📚 Nervous System: AQA A-Level Biology Revision Essentials | A-Level AQA 生物：神经系统 考点精讲
The nervous system enables rapid, precise communication between different parts of the body through electrical impulses and chemical signals. For AQA A-Level Biology, you must understand the structure of neurones, the ionic basis of the resting and action potentials, the mechanism of synaptic transmission, and the organisation of reflex arcs. This revision guide distils the essential points, pairing clear English explanations with matching Chinese translations to ensure deep, dual-language mastery of every key concept.
神经系统通过电冲动和化学信号实现身体不同部位之间快速、准确的通信。在 AQA A-Level 生物学中，你必须掌握神经元的结构、静息电位与动作电位的离子基础、突触传递的机制以及反射弧的组成。这份考点精讲提炼了核心要点，将清晰的英文解释与对应的中文译文配对，确保你以双语言充分掌握每个关键概念。
1. Overview of the Nervous System | 神经系统概述
The nervous system is divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all neurones that connect the CNS to the rest of the body. The PNS is further subdivided into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of internal organs).
神经系统分为中枢神经系统（CNS）和周围神经系统（PNS），中枢神经系统由脑和脊髓组成，周围神经系统则包括所有将中枢神经系统与身体其他部位连接起来的神经元。周围神经系统进一步分为躯体神经系统（对骨骼肌的随意控制）和自主神经系统（对内脏器官的不随意控制）。
The autonomic nervous system comprises the sympathetic and parasympathetic divisions, which generally have opposing effects on target organs. For example, sympathetic stimulation increases heart rate, while parasympathetic activity decreases it. This arrangement allows fine-tuned, rapid responses to internal and external stimuli.
自主神经系统包含交感神经和副交感神经分支，两者对靶器官通常产生相反的效应。例如，交感神经兴奋会使心率加快，而副交感神经活动则使心率减慢。这种安排使得身体能够对内外环境刺激做出精细、快速的反应。
Information flows through the nervous system along dedicated pathways. A stimulus is detected by receptors, transformed into an electrical impulse, transmitted along sensory neurones to the CNS, integrated, and then relayed via motor neurones to effectors such as muscles or glands.
信息沿着特定通路在神经系统中流动。刺激被感受器检测到，转化为电冲动，沿感觉神经元传至中枢神经系统，经过整合，再通过运动神经元传递到肌肉或腺体等效应器。
2. Structure of Neurones | 神经元的结构
A typical motor neurone possesses a large cell body (soma) containing the nucleus and major organelles, numerous branching dendrites that receive signals, a long axon that carries impulses away from the cell body, and synaptic terminals that form junctions with other cells. Many axons are wrapped in a myelin sheath, which insulates the axon and speeds up impulse conduction.
一个典型的运动神经元具有一个大型的细胞体（胞体），其中含有细胞核和主要细胞器；众多分支的树突用于接收信号；一条长轴突将冲动从细胞体传出；以及突触末梢，与其他细胞形成连接。许多轴突被髓鞘包裹，髓鞘可绝缘轴突并加快冲动传导。
Sensory neurones have the cell body located in a ganglion outside the CNS and typically have a single long dendron that carries the impulse from the receptor towards the cell body, and a shorter axon that continues into the CNS. Relay neurones (interneurones) lie entirely within the CNS and connect sensory and motor pathways.
感觉神经元的细胞体位于中枢神经系统之外的神经节中，通常具有一条长树突，将冲动从感受器传向细胞体，以及一条较短的轴突进入中枢神经系统。中间神经元（联络神经元）完全位于中枢神经系统内，连接感觉和运动通路。
The myelin sheath is formed by Schwann cells in the PNS and by oligodendrocytes in the CNS. Between adjacent Schwann cells are small gaps called nodes of Ranvier, where the axon membrane is exposed. These nodes are crucial for saltatory conduction.
周围神经系统中的髓鞘由雪旺细胞形成，中枢神经系统则由少突胶质细胞形成。相邻雪旺细胞之间的小间隙称为郎飞结，此处轴突膜裸露。这些结对于跳跃传导至关重要。
3. Resting Potential Maintenance | 静息电位的维持
When a neurone is not transmitting an impulse, the inside of its axon is negatively charged relative to the outside; this resting potential is typically about −70 mV. The potential difference is mainly due to the unequal distribution of Na⁺ and K⁺ ions across the membrane, maintained by the sodium–potassium pump and differential membrane permeability.
当神经元不传递冲动时，其轴突内部相对于外部带负电荷；这种静息电位通常约为 −70 mV。该电位差主要源于 Na⁺ 和 K⁺ 离子在膜两侧的不均匀分布，并由钠钾泵及膜通透性差异共同维持。
The sodium–potassium pump actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell, using ATP. This creates a steep concentration gradient: Na⁺ concentration is high outside and low inside, while K⁺ is high inside and low outside. The resting membrane is much more permeable to K⁺ than to Na⁺ because of the presence of many open potassium leak channels.
钠钾泵利用 ATP 主动将 3 个 Na⁺ 运出细胞，同时将 2 个 K⁺ 运入细胞。这产生了陡峭的浓度梯度：Na⁺ 外高内低，而 K⁺ 内高外低。由于有大量开放的钾泄漏通道，静息膜对 K⁺ 的通透性远高于对 Na⁺ 的通透性。
As K⁺ ions diffuse out of the cell down their concentration gradient, they leave behind impermeant negative anions (such as proteins), making the inside of the membrane increasingly negative. Eventually, the electrical gradient pulling K⁺ back in balances the chemical gradient driving K⁺ out, establishing the equilibrium potential for K⁺ and a stable resting potential.
K⁺ 离子沿浓度梯度向细胞外扩散时，留下了不可通透的负离子（如蛋白质），使得膜内侧负电性逐渐增强。最终，将 K⁺ 拉回的电化学梯度与推动 K⁺ 外流的化学梯度达到平衡，即建立了 K⁺ 的平衡电位和稳定的静息电位。
4. Generation of Action Potentials | 动作电位的产生
An action potential is a brief, rapid reversal in membrane potential that occurs when a neurone is stimulated above a threshold level, usually around −55 mV. It follows an all-or-nothing principle: once threshold is reached, a full action potential is always generated; if the stimulus is sub-threshold, no action potential occurs.
动作电位是一个短暂、快速的膜电位反转，当神经元受到的刺激超过阈值（通常约为 −55 mV）时发生。它遵循全或无定律：一旦达到阈值，必然产生一个完整的动作电位；如果刺激低于阈值，则不会产生动作电位。
When the membrane depolarises to threshold, voltage-gated Na⁺ channels open. Na⁺ ions rush into the cell down both their concentration and electrical gradients, causing rapid depolarisation. The membrane potential shoots up to about +40 mV. At the peak, Na⁺ channels inactivate and voltage-gated K⁺ channels open.
当膜去极化达到阈值时，电压门控 Na⁺ 通道开放。Na⁺ 离子沿浓度梯度和电梯度迅速涌入细胞，导致快速去极化，膜电位上升至约 +40 mV。在峰值处，Na⁺ 通道失活，电压门控 K⁺ 通道开放。
With K⁺ channels open, K⁺ ions flow out of the cell, repolarising the membrane. Because the K⁺ channels close slowly, there is often a temporary overshoot called hyperpolarisation (more negative than the resting potential). The sodium–potassium pump and the resting permeability then restore the original ionic distribution.
K⁺ 通道开放后，K⁺ 离子外流，使膜复极化。由于 K⁺ 通道关闭较慢，常会出现短暂的超极化（比静息电位更负）。随后钠钾泵和静息通透性恢复原有的离子分布。
The refractory period, during which a new action potential cannot be generated, ensures that impulses travel in one direction and limits the maximum frequency of firing. The absolute refractory period corresponds to Na⁺ channel inactivation; the relative refractory period occurs while the membrane is hyperpolarised.
不应期（在此期间不能产生新的动作电位）确保了冲动单向传导并限制了最高发放频率。绝对不应期对应于 Na⁺ 通道失活状态；相对不应期发生在膜超极化时。
5. Propagation of Action Potentials | 动作电位的传导
An action potential is propagated along an axon because the localised depolarisation triggers the opening of adjacent voltage-gated Na⁺ channels. This creates a wave of depolarisation that moves away from the cell body. In non-myelinated axons, this continuous conduction is relatively slow.
动作电位沿轴突传播，是因为局部的去极化会触发邻近的电压门控 Na⁺ 通道开放，形成去极化波从细胞体向外移动。在无髓轴突中，这种连续传导相对较慢。
In myelinated axons, ion exchange can only occur at the nodes of Ranvier, where the axon membrane is exposed. The myelin sheath insulates the internodal regions, allowing the impulse to ‘jump’ from node to node. This is called saltatory conduction and is much faster and more energy-efficient than continuous conduction.
在有髓轴突中，离子交换只能在郎飞结处发生，此处的轴突膜裸露。髓鞘使结间区绝缘，从而使冲动能够从一个结“跳跃”到下一个结。这称为跳跃传导，比连续传导快得多且更节能。
The speed of conduction also depends on axon diameter: wider axons have lower internal resistance, so impulses travel faster. Thus, myelinated axons of large diameter achieve the highest conduction velocities, essential for rapid reflex responses and coordinated movement.
传导速度也取决于轴突直径：较粗的轴突内部电阻较低，因而冲动传导更快。因此，有髓鞘的大直径轴突能达到最高的传导速度，这对快速反射反应和协调运动至关重要。
6. The Cholinergic Synapse | 胆碱能突触的结构
A synapse is the junction between two neurones, or between a neurone and an effector. A cholinergic synapse uses the neurotransmitter acetylcholine (ACh). The presynaptic knob contains many mitochondria and synaptic vesicles filled with ACh. The postsynaptic membrane carries specific receptor proteins, and the synaptic cleft is the narrow gap between them.
突触是两个神经元之间或神经元与效应器之间的连接点。胆碱能突触使用神经递质乙酰胆碱 (ACh)。突触前扣结含有大量线粒体和装满 ACh 的突触囊泡。突触后膜上带有特异性受体蛋白，两者之间的狭窄间隙为突触间隙。
Voltage-gated Ca²⁺ channels are located in the presynaptic membrane. When an action potential arrives, these channels open, allowing Ca²⁺ to enter the knob. The influx of Ca²⁺ triggers the movement of vesicles to the membrane, where they fuse and release ACh by exocytosis.
电压门控 Ca²⁺ 通道位于突触前膜上。当动作电位到达时，这些通道开放，使 Ca²⁺ 进入突触前扣结。Ca²⁺ 的内流促使囊泡向膜移动、融合，并通过胞吐作用释放 ACh。
ACh diffuses across the synaptic cleft and binds to receptor sites on the postsynaptic membrane. These receptors are associated with ligand-gated Na⁺ channels, also called nicotinic acetylcholine receptors. Binding causes the channels to open, letting Na⁺ flow into the postsynaptic cell.
ACh 扩散穿过突触间隙，与突触后膜上的受体位点结合。这些受体与配体门控 Na⁺ 通道（也称为烟碱型乙酰胆碱受体）相关联。结合导致通道开放，使 Na⁺ 流入突触后细胞。
7. Transmission Across a Synapse | 突触传递过程
Synaptic transmission begins when an action potential reaches the presynaptic terminal. Voltage-gated Ca²⁺ channels open, and Ca²⁺ ions enter. The increase in intracellular Ca²⁺ concentration triggers synaptic vesicles to dock with the presynaptic membrane and release ACh into the cleft. The amount of neurotransmitter released is proportional to the Ca²⁺ influx.
当动作电位到达突触前末梢时，突触传递开始。电压门控 Ca²⁺ 通道开放，Ca²⁺ 离子进入。细胞内 Ca²⁺ 浓度升高触发突触囊泡与突触前膜对接，并将 ACh 释放到间隙中。释放的神经递质量与 Ca²⁺ 内流量成正比。
After diffusing across the cleft, ACh binds to complementary receptors on the postsynaptic membrane. This opens chemically gated Na⁺ channels, generating an excitatory postsynaptic potential (EPSP) due to the inward Na⁺ current. If sufficient EPSPs sum to reach threshold, a new action potential is initiated in the postsynaptic neurone.
ACh 扩散穿过间隙后，与突触后膜上互补的受体结合。这打开了化学门控 Na⁺ 通道，由于 Na⁺ 内流而产生兴奋性突触后电位 (EPSP)。如果有足够多的 EPSP 总和达到阈值，就会在突触后神经元中引发新的动作电位。
To prevent continuous stimulation, the enzyme acetylcholinesterase located in the synaptic cleft rapidly hydrolyses ACh into acetate and choline. The choline is taken back into the presynaptic neurone and reused. This rapid breakdown ensures discrete, short-lived signals.
为防止持续刺激，突触间隙中的乙酰胆碱酯酶会迅速将 ACh 水解为乙酸和胆碱。胆碱被重新摄取到突触前神经元并重复利用。这种快速分解确保了信号是离散且短暂的。
Synaptic delay, the time taken for neurotransmitter release, diffusion and receptor binding (about 0.5–1 ms), accounts for slower transmission along polysynaptic pathways. However, this delay is the cost of integration, plasticity and unidirectional flow of information.
突触延搁是指神经递质释放、扩散及与受体结合所需的时间（约 0.5–1 ms），这是多突触通路传导较慢的原因。然而，这种延搁也是神经系统整合、可塑性和信息单向传递的代价。
8. Excitatory and Inhibitory Signals | 兴奋性与抑制性信号
Synapses can be excitatory or inhibitory, depending on the neurotransmitter and the ion channels it opens. Excitatory synapses, such as most cholinergic synapses, cause Na⁺ influx, depolarising the postsynaptic membrane and bringing it closer to threshold. Inhibitory synapses typically open Cl⁻ or K⁺ channels, making the inside more negative and less likely to fire.
突触可以是兴奋性的或抑制性的，取决于神经递质及其打开的离子通道。兴奋性突触（如多数胆碱能突触）引起 Na⁺ 内流，使突触后膜去极化并向阈值靠近。抑制性突触通常开放 Cl⁻ 或 K⁺ 通道，使膜内侧更负，从而更不容易产生动作电位。
A single postsynaptic neurone may receive thousands of synaptic inputs. The decision to fire an action potential is determined by the spatial and temporal summation of EPSPs and inhibitory postsynaptic potentials (IPSPs). Spatial summation adds signals from different synapses, while temporal summation adds signals arriving in rapid succession from the same synapse.
一个突触后神经元可能接收成千上万个突触输入。是否发放动作电位取决于 EPSP 和抑制性突触后电位 (IPSP) 的空间总和与时间总和。空间总和将来自不同突触的信号相加，而时间总和则将来自同一突触、快速相继到达的信号相加。
If the combined depolarisation at the axon hillock reaches the threshold voltage, an action potential is generated. This integrative ability allows the nervous system to process complex information and produce appropriate outputs, making the synapse a crucial decision-making unit.
如果轴丘处的总去极化达到阈值电压，就会产生动作电位。这种整合能力使得神经系统能够处理复杂信息并产生恰当的输出，使突触成为关键的决策单元。
9. The Neuromuscular Junction | 神经肌肉接头
The neuromuscular junction (NMJ) is a specialised cholinergic synapse between a motor neurone and a skeletal muscle fibre. While it shares many features with inter-neuronal synapses, it has several distinctive adaptations. The postsynaptic membrane, called the motor end plate, is highly folded to increase surface area and receptor density.
神经肌肉接头 (NMJ) 是运动神经元与骨骼肌纤维之间的一种特化的胆碱能突触。它与神经元间突触有许多共同特点，但也有一些独特的适应性变化。突触后膜即运动终板，高度折叠以增加表面积和受体密度。
When an action potential arrives at the presynaptic terminal of the NMJ, the mechanism of Ca²⁺ entry, vesicle fusion and ACh release is the same as in a cholinergic synapse. However, the postsynaptic response is always excitatory and large: the end-plate potential (EPP) is much larger than a typical EPSP, ensuring that it always reaches threshold and triggers a muscle action potential.
当动作电位到达 NMJ 的突触前末梢时，Ca²⁺ 进入、囊泡融合和 ACh 释放的机制与胆碱能突触相同。但突触后反应始终是兴奋性的且幅度很大：终板电位 (EPP) 远大于典型的 EPSP，确保总能达到阈值并引发肌动作电位。
The comparison between a typical cholinergic synapse and the neuromuscular junction is summarised below:
| Feature / 特征 | Cholinergic Synapse / 胆碱能突触 | Neuromuscular Junction / 神经肌肉接头 |
| Postsynaptic cell / 突触后细胞 | Another neurone or effector / 另一个神经元或效应器 | Skeletal muscle fibre / 骨骼肌纤维 |
| Postsynaptic membrane structure / 突触后膜结构 | Relatively flat, fewer receptors / 相对平坦，受体较少 | Highly folded motor end plate, high density of nicotinic receptors / 高度折叠的运动终板，高密度烟碱型受体 |
| Nature of response / 反应性质 | Excitatory or inhibitory; small EPSPs / 兴奋性或抑制性；小的 EPSP | Always excitatory; large EPP always reaches threshold / 始终兴奋性；大的 EPP 总能达到阈值 |
| Effect of ACh / ACh 的作用 | May generate EPSP or IPSP / 可产生 EPSP 或 IPSP | Always generates a muscle action potential / 总是产生肌动作电位 |
| Termination of signal / 信号终止 | Acetylcholinesterase in cleft / 间隙中的乙酰胆碱酯酶 | Acetylcholinesterase embedded in the motor end plate / 嵌入运动终板的乙酰胆碱酯酶 |
The above table highlights that while both junctions rely on ACh, the NMJ is designed for reliable 1:1 transmission, ensuring that every motor neurone impulse results in muscle contraction, whereas synaptic transmission between neurones allows for integration and modulation.
上表强调了虽然两种接头都依赖 ACh，但 NMJ 旨在实现可靠的 1:1 传递，确保每个运动神经元冲动都引起肌肉收缩；而神经元间的突触传递则允许进行整合与调控。
10. The Reflex Arc | 反射弧
A reflex arc is the simplest functional pathway in the nervous system, enabling rapid, involuntary responses to a stimulus without conscious thought. A classical spinal reflex arc involves a receptor, a sensory neurone, a relay neurone in the spinal cord, a motor neurone and an effector (usually a muscle).
反射弧是神经系统中最简单的功能通路，能在没有意识参与的情况下对刺激做出快速、不随意的反应。经典的脊髓反射弧包括感受器、感觉神经元、脊髓中的中间神经元、运动神经元和效应器（通常是肌肉）。
In a reflex such as the withdrawal reflex, a painful stimulus is detected by receptors in the skin. Impulses travel along a sensory neurone to the spinal cord, where they enter the dorsal root. The sensory neurone synapses with a relay neurone, which then synapses with a motor neurone leaving via the ventral root to stimulate an effector muscle, causing withdrawal
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