Nervous System: Key Concepts for IB & CIE Biology | 神经系统：IB与CIE考点精讲

📚 Nervous System: Key Concepts for IB & CIE Biology | 神经系统：IB与CIE考点精讲

The nervous system is a complex network of specialised cells that enables rapid communication within the body. For both IB and CIE Biology, examiners expect a detailed understanding of neurone structure, electrical and chemical signalling, synaptic transmission, and the organisation of the mammalian nervous system. Key topics also include reflex arcs and the role of sense organs such as the eye. This revision guide breaks down the core learning points with clear comparisons, step-by-step mechanisms, and essential vocabulary.

神经系统是由特化细胞构成的复杂网络，能够在体内进行快速通信。无论是 IB 还是 CIE 生物学，考官都期望学生深入理解神经元结构、电信号与化学信号、突触传递以及哺乳动物神经系统的组织架构。反射弧和诸如眼睛等感觉器官的作用也是重点。本复习指南通过清晰的对比、逐步的机制讲解和必备词汇，系统梳理了核心考点。

1. Neurone Structure | 神经元结构

Neurones are the functional units of the nervous system. Three main types are commonly examined: motor neurones, sensory neurones, and relay (intermediate) neurones. A typical motor neurone features a cell body containing the nucleus, numerous dendrites that receive signals, a long axon that conducts impulses away from the cell body, and terminal branches ending in synaptic knobs. Many vertebrate axons are insulated by a myelin sheath formed by Schwann cells, interrupted at regular intervals by nodes of Ranvier. Sensory neurones have a similar layout but the cell body is positioned off to one side of the axon, and relay neurones have many short, highly branched dendrites and a short axon.

神经元是神经系统的功能单位。常考的三种主要类型为：运动神经元、感觉神经元和联络（中间）神经元。典型的运动神经元有一个含细胞核的细胞体、多个接收信号的树突、一根将冲动传出细胞体的长轴突，以及末端分支的突触小结。许多脊椎动物的轴突由施万细胞形成的髓鞘包裹，并在一定的间隔处被郎飞氏结中断。感觉神经元的布局类似，但细胞体位于轴突一侧，而联络神经元则有大量短而高度分支的树突和较短的轴突。

2. Resting Potential | 静息电位

Even when a neurone is not transmitting an impulse, there is a potential difference across its membrane of about −70 mV, the resting potential. This is established and maintained largely 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 much more permeable to K⁺ than to Na⁺ due to open potassium leak channels. K⁺ diffuses out down its concentration gradient, making the inside of the cell negative relative to the outside. The uneven distribution of ions, with a high concentration of Na⁺ outside and K⁺ inside, stores potential energy that is essential for the generation of an action potential.

即使在神经元不传递冲动时，其膜两侧也存在约 −70 mV 的电位差，即静息电位。静息电位的建立与维持主要依靠钠-钾泵：该泵利用 ATP 将 3 个 Na⁺ 泵出细胞、将 2 个 K⁺ 泵入细胞。由于存在开放状态下的钾离子泄漏通道，膜对 K⁺ 的通透性远大于对 Na⁺ 的通透性。K⁺ 顺着浓度梯度扩散到膜外，使细胞内侧相对于外侧呈负电。Na⁺ 膜外浓度高、K⁺ 膜内浓度高的不均匀离子分布，储存了产生动作电位所必需的电势能。

3. Action Potential | 动作电位

When a stimulus depolarises the membrane to the threshold potential (around −55 mV), an action potential is fired. This all-or-nothing event can be broken into phases:

• Depolarisation: voltage-gated Na⁺ channels open, Na⁺ rushes in, and the membrane potential rises to about +40 mV.
• Repolarisation: Na⁺ channels inactivate, while voltage-gated K⁺ channels open; K⁺ efflux restores the negative internal charge.
• Hyperpolarisation: K⁺ channels are slow to close, so the potential briefly becomes more negative than the resting value.
• Refractory period: the period when the membrane is unresponsive, first absolute (no new action potential possible) then relative (a stronger stimulus is needed).

当刺激使膜去极化达到阈电位（约 −55 mV）时，便会发放一个动作电位。这一全或无事件可分为以下阶段：

• 去极化：电压门控 Na⁺ 通道打开，Na⁺ 快速内流，膜电位上升至约 +40 mV。
• 复极化：Na⁺ 通道失活，同时电压门控 K⁺ 通道打开；K⁺ 外流使膜内负电荷恢复。
• 超极化：K⁺ 通道关闭较慢，因此电位短暂比静息值更负。
• 不应期：膜处于无反应状态的时期，先是绝对不应期（不可能产生新的动作电位），后是相对不应期（需要更强的刺激）。

4. Propagation of Action Potentials | 动作电位的传导

Once an action potential is generated at one point, local circuits cause the adjacent region of the membrane to depolarise to threshold. The inward flow of Na⁺ at the active site spreads sideways, pushing positive charge along the axon interior. This passive current depolarises the neighbouring region, opening voltage-gated Na⁺ channels there. Consequently, the action potential is regenerated point by point along the entire length of an unmyelinated axon. The impulse travels in one direction because the previous segment is in its refractory period.

一旦在某一点产生动作电位，局部电流便会引起相邻膜区域去极化至阈值。活动部位的 Na⁺ 内流沿轴突内部侧向扩散，将正电荷向前推移。这一被动电流使邻近区域去极化，打开那里的电压门控 Na⁺ 通道。因此，在无髓鞘轴突的整个长度上，动作电位被逐点重新生成。冲动之所以单向传导，是因为前一节段正处于不应期。

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 Na⁺ and K⁺ channels are concentrated. The local current jumps rapidly from one node to the next, a process called saltatory conduction. This greatly increases the speed of propagation and is metabolically more efficient because fewer ions need to be pumped back after each impulse. Larger diameter axons also conduct faster.

在有髓鞘轴突中，绝缘的髓鞘阻止了离子在大部分轴突膜上的流动。动作电位只能发生在郎飞氏结处，这些部位集中了大量电压门控 Na⁺ 和 K⁺ 通道。局部电流从一个结点快速跳向下一个结点，这一过程称为跳跃传导。这大大提高了传导速度，而且由于每次冲动后需要泵回的离子较少，代谢效率也更高。直径较大的轴突传导速度也更快。

6. Synaptic Structure | 突触结构

A synapse is the junction between two neurones or between a neurone and an effector cell. The presynaptic knob contains many mitochondria and synaptic vesicles filled with neurotransmitter. The cleft is a narrow gap of about 20–30 nm separating the presynaptic and postsynaptic membranes. The postsynaptic membrane carries specific receptor proteins to which the neurotransmitter binds. The structure ensures unidirectional transmission and allows for integration of multiple signals.

突触是两个神经元之间或神经元与效应细胞之间的连接处。突触前小结含有大量线粒体和充满神经递质的突触小泡。突触间隙是将突触前膜与突触后膜隔开的约 20–30 nm 的狭窄空隙。突触后膜上携带着特定的受体蛋白，供神经递质结合。这种结构保障了传递的单向性，并允许多种信号的整合。

7. Synaptic Transmission | 突触传递

When an action potential arrives at the presynaptic knob, it opens voltage-gated Ca²⁺ channels. The influx of Ca²⁺ triggers synaptic vesicles to fuse with the membrane, releasing neurotransmitter molecules into the synaptic cleft by exocytosis. The transmitter diffuses across the cleft and binds to postsynaptic receptors. This binding may open ligand-gated Na⁺ channels, causing an excitatory postsynaptic potential (EPSP), or it may open Cl⁻ or K⁺ channels, producing an inhibitory postsynaptic potential (IPSP). The transmitter is then removed from the cleft by enzymatic breakdown, re‑uptake into the presynaptic cell, or diffusion away.

当动作电位到达突触前小结时，会打开电压门控 Ca²⁺ 通道。Ca²⁺ 的内流触发突触小泡与膜融合，通过胞吐作用将神经递质分子释放到突触间隙中。递质扩散穿过间隙并与突触后受体结合。这种结合可能打开配体门控 Na⁺ 通道，产生兴奋性突触后电位（EPSP）；也可能打开 Cl⁻ 或 K⁺ 通道，引起抑制性突触后电位（IPSP）。随后，递质通过酶促降解、被突触前细胞重摄取或扩散远离而从间隙中清除。

8. Neurotransmitters and Their Effects | 神经递质及其效应

Different neurotransmitters have distinct effects depending on the receptor type. Acetylcholine is an excitatory transmitter at the neuromuscular junction but can be inhibitory at the vagus nerve in the heart. Noradrenaline is involved in the sympathetic nervous system, preparing the body for ‘fight or flight’. Other neurotransmitters, such as GABA (gamma-aminobutyric acid), are predominantly inhibitory in the brain. The overall response of a postsynaptic neurone is determined by the sum of all EPSPs and IPSPs it receives—this is synaptic integration.

不同的神经递质因受体类型的不同而具有不同的效应。乙酰胆碱在神经肌肉接头处是兴奋性递质，但在心脏迷走神经上可以是抑制性的。去甲肾上腺素参与交感神经系统，让身体做好“战斗或逃跑”的准备。其他神经递质，如 GABA（γ-氨基丁酸），在大脑中主要起抑制作用。突触后神经元的总体反应取决于它所接收的全部 EPSP 和 IPSP 的总和——这便是突触整合。

9. The Reflex Arc | 反射弧

A reflex arc is the simplest functional circuit in the nervous system, enabling a rapid, involuntary response to a stimulus. It typically consists of a receptor, a sensory neurone, at least one synapse in the central nervous system (often via a relay neurone), a motor neurone, and an effector. Classic examples include the knee-jerk reflex (a monosynaptic stretch reflex) and the withdrawal reflex (a polysynaptic reflex involving an interneuron). Reflex arcs are important because they provide a fast, protective response without conscious thought.

反射弧是神经系统中最为简单的功能回路，能够对刺激做出迅速且不自主的反应。它通常包含感受器、感觉神经元、中枢神经系统中至少一个突触（经常通过联络神经元）、运动神经元以及效应器。典型的例子包括膝跳反射（一种单突触的牵张反射）和撤手反射（含中间神经元的多突触反射）。反射弧的重要性在于它们能提供快速且无需意识参与的自我保护性反应。

10. Organisation of the Nervous System | 神经系统的组成

The mammalian nervous system is divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), comprising all nerves outside the CNS. The PNS is further subdivided into the sensory (afferent) division, bringing impulses from receptors to the CNS, and the motor (efferent) division, carrying impulses from the CNS to effectors. The motor division includes the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of internal organs). The autonomic system has two opposing branches:

哺乳动物的神经系统分为中枢神经系统（CNS），即脑和脊髓，以及外周神经系统（PNS），即中枢神经系统以外的所有神经。PNS 进一步分为感觉（传入）部分，将冲动从感受器传至 CNS，和运动（传出）部分，将冲动从 CNS 传至效应器。运动部分包含躯体神经系统（骨骼肌的随意控制）和自主神经系统（内脏器官的不随意控制）。自主神经系统有两个作用相反的支系：

Sympathetic	Parasympathetic
Increases heart rate, dilates pupils, inhibits digestion	Decreases heart rate, constricts pupils, stimulates digestion
‘Fight or flight’	‘Rest and digest’

交感神经	副交感神经
心率加快，瞳孔放大，消化受抑	心率减慢，瞳孔缩小，消化增强
“战斗或逃跑”	“休息与消化”

11. The Eye: Photoreceptors and Visual Processing | 眼睛：光感受器与视觉处理

The retina contains two types of photoreceptor cells: rods and cones. Rods are highly sensitive to light intensity and function well in dim light, providing black-and-white vision. They contain the visual pigment rhodopsin, which breaks down upon absorbing light, initiating a signal. Cones are less sensitive but are responsible for colour vision and high visual acuity in bright light. There are three types of cones, each sensitive to a different range of wavelengths (red, green, blue). The fovea is a region of the retina packed with cones and no rods, giving the sharpest image. Impulses from photoreceptors travel via bipolar cells to ganglion cells, whose axons form the optic nerve and carry information to the brain.

视网膜含有两类感光细胞：视杆细胞和视锥细胞。视杆细胞对光强度高度敏感，在弱光下工作良好，提供黑白视觉。它们含有视觉色素视紫红质，吸收光后分解并启动信号。视锥细胞的敏感性较低，但负责强光下的色觉和高视觉敏锐度。视锥细胞有三种类型，各自对不同波长范围（红、绿、蓝）敏感。黄斑区是视网膜上密集分布视锥细胞、没有视杆细胞的区域，形成最清晰的图像。来自感光细胞的冲动通过双极细胞传递给神经节细胞，后者的轴突汇聚形成视神经，将信息传递至大脑。

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