📚 A-Level WJEC Biology: Nervous System Key Points Review | A-Level WJEC 生物：神经系统 考点精讲
The nervous system is a central topic in the WJEC A-Level Biology specification, covering neuronal structure, impulse generation, synaptic transmission and organisation of the nervous system. This article condenses the key concepts you must understand, linking structure to function and presenting them in a clear, exam-focused manner.
神经系统是 WJEC A-Level 生物考纲中的核心主题，涵盖神经元结构、冲动产生、突触传递以及神经系统的组织方式。本文提炼了必须掌握的关键概念，将结构与功能联系起来，并以清晰、紧扣考点的方式呈现。
1. Overview of the Nervous System | 神经系统概述
The nervous system is a complex communication network that enables organisms to detect changes in their environment and coordinate rapid responses. It consists of the central nervous system (CNS) – the brain and spinal cord – and the peripheral nervous system (PNS), which connects the CNS to receptors and effectors. Neurones are the functional units, carrying electrical impulses swiftly and accurately.
神经系统是一个复杂的通讯网络，使生物体能感知环境变化并协调快速反应。它由中枢神经系统（CNS，即脑和脊髓）和周围神经系统（PNS，将中枢与感受器和效应器相连）组成。神经元是功能单位，快速而准确地传导电信号。
2. Neurone Structure and Types | 神经元结构与类型
A typical motor neurone has a cell body containing the nucleus, many short dendrites that receive signals, and a long axon that transmits impulses away from the cell body. The axon is surrounded by a myelin sheath formed by Schwann cells, which insulates the axon and speeds up conduction. Gaps in the myelin sheath are called nodes of Ranvier. Sensory neurones carry impulses from receptors to the CNS, relay neurones connect sensory and motor neurones within the CNS, and motor neurones carry impulses from the CNS to effectors (muscles or glands).
一个典型的运动神经元具有含细胞核的胞体、许多接收信号的短树突以及一条将冲动传离胞体的长轴突。轴突被由施万细胞形成的髓鞘包裹，这起到绝缘作用并加快传导速度。髓鞘上的间断处称为郎飞氏结。感觉神经元将冲动从感受器传至中枢神经系统，联络神经元在中枢神经系统内连接感觉和运动神经元，运动神经元则将冲动从中枢神经系统传至效应器（肌肉或腺体）。
3. Resting Potential: Establishing the Potential Difference | 静息电位：建立电位差
All neurones maintain a resting potential of about –70 mV across the cell membrane (inside negative relative to outside). This is established and maintained mainly by the sodium–potassium pump, which actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell per ATP hydrolysed. The membrane is also more permeable to K⁺ than to Na⁺ at rest due to more open potassium leak channels, so K⁺ diffuses out down its concentration gradient. This movement of positive charge makes the inside more negative. The unequal distribution of ions thus creates an electrochemical gradient.
所有神经元在细胞膜两侧维持约 –70 mV 的静息电位（膜内相对膜外为负）。该电位主要由钠钾泵建立和维持，该泵每水解一分子 ATP 便将 3 个 Na⁺ 泵出细胞、2 个 K⁺ 泵入细胞。静息时膜对 K⁺ 的通透性高于 Na⁺，因为更多钾漏通道开放，因此 K⁺ 顺浓度梯度外流。这种正电荷的外移使膜内变得更负。离子的不均匀分布从而建立起电化学梯度。
4. Action Potential: Generation of the Nerve Impulse | 动作电位：神经冲动的产生
When a neurone is stimulated, the membrane depolarises. If the stimulus reaches the threshold potential (around –55 mV), voltage-gated sodium channels open. Na⁺ rushes into the axon, raising the membrane potential to about +40 mV. This is depolarisation. The sodium channels then inactivate, and voltage-gated potassium channels open, allowing K⁺ to leave the axon. This repolarises the membrane. Often the membrane briefly hyperpolarises (becomes more negative than resting potential) before the sodium–potassium pump restores resting ionic distributions. This entire sequence is an action potential, obeying the all-or-nothing law.
当神经元受到刺激，膜将去极化。若刺激达到阈电位（约 –55 mV），电压门控钠通道开放。Na⁺ 涌入轴突，使膜电位升至约 +40 mV，此为去极化。随后钠通道失活，电压门控钾通道开放，K⁺ 离开轴突，使膜复极化。通常膜会短暂地超极化（比静息电位更负），之后钠钾泵恢复静息时的离子分布。这整个序列就是动作电位，遵循全或无定律。
5. Propagation of the Action Potential | 动作电位的传播
In unmyelinated neurones, an action potential generates local currents in the adjacent region, causing depolarisation to threshold and opening of sodium channels. This results in continuous propagation. In myelinated neurones, the insulating myelin sheath prevents ion flow except at the nodes of Ranvier, where there is a high density of voltage-gated Na⁺ channels. The action potential jumps from node to node – a process called saltatory conduction – which significantly increases the speed of transmission and conserves energy.
在无髓神经元中，动作电位在相邻区域产生局部电流，引起去极化达到阈值并打开钠通道，形成连续传播。在有髓神经元中，绝缘的髓鞘阻止离子流动，只在郎飞氏结处有高密度的电压门控 Na⁺ 通道。动作电位从一个结跳跃到下一个结——这一过程称为跳跃传导——大大提高了传递速度并节省能量。
6. The Refractory Period and Its Importance | 不应期及其重要意义
The refractory period is the time during which it is impossible or more difficult to generate a new action potential. The absolute refractory period occurs while voltage-gated sodium channels are inactivated; another impulse cannot be generated no matter how strong the stimulus. The relative refractory period follows, when potassium channels are still open and the membrane is hyperpolarised; only a stronger-than-normal stimulus can elicit a new impulse. The refractory period ensures that action potentials are discrete events, limits the maximum frequency of impulses, and ensures unidirectional propagation.
不应期是指不可能或更难产生新动作电位的时间段。绝对不应期发生在电压门控钠通道失活期间，无论刺激多强都不能产生新的冲动。随后是相对不应期，此时钾通道仍开放且膜超极化，只有强于正常的刺激才能引发新冲动。不应期确保动作电位是离散事件，限制最大冲动频率，并保证单向传播。
7. Synaptic Transmission: Cholinergic Synapses | 突触传递：胆碱能突触
A synapse is the junction between two neurones (or between a neurone and an effector). At a cholinergic synapse, the arrival of an action potential at the presynaptic terminal opens voltage-gated Ca²⁺ channels. Calcium ions enter the presynaptic knob, causing synaptic vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis. ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane. This opens ligand-gated Na⁺ channels, leading to influx of sodium and depolarisation of the postsynaptic membrane. If the threshold is reached, an action potential is triggered in the postsynaptic neurone.
突触是两个神经元之间（或神经元与效应器之间）的连接点。在胆碱能突触中，动作电位到达突触前末梢，打开电压门控 Ca²⁺ 通道。钙离子进入突触前小体，引起含有乙酰胆碱（ACh）的突触小泡与突触前膜融合，通过胞吐作用将 ACh 释放到突触间隙。ACh 扩散过间隙并与突触后膜上的受体蛋白结合。这打开配体门控 Na⁺ 通道，导致钠内流和突触后膜去极化。若达到阈值，则在突触后神经元触发动作电位。
8. Roles of Acetylcholinesterase and Synaptic Integration | 乙酰胆碱酯酶的作用与突触整合
To stop the signal, the enzyme acetylcholinesterase, located in the synaptic cleft, rapidly hydrolyses ACh into acetate and choline. The choline is taken back into the presynaptic neurone and reused. This rapid breakdown prevents continuous stimulation of the postsynaptic cell. At many synapses, a single presynaptic impulse may produce only a small excitatory postsynaptic potential (EPSP) that fails to reach threshold. The postsynaptic neurone integrates many inputs through summation: temporal summation (several impulses arriving in quick succession) and spatial summation (impulses from several presynaptic neurones arriving simultaneously) can combine to trigger an action potential.
为了终止信号，位于突触间隙的乙酰胆碱酯酶迅速将 ACh 水解为乙酸和胆碱。胆碱被回收至突触前神经元并重新利用。这种快速分解防止了对突触后细胞的持续刺激。在许多突触中，单个突触前冲动可能只产生一个微小的兴奋性突触后电位（EPSP），达不到阈值。突触后神经元通过总和作用整合多种输入：时间总和（多个冲动快速连续到达）和空间总和（多个突触前神经元的冲动同时到达）可以组合起来触发动作电位。
9. Inhibitory Synapses and Modulation | 抑制性突触与调控
Not all synapses are excitatory. Inhibitory synapses release neurotransmitters (such as GABA) that bind to receptors and open ligand-gated channels allowing Cl⁻ influx or K⁺ efflux. This makes the inside of the postsynaptic membrane more negative – an inhibitory postsynaptic potential (IPSP) – and makes it harder for the cell to reach threshold. The balance of EPSPs and IPSPs determines whether the postsynaptic neurone fires. Many drugs and toxins act by altering synaptic transmission, e.g. by blocking receptors, inhibiting breakdown enzymes, or mimicking neurotransmitters.
并非所有突触都是兴奋性的。抑制性突触释放神经递质（如 GABA），与受体结合后打开配体门控通道，允许 Cl⁻ 内流或 K⁺ 外流。这使突触后膜内部变得更负——即抑制性突触后电位（IPSP），使细胞更难达到阈值。EPSP 和 IPSP 的平衡决定突触后神经元是否发放。许多药物和毒素通过改变突触传递发挥作用，例如阻断受体、抑制分解酶或模拟神经递质。
10. The Reflex Arc and Survival Value | 反射弧与生存价值
A reflex arc is the simplest functional neural pathway, responsible for rapid, involuntary responses to stimuli. For example, in the spinal withdrawal reflex (such as touching a hot object), sensory neurones carry impulses from pain receptors to the spinal cord, where they synapse with relay neurones, which in turn synapse with motor neurones. The motor neurones stimulate effectors (biceps muscle contracts to withdraw the hand). The absence of brain involvement explains the speed of the response – it is innate and automatic. Reflexes have survival value by protecting the body from injury and enabling fast corrections to maintain posture.
反射弧是最简单的功能性神经通路，负责对刺激作出快速、不自主的反应。例如，在脊髓屈曲反射（如触摸高温物体）中，感觉神经元将冲动从痛觉感受器传至脊髓，这里它们与联络神经元形成突触，后者再与运动神经元形成突触。运动神经元刺激效应器（肱二头肌收缩以缩手）。由于不涉及大脑，这种反应速度极快——它是天生的、自动的。反射通过保护身体免受伤害并能快速矫正以维持姿势，具有生存价值。
11. Structure and Function of the Central and Peripheral Nervous Systems | 中枢和周围神经系统的结构与功能
The CNS is protected by the skull and vertebral column and surrounded by meninges. White matter contains mostly myelinated axons, while grey matter contains cell bodies and synapses. In the spinal cord, grey matter is butterfly-shaped and surrounded by white matter; this arrangement facilitates integration of incoming information and outgoing motor commands. The PNS consists of cranial and spinal nerves. The somatic nervous system controls voluntary movements, while the autonomic nervous system (ANS) regulates involuntary functions. The ANS divides into sympathetic and parasympathetic divisions, which often have antagonistic effects: sympathetic activity prepares the body for ‘fight or flight’ (increasing heart rate, dilating pupils), while parasympathetic activity promotes ‘rest and digest’ (slowing heart rate, stimulating digestion).
中枢神经系统受颅骨和脊柱保护，并被脑脊膜包裹。白质主要包含有髓轴突，灰质包含细胞体和突触。在脊髓中，灰质呈蝴蝶形并被白质包围；这一排列便于整合传入信息和传出运动指令。周围神经系统由脑神经和脊神经组成。躯体神经系统控制随意运动，自主神经系统（ANS）调节不随意功能。自主神经系统分为交感神经和副交感神经，常具有拮抗作用：交感活动使身体准备好“搏斗或逃跑”（心率上升、瞳孔扩大），副交感活动则促进“休息和消化”（心率减慢、刺激消化）。
12. Exam Tips and Common Pitfalls | 考试技巧与常见误区
Be precise with terminology: distinguish between ‘resting potential’ and ‘equilibrium potential’, and between ‘depolarisation’, ‘repolarisation’ and ‘hyperpolarisation’. When describing the action potential, always emphasise the role of voltage-gated channels and the correct sequence of ion movements. For synapses, ensure you use the term ‘neurotransmitter’ and not ‘hormone’; remember that vesicles fuse with the presynaptic membrane, not the postsynaptic. In summation questions, clearly explain EPSP and IPSP and how they interact. Accurate spelling of key terms (e.g. acetylcholinesterase, saltatory) and using correct units (mV) can earn extra marks.
术语要准确：区分“静息电位”和“平衡电位”，区分“去极化”“复极化”“超极化”。描述动作电位时，始终强调电压门控通道的作用以及离子流动的正确顺序。对于突触，务必使用“神经递质”而非“激素”；记住小泡与突触前膜融合，而非突触后膜。在关于总和的题目中，清晰解释 EPSP 和 IPSP 以及它们如何相互作用。正确拼写关键术语（如 acetylcholinesterase, saltatory）并使用正确的单位（mV）可以多得几分。
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