📚 The Nervous System: Neurones, Reflexes and Coordination | 神经系统：神经元、反射与协调 考点精讲
The nervous system is the body's rapid communication network. It detects changes in the internal and external environment — called stimuli — and coordinates appropriate responses. For IGCSE CCEA IGCSE Biology, mastering this topic means understanding how nerve cells (neurones) transmit signals, how reflex arcs produce quick involuntary reactions, and how sense organs such as the eye convert stimuli into electrical impulses, and how nervous control compares with hormonal control.
神经系统是人体内快速的通讯网络。它探测体内外环境的变化——称为刺激——并协调合适的反应。对于 CCEA IGCSE 生物学考试，掌握这一主题意味着要理解神经细胞（神经元）如何传递信号、反射弧如何产生快速的不随意反应、像眼睛这样的感觉器官如何将刺激转化为电冲动，以及神经控制如何与激素控制进行比较。
This article keeps every point from the original revision note and expands each one with the detail, correct terminology and exam phrasing you need. You will find the organisation of the nervous system, the structure and types of neurones, the nerve impulse and synapses, receptors and sense organs, the eye, reflex arcs, the brain and spinal cord, voluntary versus involuntary action, a full comparison of nervous and hormonal control, the effect of drugs on synapses, and finally exam skills and a revision checklist.
本文保留原复习笔记中的全部信息点，并逐一扩展，补充你所需要的细节、正确术语和考试表述。你将看到：神经系统的组织、神经元的结构与类型、神经冲动与突触、感受器与感觉器官、眼、反射弧、脑与脊髓、随意动作与不随意动作、神经控制与激素控制的完整比较、药物对突触的影响，以及最后的考试技巧与复习清单。
1. Organisation of the Nervous System | 神经系统的组织
The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, which act as the coordination and processing centres. The PNS comprises all the nerves that branch out from the CNS to the rest of the body, carrying messages to and from the CNS. In the PNS, nerves are bundles of many neurones wrapped in connective tissue.
神经系统分为中枢神经系统（CNS）和周围神经系统（PNS）。中枢神经系统由脑和脊髓组成，起到协调和处理中心的作用。周围神经系统则包含从 CNS 发出并连接到身体其余部分的所有神经，负责将信息传入和传出 CNS。在周围神经系统中，神经是由许多神经元组成的束，外面包裹着结缔组织。
The neurones that carry impulses from sense organs to the CNS are sensory neurones; those that carry impulses from the CNS to muscles or glands are motor neurones. The cell bodies of sensory neurones are located in small swellings called ganglia just outside the spinal cord. Relay neurones (also called intermediate neurones) lie entirely inside the CNS and link sensory input to motor output.
将冲动从感觉器官传送到 CNS 的神经元称为感觉神经元；将冲动从 CNS 传送到肌肉或腺体的则是运动神经元。感觉神经元的细胞体位于脊髓外称为神经节的小膨大中。中间神经元（也称联络神经元）完全位于 CNS 内部，把感觉输入与运动输出连接起来。
- Central nervous system (CNS) — brain and spinal cord; 中枢神经系统——脑与脊髓
- Peripheral nervous system (PNS) — cranial and spinal nerves; 周围神经系统——脑神经与脊神经
- Sensory (afferent) neurone — impulse travels towards the CNS; 感觉（传入）神经元——冲动传向 CNS
- Relay (intermediate) neurone — inside the CNS, links sensory to motor; 中间神经元——位于 CNS 内，连接感觉与运动
- Motor (efferent) neurone — impulse travels away from the CNS to an effector; 运动（传出）神经元——冲动离开 CNS 传向效应器
- Effector — a muscle or a gland that responds; 效应器——作出反应的肌肉或腺体
The CNS is protected by the skull and the vertebral column, and by three membranes called meninges. This protection matters because neurones of the CNS cannot normally be replaced if they are damaged, whereas peripheral neurones can sometimes regenerate.
CNS 受到颅骨和脊柱的保护，还受到三层称为脑脊膜（脑膜）的膜保护。这种保护很重要，因为 CNS 的神经元一旦受损通常无法替换，而周围神经元的轴突有时可以再生。
2. Structure and Types of Neurones | 神经元的结构与类型
A typical neurone has a cell body containing a nucleus, several branching dendrites that receive impulses, and a long axon that carries impulses away from the cell body. Many axons are surrounded by a fatty myelin sheath formed by Schwann cells, which insulates the axon and greatly increases the speed of conduction. Gaps in the sheath are called nodes of Ranvier, and the impulse effectively jumps from node to node.
一个典型的神经元具有含细胞核的细胞体、若干接收冲动的分枝状树突，以及一条把冲动传离细胞体的长轴突。许多轴突外面包裹着由施万细胞形成的脂肪性髓鞘，它使轴突绝缘并大幅提高传导速度。髓鞘上的间隙称为郎飞结，冲动实际上是从一个结跳跃到下一个结。
The end of the axon branches into many small swellings called synaptic knobs (axon terminals), which form synapses with the next cell. The cytoplasm of a neurone is packed with mitochondria, because transmitting impulses and releasing neurotransmitter require a great deal of energy. The long, thin shape of a neurone gives it a very large surface area relative to its volume, which helps it maintain the ion gradients needed for conduction.
轴突末端分支成许多称为突触小体（轴突终末）的小膨大，与下一个细胞形成突触。神经元的细胞质中富含线粒体，因为传递冲动和释放神经递质需要大量能量。神经元细长的形状使其相对体积而言具有非常大的表面积，这有助于维持传导所需的离子浓度梯度。
| Feature 特征 | Sensory neurone 感觉神经元 | Relay neurone 中间神经元 | Motor neurone 运动神经元 |
| Direction 传导方向 | Receptor → CNS 感受器→CNS | Within CNS CNS 内部 | CNS → effector CNS→效应器 |
| Cell body 细胞体位置 | In a ganglion outside the spinal cord 脊髓外的神经节中 | Inside the CNS CNS 内 | Inside the CNS (grey matter of spinal cord) 脊髓灰质内 |
| Axon length 轴突 | Long, with a short dendron 长，具短树突 | Short 短 | Long, myelinated 长且有髓鞘 |
A useful exam habit is to name the type of neurone before you describe its function. Mark schemes often award one mark for the correct name and a second mark for the direction of the impulse, so never leave out the direction.
一个有用的考试习惯是：在描述功能之前先点明神经元类型。评分标准常常对正确名称给 1 分，对冲动方向给 1 分，所以绝不要漏掉方向。
3. The Nerve Impulse | 神经冲动
An impulse is a wave of electrical change that travels along the membrane of a neurone. At rest, the inside of the axon is negatively charged relative to the outside, at about −70 mV. This resting potential is maintained by the movement of sodium ions (Na⁺) out of the axon and potassium ions (K⁺) into the axon, which requires ATP. The membrane is said to be polarised.
冲动是沿神经元膜传播的电变化波。在静息状态下，轴突内部相对外部带负电，约为 −70 mV。该静息电位通过钠离子（Na⁺）外流和钾离子（K⁺）内流来维持，此过程需要消耗 ATP。此时膜被称为极化的（有极性的）。
When a stimulus is strong enough to reach the threshold, voltage-gated sodium channels open and Na⁺ floods in. The inside becomes positive: this is depolarisation, and it produces the action potential. Potassium channels then open, K⁺ leaves the axon, and the membrane repolarises. For a fraction of a second the neurone cannot be stimulated again — the refractory period, which keeps the impulse travelling in one direction.
当刺激强到达到阈值时，电压门控钠通道打开，Na⁺ 大量内流。内部变为正电：这就是去极化，并产生动作电位。随后钾通道打开，K⁺ 流出轴突，膜重新极化。在极短的一瞬内神经元不能再次被刺激——这就是不应期，它保证冲动只向一个方向传播。
ial −70 mV → Depolarisation → Action potential +40 mV → Repolarisation → Refractory period
An impulse obeys the all-or-nothing law: any stimulus above the threshold produces a full-sized impulse, and a stronger stimulus does not produce a bigger impulse, only a greater frequency of impulses. Myelinated axons conduct much faster than unmyelinated ones because the impulse jumps between nodes of Ranvier; thinner axons conduct more slowly than thicker ones.
冲动遵循"全或无"定律：任何高于阈值的刺激都会产生一个完整大小的冲动，更强的刺激不会产生更大的冲动，只是使冲动频率更高。有髓轴突的传导速度远快于无髓轴突，因为冲动在郎飞结之间跳跃传导；较细的轴突比较粗的传导更慢。
Note that the impulse is not a flow of electricity along a wire, and it is not a flow of neurotransmitter. It is a localised change in the permeability of the membrane that is passed along the axon, with the same amplitude at every point.
请注意：冲动不是在导线中流动的电流，也不是神经递质的流动。它是膜通透性发生的局部改变，沿着轴突被依次传递，且每一点的幅度都相同。
4. Synapses and Transmission | 突触与传递
A synapse is the junction between two neurones, or between a neurone and an effector. It consists of the presynaptic membrane, a narrow synaptic cleft of about 20 nm filled with tissue fluid, and the postsynaptic membrane. The two cells do not touch, so the electrical impulse cannot jump across the gap directly.
突触是两个神经元之间，或神经元与效应器之间的连接处。它由突触前膜、充满组织液的约 20 nm 宽的突触间隙，以及突触后膜组成。两个细胞并不接触，因此电冲动不能直接跨过间隙。
When an impulse arrives at the synaptic knob, calcium ions (Ca²⁺) enter and cause vesicles containing neurotransmitter, such as acetylcholine, to fuse with the presynaptic membrane and release their contents into the cleft. The transmitter diffuses across the cleft and binds to receptor molecules on the postsynaptic membrane, which triggers a new impulse. Enzymes then break the transmitter down so the synapse can be reused.
当冲动到达突触小体时，钙离子（Ca²⁺）内流，使含有神经递质（如乙酰胆碱）的囊泡与突触前膜融合，并将其内容物释放到间隙中。递质扩散穿过间隙，与突触后膜上的受体分子结合，从而触发一个新的冲动。随后酶将递质分解，使突触可以再次使用。
enters → Vesicle fuses → Transmitter released → Diffusion → Receptor binding → New impulse
Synapses have three important properties for exam answers. First, they are unidirectional: transmission only happens from the presynaptic to the postsynaptic neurone because receptors are only on the postsynaptic membrane and transmitter is only stored in the presynaptic knob. Second, they slow the impulse down slightly, so a synapse acts like a valve or relay station. Third, they allow many neurones to converge on one, which is how the nervous system integrates information.
突触有三个对答题很重要的特性。第一，它们是单向的：传递只从突触前神经元到突触后神经元，因为受体只存在于突触后膜上，而递质只贮存在突触前小体中。第二，它们会使冲动略微减慢，因此突触像阀门或中继站。第三，它们允许许多神经元汇聚到同一个神经元上，这正是神经系统整合信息的方式。
- Excitatory synapse — transmitter makes the next neurone more likely to fire; 兴奋性突触——递质使下一个神经元更易发放冲动
- Inhibitory synapse — transmitter makes firing less likely; 抑制性突触——递质使发放冲动更难发生
- Summation — many small inputs add up to reach the threshold; 总和效应——许多小的输入叠加达到阈值
- Neurotransmitter is a chemical; the impulse is electrical. Never mix the two in an answer. 神经递质是化学物质，冲动是电信号，答题时切勿混淆两者
5. Receptors and Sense Organs | 感受器与感觉器官
A receptor is a cell or a nerve ending that detects a specific stimulus and produces an electrical impulse in response. A sense organ is a structure containing receptors together with accessory tissues that protect or assist them. The eye, the ear, the nose, the tongue and the skin are all sense organs.
感受器是能探测特定刺激并由此产生电冲动的细胞或神经末梢。感觉器官是包含感受器及其保护或辅助它们的附属组织的结构。眼、耳、鼻、舌和皮肤都是感觉器官。
Different receptors respond to different stimuli, and this specificity is a common exam point. The skin contains temperature receptors, pain receptors, touch receptors and pressure receptors; the eye contains light receptors (rods and cones); the ear contains receptors for sound and for balance; the tongue and nose contain chemoreceptors for dissolved and volatile chemicals.
不同的感受器对不同刺激作出反应，这种专一性是常见考点。皮肤中含有温度感受器、痛觉感受器、触觉感受器和压觉感受器；眼含有光感受器（视杆细胞和视锥细胞）；耳含有感受声音和平衡的感受器；舌和鼻含有对溶解态和挥发性化学物质敏感的化学感受器。
| Sense organ 感觉器官 | Stimulus 刺激 | Receptor 感受器 |
| Eye 眼 | Light 光 | Rods and cones 视杆与视锥细胞 |
| Ear 耳 | Sound, gravity 声音、重力 | Hair cells in cochlea, semicircular canals 耳蜗与前庭毛细胞 |
| Skin 皮肤 | Temperature, pressure, touch, pain 温度、压力、触摸、疼痛 | Free nerve endings and encapsulated endings 游离与囊状神经末梢 |
| Tongue 舌 | Dissolved chemicals 溶解的化学物质 | Taste buds 味蕾 |
The energy of the stimulus is converted into the energy of an electrical impulse, and this conversion is called transduction. The impulse then travels along a sensory neurone to the CNS, where it may produce a sensation or a response.
刺激的能量被转换为电冲动的能量，这种转换称为转导。冲动随后沿感觉神经元传到 CNS，在那里可能产生感觉或引起反应。
6. The Eye: Structure and Function | 眼的结构与功能
The eye is a sense organ specialised for detecting light. Light enters through the cornea, which refracts it, then passes through the pupil. The iris controls the diameter of the pupil using circular and radial muscles, and the lens, held in place by suspensory ligaments, fine-tunes the focus onto the retina.
眼是专门用于探测光的感觉器官。光通过角膜进入并被其折射，然后穿过瞳孔。虹膜利用环状肌和辐射状肌控制瞳孔的直径，而由悬韧带固定位置的晶状体则对焦到视网膜上进行精细调节。
| Part 结构 | Function 功能 |
| Cornea 角膜 | Transparent, refracts light into the eye 透明，使光折射进入眼内 |
| Iris 虹膜 | Muscular, controls pupil diameter 肌肉组织，控制瞳孔大小 |
| Lens 晶状体 | Transparent, elastic, focuses light on the retina 透明有弹性，将光聚焦到视网膜 |
| Retina 视网膜 | Contains rods and cones; light receptor layer 含视杆与视锥细胞，光感受层 |
| Optic nerve 视神经 | Carries impulses from retina to brain 将冲动由视网膜传至脑 |
| Blind spot 盲点 | No rods or cones where optic nerve leaves 视神经穿出处无视杆与视锥细胞 |
Rods work in dim light and are sensitive to black and white; cones work in bright light and detect colour, and they are concentrated in the fovea. This is why colour is hard to see in dim light, and why looking slightly to one side of a faint object helps you see it.
视杆细胞在暗光下工作，对黑白色敏感；视锥细胞在亮光下工作并能分辨颜色，且集中在中央凹。这就是为什么在暗光下难以分辨颜色，也是为什么稍微偏离暗淡物体的中心去看反而更容易看清它。
The pupil reflex protects the retina. In bright light, circular muscles of the iris contract and radial muscles relax, so the pupil constricts; in dim light the reverse happens and the pupil dilates. This reflex is involuntary and is coordinated by the brain.
瞳孔反射可以保护视网膜。强光下，虹膜的环状肌收缩、辐射状肌放松，瞳孔缩小；暗光下情况相反，瞳孔放大。该反射是不随意的，由脑协调。
Accommodation is the focusing of light from objects at different distances. To look at a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more curved, so its refractive power increases. To look at a distant object, the ciliary muscles relax, the ligaments are pulled tight, and the lens becomes thinner and flatter.
调节（视觉调节）是指对不同距离物体所发出的光进行聚焦。看近物时，睫状肌收缩，悬韧带松弛，晶状体变厚、曲率增大，折光能力增强。看远物时，睫状肌放松，悬韧带被拉紧，晶状体变薄、变平。
Near object: ciliary muscle contracts → lens thicker → more refraction
Short sight (myopia) occurs when the eyeball is too long or the lens too strong, so the image forms in front of the retina; it is corrected with a concave (diverging) lens. Long sight (hyperopia) occurs when the eyeball is too short or the lens too weak, so the image forms behind the retina; it is corrected with a convex (converging) lens.
近视（myopia）是由于眼球太长或晶状体过强，使像成在视网膜之前；用凹（发散）透镜矫正。远视（hyperopia）是由于眼球太短或晶状体太弱，使像成在视网膜之后；用凸（会聚）透镜矫正。
7. Reflex Arcs and Reflex Actions | 反射弧与反射动作
A reflex action is a rapid, automatic and involuntary response to a stimulus, and it does not require conscious thought. Reflexes are protective: they allow the body to respond to danger before the brain has processed the information, which reduces the chance of injury.
反射动作是对刺激作出的快速、自动、不随意的反应，不需要有意识的思考。反射具有保护作用：它们使身体在脑处理信息之前就对危险作出反应，从而降低受伤的可能性。
The pathway taken by the impulse is called a reflex arc. It always follows the same sequence, and this sequence is one of the most frequently examined facts in the whole topic.
冲动经过的路径称为反射弧。它总是遵循相同的顺序，而这一顺序是整个主题中最常考的要点之一。
s → Receptor → Sensory neurone → Relay neurone (CNS) → Motor neurone → Effector → Response
- Receptor — detects the stimulus; 感受器——探测刺激
- Sensory neurone — carries the impulse to the spinal cord; 感觉神经元——将冲动传入脊髓
- Relay neurone — inside the CNS, passes the impulse to the motor neurone; 中间神经元——位于 CNS 内，将冲动传给运动神经元
- Motor neurone — carries the impulse to the effector; 运动神经元——将冲动传到效应器
- Effector — a muscle (contracts) or a gland (secretes); 效应器——肌肉（收缩）或腺体（分泌）
The classic example is the knee-jerk reflex, in which
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