📚 Mastering the Nervous System for IB & CCEA Biology | IB CCEA 生物：神经系统 考点精讲
The nervous system allows organisms to detect changes in their environment and coordinate rapid responses. From the simplest reflex to complex thought, nerve cells (neurons) generate and transmit electrochemical signals. This article covers the core concepts of neural structure, action potentials, synaptic transmission, and reflex arcs, providing a dual-language revision guide tailored to IB and CCEA Biology specifications.
神经系统使生物能够感知环境变化并协调快速反应。从最简单的反射到复杂的思维，神经细胞（神经元）产生并传导电化学信号。本文涵盖神经元结构、动作电位、突触传递和反射弧等核心概念，提供一份针对IB和CCEA生物考试要求的双语复习指南。
1. Neurons: Structure and Types | 神经元：结构与类型
A typical neuron consists of a cell body (soma), dendrites, and an axon. Dendrites receive signals and convey them towards the cell body, while the axon transmits impulses away from the cell body to other neurons or effectors. Many axons are insulated by a myelin sheath formed by Schwann cells, which speeds up signal conduction.
一个典型的神经元由细胞体（胞体）、树突和轴突组成。树突接收信号并将其传向细胞体，而轴突则将冲动从细胞体传向其他神经元或效应器。许多轴突被施万细胞形成的髓鞘包裹，从而加快信号传导速度。
Neurons are classified into three functional types: sensory neurons carry impulses from receptors to the central nervous system (CNS); motor neurons carry impulses from the CNS to effectors (muscles or glands); relay neurons (interneurons) connect sensory and motor neurons within the CNS.
神经元按功能分为三类：感觉神经元将冲动从感受器传向中枢神经系统（CNS）；运动神经元将冲动从中枢神经系统传向效应器（肌肉或腺体）；联络神经元（中间神经元）在中枢神经系统内连接感觉神经元和运动神经元。
- Sensory neuron: long dendrite, short axon, cell body located outside the CNS.
- 感觉神经元：树突长，轴突短，胞体位于中枢神经系统之外。
- Motor neuron: short dendrites, long axon, cell body inside the CNS.
- 运动神经元：树突短，轴突长，胞体位于中枢神经系统内。
2. Resting Potential: The Polarised Membrane | 静息电位：极化膜
When a neuron is not transmitting an impulse, its membrane is polarised, with a resting potential of approximately −70 mV inside relative to the outside. This potential is maintained by the uneven distribution of ions across the membrane and selective permeability.
当神经元不传递冲动时，其膜处于极化状态，膜内相对于膜外的静息电位约为 −70 mV。这种电位由离子在膜两侧的不均匀分布以及膜的选择通透性维持。
The sodium–potassium pump actively transports 3 Na⁺ out of the axon for every 2 K⁺ it brings in, using ATP. This creates concentration gradients: Na⁺ is much higher outside, K⁺ higher inside. At rest, the membrane is more permeable to K⁺ than to Na⁺ due to more open potassium leak channels. K⁺ diffuses out, leaving behind negatively charged proteins and organic anions, making the inside negative.
钠钾泵利用ATP，每主动转运出3个Na⁺就运入2个K⁺，从而建立起浓度梯度：膜外Na⁺浓度高，膜内K⁺浓度高。静息状态下，由于钾泄漏通道较多开放，膜对K⁺的通透性远高于Na⁺。K⁺外流，将带负电的蛋白质和有机阴离子留在膜内，使膜内呈负电。
3. 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 that travels along an axon. It occurs when the membrane depolarises to a threshold of about −55 mV. If the stimulus is strong enough, voltage-gated sodium channels open, and Na⁺ rushes in, causing the potential to rise to around +40 mV.
动作电位是沿轴突传导的膜电位的快速而短暂的变化。当膜去极化达到约−55 mV的阈电位时发生。如果刺激足够强，电压门控钠通道打开，Na⁺迅速内流，使电位升至约+40 mV。
At the peak, voltage-gated sodium channels inactivate and voltage-gated potassium channels open. K⁺ rushes out, repolarising the membrane. There may be a temporary hyperpolarisation (undershoot) as K⁺ channels close slowly. The sodium–potassium pump then restores the original ion distribution.
在峰值处，电压门控钠通道失活，电压门控钾通道打开。K⁺迅速外流，使膜复极化。由于钾通道关闭较慢，可能出现短暂的超极化（下冲）。随后钠钾泵恢复离子原有的分布。
The action potential follows the all-or-nothing law: once the threshold is reached, a full action potential is generated; otherwise, none occurs. The refractory period (absolute and relative) ensures unidirectional propagation and limits the frequency of impulses.
动作电位遵循“全或无”定律：一旦达到阈值，就会产生一个完整的动作电位；否则无动作电位。不应期（绝对和相对）保证了冲动单向传导并限制了冲动的频率。
4. Propagation of Action Potentials | 动作电位的传导
An action potential generated in one region of the axon depolarises the adjacent membrane, triggering voltage-gated sodium channels to open. This sets up local circuits that propagate the impulse along the axon. In unmyelinated neurons, conduction is continuous and relatively slow.
轴突某一区域产生的动作电位使邻近膜去极化，从而触发电压门控钠通道开放。这建立起局部电流，使冲动沿轴突传播。在无髓鞘神经元中，传导是连续的，速度相对较慢。
The speed of propagation depends on axon diameter (larger diameter gives less resistance, so faster conduction) and myelination. In vertebrates, myelination dramatically increases conduction velocity.
传导速度取决于轴突直径（直径越大，电阻越小，传导越快）和髓鞘化程度。在脊椎动物中，髓鞘化显著提高了传导速度。
5. Saltatory Conduction | 跳跃传导
In myelinated axons, the insulating myelin sheath prevents ion flow across most of the axonal membrane. Action potentials can only occur at the nodes of Ranvier, where voltage-gated sodium channels are concentrated. The local current ‘jumps’ from one node to the next, a process called saltatory conduction.
在有髓鞘轴突中，绝缘的髓鞘阻止了离子在大部分膜区域的流动。动作电位只发生在朗飞结处，此处聚集着电压门控钠通道。局部电流从一个郎飞结“跳跃”到下一个，这一过程称为跳跃传导。
Saltatory conduction is much faster than continuous conduction and conserves energy because less active ion transport is needed to restore gradients after each impulse.
跳跃传导远比连续传导快，且节省能量，因为每次冲动后恢复离子梯度所需的活动离子转运较少。
6. Synaptic Transmission | 突触传递
Neurons communicate at synapses. A cholinergic synapse uses acetylcholine (ACh) as the neurotransmitter. When an action potential arrives at the presynaptic terminal, it opens voltage-gated calcium channels. Ca²⁺ ions enter the knob, causing synaptic vesicles to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis.
神经元在突触处进行通讯。胆碱能突触使用乙酰胆碱（ACh）作为神经递质。动作电位到达突触前末梢时，打开电压门控钙通道。Ca²⁺内流进入突触小体，促使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 diffuses across the cleft and binds to specific receptor proteins on the postsynaptic membrane. This opens ligand-gated sodium channels, allowing Na⁺ to enter, which depolarises the postsynaptic membrane and may initiate an action potential if threshold is reached. ACh is quickly broken down by the enzyme acetylcholinesterase to prevent continuous stimulation.
ACh扩散通过间隙，与突触后膜上的特异性受体蛋白结合。这打开了配体门控钠通道，允许Na⁺内流，使突触后膜去极化，如果达到阈值则可能引发动作电位。ACh很快被乙酰胆碱酯酶分解，以防止持续刺激。
7. Neurotransmitters and Summation | 神经递质与总和效应
Excitatory neurotransmitters (e.g., ACh, glutamate) depolarise the postsynaptic membrane, bringing it closer to threshold. Inhibitory neurotransmitters (e.g., GABA) hyperpolarise the membrane, making an action potential less likely. The overall effect depends on the balance of excitatory and inhibitory inputs.
兴奋性神经递质（如ACh、谷氨酸）使突触后膜去极化，更接近阈值。抑制性神经递质（如GABA）使膜超极化，降低动作电位产生的可能性。总体效果取决于兴奋性输入和抑制性输入的平衡。
Spatial summation occurs when several presynaptic neurons release neurotransmitter at different locations on the same postsynaptic cell at the same time, combining to reach threshold. Temporal summation happens when a single presynaptic neuron fires rapidly, releasing neurotransmitter repeatedly before the previous effect fades, causing a cumulative depolarisation.
空间总和指多个突触前神经元同时在同一突触后细胞的不同位置释放神经递质，叠加达到阈值。时间总和指单个突触前神经元快速发放冲动，在上一次效应消退前反复释放递质，引起累积性去极化。
8. Central and Peripheral Nervous Systems | 中枢与周围神经系统
The human nervous system consists of the CNS (brain and spinal cord) and the PNS (cranial and spinal nerves). The PNS is further divided into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of internal organs).
人类神经系统由中枢神经系统（CNS）（脑和脊髓）和周围神经系统（PNS）（脑神经和脊神经）组成。PNS进一步分为躯体神经系统（骨骼肌的随意控制）和自主神经系统（内脏器官的非随意控制）。
The autonomic system has two antagonistic branches: the sympathetic division prepares the body for ‘fight or flight’ (increases heart rate, dilates pupils), while the parasympathetic division promotes ‘rest and digest’ (slows heart rate, stimulates digestion).
自主神经系统有两个拮抗的分支：交感神经系统使身体准备“战斗或逃跑”（心率加快、瞳孔扩大），副交感神经系统促进“休息和消化”（心率减慢、消化活动增强）。
9. Reflex Arc: A Simple Neural Pathway | 反射弧：简单的神经通路
A reflex arc is the simplest functional unit of the nervous system, involving a sensory neuron, relay neuron, and motor neuron. The pathway does not require conscious input from the brain, enabling rapid, involuntary responses to protect the body from harm.
反射弧是神经系统最简单的功能单位，涉及感觉神经元、联络神经元和运动神经元。该通路不需要大脑的有意识参与，从而实现快速的非随意反应以保护身体免受伤害。
The sequence of a spinal reflex (e.g., withdrawal reflex): stimulus → receptor → sensory neuron → relay neuron in spinal cord → motor neuron → effector (muscle). The synapse in the relay neuron introduces a short synaptic delay, but the response is still extremely fast.
脊反射（如缩手反射）的顺序为：刺激 → 感受器 → 感觉神经元 → 脊髓中的联络神经元 → 运动神经元 → 效应器（肌肉）。联络神经元中的突触引入了很短的突触延搁，但反应仍然极快。
10. Drugs and the Synapse | 药物与突触
Many drugs affect synaptic transmission by mimicking neurotransmitters or altering their release, reuptake, or breakdown. Nicotine mimics ACh and binds to nicotinic receptors, causing an initial stimulation followed by desensitisation. Cocaine blocks the reuptake of dopamine, prolonging its excitatory effect.
许多药物通过模拟神经递质或改变其释放、再摄取或分解来影响突触传递。尼古丁模拟ACh并与烟碱受体结合，先引起兴奋，随后导致受体失敏。可卡因阻断多巴胺的再摄取，延长其兴奋效应。
Organophosphates (found in some insecticides and nerve gases) inhibit acetylcholinesterase, leading to accumulation of ACh in the synapse, causing continuous depolarisation and eventually paralysis. Botulinum toxin blocks the release of ACh at neuromuscular junctions, causing muscle paralysis. These examples illustrate how drugs can be excitatory or inhibitory and are often tested in exams.
有机磷化合物（存在于某些杀虫剂和神经毒气中）抑制乙酰胆碱酯酶，导致ACh在突触中积累，引起持续去极化并最终导致麻痹。肉毒杆菌毒素阻断神经肌肉接头处ACh的释放，导致肌肉麻痹。这些例子展示了药物如何发挥兴奋或抑制作用，是考试中常见考点。
11. Comparing Nervous and Hormonal Communication | 神经与激素通讯的比较
Both systems enable cell signalling, but they differ significantly in speed, duration, and mode of transmission. Nervous communication uses electrical impulses and chemical neurotransmitters, transmitting signals in milliseconds, and the effects are short-lived and localised. Hormonal communication uses chemical messengers transported in the blood, takes seconds to hours to act, but the effects are often longer-lasting and more widespread.
两个系统都能传递细胞信号，但在速度、持续时间和传递方式上有显著差异。神经通讯使用电冲动和化学神经递质，信号在毫秒内传递，效应短暂且局部。激素通讯使用经血液运输的化学信使，作用时间从数秒到数小时，但效应通常更持久且范围更广。
| Feature | Nervous System | Endocrine System |
| Speed | Rapid (ms) | Slower (s–h) |
| Duration | Short-lived | Long-lasting |
| Transmission | Neurones, electrical & chemical | Blood, chemical only |
| Target | Specific cells (synapses) | Any cell with receptor |
12. Key Diagrams and Exam Tips | 关键图解与应试技巧
Examiners frequently ask students to label diagrams of a motor neuron, synapse, or reflex arc. Be able to identify and annotate: dendrites, cell body, axon, myelin sheath, nodes of Ranvier, synaptic knob, vesicles, neurotransmitter receptors, and acetylcholinesterase. Practice drawing and explaining the action potential graph, marking resting potential, threshold, depolarisation, repolarisation, and hyperpolarisation.
考官经常要求学生在运动神经元、突触或反射弧的示意图上标注结构。要能识别并注释：树突、细胞体、轴突、髓鞘、朗飞结、突触小体、突触小泡、神经递质受体和乙酰胆碱酯酶。练习绘制并解释动作电位曲线图，标出静息电位、阈电位、去极化、复极化和超极化。
When describing action potentials, always mention the movement of specific ions (Na⁺ influx, K⁺ efflux), the role of voltage-gated channels, and the all-or-nothing principle. Use correct terminology: ‘depolarisation’ not ‘positive charge’, ‘repolarisation’ not ‘returning to normal’. Link structure to function, e.g., ‘myelination allows saltatory conduction, which increases speed while conserving energy’. Review how drugs alter synaptic events, as this is a classic application question.
描述动作电位时，务必提到具体离子的移动（Na⁺内流、K⁺外流）、电压门控通道的作用以及全或无原理。使用正确术语：“去极化”而非“带正电”，“复极化”而非“恢复常态”。将结构与其功能相联系，例如“髓鞘化使跳跃传导成为可能，从而加快速度并节省能量”。复习药物如何改变突触事件，因为这是经典的应用题。
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