Coordination in Living Organisms | 生物体内的协调作用

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

Coordination is essential for survival because living organisms must monitor their internal environment and the external world, then adjust their activities accordingly. In Cambridge A Level Biology, coordination brings together the nervous system, the endocrine system and plant growth regulators to explain how signals are generated, transmitted and converted into responses.

协调作用对生存至关重要，因为生物体必须监测内环境和外界变化，并相应调整自身活动。在剑桥A Level生物课程中，协调作用综合了神经系统、内分泌系统和植物生长调节物质，解释信号如何产生、传递并转化为反应。

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1. Overview of Coordination Systems | 协调系统概述

Mammals possess two major coordination systems: the nervous system and the endocrine system. The nervous system uses electrical impulses transmitted along neurones, while the endocrine system uses chemical hormones carried in the blood.

哺乳动物拥有两大协调系统：神经系统和内分泌系统。神经系统通过神经元传导电冲动，内分泌系统则通过血液运输化学激素。

The nervous system provides rapid, short-lived and precise responses, for example a reflex action. The endocrine system is slower to act but produces longer-lasting and often widespread effects, such as control of blood glucose concentration.

神经系统产生快速、短暂且精确的反应，例如反射动作。内分泌系统作用较慢，但反应持续时间更长，影响往往更广泛，例如血糖浓度的控制。

Although the two systems differ, they interact closely in processes such as stress responses and homeostasis.

尽管两个系统机制不同，但它们在应激反应和稳态等过程中紧密合作。

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2. Neurone Structure and Types | 神经元结构与类型

Neurones are specialised cells that transmit electrical impulses. A typical motor neurone consists of a cell body, dendrites, an axon, a myelin sheath and synaptic knobs.

神经元是传递电冲动的特化细胞。典型的运动神经元由细胞体、树突、轴突、髓鞘和突触小体组成。

Sensory neurones carry impulses from receptor cells to the central nervous system (CNS). Relay neurones are found inside the CNS and connect sensory neurones to motor neurones. Motor neurones carry impulses from the CNS to effector organs such as muscles and glands.

感觉神经元将冲动从感受器细胞传至中枢神经系统。中间神经元位于中枢神经系统内部，连接感觉神经元和运动神经元。运动神经元将冲动从中枢神经系统传至效应器，如肌肉和腺体。

Schwann cells wrap around the axon to form the myelin sheath, which insulates the axon and speeds up impulse transmission. Gaps between Schwann cells are called nodes of Ranvier.

施万细胞包裹轴突形成髓鞘，髓鞘起绝缘作用并加快冲动传导。施万细胞之间的间隙称为郎飞结。

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3. Resting Potential | 静息电位

When a neurone is not transmitting an impulse, the inside of the axon is about −70 mV compared with the outside. This potential difference is called the resting potential.

当神经元不传导冲动时，轴突内部相对外部约为−70 mV。这种电位差称为静息电位。

The resting potential is established by the sodium-potassium pump, which actively transports 3 Na⁺ out of the axon and 2 K⁺ into the axon for each ATP molecule hydrolysed. The membrane also contains open potassium leak channels, making it more permeable to K⁺ than to Na⁺.

静息电位由钠钾泵建立，该泵每水解一个ATP主动将3个Na⁺运出轴突，同时将2个K⁺运入轴突。膜上还存在开放的钾泄漏通道，使膜对K⁺的通透性大于对Na⁺的通透性。

As a result, more positive ions leave the cell than enter, and the inner surface of the membrane becomes negative relative to the outer surface.

因此，离开细胞的正离子多于进入细胞的正离子，膜内表面相对膜外表面呈负电性。

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4. Action Potential | 动作电位

When a stimulus depolarises the membrane to the threshold potential, typically about −55 mV, voltage-gated sodium channels open. Sodium ions flood into the axon, causing rapid depolarisation to around +30 mV.

当刺激使膜去极化达到阈电位（通常约−55 mV）时，电压门控钠通道打开。钠离子大量涌入轴突，使膜迅速去极化至约+30 mV。

Sodium channels then inactivate, and voltage-gated potassium channels open. Potassium ions move out of the axon, repolarising the membrane. There is often a brief hyperpolarisation because potassium channels close slowly.

随后钠通道失活，电压门控钾通道打开。钾离子移出轴突，使膜复极化。由于钾通道关闭较慢，常出现短暂的超极化。

During the refractory period, the neurone cannot generate another action potential. This ensures impulses travel in one direction and limits their frequency.

在不应期内，神经元不能产生新的动作电位。这确保了冲动单向传导并限制了冲动频率。

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5. Propagation of Action Potentials | 动作电位的传导

An action potential acts as a local electrical event. The influx of Na⁺ at one point creates local currents that depolarise the adjacent section of axon membrane, opening voltage-gated sodium channels and generating a new action potential.

动作电位是一个局部电事件。某一点Na⁺内流产生局部电流，使相邻轴突膜区域去极化，打开电压门控钠通道并产生新的动作电位。

In myelinated neurones, action potentials occur only at nodes of Ranvier because the myelin sheath prevents ion movement. The impulse therefore jumps from node to node; this is called saltatory conduction and is much faster than continuous conduction in unmyelinated neurones.

在有髓神经元中，动作电位只在郎飞结处发生，因为髓鞘阻止离子跨膜移动。因此冲动从一个结跳到下一个结，这称为跳跃传导，比无髓神经元中的连续传导快得多。

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6. Synaptic Transmission | 突触传递

A synapse is the junction between two neurones or between a neurone and an effector cell. The presynaptic knob contains vesicles filled with neurotransmitter, such as acetylcholine.

突触是两个神经元之间或神经元与效应细胞之间的连接部位。突触前小体含有充满神经递质（如乙酰胆碱）的小泡。

When an action potential arrives at the presynaptic membrane, voltage-gated calcium ion channels open, allowing Ca²⁺ to enter. This causes vesicles to fuse with the presynaptic membrane and release neurotransmitter by exocytosis.

当动作电位到达突触前膜时，电压门控钙离子通道打开，Ca²⁺进入。这使小泡与突触前膜融合，通过胞吐释放神经递质。

The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane, opening ligand-gated Na⁺ channels. If enough Na⁺ enters to depolarise the postsynaptic membrane to threshold, an action potential is generated in the postsynaptic neurone.

神经递质扩散穿过突触间隙，与突触后膜上的受体蛋白结合，打开配体门控Na⁺通道。如果足够多的Na⁺进入使突触后膜去极化到阈电位，突触后神经元便产生动作电位。

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7. The Cholinergic Synapse and Its Significance | 胆碱能突触及其重要性

Acetylcholine is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase into acetate and choline. The products are reabsorbed by the presynaptic neurone and used to reform acetylcholine.

乙酰胆碱在突触间隙被乙酰胆碱酯酶迅速分解为乙酸和胆碱。产物被突触前神经元重新吸收，用于重新合成乙酰胆碱。

This breakdown prevents continuous stimulation of the postsynaptic neurone and allows synapses to transmit information precisely. Many drugs and toxins affect cholinergic synapses; for example, nicotine mimics acetylcholine and binds to its receptors, while some insecticides inhibit acetylcholinesterase, causing overstimulation.

这种分解防止突触后神经元被持续刺激，并使突触能够精确传递信息。许多药物和毒素影响胆碱能突触；例如，尼古丁模拟乙酰胆碱并与其受体结合，而某些杀虫剂抑制乙酰胆碱酯酶，导致过度刺激。

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8. Endocrine System and Hormone Action | 内分泌系统与激素作用

Endocrine glands secrete hormones directly into the bloodstream. Hormones travel throughout the body but only affect target cells that possess specific receptor proteins for that hormone.

内分泌腺将激素直接分泌到血液中。激素随血液流遍全身，但只作用于具有该激素特异性受体蛋白的靶细胞。

Peptide hormones, such as insulin and glucagon, are water-soluble and bind to cell surface receptors. They often activate a second messenger, such as cyclic AMP, inside the target cell, which brings about the cellular response.

肽类激素（如胰岛素和胰高血糖素）是水溶性的，与细胞表面受体结合。它们通常激活靶细胞内的第二信使（如环磷酸腺苷cAMP），从而引起细胞反应。

Steroid hormones, such as oestrogen and testosterone, are lipid-soluble and pass through the cell membrane. They bind to intracellular receptors and directly influence gene transcription.

类固醇激素（如雌激素和睾酮）是脂溶性的，可穿过细胞膜。它们与细胞内受体结合，直接影响基因转录。

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9. Regulation of Blood Glucose Concentration | 血糖浓度的调节

The pancreas contains endocrine tissue called the islets of Langerhans. Alpha cells secrete glucagon, while beta cells secrete insulin.

胰腺含有称为胰岛的内分泌组织。α细胞分泌胰高血糖素，β细胞分泌胰岛素。

After a meal, blood glucose concentration rises. Beta cells detect the rise and release insulin. Insulin binds to receptors on liver and muscle cells, increasing glucose uptake and promoting conversion of glucose to glycogen (glycogenesis). This lowers blood glucose back to normal.

进食后，血糖浓度升高。β细胞检测到升高并释放胰岛素。胰岛素与肝细胞和肌肉细胞上的受体结合，增加葡萄糖摄取，促进葡萄糖转化为糖原（糖原生成），使血糖回落到正常水平。

If blood glucose falls, alpha cells release glucagon. Glucagon stimulates the liver to break down glycogen

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