📚 Nervous System | A-Level OCR 生物：神经系统 考点精讲
The nervous system is a complex network of specialised cells that enables rapid communication between different parts of the body. In the OCR A-Level Biology specification, understanding the structure and function of neurones, the generation and propagation of action potentials, synaptic transmission, and the organisation of the human nervous system is essential. This article covers all key points in detail, pairing English explanations with Chinese translations to support bilingual learners.
神经系统是一个由特化细胞组成的复杂网络，能够在身体不同部位之间实现快速通信。在 OCR A-Level 生物考试大纲中，理解神经元的结构与功能、动作电位的产生与传导、突触传递以及人类神经系统的组织方式是至关重要的。本文详细讲解所有考点，中英对照，帮助双语学习者掌握要点。
1. Overview of the Nervous System | 神经系统概述
The nervous system is divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all nerves outside the CNS. The PNS is further subdivided into the sensory (afferent) division and the motor (efferent) division. The motor division is split into the somatic nervous system, controlling voluntary movements, and the autonomic nervous system, regulating involuntary functions such as heart rate and digestion.
神经系统分为中枢神经系统（CNS），包括脑和脊髓，以及周围神经系统（PNS），包含所有CNS以外的神经。PNS进一步分为感觉（传入）部分和运动（传出）部分。运动部分又分为躯体神经系统，控制随意运动，和自主神经系统，调节心率、消化等不自主功能。
The autonomic nervous system consists of two branches: the sympathetic and parasympathetic systems, which often have antagonistic effects. The sympathetic system prepares the body for 'fight or flight', increasing heart rate and diverting blood to muscles, while the parasympathetic system promotes 'rest and digest', slowing heart rate and stimulating digestion.
自主神经系统包含两个分支：交感神经和副交感神经，它们通常具有拮抗作用。交感神经系统让身体准备“战斗或逃跑”，增加心率并将血液导向肌肉；副交感神经系统则促进“休息与消化”，减缓心率并刺激消化。
2. Structure of Neurones | 神经元的结构
Neurones are the functional units of the nervous system. They have a cell body containing the nucleus, dendrites that receive signals, and an axon that transmits impulses away from the cell body. The axon may be covered by a myelin sheath, formed by Schwann cells in the PNS, which insulates the axon and speeds up impulse conduction. Nodes of Ranvier are gaps in the myelin sheath where depolarisation occurs.
神经元是神经系统的功能单位。它们含有一个包含细胞核的细胞体、接收信号的树突，以及将冲动传离细胞体的轴突。轴突可能被髓鞘包裹，髓鞘由周围神经系统中的施万细胞形成，起到绝缘轴突和加速冲动传导的作用。郎飞结是髓鞘上的间隙，是去极化发生的位置。
There are three main types of neurones: sensory neurones carry impulses from receptors to the CNS; motor neurones carry impulses from the CNS to effectors; and relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones.
神经元主要有三种类型：感觉神经元将冲动从感受器传向中枢神经系统；运动神经元将冲动从中枢神经系统传向效应器；中间神经元完全位于中枢神经系统内，连接感觉和运动神经元。
3. Resting Potential | 静息电位
When a neurone is not transmitting an impulse, the inside of the axon is negatively charged relative to the outside, typically around −70 mV. This is the resting potential, maintained by the sodium-potassium pump, which actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell, using ATP. The membrane is more permeable to K⁺ than Na⁺ due to more potassium leak channels, so K⁺ diffuses out, making the interior negative.
当神经元不传导冲动时，轴突内部相对于外部带负电，通常约为 −70 mV。这就是静息电位，由钠钾泵维持，该泵利用 ATP 主动转运 3 个 Na⁺ 出细胞和 2 个 K⁺ 入细胞。膜对 K⁺ 的通透性高于 Na⁺，因为存在更多的钾泄漏通道，所以 K⁺ 扩散出去，使内部带负电。
Key ions involved: Na⁺ concentration is higher outside, K⁺ concentration is higher inside. The uneven distribution creates an electrochemical gradient. The resting potential can be recorded using a microelectrode and an oscilloscope.
涉及的关键离子：Na⁺ 浓度胞外高于胞内，K⁺ 浓度胞内高于胞外。这种不均匀分布形成了电化学梯度。静息电位可以用微电极和示波器记录。
4. Action Potential: Depolarisation and Repolarisation | 动作电位：去极化与复极化
An action potential is a rapid, temporary change in membrane potential. When a stimulus causes the membrane to reach the threshold potential (around −55 mV), voltage-gated Na⁺ channels open, allowing Na⁺ to rush in. This causes depolarisation, where the inside becomes positive (up to +40 mV).
动作电位是膜电位的快速、暂时性变化。当刺激使膜电位达到阈电位（约 −55 mV）时，电压门控 Na⁺ 通道打开，Na⁺ 大量涌入。这导致去极化，内部变为正电（可达 +40 mV）。
At the peak, Na⁺ channels inactivate and voltage-gated K⁺ channels open, allowing K⁺ to leave the cell. This causes repolarisation, returning the membrane potential towards the resting level. Often there is a temporary overshoot called hyperpolarisation before the resting potential is restored.
在峰值时，Na⁺ 通道失活，电压门控 K⁺ 通道打开，K⁺ 离开细胞。这导致复极化，膜电位回到静息水平。在静息电位恢复之前，通常会出现短暂的超射，称为超极化。
The action potential follows the all-or-nothing law: if the threshold is reached, an action potential of fixed magnitude occurs; if not, no impulse is generated.
动作电位遵循全或无定律：如果达到阈值，就会产生一个固定大小的动作电位；如果没有达到，则不产生冲动。
5. Propagation of Action Potentials | 动作电位的传导
Action potentials travel along the axon from the cell body to the synaptic terminals. In unmyelinated neurones, propagation is continuous and relatively slow. Local electrical currents cause adjacent regions of the membrane to depolarise, generating a new action potential slightly further along.
动作电位沿轴突从细胞体传向突触末梢。在无髓神经元中，传导是连续的，相对较慢。局部电流使邻近膜区域去极化，从而在稍远处产生新的动作电位。
In myelinated neurones, the myelin sheath acts as an insulator, preventing ion flow. Depolarisation only occurs at the nodes of Ranvier, where the axon membrane is exposed. The action potential jumps from node to node in a process called saltatory conduction, which greatly increases speed and is energy-efficient because fewer ions need to be pumped back.
在有髓神经元中，髓鞘作为绝缘体，阻止离子流动。去极化只发生在郎飞结处，那里轴突膜裸露。动作电位从一结跳到下一结，称为跳跃传导，大大提高了速度，而且节能，因为需要泵回的离子较少。
6. The Refractory Period | 不应期
The refractory period is the time after an action potential during which a new action potential cannot be initiated or requires a stronger stimulus. There are two phases: the absolute refractory period, when Na⁺ channels are inactivated and no stimulus can generate another action potential; and the relative refractory period, when K⁺ channels are still open and the membrane is hyperpolarised, requiring a larger-than-normal stimulus.
不应期是指动作电位之后，无法产生新的动作电位或需要更强刺激的时期。有两个阶段：绝对不应期，此时 Na⁺ 通道失活，任何刺激都不能产生另一个动作电位；相对不应期，此时 K⁺ 通道仍开放，膜超极化，需要比正常更大的刺激。
The refractory period ensures that action potentials are discrete, unidirectional, and limits the maximum frequency of impulse conduction.
不应期确保了动作电位是独立的、单向的，并限制了冲动传导的最高频率。
7. Synaptic Transmission | 突触传递
Synapses are junctions between neurones or between a neurone and an effector. Most are chemical synapses. When an action potential arrives at the presynaptic terminal, voltage-gated Ca²⁺ channels open, calcium ions enter, causing synaptic vesicles to fuse with the membrane and release neurotransmitter into the synaptic cleft.
突触是神经元之间或神经元与效应器之间的连接点。大多数是化学突触。当动作电位到达突触前末梢时，电压门控 Ca²⁺ 通道打开，钙离子进入，导致突触囊泡与膜融合，将神经递质释放到突触间隙中。
The neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane. This opens ligand-gated ion channels, allowing Na⁺ to enter (excitatory synapse) or Cl⁻ to enter / K⁺ to leave (inhibitory synapse). The resulting change in postsynaptic potential determines whether the postsynaptic neurone reaches threshold.
神经递质扩散通过间隙，与突触后膜上的特异性受体结合。这打开了配体门控离子通道，允许 Na⁺ 进入（兴奋性突触）或 Cl⁻ 进入 / K⁺ 离开（抑制性突触）。突触后电位的变化决定了突触后神经元是否达到阈值。
To prevent continuous stimulation, the neurotransmitter is removed by reuptake into the presynaptic neurone or broken down by enzymes, e.g., acetylcholinesterase breaks down acetylcholine.
为防止持续刺激，神经递质通过重新摄取到突触前神经元或通过酶降解而被清除，例如乙酰胆碱酯酶水解乙酰胆碱。
8. Summation | 总和效应
Summation is the process by which the effects of multiple synaptic inputs are combined to determine whether an action potential is generated. Spatial summation occurs when several presynaptic neurones release neurotransmitter simultaneously at different sites on the same postsynaptic neurone, combining their effects. Temporal summation involves a single presynaptic neurone releasing neurotransmitter multiple times in rapid succession, so the effects add up over time.
总和效应是指多个突触输入的效果被结合起来决定是否产生动作电位的过程。空间总和是指多个突触前神经元在同一突触后神经元的不同部位同时释放神经递质，合并它们的效果。时间总和是指单个突触前神经元在短时间内多次释放神经递质，使效果随时间累加。
Inhibitory postsynaptic potentials (IPSPs) can cancel out excitatory ones (EPSPs). The net sum of EPSPs and IPSPs determines whether the postsynaptic membrane reaches threshold at the axon hillock.
抑制性突触后电位（IPSPs）可以抵消兴奋性突触后电位（EPSPs）。EPSPs和IPSPs的总和决定了突触后膜在轴突起始段是否达到阈值。
9. Structure and Function of the Brain | 脑的结构与功能
In the OCR specification, you should know the main regions of the human brain: the cerebrum (controls voluntary actions, memory, speech, and conscious thought), the cerebellum (coordinates movement and balance), the medulla oblongata (controls involuntary activities like breathing and heart rate), and the hypothalamus (regulates homeostasis and links the nervous and endocrine systems).
根据OCR大纲，你需要了解人类大脑的主要区域：大脑（控制随意运动、记忆、语言和有意识思维），小脑（协调运动和平衡），延髓（控制呼吸和心率等不自主活动），以及下丘脑（调节稳态并连接神经系统和内分泌系统）。
Brain structure can be studied using techniques such as MRI, CT scans, and functional imaging (fMRI). The brain is protected by the skull, meninges, and cerebrospinal fluid.
大脑的结构可以使用MRI、CT扫描和功能性成像（fMRI）等技术研究。大脑受到颅骨、脑膜和脑脊液的保护。
10. Reflex Arcs | 反射弧
A reflex is a rapid, involuntary response to a stimulus, often protective. The pathway is called a reflex arc. A typical spinal reflex involves a receptor, sensory neurone, relay neurone (in the CNS), motor neurone, and effector. This bypasses conscious brain areas for speed. An example is the withdrawal reflex when touching a hot object.
反射是对刺激的快速、不自主的反应，通常具有保护性。其通路称为反射弧。典型的脊髓反射包括感受器、感觉神经元、中间神经元（在中枢神经系统中）、运动神经元和效应器。这绕过了有意识的大脑区域以提高速度。例如，接触热物体时的缩回反射。
In OCR exams, you may be asked to describe the knee-jerk reflex or the blinking reflex. The knee-jerk reflex is monosynaptic with no relay neurone; the synapse is directly between sensory and motor neurones.
在OCR考试中，你可能需要描述膝跳反射或眨眼反射。膝跳反射是单突触的，没有中间神经元；突触直接在感觉和运动神经元之间。
11. Drugs and Synaptic Transmission | 药物与突触传递
Many drugs affect synaptic transmission by acting as agonists or antagonists. For example, nicotine mimics acetylcholine and binds to nicotinic receptors, causing overstimulation. Curare blocks acetylcholine receptors, preventing muscle contraction. Organophosphates inhibit acetylcholinesterase, causing acetylcholine to accumulate and leading to continuous muscle stimulation.
许多药物通过作为激动剂或拮抗剂影响突触传递。例如，尼古丁模仿乙酰胆碱并与烟碱受体结合，导致过度刺激。箭毒阻断乙酰胆碱受体，阻止肌肉收缩。有机磷抑制乙酰胆碱酯酶，导致乙酰胆碱积聚，引起持续的肌肉刺激。
Understanding these mechanisms is important for OCR Biology, as it links molecular neurobiology to real-world applications and toxin effects.
理解这些机制对于OCR生物很重要，因为它将分子神经生物学与现实世界的应用和毒素效应联系起来。
12. Key Exam Tips and Common Misconceptions | 关键考试技巧与常见误解
Many students confuse the roles of Na⁺ and K⁺ during the action potential. Remember, depolarisation is due to Na⁺ influx, repolarisation due to K⁺ efflux. Another common error is describing the myelin sheath as increasing the speed by 'insulating' alone; you must mention saltatory conduction and nodes of Ranvier. Always be precise about the direction of ion movement and the type of gating.
许多学生混淆了动作电位期间Na⁺和K⁺的作用。记住，去极化是因为Na⁺内流，复极化是因为K⁺外流。另一个常见错误是仅说髓鞘通过“绝缘”提高速度；你必须提到跳跃传导和郎飞结。对离子移动的方向和门控类型要始终精确。
When explaining summation, clearly distinguish spatial from temporal and link it to postsynaptic potentials. Use correct terminology: EPSP, IPSP, threshold, axon hillock. Practice diagram drawing of an axon potential and label the stages.
在解释总和时，要清楚区分空间总和与时间总和，并将其与突触后电位联系起来。使用正确的术语：EPSP、IPSP、阈值、轴突起始段。练习绘制轴突电位图并标注各个阶段。
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