Endocrine System: Hormones | 内分泌系统：激素

The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. Hormones are chemical messengers that travel throughout the body but only affect target cells possessing the appropriate receptor proteins. They can be peptides, proteins, amino acid derivatives, or steroids. Well-known endocrine glands include the pituitary (master gland), thyroid, adrenal glands, pancreas, and gonads. Hormonal coordination tends to be slower than nervous coordination because it relies on transport via blood circulation, but the effects are generally longer-lasting and often regulate long-term processes such as growth, metabolism, and reproduction.

内分泌系统由无导管腺体组成，它们将激素直接分泌到血液中。激素是化学信使，随血液流遍全身，但只作用于拥有相应受体蛋白的靶细胞。激素可以是多肽、蛋白质、氨基酸衍生物或类固醇。著名的内分泌腺包括垂体（主腺）、甲状腺、肾上腺、胰腺和性腺。激素协调通常比神经协调慢，因为需要依靠血液循环运输，但效果通常更持久，往往调控生长、代谢和繁殖等长期过程。

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6. Mechanism of Hormone Action | 激素作用机制

Hormones exert their effects by binding to receptors. Lipid-soluble steroid hormones (e.g., oestrogen) can pass through the plasma membrane and bind to intracellular receptors, forming a hormone–receptor complex that acts as a transcription factor, directly regulating gene expression. In contrast, many peptide and amino acid-derived hormones are water-soluble and cannot cross the membrane, so they bind to cell-surface receptors. This often activates a second messenger system. For instance, glucagon or adrenaline binds to G-protein-coupled receptors, activating adenylate cyclase, which converts ATP into cyclic AMP (cAMP). cAMP then activates protein kinase A, which phosphorylates specific enzymes, triggering a cascade of intracellular responses. This amplification allows a tiny concentration of hormone to produce a large effect.

激素通过与受体结合发挥作用。脂溶性的类固醇激素（如雌激素）可以穿过细胞膜并与胞内受体结合，形成的激素-受体复合物充当转录因子，直接调控基因表达。相反，许多多肽和氨基酸衍生的激素是水溶性的，无法穿过细胞膜，因此与细胞表面受体结合。这一结合通常会激活第二信使系统。例如，胰高血糖素或肾上腺素与 G 蛋白偶联受体结合，激活腺苷酸环化酶，该酶将 ATP 转化为环磷酸腺苷 (cAMP)。cAMP 进而激活蛋白激酶 A，后者使特定酶磷酸化，引发级联的胞内反应。这种放大效应使得微量的激素即可产生巨大影响。

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7. Control of Blood Glucose – A Hormonal Coordination Example | 血糖调节——激素协调实例

Blood glucose concentration is maintained within a narrow range (about 4–6 mmol dm⁻³) by the antagonistic actions of insulin and glucagon, produced by the β and α cells of the pancreatic islets of Langerhans, respectively. When blood glucose rises after a meal, β-cells secrete insulin, which increases the permeability of target cells (especially muscle and liver) to glucose, stimulates glycogenesis (conversion of glucose to glycogen), and enhances glucose use in respiration. When blood glucose falls, α-cells secrete glucagon, which activates glycogenolysis (breakdown of glycogen to glucose) and gluconeogenesis (formation of glucose from non-carbohydrate sources) in the liver. Adrenaline also promotes glycogenolysis and is released during stress. This negative feedback loop ensures a steady supply of glucose to the brain and tissues.

血糖浓度通过胰岛素和胰高血糖素的拮抗作用维持在一个狭窄范围（约 4–6 mmol dm⁻³）。这两种激素分别由胰岛的 β 细胞和 α 细胞分泌。餐后血糖升高时，β 细胞分泌胰岛素，增加靶细胞（特别是肌肉和肝脏细胞）对葡萄糖的通透性，促进糖原生成（葡萄糖转化为糖原），并增强呼吸作用中葡萄糖的利用。血糖下降时，α 细胞分泌胰高血糖素，激活肝脏中的糖原分解（糖原分解为葡萄糖）和糖异生（由非糖物质生成葡萄糖）。肾上腺素也可促进糖原分解并在应激时释放。这一负反馈回路确保大脑和组织获得稳定的葡萄糖供应。

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8. Plant Responses: Tropisms | 植物反应：向性

Plants coordinate their responses to directional stimuli through growth movements called tropisms. A tropism is a growth response in which the direction of growth is determined by the direction of the external stimulus. Phototropism is growth towards or away from light; geotropism (gravitropism) is growth in response to gravity; and hydrotropism is growth towards water. Shoots typically show positive phototropism and negative geotropism, enabling them to reach light for photosynthesis. Roots exhibit positive geotropism and positive hydrotropism, anchoring the plant and absorbing water and minerals. These responses are controlled by the uneven distribution of the plant hormone auxin (indole-3-acetic acid, IAA).

植物通过称为向性的生长运动来协调对方向性刺激的反应。向性是一种生长反应，其生长方向由外界刺激的方向决定。向光性是朝向或背离光源的生长；向地性（向重力性）是对重力的生长反应；向水性是朝向水分的生长。通常地上茎表现出正向光性和负向地性，以便获取光照用于光合作用。根则表现出正向地性和正向水性，以固定植物并吸收水分和矿物质。这些反应由植物激素生长素（吲哚-3-乙酸，IAA）的不均匀分布控制。

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9. Role of Auxin (IAA) in Phototropism | 生长素在向光性中的作用

When a shoot tip receives unilateral light, auxin synthesised in the tip moves laterally to the shaded side. This creates a higher concentration of IAA on the side away from the light. In shoots, a higher auxin concentration promotes cell elongation; therefore, the cells on the shaded side elongate more rapidly than those on the illuminated side. This differential growth causes the shoot to bend towards the light source. In roots, high auxin concentrations inhibit cell elongation, so they grow away from the side with more auxin, resulting in negative phototropism and positive gravitropism. The acid growth hypothesis explains auxin action: auxin stimulates proton pumps, lowering cell wall pH, activating expansins, and increasing cell wall extensibility, leading to water uptake and elongation.

当茎尖受到单侧光照时，在顶端合成的生长素向背光侧横向运输，导致背光侧 IAA 浓度更高。在茎中，高浓度生长素促进细胞伸长，因此背光侧细胞比向光侧伸长更快。这种差异生长使得茎弯向光源。在根中，高浓度生长素抑制细胞伸长，因此根背离生长素多的一侧生长，表现为负向光性和正向地性。酸生长假说解释了生长素的作用：生长素刺激质子泵，降低细胞壁 pH，激活扩展蛋白，增加细胞壁伸展性，导致细胞吸水并伸长。

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10. Comparison of Nervous and Hormonal Coordination | 神经与激素协调的比较

Nervous and hormonal systems are complementary. Nerves transmit impulses rapidly (up to 100 m s⁻¹) and responses are immediate but short-lived. The signal is confined to specific pathways and acts on muscles, glands, or other neurones. Hormonal coordination involves slower signal transmission (via blood) with responses taking seconds to days, but the effects are longer-lasting and often widespread. Hormones can influence virtually all tissues that bear receptors. The nervous system is ideal for immediate adjustments to changes (e.g., reflex actions), while the endocrine system is suited for sustained regulation of internal conditions (e.g., growth, metabolism, and blood glucose homeostasis).

神经系统和激素系统是互补的。神经传递冲动迅速（可达 100 m s⁻¹），反应即时但短暂。信号局限于特定通路，作用于肌肉、腺体或其他神经元。激素协调则通过血液传导较慢的信号，反应耗时数秒到数天，但效果持久且通常广泛。激素可以影响几乎所有带有受体的组织。神经系统适合对外界变化作出即时调整（如反射动作），而内分泌系统适合对内环境进行持续调节（如生长、代谢和血糖稳态）。

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