📚 IB Biology: The Nervous System Key Concepts | IB 生物：神经系统 考点精讲
The nervous system is the body’s primary communication and control network, responsible for detecting stimuli, integrating information, and coordinating rapid responses. In IB Biology, mastering this topic means understanding the structure and function of neurons, the propagation of nerve impulses, synaptic transmission, and the organisation of the human nervous system. This article unpacks all the essential concepts you need to know, with clear explanations, useful diagrams in words, and exam-focused tips.
神经系统是人体主要的通信和控制网络，负责探测刺激、整合信息并协调快速反应。在 IB 生物学中，掌握这一主题意味着要理解神经元的结构与功能、神经冲动的传播、突触传递以及人体神经系统的组织。本文拆解了所有你需要掌握的核心概念，配有清晰的解释、文字图示和考试重点提示。
1. Neurons: Structure and Types | 神经元：结构与类型
Neurons are specialised cells that transmit electrical and chemical signals. A typical motor neuron consists of a cell body (soma) containing the nucleus, numerous short dendrites that receive signals, and a long axon that conducts impulses away from the cell body. The axon is often insulated by a myelin sheath formed by Schwann cells, with gaps called nodes of Ranvier. Axon terminals at the end form synapses with other neurons or effectors.
神经元是传递电信号和化学信号的特化细胞。一个典型的运动神经元由含有细胞核的胞体、大量接收信号的短树突，以及一根将冲动传离胞体的长轴突组成。轴突通常由施万细胞形成的髓鞘绝缘，中间有朗飞氏结的间隙。轴突末梢与其他神经元或效应器形成突触。
There are three main functional types: sensory neurons carry impulses from receptors to the central nervous system (CNS); relay neurons (interneurons) are found entirely within the CNS and connect sensory to motor neurons; motor neurons transmit impulses from the CNS to effectors such as muscles or glands.
根据功能主要有三种类型：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（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；运动神经元将冲动从中枢神经系统传至效应器，如肌肉或腺体。
In diagrams, you must be able to label: dendrites, cell body, nucleus, axon, myelin sheath, nodes of Ranvier, axon terminal, and synapse. Also note that myelination increases the speed of impulse conduction through saltatory conduction.
在示意图中你必须能够标注：树突、胞体、细胞核、轴突、髓鞘、朗飞氏结、轴突末梢和突触。还要注意髓鞘化通过跳跃传导增加了冲动传导的速度。
2. Resting Potential | 静息电位
When a neuron is not transmitting an impulse, the inside of the axon is negatively charged relative to the outside. This resting potential is typically around -70 mV. It is established and maintained by the sodium-potassium pump (Na⁺/K⁺ ATPase), which actively transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, using ATP. This creates concentration gradients: Na⁺ is higher outside, K⁺ is higher inside.
当神经元不传递冲动时，轴突内部相对于外部带负电。这一静息电位通常在 -70 mV 左右。它由钠钾泵（Na⁺/K⁺ ATP 酶）建立并维持，该泵利用 ATP 主动将 3 个钠离子运出细胞，并将 2 个钾离子运入细胞。这形成了浓度梯度：膜外 Na⁺ 浓度较高，膜内 K⁺ 浓度较高。
The membrane is more permeable to K⁺ than to Na⁺ due to more open potassium leak channels. K⁺ diffuses out down its concentration gradient, making the inside negative. The equilibrium potential for K⁺ is around -90 mV, but the slight inward leak of Na⁺ keeps the resting potential at about -70 mV. The Na⁺/K⁺ pump counteracts this leakage to maintain gradients.
由于开放更多的钾漏通道，膜对 K⁺ 的通透性比对 Na⁺ 更高。K⁺ 沿浓度梯度外流，使膜内变负。K⁺ 的平衡电位约为 -90 mV，但由于少量 Na⁺ 向内漏入，静息电位维持在约 -70 mV。钠钾泵通过抵消这种泄漏来维持离子梯度。
3. Action Potential | 动作电位
An action potential is a rapid, transient reversal of membrane potential that propagates along the axon. It follows an all-or-none law: once the threshold potential (around -55 mV) is reached, an action potential is triggered without variation in amplitude.
动作电位是膜电位的快速、短暂逆转，沿轴突传播。它遵循全或无定律：一旦到达阈电位（约 -55 mV），就会触发一个幅度不变的动作电位。
The sequence involves: (1) Depolarisation: voltage-gated Na⁺ channels open; Na⁺ rushes in, making the inside positive (up to +40 mV). (2) Repolarisation: Na⁺ channels inactivate, and voltage-gated K⁺ channels open; K⁺ flows out, restoring negative charge inside. (3) Hyperpolarisation: K⁺ channels are slow to close, so the potential briefly becomes more negative than resting. (4) Refractory period: the absolute refractory period occurs when Na⁺ channels are inactivated, preventing another action potential; the relative refractory period follows, when a stronger stimulus is needed.
其过程包括：（1）去极化：电压门控 Na⁺ 通道开放；Na⁺ 内流，使膜内变正（可达 +40 mV）。（2）复极化：Na⁺ 通道失活，电压门控 K⁺ 通道开放；K⁺ 外流，恢复膜内负电。（3）超极化：K⁺ 通道关闭缓慢，电位短暂地变得比静息更负。（4）不应期：绝对不应期发生在 Na⁺ 通道失活时，不能产生新的动作电位；随后是相对不应期，需要更强的刺激才能触发。
During depolarisation, the membrane potential moves towards the Na⁺ equilibrium potential; during repolarisation, it moves towards K⁺ equilibrium potential. The local currents caused by ion movements depolarise adjacent regions of the axon, propagating the impulse.
去极化时，膜电位趋向 Na⁺ 平衡电位；复极化时，膜电位趋向 K⁺ 平衡电位。离子运动产生的局部电流使轴突的邻近区域去极化，从而传播冲动。
4. Saltatory Conduction | 跳跃传导
In myelinated axons, the insulating myelin sheath prevents ion flow across the membrane except at the nodes of Ranvier, where there is a high density of voltage-gated Na⁺ and K⁺ channels. Action potentials therefore jump from node to node, a process called saltatory conduction. This increases conduction speed (up to 120 m/s) compared to unmyelinated axons (about 1 m/s) and is more energy-efficient because the Na⁺/K⁺ pump only needs to restore gradients at the nodes.
在有髓鞘的轴突中，绝缘的髓鞘阻止了离子跨膜流动，只有在朗飞氏结处存在高密度的电压门控 Na⁺ 和 K⁺ 通道。因此动作电位从一个结跳到下一个结，这一过程称为跳跃传导。与无髓鞘轴突（约 1 m/s）相比，这增加了传导速度（可达 120 m/s），并且更加节能，因为钠钾泵只需在朗飞氏结处恢复离子梯度。
Factors that affect speed include axon diameter (larger diameter reduces resistance, increasing speed) and myelination. In multiple sclerosis, the immune system attacks the myelin sheath, disrupting saltatory conduction and leading to loss of coordination and muscle control.
影响速度的因素包括轴突直径（直径越大，电阻越小，速度越快）和髓鞘化。在多发性硬化症中，免疫系统攻击髓鞘，破坏跳跃传导，导致协调和肌肉控制能力的丧失。
5. Synaptic Transmission | 突触传递
A synapse is a junction between two neurons or between a neuron and an effector. The presynaptic neuron releases neurotransmitter from its axon terminal into the synaptic cleft, a narrow gap of about 20 nm. The neurotransmitter diffuses across and binds to specific receptors on the postsynaptic membrane, opening ligand-gated ion channels and generating a postsynaptic potential.
突触是两个神经元之间或神经元与效应器之间的连接点。突触前神经元从其轴突末梢释放神经递质到宽约 20 nm 的突触间隙。神经递质扩散过去并绑定到突触后膜上的特异性受体，打开配体门控离子通道，产生突触后电位。
The process begins when an action potential arrives at the presynaptic terminal, causing voltage-gated Ca²⁺ channels to open. Ca²⁺ influx triggers synaptic vesicles containing neurotransmitter (e.g., acetylcholine) to fuse with the presynaptic membrane and release their contents by exocytosis. The neurotransmitter is then broken down (e.g., acetylcholinesterase breaks down acetylcholine) or reabsorbed to terminate the signal.
这一过程始于动作电位到达突触前末梢，导致电压门控 Ca²⁺ 通道开放。Ca²⁺ 内流触发含有神经递质（例如乙酰胆碱）的突触囊泡与突触前膜融合，通过胞吐作用释放内容物。随后神经递质被分解（如乙酰胆碱酯酶分解乙酰胆碱）或被重吸收以终止信号。
Excitatory neurotransmitters (e.g., acetylcholine at neuromuscular junctions) cause depolarisation (excitatory postsynaptic potential, EPSP) by opening Na⁺ channels. Inhibitory neurotransmitters (e.g., GABA) cause hyperpolarisation (inhibitory postsynaptic potential, IPSP) by opening Cl⁻ or K⁺ channels. Summation of EPSPs and IPSPs at the axon hillock determines whether an action potential is initiated.
兴奋性神经递质（如神经肌肉接头处的乙酰胆碱）通过打开 Na⁺ 通道引起去极化（兴奋性突触后电位，EPSP）。抑制性神经递质（如 GABA）通过打开 Cl⁻ 或 K⁺ 通道引起超极化（抑制性突触后电位，IPSP）。在轴丘处 EPSP 和 IPSP 的叠加决定是否触发动作电位。
6. The Human Nervous System: CNS and PNS | 人体神经系统：中枢与外周
The human nervous system is divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), made up of nerves and ganglia outside the CNS. The PNS is further divided into the sensory (afferent) division carrying impulses towards the CNS, and the motor (efferent) division carrying impulses away from the CNS.
人体神经系统分为中枢神经系统（CNS）和外周神经系统（PNS）。CNS 由脑和脊髓组成，PNS 由 CNS 以外的神经和神经节构成。PNS 进一步分为将冲动传向 CNS 的感觉（传入）部分，以及将冲动从 CNS 传出的运动（传出）部分。
The motor division includes the somatic nervous system, which controls voluntary movements of skeletal muscles, and the autonomic nervous system, which regulates involuntary functions such as heart rate, digestion, and glandular secretion. The autonomic system is subdivided into the sympathetic and parasympathetic divisions, which generally have antagonistic effects.
运动部分包括控制骨骼肌随意运动的躯体神经系统，以及调节心率、消化和腺体分泌等不自主功能的自主神经系统。自主神经系统又分为交感神经和副交感神经两部分，它们通常具有拮抗作用。
7. The Brain: Key Structures and Functions | 脑：关键结构与功能
The brain acts as the integration centre. Key parts include the cerebral hemispheres (cerebrum), which are responsible for higher-order functions such as reasoning, memory, language, and voluntary motor control. The cerebral cortex is highly folded, increasing surface area. The left hemisphere generally controls the right side of the body and vice versa.
脑是整合中心。关键部分包括大脑半球（大脑），负责推理、记忆、语言和随意运动控制等高级功能。大脑皮层高度折叠，增加了表面积。左半球通常控制身体右侧，反之亦然。
The cerebellum coordinates balance, posture, and fine motor movements. The medulla oblongata (part of the brain stem) controls autonomic functions like breathing, heart rate, and blood pressure. The hypothalamus is a small region that maintains homeostasis by regulating body temperature, thirst, hunger, and linking the nervous system to the endocrine system via the pituitary gland.
小脑协调平衡、姿势和精细运动。延髓（脑干的一部分）控制呼吸、心率和血压等自主功能。下丘脑是一个小区域，通过调节体温、口渴、饥饿以及通过垂体将神经系统与内分泌系统联系起来，维持稳态。
You should be able to identify these structures on a diagram of the brain and explain their roles. The corpus callosum, a thick band of nerve fibres connecting the two hemispheres, is also relevant for communication between hemispheres.
你应该能够在脑的示意图上识别这些结构并解释其作用。胼胝体是一条连接两个半球的厚神经纤维束，也与半球间的通信有关。
8. Reflex Arc | 反射弧
A reflex arc is the simplest functional unit of the nervous system, providing a rapid, involuntary response to a stimulus without conscious brain involvement. The basic pathway involves: receptor → sensory neuron → relay neuron (in the spinal cord or brain stem) → motor neuron → effector.
反射弧是神经系统最简单的功能单元，提供了一个对刺激的快速、不随意反应，无需意识参与。基本通路为：感受器 → 感觉神经元 → 联络神经元（在脊髓或脑干） → 运动神经元 → 效应器。
A classic example is the withdrawal reflex: touching a hot object activates pain receptors in the skin; impulses travel along a sensory neuron to the spinal cord, where the sensory neuron synapses with a relay neuron, which in turn synapses with a motor neuron, causing the biceps muscle to contract and withdraw the hand. Simultaneously, other interneurons inhibit the motor neurons to the triceps (reciprocal inhibition) and send signals to the brain to register pain.
一个经典例子是屈曲反射：触碰高温物体激活皮肤痛觉感受器；冲动沿感觉神经元传到脊髓，感觉神经元与联络神经元形成突触，联络神经元再与运动神经元形成突触，使肱二头肌收缩、手缩回。同时，其他中间神经元抑制肱三头肌的运动神经元（交互抑制），并向脑发出信号以感知疼痛。
Reflexes are fast because the pathway involves only a few synapses, avoiding the longer processing time of the brain. This is important for protecting the body from harm. The knee-jerk reflex is another simple monosynaptic reflex used to test neurological function.
反射之所以快，是因为通路只涉及少数突触，避免了脑处理所需的更长时间。这对保护身体免受伤害很重要。膝跳反射是另一种简单的单突触反射，用于测试神经功能。
9. Autonomic Nervous System: Sympathetic vs Parasympathetic | 自主神经系统：交感与副交感
The autonomic nervous system controls involuntary internal organs and is critical for homeostasis. The sympathetic division prepares the body for ‘fight or flight’ responses: increasing heart and breathing rates, dilating pupils, inhibiting digestion, and stimulating glucose release. Its preganglionic neurons are short, releasing acetylcholine, and postganglionic neurons are long, releasing noradrenaline (norepinephrine).
自主神经系统控制不随意的内脏器官，对稳态至关重要。交感神经部分使身体做好“战斗或逃跑”的准备：加快心率和呼吸频率、扩大瞳孔、抑制消化、刺激葡萄糖释放。其节前神经元较短，释放乙酰胆碱，节后神经元较长，释放去甲肾上腺素。
The parasympathetic division dominates during ‘rest and digest’ conditions: slowing heart rate, stimulating digestion, constricting pupils, and promoting energy storage. Its preganglionic neurons are long, releasing acetylcholine, and postganglionic neurons are short, also releasing acetylcholine.
副交感神经部分在“休息与消化”状态下占主导：减慢心率、促进消化、缩小瞳孔、促进能量储存。其节前神经元较长，释放乙酰胆碱，节后神经元较短，也释放乙酰胆碱。
Many organs receive dual innervation, with the two divisions acting antagonistically to fine-tune function. For example, heart rate is increased by sympathetic stimulation and decreased by parasympathetic stimulation via the vagus nerve.
许多器官接受双重支配，两个部分拮抗作用以精细调节功能。例如，心率通过交感神经刺激升高，通过迷走神经的副交感刺激降低。
10. Neurotransmitters and Pharmacology | 神经递质与药理学
Neurotransmitters are chemical messengers that transmit signals across synapses. Acetylcholine (ACh) is used at neuromuscular junctions and in the autonomic and CNS. ACh is broken down by acetylcholinesterase in the cleft. Neonicotinoid pesticides bind irreversibly to insect ACh receptors, causing paralysis and death, but have lower affinity for mammalian receptors, offering selective toxicity.
神经递质是跨突触传递信号的化学信使。乙酰胆碱（ACh）用于神经肌肉接头以及自主神经和中枢神经系统。ACh 在间隙中被乙酰胆碱酯酶分解。新烟碱类杀虫剂不可逆地与昆虫 ACh 受体结合，导致麻痹和死亡，但对哺乳动物受体的亲和力较低，具有选择性毒性。
GABA (gamma-aminobutyric acid) is the main inhibitory neurotransmitter in the brain. Drugs like benzodiazepines enhance GABA receptor activity, reducing anxiety. Dopamine is involved in reward and motor control; loss of dopamine-producing neurons leads to Parkinson’s disease. Serotonin influences mood, sleep, and appetite; low levels are linked to depression.
GABA（γ-氨基丁酸）是脑内主要的抑制性神经递质。苯二氮卓类药物增强 GABA 受体活性，减轻焦虑。多巴胺参与奖赏和运动控制；产生多巴胺的神经元死亡会导致帕金森病。血清素影响情绪、睡眠和食欲；水平低与抑郁症有关。
Exam questions often ask you to explain how a drug or toxin affects synaptic transmission. Always describe the precise mechanism: e.g., inhibiting release, blocking receptors, preventing reuptake, or interfering with enzymatic breakdown.
考试题目经常要求解释药物或毒素如何影响突触传递。一定要描述精确的机制：例如抑制释放、阻断受体、阻止重摄取或干扰酶解。
11. Sensory Receptors and Perception | 感受器与感知
Sensory receptors are transducers that convert a specific form of stimulus energy into an electrical signal (generator or receptor potential). Mechanoreceptors (touch, pressure, sound), photoreceptors (light), chemoreceptors (taste, smell, blood pH), thermoreceptors (temperature), and nociceptors (pain) are classified by the energy they detect.
感受器是换能器，将特定形式的刺激能量转换为电信号（发生器电位或感受器电位）。根据检测的能量类型，可分为机械感受器（触觉、压力、声音）、光感受器（光）、化学感受器（味觉、嗅觉、血液 pH）、温度感受器（温度）和伤害性感受器（疼痛）。
A receptor potential is a graded potential; if it reaches threshold, it triggers an action potential in the associated sensory neuron. The frequency of action potentials encodes stimulus intensity. Adaptation is the decrease in response to a constant stimulus; for example, pressure receptors adapt quickly, while pain receptors adapt little or not at all.
感受器电位是等级电位；如果达到阈值，就会在关联的感觉神经元中触发动作电位。动作电位的频率编码刺激强度。适应是指对恒定刺激的反应减弱；例如压力感受器适应很快，而伤害性感受器几乎不或完全不适应。
The structure of the human eye is often covered: the retina contains rod cells (sensitive to low light, black/white) and cone cells (responsible for colour vision). Rods contain the pigment rhodopsin; light causes rhodopsin to break down, hyperpolarising the rod cell and reducing neurotransmitter release, which indirectly initiates signals to the optic nerve.
人眼的结构经常涉及：视网膜含有视杆细胞（对弱光敏感，负责黑白视觉）和视锥细胞（负责色觉）。视杆细胞含有视紫红质色素；光照使视紫红质分解，导致视杆细胞超极化，减少神经递质释放，从而间接启动信号传递到视神经。
12. Common Exam Questions and Tips | 常见考题与技巧
IB exam questions on the nervous system range from objective multiple-choice to extended-response questions. You must be confident in drawing and annotating an action potential graph with labelled axes (membrane potential in mV vs time in ms), indicating resting potential, threshold, depolarisation, repolarisation, hyperpolarisation, and refractory periods.
神经系统部分的 IB 考题范围从客观选择题到扩展简答题。你必须能够自信地绘制并注释动作电位图，标注坐标轴（膜电位 mV 对时间 ms），标出静息电位、阈值、去极化、复极化、超极化和不应期。
Another classic question is explaining the propagation of a nerve impulse along a non-myelinated vs myelinated axon. Use the terms local currents, nodes of Ranvier, and saltatory conduction. Also be prepared to compare synaptic transmission at cholinergic synapses with neuromuscular junctions.
另一个经典考题是解释神经冲动沿无髓鞘与有髓鞘轴突的传播。要使用局部电流、朗飞氏结和跳跃传导等术语。还要准备好比较胆碱能突触与神经肌肉接头的突触传递。
Data-based questions may present an oscilloscope trace showing action potentials under different conditions (e.g., after applying a drug). Interpret the changes: if the amplitude decreases, perhaps Na⁺ channels are blocked; if the depolarisation slope is less steep, perhaps some Na⁺ channels are inactive or the concentration gradient is altered.
数据题可能会给出不同条件下（如施加药物后）显示动作电位的示波器轨迹。要解释变化：如果幅度减小，可能是 Na⁺ 通道被阻断；如果去极化斜率变平缓，可能是一些 Na⁺ 通道失活或浓度梯度发生了改变。
Always link structure to function: for example, explain how the extensive folding of the cerebral cortex increases the number of neurons that can be packed, enabling complex processing. Mention specific examples of reflex arcs and neurotransmitters to strengthen your answers.
始终将结构与功能联系起来：例如解释大脑皮层的大量折叠如何增加可容纳的神经元数量，从而实现复杂加工。引用具体的反射弧和神经递质实例来加强你的答案。
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