Nervous System: IB & WJEC Biology Revision Guide | 神经系统：IB WJEC 生物考点精讲

📚 Nervous System: IB & WJEC Biology Revision Guide | 神经系统：IB WJEC 生物考点精讲

The nervous system coordinates rapid, short-term responses to stimuli using electrical impulses. It is a core topic in IB and WJEC Biology, requiring a clear understanding of neuron structure, membrane potentials, synaptic transmission, and the organisation of the central and peripheral nervous systems. This guide breaks down each key concept with bilingual explanations to support your revision.

神经系统利用电冲动协调对刺激的快速、短期反应。这是 IB 和 WJEC 生物学中的核心主题，需要清晰理解神经元结构、膜电位、突触传递以及中枢和外周神经系统的组织。本指南以双语解释逐一分解每个关键概念，助力你的复习。

1. Introduction to the Nervous System | 神经系统概论

The nervous system enables organisms to detect environmental changes and coordinate appropriate responses with speed and precision. It consists of the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), which includes all nerves outside the CNS. The fundamental functional unit is the neurone, supported by glial cells that provide insulation, nutrients, and immune defence.

神经系统使生物体能够检测环境变化，并迅速而精确地协调适当的反应。它由中枢神经系统（CNS，包括脑和脊髓）和外周神经系统（PNS，包括所有 CNS 之外的神经）组成。基本功能单位是神经元，由胶质细胞支持，提供绝缘、营养和免疫防御。

Compared to the endocrine system, nervous communication is faster, uses electrical and chemical signals, and has localised, short-lived effects. It operates through three main processes: sensory input (reception of stimuli), integration (processing in the CNS), and motor output (activation of effectors such as muscles and glands).

与内分泌系统相比，神经通信速度更快，使用电信号和化学信号，且作用局部而短暂。它通过三个主要过程运行：感觉输入（接受刺激）、整合（在 CNS 中处理）和运动输出（激活效应器，如肌肉和腺体）。

2. Neurone Structure and Classification | 神经元结构与分类

A typical neurone has a cell body (soma) containing the nucleus, dendrites that receive signals from other neurones, and an axon that conducts impulses away from the cell body toward other neurones or effectors. The axon is often covered by a myelin sheath, a lipid-rich insulating layer formed by Schwann cells in the PNS and oligodendrocytes in the CNS. Gaps in the sheath are called nodes of Ranvier, which are critical for rapid impulse conduction.

一个典型的神经元有一个含有细胞核的胞体、接收其他神经元信号的树突以及将冲动传离胞体至其他神经元或效应器的轴突。轴突通常被髓鞘覆盖，髓鞘是一种富含脂质的绝缘层，由 PNS 中的施万细胞和 CNS 中的少突胶质细胞形成。髓鞘上的间隙称为郎飞氏结，对快速的冲动传导至关重要。

Neurones are functionally classified into three types: sensory neurones carry impulses from receptors to the CNS, motor neurones transmit commands from the CNS to effectors (muscles or glands), and relay (intermediate) neurones interconnect sensory and motor pathways within the CNS. In a reflex arc, all three types work together to produce a rapid, involuntary response.

神经元按功能分为三类：感觉神经元将冲动从感受器传至 CNS，运动神经元将指令从 CNS 传到效应器（肌肉或腺体），联络（中间）神经元在 CNS 内连接感觉和运动通路。在反射弧中，这三种类型协同工作，产生快速、非自主的反应。

3. Establishing the Resting Potential | 静息电位的建立

A non-conducting neurone maintains a resting membrane potential of approximately −70 mV, meaning the inside of the axon is negatively charged relative to the outside. This polarisation is maintained by two principal mechanisms: the sodium–potassium pump (Na⁺/K⁺-ATPase) actively transports 3 Na⁺ out of the cell and 2 K⁺ in, building electrochemical gradients; and the membrane shows greater permeability to K⁺ than Na⁺ at rest due to the presence of non-gated (leak) K⁺ channels.

一个未传导冲动的神经元维持约 −70 mV 的静息膜电位，这意味着轴突内部相对于外部带负电。这种极化由两个主要机制维持：钠钾泵（Na⁺/K⁺-ATP 酶）主动将 3 个 Na⁺ 运出细胞、2 个 K⁺ 运入，建立电化学梯度；同时，由于存在非门控（漏）钾通道，静息状态下膜对 K⁺ 的通透性高于 Na⁺。

K⁺ diffuses out of the cell down its concentration gradient, but large intracellular anions (e.g. negatively charged proteins) cannot follow, creating a net negative charge inside. This equilibrium is described by the Goldman equation, but for exam purposes remember that the resting potential is close to the equilibrium potential for K⁺ because the membrane is most permeable to that ion.

K⁺ 沿浓度梯度扩散出细胞，而胞内的大分子阴离子（如带负电的蛋白质）不能随之外出，从而产生净负电荷。该平衡由 Goldman 方程描述，但在考试中记住，由于膜对钾离子的通透性最高，静息电位接近 K⁺ 的平衡电位。

4. The Action Potential in Detail | 动作电位详解

An action potential is a brief, regenerative electrical impulse triggered when the membrane potential reaches a threshold of about −55 mV. It follows an all-or-nothing law: once threshold is met, a full action potential is produced; sub-threshold stimuli generate no impulse. The process involves sequential changes in membrane permeability to Na⁺ and K⁺ governed by voltage-gated channels.

动作电位是一种短暂的、可再生的电冲动，当膜电位达到约 −55 mV 的阈电位时触发。它遵循全或无定律：一旦达到阈值，就会产生一个完整的动作电位；阈下刺激不产生冲动。该过程涉及由电压门控通道调控的膜对 Na⁺ 和 K⁺ 通透性的有序变化。

Depolarisation: voltage-gated Na⁺ channels open, causing a massive influx of Na⁺ that reverses the membrane potential to around +40 mV. Repolarisation: Na⁺ channels inactivate and voltage-gated K⁺ channels open, allowing K⁺ efflux that drives the potential back toward negative values. Hyperpolarisation (undershoot): K⁺ channels close slowly, so the membrane briefly becomes more negative than resting; the Na⁺/K⁺ pump then restores the ionic gradients. During the absolute refractory period, no new action potential can be generated, ensuring one-way propagation; the relative refractory period requires a stronger stimulus.

去极化：电压门控 Na⁺ 通道开放，大量 Na⁺ 内流，使膜电位反转为约 +40 mV。复极化：Na⁺ 通道失活，电压门控 K⁺ 通道开放，K⁺ 外流将电位推回负值。超极化（下冲）：K⁺ 通道关闭较慢，膜电位暂时比静息电位更负；随后 Na⁺/K⁺ 泵恢复离子梯度。在绝对不应期内，不能产生新的动作电位，保证了单向传导；相对不应期则需要更强的刺激。

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

Once an action potential is generated in one region of the axon, it acts as a stimulus for the adjacent region. Local current flows between the depolarised zone and the neighbouring resting membrane, causing the resting zone to reach threshold and fire its own action potential. This process repeats along the entire length of the axon.

一旦轴突某一区域产生动作电位，它会作为相邻区域的刺激。局部电流在去极化区与邻近的静息膜之间流动，使静息区达到阈值，并触发自身的动作电位。这一过程沿整个轴突长度重复进行。

In unmyelinated neurones, this is continuous conduction, which is relatively slow. In myelinated neurones, saltatory conduction occurs because the insulating myelin prevents ion flow except at nodes of Ranvier. The impulse jumps from node to node, greatly accelerating conduction speed. Larger axon diameter reduces internal resistance, also contributing to faster conduction. This is why vertebrates rely on myelination to achieve rapid nerve impulses without needing giant axons.

在无髓神经元中，这种连续传导相对缓慢。在有髓神经元中，由于绝缘的髓鞘阻止离子流动（除郎飞氏结外），发生跳跃传导。冲动从一个结

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