📚 The Nervous System | 神经系统
The nervous system allows the body to detect changes in the environment and to respond rapidly. In IGCSE Edexcel Biology, you need to understand that the central nervous system consists of the brain and spinal cord and is connected to sense organs by nerves. This article covers the structure and function of the entire human nervous system, including neurones, reflex arcs, and synapses.
神经系统使身体能够检测环境中的变化并作出快速反应。在IGCSE Edexcel 生物学中，你需要理解中枢神经系统由脑和脊髓组成，并通过神经与感觉器官相连。本文涵盖整个人体神经系统的结构和功能，包括神经元、反射弧和突触。
1. What is the Nervous System? | 什么是神经系统？
The nervous system is a communication network that uses electrical impulses to transmit signals very quickly. It enables an organism to react to stimuli (changes in the internal or external environment) and coordinates the activities of different organs.
神经系统是一个使用电冲动快速传递信号的通讯网络。它使生物体能够对刺激（内部或外部环境的变化）作出反应，并协调不同器官的活动。
In humans, the nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS processes information and makes decisions, while the PNS carries messages to and from the CNS.
在人体中，神经系统分为两个主要部分：中枢神经系统（CNS）和周围神经系统（PNS）。中枢神经系统处理信息并做出决策，而周围神经系统负责将信息传入和传出中枢神经系统。
2. The Central Nervous System (CNS) | 中枢神经系统
The CNS consists of the brain and the spinal cord. These structures are protected by bone: the brain is enclosed by the skull (cranium), and the spinal cord runs through the vertebral column.
中枢神经系统由脑和脊髓组成。这些结构受到骨骼的保护：脑由颅骨包裹，脊髓穿过脊柱。
The brain is the control centre. It interprets sensory information, stores memory, generates thoughts and emotions, and sends instructions to muscles and glands. The spinal cord acts as a major highway for nerve impulses travelling to and from the brain, and it also controls some reflex actions independently.
脑是控制中心。它解析感觉信息，存储记忆，产生思维和情绪，并向肌肉和腺体发送指令。脊髓是神经冲动进出大脑的主要通道，并且还能独立控制某些反射动作。
In an exam, you should be able to label a simple diagram of the brain and spinal cord and state that they form the CNS.
在考试中，你应该能够标记脑和脊髓的简图，并说明它们构成了中枢神经系统。
3. The Peripheral Nervous System (PNS) | 周围神经系统
The PNS consists of all the nerves that branch out from the CNS to the rest of the body. These nerves are bundles of hundreds or thousands of neurones (nerve cells).
周围神经系统由从中枢神经系统分支到身体其余部分的所有神经组成。这些神经是由成百上千个神经元（神经细胞）组成的束。
The PNS links the CNS to sense organs (such as the eyes, ears and skin) and to effectors (muscles and glands). It can be subdivided into the somatic nervous system, which controls voluntary movements, and the autonomic nervous system, which controls involuntary actions like heart rate and digestion.
周围神经系统将中枢神经系统与感觉器官（如眼、耳和皮肤）以及效应器（肌肉和腺体）联系起来。它可以进一步分为控制随意运动的躯体神经系统和控制心跳、消化等非随意动作的自主神经系统。
4. Neurones: The Building Blocks | 神经元：基本单位
Neurones are specialised cells that carry electrical impulses. A typical neurone has a cell body containing the nucleus and cytoplasm, dendrites that receive signals, and a long axon that carries impulses away from the cell body.
神经元是能够传导电冲动的特化细胞。一个典型的神经元有一个包含细胞核和细胞质的细胞体、接收信号的树突，以及将冲动从细胞体传走的长轴突。
Many axons are surrounded by a fatty myelin sheath, which insulates the axon and speeds up the transmission of nerve impulses. In between the myelin sheaths are gaps called nodes of Ranvier, where the impulse jumps – a process known as saltatory conduction.
许多轴突被脂肪性的髓鞘包裹，髓鞘能使轴突绝缘并加快神经冲动的传递。在髓鞘之间有一些间隙，称为郎飞氏结，冲动在这里跳跃传导——这一过程被称为跳跃传导。
The IGCSE specification requires you to know three types of neurone: sensory, relay and motor. All three have the same basic parts but differ in shape and position.
IGCSE 大纲要求你了解三类神经元：感觉神经元、中间神经元和运动神经元。这三类神经元具有相同的基本结构，但在形状和位置上有所不同。
5. Sensory Neurones | 感觉神经元
Sensory neurones transmit impulses from receptors (in sense organs) towards the CNS. A receptor is a specialised cell that detects a stimulus, such as light, sound, temperature or pressure.
感觉神经元将冲动从感受器（位于感觉器官内）传向中枢神经系统。感受器是一种能够检测刺激的特化细胞，例如光、声音、温度或压力。
The cell body of a sensory neurone is located outside the CNS, in a swelling called a ganglion, and it has a long dendrite carrying the impulse from the receptor to the cell body. The axon then carries the impulse into the spinal cord.
感觉神经元的细胞体位于中枢神经系统之外的一个膨大结构中，称为神经节，它有一条长长的树突将冲动从感受器传到细胞体。轴突随后将冲动传入脊髓。
6. Relay Neurones | 中间神经元
Relay neurones (also called interneurones) are found entirely within the CNS. They connect sensory neurones to motor neurones and are involved in processing information and making decisions.
中间神经元（也称联络神经元）完全位于中枢神经系统内部。它们将感觉神经元与运动神经元连接起来，并参与信息处理和决策。
A relay neurone has a short axon and many branching dendrites, allowing it to form connections with many other neurones. It does not have a myelin sheath – impulses travel more slowly, but this is acceptable because distances inside the CNS are tiny.
中间神经元的轴突较短，树突分枝众多，这使它能够与许多其他神经元建立联系。它没有髓鞘——冲动传导稍慢一些，但因为在中枢神经系统内部的距离很短，这完全可行。
7. Motor Neurones | 运动神经元
Motor neurones carry impulses away from the CNS to effectors – either muscles (causing contraction) or glands (causing secretion). Their cell bodies are located inside the CNS, and their long axons extend out to the target organ.
运动神经元将冲动从中枢神经系统传向效应器——肌肉（引起收缩）或腺体（引起分泌）。它们的细胞体位于中枢神经系统内，长轴突延伸到靶器官。
The motor neurone has a large cell body with many short dendrites in the CNS and a very long axon covered with myelin sheath. When an impulse reaches the neuromuscular junction, it triggers muscle contraction.
运动神经元有一个带有许多短树突的较大细胞体（位于中枢神经系统内），以及一条非常长、被髓鞘包裹的轴突。当冲动到达神经肌肉接头时，会引发肌肉收缩。
8. The Reflex Arc | 反射弧
A reflex action is an automatic, rapid and involuntary response to a stimulus that does not involve conscious thought. The pathway taken by the nerve impulses in a reflex is called a reflex arc.
反射动作是对刺激的一种自动、快速且不涉及有意识思考的非随意反应。反射中神经冲动所经过的通路称为反射弧。
sponse. The brain is sometimes informed afterwards, but the reflex is coordinated by the spinal cord or brain stem to save time.
一个典型的反射弧包括以下顺序：刺激 → 感受器 → 感觉神经元 → 中枢神经系统中的中间神经元 → 运动神经元 → 效应器 → 反应。大脑有时会在之后获得信息，但反射是由脊髓或脑干协调完成的，以节省时间。
9. Synapses and Neurotransmitters | 突触与神经递质
A synapse is the tiny gap between the end of one neurone and the next neurone. The electrical impulse cannot cross the synapse directly, so a chemical messenger called a neurotransmitter is used.
突触是一个神经元末端与下一个神经元之间的微小间隙。电冲动无法直接跨过突触，因此需要使用一种称为神经递质的化学信使。
When an impulse arrives at the pre-synaptic neurone, it triggers vesicles to release neurotransmitter molecules into the synaptic cleft. These diffuse across and bind to specific receptors on the post-synaptic neurone, generating a new electrical impulse. The neurotransmitter is then broken down or reabsorbed to stop the signal.
当冲动到达突触前神经元时，它会引发囊泡向突触间隙释放神经递质分子。这些分子扩散并通过突触间隙，与突触后神经元上的特异受体结合，产生新的电冲动。随后神经递质被分解或重吸收，以终止信号。
The synapse ensures that impulses travel in one direction only and allows for the integration of signals from multiple neurones. Drugs and toxins can interfere with synaptic transmission, either enhancing or blocking the effects of neurotransmitters.
突触确保冲动只能沿一个方向传递，并允许整合来自多个神经元的信号。药物和毒素可以干扰突触传递，增强或阻断神经递质的作用。
10. A Worked Example: The Withdrawal Reflex | 实例分析：缩回反射
If you touch a hot object, heat receptors in your skin generate impulses. These travel along a sensory neurone to the spinal cord.
如果你碰到一个热的物体，皮肤中的热感受器会产生冲动。这些冲动沿感觉神经元传到脊髓。
In the spinal cord, the sensory neurone synapses with a relay neurone, which in turn synapses with a motor neurone. The motor neurone sends impulses to the biceps muscle in your arm, causing it to contract and pull your hand away – all before you consciously feel the pain.
在脊髓中，感觉神经元与中间神经元形成突触连接，中间神经元又与运动神经元形成突触连接。运动神经元将冲动发送到手臂的肱二头肌，引起肌肉收缩，将手拉回——这一切都在你有意识地感受到疼痛之前完成。
This reflex is a survival mechanism that minimises damage. In an exam, you should be able to label the components of a reflex arc on a diagram of the spinal cord and explain the gap (synapse) between neurones.
这种反射是一种将伤害降至最低的生存机制。在考试中，你应该能够在脊髓示意图上标记反射弧的各个组成部分，并解释神经元之间的间隙（突触）。
11. Importance of Reflex Actions | 反射动作的重要性
Reflex actions protect the body from harm. Because they are processed in the spinal cord, they are extremely fast, avoiding the delay that would occur if signals had to travel all the way to the brain and back.
反射动作保护身体免受伤害。由于它们在脊髓中处理，所以速度极快，避免了信号必须一路传到大脑再返回所导致的延迟。
In addition to protective reflexes like the withdrawal reflex, the body has many innate reflexes such as the pupil reflex (controlling the amount of light entering the eye) and the knee-jerk reflex. These are used in medical examinations to test the health of the nervous system.
除了像缩回反射这样的保护性反射外，身体还有许多先天反射，如瞳孔反射（控制进入眼睛的光量）和膝跳反射。这些反射在医学检查中用于测试神经系统的健康状况。
12. Comparison: Nervous System vs Endocrine System | 神经系统与内分泌系统的对比
Both the nervous system and the endocrine system are communication systems in the body, but they work very differently. The IGCSE course often asks you to compare the two.
神经系统和内分泌系统都是体内的通讯系统，但它们的工作方式非常不同。IGCSE 课程经常会要求你对两者进行比较。
The table below summarises the key differences:
下表总结了主要差异：
| Feature | 特征 | Nervous System | 神经系统 | Endocrine System | 内分泌系统 |
|---|---|---|
| Signal type | 信号类型 | Electrical impulses (and chemical at synapses) | 电冲动（突触处为化学信号） | Chemical messengers (hormones) | 化学信使（激素） |
| Speed | 速度 | Very fast (milliseconds) | 非常快（毫秒级） | Slower (seconds to hours) | 较慢（数秒至数小时） |
| Pathway | 传递途径 | Specific neurones (wired network) | 特定神经元（有线网络） | Bloodstream (broadcast) | 血液循环（广播式） |
| Duration | 持续时间 | Short-lived | 短暂 | Longer-lasting | 较持久 |
| Function | 功能 | Rapid responses, e.g. reflexes, movement | 快速反应，如反射、运动 | Slow, long-term changes, e.g. growth, metabolism | 缓慢、长期的变化，如生长、代谢 |
In some situations, the two systems work together. For example, the nervous system can stimulate the release of the hormone adrenaline from the adrenal glands during the 'fight or flight' response.
在某些情况下，这两个系统会协同工作。例如，在“战斗或逃跑”反应中，神经系统可以刺激肾上腺释放激素肾上腺素。
13. Summary of Key Points | 关键点总结
The CNS comprises the brain and spinal cord; the PNS comprises nerves linking the CNS to receptors and effectors. Sensory neurones carry impulses towards the CNS, relay neurones connect within the CNS, and motor neurones carry impulses away to effectors.
中枢神经系统由脑和脊髓组成；周围神经系统由连接中枢神经系统与感受器和效应器的神经组成。感觉神经元将冲动传向中枢神经系统，中间神经元在中枢神经系统内部进行连接，运动神经元将冲动传出至效应器。
Reflex arcs provide rapid, involuntary protective responses. Synapses use neurotransmitters to ensure one-way transmission and can be affected by drugs. Mastery of the labelled diagram of a reflex arc and the comparison with the endocrine system is essential for top marks in IGCSE.
反射弧提供快速、非随意的保护性反应。突触利用神经递质确保单向传递，并可能受到药物的影响。掌握反射弧的标注图以及与内分泌系统的对比，对于在 IGCSE 中取得高分至关重要。
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