📚 A-Level CCEA Biology: Nervous System | 神经系统考点精讲
The nervous system is a fundamental topic in CCEA A-Level Biology, enabling rapid communication and coordination within the body. This revision guide distils the essential knowledge on neuron structure, action potentials, synaptic transmission, reflex arcs, and the autonomic nervous system. Understanding these concepts is vital for tackling data-analysis questions and extended responses in your exams.
神经系统是CCEA A-Level生物学的核心主题，负责体内快速通讯与协调。本篇考点精讲浓缩了神经元结构、动作电位、突触传递、反射弧及自主神经系统等必备知识。掌握这些概念对于应对数据分析题和扩展回答题至关重要。
1. Neuron Structure and Function | 神经元的结构与功能
Neurones are the functional units of the nervous system. A typical motor neurone consists of a cell body (soma) containing the nucleus, branched dendrites that receive signals, and a long axon that transmits impulses away from the cell body. Many axons are wrapped in a myelin sheath formed by Schwann cells, which insulates the axon and speeds up transmission. Gaps in the sheath are called nodes of Ranvier.
神经元是神经系统的功能单位。典型的运动神经元由含细胞核的胞体、接受信号的树突、以及将冲动传离胞体的长轴突组成。许多轴突被施万细胞形成的髓鞘包裹，起绝缘作用并加速传导。髓鞘上不连续的裸露区称为朗飞结。
ic terminal. Sensory neurones carry impulses from receptors to the central nervous system (CNS), relay neurones connect sensory and motor neurones within the CNS, and motor neurones convey impulses from the CNS to effectors such as muscles or glands.
冲动传导方向为树突端 → 胞体 → 轴突 → 突触末梢。感觉神经元将冲动从感受器传至中枢神经系统（CNS），联络神经元在CNS内连接感觉与运动神经元，而运动神经元将冲动从CNS传至效应器，如肌肉或腺体。
- Sensory neurone: long dendron from receptor to cell body, short axon.
- 感觉神经元：自感受器至胞体的长树突，短轴突。
- Motor neurone: short dendrites, long axon to effector.
- 运动神经元：短树突，长轴突至效应器。
- Relay neurone: entirely within CNS, many short dendrites and a short axon.
- 联络神经元：完全位于CNS内，许多短树突和一条短轴突。
2. Resting Potential | 静息电位
When a neurone is not transmitting an impulse, the inside of the axon is negatively charged relative to the outside. This resting potential is approximately −70 mV. It is established and maintained by the unequal distribution of ions and the selective permeability of the membrane.
当神经元不传导冲动时，轴突内部相对外部带负电，这种静息电位约−70 mV。它由离子不均衡分布和膜的选择通透性建立并维持。
Sodium-potassium pumps actively transport 3 Na⁺ out of the axon for every 2 K⁺ brought in, using ATP. The membrane is much more permeable to K⁺ than to Na⁺ at rest, so K⁺ diffuses out down its concentration gradient through potassium leak channels. Large, negatively charged proteins inside the axon cannot cross the membrane, contributing to the negative internal potential.
钠-钾泵利用ATP主动转运，每泵出3个Na⁺便泵入2个K⁺。静息时膜对K⁺的通透性远高于Na⁺，因此K⁺经钾渗漏通道顺浓度梯度外流。轴突内部带负电的大分子蛋白质无法透过膜，进一步巩固负电内环境。
Resting potential = −70 mV, maintained by Na⁺/K⁺-ATPase and K⁺ leak channels
静息电位 = −70 mV，由Na⁺/K⁺-ATP酶和K⁺渗漏通道维持
3. Action Potential Generation | 动作电位的产生
An action potential is a rapid, temporary change in membrane potential that propagates along the axon. It is triggered when the membrane potential reaches a threshold (around −55 mV), causing voltage-gated sodium channels to open.
动作电位是膜电位沿轴突快速而暂时的变化。当膜电位达到阈电位（约−55 mV）时，电压门控钠通道开放，动作电位即被触发。
Depolarisation: Voltage-gated Na⁺ channels open, Na⁺ rushes into the axon down its electrochemical gradient. The inside becomes positive (up to +40 mV). This is the rising phase. Repolarisation: Na⁺ channels inactivate, and voltage-gated K⁺ channels open. K⁺ efflux restores the negative interior. Hyperpolarisation: K⁺ channels are slow to close, so the potential overshoots slightly below resting. The sodium-potassium pump restores the original ion distribution.
去极化：电压门控Na⁺通道开放，Na⁺顺电化学梯度涌入轴突，内部变为正电（可达+40 mV），形成升支。复极化：Na⁺通道失活，电压门控K⁺通道开放，K⁺外流使内部恢复负电。超极化：K⁺通道关闭较慢，电位稍低于静息值。随后钠-钾泵恢复离子分布。
The action potential obeys the all-or-nothing law: once threshold is reached, the full action potential is fired; sub-threshold stimuli produce no propagation. The refractory period (absolute and relative) ensures impulses travel in one direction and limits their frequency.
动作电位遵循全或无原理：一旦达到阈值，一个完整的动作电位便产生；阈下刺激不会引起传导。不应期（绝对和相对）确保冲动单向传导并限制其频率。
| Phase | Ion Movement | Membrane Potential |
|---|---|---|
| Depolarisation | Na⁺ influx | −70 mV → +40 mV |
| Repolarisation | K⁺ efflux | +40 mV → −70 mV |
| Hyperpolarisation | K⁺ efflux (continued) | < −70 mV |
| Restoration | Na⁺/K⁺ pump | −70 mV |
4. Propagation of Action Potentials | 动作电位的传导
Once generated, an action potential triggers the adjacent region of the membrane to depolarise through local currents. These are movement of ions between depolarised and resting zones, setting up a circuit that opens voltage-gated Na⁺ channels further along the axon.
动作电位一旦产生，即通过局部电流引发相邻膜区域去极化。局部电流是去极化区域与静息区域之间的离子移动，形成回路，并开启轴突下游的电压门控Na⁺通道。
In non-myelinated neurones, the action potential propagates as a continuous wave, which is relatively slow (0.5–2 m/s). In myelinated neurones, the myelin sheath acts as an insulator. Action potentials occur only at the nodes of Ranvier, where ion channels are concentrated. The impulse appears to jump from node to node — saltatory conduction — dramatically increasing speed (up to 120 m/s).
在无髓神经元中，动作电位以连续波的方式传导，速度较慢（0.5–2 m/s）。在有髓神经元中，髓鞘充当绝缘体。动作电位仅发生在离子通道集中的朗飞结，冲动呈跳跃式传递——跳跃传导——速度大幅提升（可达120 m/s）。
Factors increasing speed: larger axon diameter (lower internal resistance) and tighter myelination. Saltatory conduction is energetically efficient because fewer Na⁺/K⁺ pump actions are needed to re-establish gradients.
提升传导速度的因素：轴突直径较大（内部电阻低）以及髓鞘包裹致密。跳跃传导更为节能，因为恢复离子梯度所需的Na⁺/K⁺泵运作次数更少。
5. Synaptic Transmission | 突触传递
Synapses are junctions where a neurone communicates with another neurone or an effector. The most common type in vertebrates is the chemical synapse. The presynaptic neurone contains synaptic vesicles filled with neurotransmitter.
突触是一个神经元与另一个神经元或效应器通讯的连接处。脊椎动物中最常见的是化学突触。突触前神经元内含有充满神经递质的突触小泡。
Sequence of events:
1. An action potential arrives at the presynaptic terminal and depolarises the membrane.
2. Voltage-gated Ca²⁺ channels open, Ca²⁺ ions enter the cytoplasm.
3. Influx of Ca²⁺ triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (e.g., acetylcholine) by exocytosis into the synaptic cleft.
4. Neurotransmitter diffuses across the cleft and binds to specific receptor proteins on the postsynaptic membrane.
5. Ligand-gated ion channels (e.g., Na⁺ channels) open, allowing Na⁺ to enter, causing depolarisation — an excitatory postsynaptic potential (EPSP).
6. If the EPSP reaches threshold, a new action potential is initiated in the postsynaptic neurone.
事件顺序：
1. 动作电位到达突触前末梢并引发膜去极化。
2. 电压门控Ca²⁺通道开放，Ca²⁺进入胞质。
3. Ca²⁺内流促使突触小泡与突触前膜融合，通过胞吐作用将神经递质（如乙酰胆碱）释放到突触间隙。
4. 神经递质扩散越过间隙，与突触后膜上的特异受体蛋白结合。
5. 配体门控离子通道（如Na⁺通道）开放，Na⁺进入引起去极化，即兴奋性突触后电位（EPSP）。
6. 若EPSP达到阈值，则在突触后神经元上触发新的动作电位。
Synapses ensure unidirectional transmission because neurotransmitter receptors are only on the postsynaptic membrane. They can also integrate excitatory and inhibitory inputs (summation), enabling complex processing.
突触确保单向传递，因为神经递质受体仅位于突触后膜。它们还可整合兴奋性和抑制性输入（总和作用），实现复杂的信息处理。
6. Neurotransmitters and Cholinergic Synapses | 神经递质与胆碱能突触
Acetylcholine (ACh) is the neurotransmitter at neuromuscular junctions and many synapses in the brain and autonomic nervous system. A synapse that uses ACh is termed a cholinergic synapse.
乙酰胆碱（ACh）是神经肌肉接头以及脑、自主神经系统中许多突触使用的神经递质。使用ACh的突触称为胆碱能突触。
After ACh binds to receptors and produces an EPSP, it must be removed rapidly to stop continuous stimulation. The enzyme acetylcholinesterase, present in the synaptic cleft, hydrolyses ACh into acetate and choline. Choline is reabsorbed by the presynaptic neurone and recycled with acetyl CoA to form more ACh.
ACh与受体结合产生EPSP后，必须迅速被清除以终止持续刺激。突触间隙中的乙酰胆碱酯酶将ACh水解为乙酸和胆碱。胆碱被突触前神经元重吸收，与乙酰辅酶A再循环生成更多ACh。
Other neurotransmitters include noradrenaline, dopamine, serotonin, and GABA. GABA is typically an inhibitory neurotransmitter: its binding opens Cl⁻ channels, hyperpolarising the postsynaptic membrane (IPSP) and making it less likely to fire an action potential.
其他神经递质包括去甲肾上腺素、多巴胺、5-羟色胺和GABA。GABA通常为抑制性递质：其结合打开Cl⁻通道，使突触后膜超极化（IPSP），降低兴奋性。
Drugs and toxins can affect synaptic transmission. For example, organophosphate pesticides inhibit acetylcholinesterase, leading to excess ACh accumulation and overstimulation. Strychnine blocks inhibitory glycine receptors.
药物和毒素可影响突触传递。例如，有机磷农药抑制乙酰胆碱酯酶，导致ACh过多积累和过度刺激。士的宁阻断抑制性甘氨酸受体。
| Neurotransmitter | Type | Receptor/Effect |
|---|---|---|
| Acetylcholine | Excitatory | Nicotinic receptors, Na⁺ influx |
| GABA | Inhibitory | Cl⁻ influx, hyperpolarisation |
| Noradrenaline | Excitatory/Inhibitory | G-protein coupled receptors |
7. Central and Peripheral Nervous System | 中枢神经系统与外周神经系统
The central nervous system (CNS) consists of the brain and spinal cord. The spinal cord contains grey matter (mainly cell bodies and synapses) in the centre and white matter (myelinated axons) on the outside. It is protected by the meninges and cerebrospinal fluid.
中枢神经系统（CNS）由脑和脊髓组成。脊髓中央为灰质（主要为胞体和突触），外周为白质（有髓轴突）。脑脊膜和脑脊液提供保护。
The peripheral nervous system (PNS) comprises all nerves radiating from the CNS. It is divided into:
- Somatic nervous system: controls voluntary movements via motor neurones to skeletal muscles.
- Autonomic nervous system: regulates involuntary functions (heart rate, digestion, glandular secretion) and is further split into sympathetic and parasympathetic divisions.
外周神经系统（PNS）包括所有发自CNS的神经。它分为：
- 躯体神经系统：通过运动神经元控制骨骼肌的随意运动。
- 自主神经系统：调节不随意功能（心率、消化、腺体分泌），并进一步分为交感神经和副交感神经分支。
Sensory nerves carry impulses to the CNS, and mixed nerves contain both sensory and motor fibres. The dorsal root of a spinal nerve contains sensory neurones, while the ventral root contains motor neurones.
感觉神经将冲动传入CNS，而混合神经包含感觉和运动纤维。脊神经的背根含有感觉神经元，腹根含有运动神经元。
8. Reflex Arc | 反射弧
A reflex arc is the simplest functional pathway of the nervous system, producing a rapid, involuntary response to a stimulus without conscious thought. It consists of:
- Receptor detects stimulus.
- Sensory neurone transmits impulse to CNS.
- Relay neurone (intermediate) in spinal cord or brain stem.
- Motor neurone carries impulse to effector.
- Effector (muscle or gland) brings about the response.
反射弧是神经系统最简单的功能性通路，能对刺激产生快速、无意识的非自主反应。它包含：
- 感受器检测刺激。
- 感觉神经元将冲动传入CNS。
- 脊髓或脑干中的联络神经元（中间神经元）。
- 运动神经元将冲动传至效应器。
- 效应器（肌肉或腺体）作出反应。
A classic example is the knee-jerk reflex: tapping the patellar tendon stretches the quadriceps muscle spindle (receptor), generating an impulse that travels via a sensory neurone directly to the motor neurone in the spinal cord (monosynaptic arc), causing contraction of the quadriceps and extension of the leg. The brain is informed after the response occurs.
经典例子是膝跳反射：叩击髌腱牵拉股四头肌肌梭（感受器），产生的冲动经感觉神经元直达脊髓中的运动神经元（单突触反射弧），引起股四头肌收缩和小腿伸展。大脑在反应发生后获得通知。
Reflex arcs are protective — they minimise damage by bypassing higher brain centres. The withdrawal reflex (e.g., touching a hot object) is polysynaptic, incorporating relay neurones and often contralateral coordination (flexor contracts, extensor relaxes on opposite limb).
反射弧具有保护作用——绕过高级脑中枢以最小化损伤。缩手反射（如触摸热物）是多突触的，包含联络神经元，常伴有对侧协调（屈肌收缩、对侧肢体的伸肌松弛）。
9. Autonomic Nervous System | 自主神经系统
The autonomic nervous system (ANS) controls internal organs and glands. It has two antagonistic branches:
- Sympathetic system: prepares the body for 'fight or flight' — increases heart rate, dilates pupils, inhibits digestion, stimulates adrenaline release.
- Parasympathetic system: promotes 'rest and digest' — decreases heart rate, constricts pupils, stimulates digestion and salivation.
自主神经系统（ANS）调控内脏和腺体。它包含两个拮抗分支：
- 交感神经：使身体进入“战或逃”状态——加快心率、放大瞳孔、抑制消化、刺激肾上腺素分泌。
- 副交感神经：促进“休息与消化”——减慢心率、缩小瞳孔、刺激消化和唾液分泌。
Autonomic pathways use two neurones to reach the effector: a preganglionic neurone (from CNS) and a postganglionic neurone (to target organ). In sympathetic pathways, preganglionic neurones secrete ACh, while most postganglionic neurones release noradrenaline. In parasympathetic pathways, both preganglionic and postganglionic neurones release ACh.
自主通路使用两个神经元到达效应器：节前神经元（自CNS）和节后神经元（至靶器官）。交感通路中，节前神经元分泌ACh，多数节后神经元释放去甲肾上腺素。副交感通路中，节前和节后神经元均释放ACh。
| Parameter | Sympathetic | Parasympathetic |
|---|---|---|
| Heart rate | Increases | Decreases |
| Bronchioles | Dilate | Constrict |
| Digestion | Inhibited | Stimulated |
| Pupil | Dilates | Constricts |
| Saliva | Thick, viscous | Copious, watery |
| CNS origin | Thoraco-lumbar | Cranio-sacral |
10. Common Exam Pitfalls and Tips | 常见考试误区与技巧
Mistake 1: Confusing depolarisation and repolarisation ion movements. Always specify that depolarisation is due to Na⁺ influx through voltage-gated Na⁺ channels, while repolarisation is K⁺ efflux. Examiners expect precise ion names and direction.
误区1：混淆去极化与复极化的离子移动。务必指明去极化为Na⁺经电压门控Na⁺通道内流，复极化为K⁺外流。考官期望精确的离子名称和流动方向。
Mistake 2: Forgetting the refractory period role. The absolute refractory period prevents overlap of impulses and ensures unidirectional propagation. Mention it when explaining discrete impulses.
误区2：忽略不应期的作用。绝对不应期防止冲动重叠并确保单向传导。解释离散性冲动时务必提及。
Mistake 3: Omitting the role of Ca²⁺ in synaptic vesicles. Many students say 'action potential causes vesicle release' sequence must include depolarisation → Ca²⁺ channels open → Ca²⁺ influx → vesicle fusion.
误区3：遗漏Ca²⁺在突触小泡中的作用。许多学生说“动作电位引起小泡释放”，却未提Ca²⁺进入。必须包括：去极化 → Ca²⁺通道开放 → Ca²⁺内流 → 小泡融合的顺序。
Mistake 4: Mixing up myelinated and non-myelinated speed. Myelination increases propagation speed by saltatory conduction; larger diameter also helps. Cite numerical values if needed.
误区4：混淆有髓与无髓的速度。髓鞘通过跳跃传导提高传导速度；轴突直径较大也有帮助。必要时可引用数值。
Mistake 5: Neglecting the integration role of synapses. Synapses can summate EPSPs and IPSPs (temporal and spatial summation), allowing decision-making. Use appropriate terms.
误区5：忽视突触的整合功能。突触可总和EPSP和IPSP（时间和空间总和），实现决策。使用恰当术语。
For data-response questions, practice interpreting graphs of membrane potential changes and tables comparing autonomic effects. Always link structure to function — for example, explain how numerous voltage-gated Na⁺ channels at nodes allow saltatory conduction.
对于数据应答题，练习解读膜电位变化图表和比较自主神经效应的表格。始终联系结构与功能——例如，解释朗飞结处大量电压门控Na⁺通道如何实现跳跃传导。
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