IB & OCR Biology: Nervous System Key Points | IB & OCR 生物：神经系统考点精讲

📚 IB & OCR Biology: Nervous System Key Points | IB & OCR 生物：神经系统考点精讲

The nervous system is one of the most high‑yield topics in both IB and OCR A‑level Biology, linking cellular physiology to whole‑body coordination. Mastering the structure of neurones, the mechanism of action potentials, synaptic transmission and the organisation of the nervous system is essential for top marks. This article distils the core concepts, common trap areas and exam‑style reasoning you need.

神经系统是 IB 和 OCR A‑level 生物学中分值最高的主题之一，将细胞生理与全身协调联系起来。掌握神经元结构、动作电位机制、突触传递以及神经系统的组织方式是获得高分的关键。本文浓缩了核心概念、常见易错点以及所需的考试思维。

1. Structure of a motor neurone | 运动神经元的结构

A motor neurone carries impulses from the central nervous system to an effector, such as a muscle. Its key components are the dendrites, cell body, axon, myelin sheath and synaptic knobs. The myelin sheath is formed by Schwann cells and acts as an electrical insulator, greatly increasing the speed of impulse conduction.

运动神经元将冲动从中枢神经系统传递至效应器（如肌肉）。其关键组成包括树突、细胞体、轴突、髓鞘和突触小结。髓鞘由施万细胞构成，起电绝缘作用，可大幅提高冲动传导速度。

Between adjacent Schwann cells there are small gaps called nodes of Ranvier. In myelinated fibres the action potential ‘jumps’ from node to node — a phenomenon known as saltatory conduction. This is a favourite comparison point with unmyelinated neurones, where the impulse travels more slowly by continuous propagation.

相邻施万细胞之间的小间隙称为朗飞结。在有髓纤维中，动作电位在结间「跳跃」传导，这种现象称为跳跃传导。这是最常与无髓神经元进行比较的一点，无髓纤维中冲动通过连续传播，速度较慢。

2. Resting potential: the polarised membrane | 静息电位：极化膜

All neurones maintain a resting potential of about −70 mV, meaning the inside of the axon is negative relative to the outside. This is established and maintained by the sodium‑potassium pump (Na⁺/K⁺ ATPase), which actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell for each ATP hydrolysed.

所有神经元维持约 –70 mV 的静息电位，即轴突内部相对于外部为负电。该电位由钠钾泵（Na⁺/K⁺ ATPase）建立与维持：每水解一个 ATP，主动转运 3 个 Na⁺ 出胞、2 个 K⁺ 入胞。

The membrane is also more permeable to K⁺ than to Na⁺ because of the presence of non‑gated potassium leak channels. As a result, K⁺ diffuses out down its concentration gradient, making the inside more negative. This equilibrium between electrical and chemical gradients is described by the Goldman equation, but for IB/OCR you typically need to explain the principle qualitatively.

膜对 K⁺ 的通透性远高于 Na⁺，因为存在非门控的钾漏通道。因此，K⁺ 沿浓度梯度外流，使膜内更负。这种电化学梯度间的平衡可由 Goldman 方程描述，但在 IB/OCR 考试中通常要求定性解释原理。

3. The action potential: depolarisation, repolarisation and hyperpolarisation | 动作电位：去极化、复极化和超极化

When a neurone is stimulated and reaches the threshold potential (around −55 mV), voltage‑gated Na⁺ channels open rapidly. Na⁺ floods in, causing depolarisation and even a brief overshoot where the membrane potential becomes positive (up to about +40 mV).

当神经元受刺激并达到阈电位（约 –55 mV），电压门控 Na⁺ 通道迅速打开。Na⁺ 大量内流，引致去极化甚至短暂超射，膜电位变为正电（最高约 +40 mV）。

Almost immediately, voltage‑gated Na⁺ channels inactivate and voltage‑gated K⁺ channels open. K⁺ exits the cell, bringing the membrane potential back down towards the resting level — this is repolarisation. Because K⁺ channels are slow to close, the potential often dips below the resting value (hyperpolarisation) before the pump and leak channels restore the resting state.

几乎同时，电压门控 Na⁺ 通道失活，电压门控 K⁺ 通道打开。K⁺ 外流使膜电位回落至静息水平，即复极化。因 K⁺ 通道关闭较慢，电位常降至静息值以下（超极化），随后由泵和漏通道恢复静息状态。

Exam tip: always state that the Na⁺/K⁺ pump restores ionic gradients but does not directly create the action potential; the steep changes are entirely due to ion movement through voltage‑gated channels.

考试提示：务必说明 Na⁺/K⁺ 泵恢复离子浓度梯度，但并不直接产生动作电位；陡峭变化完全源于离子经电压门控通道的流动。

4. All‑or‑nothing law and the refractory period | 全或无定律与不应期

An action potential either fires fully or not at all once threshold is reached. The magnitude of the peak is always the same regardless of stimulus strength; stronger stimuli encode information by increasing the frequency of action potentials, not their amplitude.

一旦达到阈值，动作电位要么完全触发，要么完全不触发。无论刺激强度如何，峰值幅度始终相同；更强刺激通过提高动作电位的频率来编码信息，而非改变其幅度。

The absolute refractory period occurs while voltage‑gated Na⁺ channels are inactivated; no new action potential can be generated. The relative refractory period follows, when some Na⁺ channels have recovered but the membrane is hyperpolarised — a stronger‑than‑threshold stimulus is needed. The refractory period ensures unidirectional propagation and limits maximum firing frequency.

绝对不应期出现在电压门控 Na⁺ 通道失活期间，无法产生新动作电位。随之而来的是相对不应期，此时部分 Na⁺ 通道已恢复，但膜处于超极化状态，需高于阈值的刺激才能触发。不应期确保了单向传播并限制最大放电频率。

5. Conduction velocity and factors that affect it | 传导速度及其影响因素

Three main factors influence how fast an action potential travels: axon diameter, myelination and temperature. A larger diameter reduces internal resistance, allowing faster local‑circuit current spread. Myelination introduces saltatory conduction, dramatically speeding up transmission — in humans, up to 120 m s⁻¹ compared with only ~1 m s⁻¹ in unmyelinated fibres.

影响动作电位传播速度的主要因素有三：轴突直径、髓鞘化和温度。更大直径降低内阻，使局部电流更快扩散。髓鞘化引入跳跃传导，大幅提高传导速度——在人类可达 120 m s⁻¹，而无髓纤维仅约 1 m s⁻¹。

Temperature affects the kinetic energy of ions and the speed of conformational changes in channel proteins. Moderate warming increases speed, but very high temperatures denature proteins and inhibit function. This is often tested with a graph interpretation question.

温度影响离子的动能和通道蛋白构象变化的速率。适度升温可提高速度，但过高温度会使蛋白变性、抑制功能。这一考点常以图表解释题出现。

6. Synaptic structure and neurotransmitter release | 突触结构与神经递质释放

A chemical synapse consists of a presynaptic terminal, a synaptic cleft (about 20 nm wide) and a postsynaptic membrane. The arrival of an action potential opens voltage‑gated Ca²⁺ channels in the presynaptic knob. The influx of Ca²⁺ causes synaptic vesicles containing neurotransmitters (e.g. acetylcholine) to fuse with the membrane and release their contents by exocytosis.

化学突触由突触前末梢、突触间隙（约 20 nm 宽）和突触后膜组成。动作电位到达时，突触前小结的电压门控 Ca²⁺ 通道打开。Ca²⁺ 内流使含有神经递质（如乙酰胆碱）的突触小泡与膜融合，通过胞吐释放递质。

Neurotransmitter molecules diffuse across the cleft and bind to specific receptor proteins on the postsynaptic membrane. This binding opens ligand‑gated ion channels (e.g. Na⁺ channels), leading to a local excitatory or inhibitory postsynaptic potential (EPSP or IPSP). Acetylcholine in the neuromuscular junction is always excitatory, whereas GABA is inhibitory in the brain.

神经递质分子在间隙内扩散，与突触后膜上的特异性受体蛋白结合。结合打开配体门控离子通道（如 Na⁺ 通道），产生局部兴奋性或抑制性突触后电位（EPSP 或 IPSP）。神经‑肌肉接头处的乙酰胆碱始终为兴奋性，而大脑中的 GABA 则为抑制性。

7. Summation and integration of signals | 信号的总和与整合

A single presynaptic release usually generates an EPSP smaller than the threshold. To fire an action potential, the postsynaptic neurone must receive multiple inputs. Spatial summation occurs when several presynaptic neurones release neurotransmitter simultaneously onto the same postsynaptic cell. Temporal summation happens when the same presynaptic neurone fires repeatedly in quick succession, allowing EPSPs to build up before they decay.

单次突触前释放通常产生的 EPSP 远低于阈值。要使突触后神经元发放动作电位，须接受多个输入。空间总和指多个突触前神经元同时向同一突触后细胞释放递质；时间总和指同一突触前神经元快速连续放电，使 EPSP 在其衰减前累加起来。

Inhibitory synapses release neurotransmitters (like GABA) that open Cl⁻ or K⁺ channels, hyperpolarising the membrane or stabilising it near the resting potential. The overall outcome at the axon hillock depends on the integration of all excitatory and inhibitory inputs — often called synaptic integration.

抑制性突触释放递质（如 GABA），打开 Cl⁻ 或 K⁺ 通道，使膜超极化或稳定在静息电位附近。轴丘处的最终结果取决于所有兴奋性与抑制性输入的综合——常被称为突触整合。

8. Cholinergic synapses and their modulation | 胆碱能突触及其调控

At cholinergic synapses, acetylcholine (ACh) is the transmitter. Once released and bound to receptors, ACh is rapidly broken down by acetylcholinesterase (AChE) in the cleft. The products, acetate and choline, are reuptaken by the presynaptic terminal and recycled. This rapid inactivation prevents continuous stimulation and ensures the signal is brief and precise.

在胆碱能突触中，乙酰胆碱 (ACh) 作为递质。释放并与受体结合后，ACh 迅速被间隙中的乙酰胆碱酯酶 (AChE) 分解。产物乙酸和胆碱被突触前末梢回收再利用。这种快速失活可防止持续刺激，确保信号短暂而精确。

Many drugs and toxins target cholinergic synapses. For example, neonicotinoids are insecticides that bind irreversibly to postsynaptic ACh receptors in insects, causing paralysis. Botulinum toxin prevents exocytosis of ACh, leading to flaccid paralysis. Sarin, a nerve gas, inhibits AChE, causing ACh to accumulate and produce sustained muscle contraction — a classic exam application.

许多药物和毒素以胆碱能突触为靶点。例如，新烟碱类杀虫剂不可逆地与昆虫突触后 ACh 受体结合，导致麻痹。肉毒杆菌毒素阻止 ACh 的胞吐，引致弛缓性麻痹。神经毒剂沙林抑制 AChE，致 ACh 积累并引起持续肌肉收缩——这是典型的考试应用题。

9. The mammalian nervous system: CNS and PNS | 哺乳动物神经系统：中枢与周围神经系统

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 cranial and spinal nerves. The PNS is further subdivided into the sensory (afferent) pathway, carrying impulses towards the CNS, and the motor (efferent) pathway, transmitting impulses away from the CNS to effectors.

人体神经系统分为中枢神经系统 (CNS)，包括脑和脊髓，以及周围神经系统 (PNS)，由脑神经和脊神经组成。PNS 进一步分为将冲动传往 CNS 的感觉（传入）通路，和将冲动从 CNS 传到效应器的运动（传出）通路。

The motor pathway comprises the somatic nervous system (voluntary control of skeletal muscle) and the autonomic nervous system (involuntary regulation of internal organs). The latter is split into the sympathetic and parasympathetic divisions, which generally have antagonistic effects — e.g. sympathetic activation increases heart rate, while parasympathetic activation decreases it.

运动通路包括躯体神经系统（骨骼肌的随意控制）和自主神经系统（内脏器官的非随意调节）。后者分为交感神经和副交感神经分支，通常作用相反——例如，交感神经激活可加快心率，而副交感神经则减慢心率。

10. Reflex arcs and their adaptive value | 反射弧及其适应价值

A reflex arc is the simplest neural pathway mediating a rapid, automatic response to a stimulus. The classic knee‑jerk reflex involves sensory neurones detecting stretch in the quadriceps muscle, transmitting an impulse directly to a motor neurone in the spinal cord (via a single synapse), which then stimulates muscle contraction. This is an example of a monosynaptic reflex.

反射弧是最简单的神经通路，介导对刺激的快速自动反应。经典的膝跳反射涉及感觉神经元检测股四头肌的牵拉，将冲动直接通过单突触传递给脊髓中的运动神经元，后者刺激肌肉收缩。此为单突触反射的例子。

More complex reflexes, such as the withdrawal reflex, involve connector (relay) neurones in the spinal cord that integrate information and coordinate responses — e.g. simultaneously stimulating flexor muscles and inhibiting extensors (reciprocal inhibition). Reflexes are not dependent on brain input, which allows for extremely rapid responses essential for survival.

更复杂的反射，如缩手反射，涉及脊髓中的连接（中间）神经元整合信息并协调反应——例如同时刺激屈肌、抑制伸肌（交互抑制）。反射不依赖大脑参与，可产生极快速反应，对生存至关重要。

11. Recap of key equations and values | 核心公式与数值回顾

For IB and OCR, you are not required to perform complex calculations on the Goldman equation, but you should know the resting membrane potential value and typical changes during an action potential. The Nernst equation for equilibrium potential of an ion X is sometimes introduced conceptually:

IB 和 OCR 考试不要求对 Goldman 方程进行复杂计算，但应掌握静息膜电位值及动作电位期间的典型变化。有时会概念性引入某离子 X 平衡电位的 Nernst 方程：

Eₓ = (RT / zF) ln([X⁺]ₒᵤₜ / [X⁺]ᵢₙ)

However, emphasis is placed on describing the contributions of the Na⁺/K⁺ pump and differential permeability. Key numerical facts to remember include: resting potential around −70 mV; threshold around −55 mV; peak action potential about +40 mV; and the duration of an action potential in neurones typically 1–2 ms.

但重点在于描述 Na⁺/K⁺ 泵和差异通透性的贡献。需记忆的关键数值包括：静息电位约 –70 mV；阈电位约 –55 mV；动作电位峰值约 +40 mV；神经元动作电位持续时长通常 1–2 ms。

12. Common pitfalls and exam strategies | 常见误区与应试策略

The most frequent mistake is to claim that the sodium‑potassium pump is directly responsible for depolarisation or repolarisation. Remember: the pump sets up the gradients and restores them over the long term, but the rapid electrical events are due to the passive opening of voltage‑gated channels. Another common error is confusing ‘saltatory conduction’ with ‘simple diffusion’ — always reference the nodes of Ranvier and the absence of myelin at these points.

最常犯的错误是声称钠钾泵直接导致去极化或复极化。请记住：泵建立浓度梯度并长期维持，但快速的电事件却因电压门控通道被动打开所致。另一常见误点是将「跳跃传导」与「简单扩散」混淆——必须提及朗飞结及此处无髓鞘。

When interpreting graphs of membrane potential change, always label the phases and state exactly which channels open, inactivate or close. In synaptic questions, use precise language: ‘neurotransmitter binds to post‑synaptic receptors’ and ’causes ligand‑gated ion channels to open’, not ‘it opens channels’. Finally, practise applying your knowledge to unfamiliar contexts, such as toxin action or genetic disorders affecting ion channels — these are favourite IB Paper 2 and OCR application questions.

在解读膜电位变化曲线图时，务必标注各阶段并准确说明何种通道打开、失活或关闭。作答突触问题时，使用精准语言：「神经递质与突触后受体结合」并「引起配体门控离子通道打开」，而非「打开通道」。最后，练习将知识应用于陌生情景，如毒素作用或影响离子通道的遗传疾病——这些是 IB Paper 2 和 OCR 应用题的常客。

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IB & OCR Biology: Nervous System Key Points | IB & OCR 生物：神经系统 考点精讲