📚 A-Level Biology: Generation and Conduction of Nerve Impulses | A-Level 生物：神经冲动的产生与传导
Nerve impulses are the fundamental language of the nervous system. Understanding how a neuron generates and conducts an action potential is a core topic in CIE A-Level Biology, linking membrane transport, ion channels, and bioelectrical properties.
神经冲动是神经系统的基本语言。理解神经元如何产生并传导动作电位，是 CIE A-Level 生物学的核心内容，它将膜运输、离子通道与生物电特性联系在一起。
1. Resting Potential | 静息电位
In an unstimulated neuron, the membrane is polarised, meaning there is a potential difference across it. The resting potential is typically about -70 mV, with the inside of the axon being negatively charged relative to the outside.
在未受刺激的神经元中，膜处于极化状态，即膜两侧存在电位差。静息电位通常约为 -70 mV，轴突内部相对于外部带负电。
This resting potential is established and maintained by three key mechanisms:
静息电位由三个关键机制建立并维持：
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The sodium-potassium pump (Na⁺/K⁺ ATPase) actively transports 3 Na⁺ ions out of the axon and 2 K⁺ ions into the axon, using energy from ATP. This creates unequal ion concentrations.
钠钾泵（Na⁺/K⁺ ATPase）利用 ATP 水解释放的能量，主动将 3 个 Na⁺ 泵出轴突，同时将 2 个 K⁺ 泵入轴突，从而建立离子浓度差。
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The axon membrane has many potassium leak channels but relatively few sodium leak channels, so the membrane is much more permeable to K⁺ than to Na⁺. Potassium ions diffuse out of the cell down their concentration gradient, carrying positive charge out and making the inside negative.
轴突膜上存在大量钾漏通道，而钠漏通道相对较少，因此膜对 K⁺ 的通透性远高于 Na⁺。钾离子顺着浓度梯度扩散出细胞，带走正电荷，使细胞内相对变负。
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Large anions such as negatively charged proteins and organic phosphates remain inside the axon and cannot cross the membrane, contributing further to the negative charge inside.
大的阴离子如带负电的蛋白质和有机磷酸根无法穿过膜而留在轴突内，进一步增加了细胞内的负电荷。
At rest, the outward leak of K⁺ is balanced by the electrical attraction of the negative interior, so the membrane potential remains stable at -70 mV.
静息时，K⁺ 的外漏与细胞内部负电荷对 K⁺ 的静电吸引力相平衡，因此膜电位稳定在 -70 mV。
2. The Action Potential | 动作电位
An action potential is a rapid, temporary change in membrane potential, from about -70 mV to approximately +40 mV and back. It occurs when a neuron is stimulated past a threshold value of about -55 mV.
动作电位是膜电位的快速而短暂的变化，从约 -70 mV 变为约 +40 mV，然后再恢复。当神经元受到超过阈值约 -55 mV 的刺激时，就会发生动作电位。
The generation of an action potential involves several stages:
动作电位的产生涉及以下几个阶段：
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Depolarisation: When the stimulus reaches threshold, voltage-gated sodium channels open. Na⁺ flows rapidly into the axon down both the concentration and electrochemical gradients. This influx of positive charge makes the inside less negative, and the membrane potential rapidly rises to about +40 mV.
去极化：当刺激达到阈值时，电压门控钠通道打开。Na⁺ 顺着浓度梯度和电化学梯度快速流入轴突。正电荷的内流使细胞内负电荷减少，膜电位迅速升高至约 +40 mV。
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Overshoot: The membrane potential briefly reverses polarity, with the inside becoming positive relative to the outside.
超射：膜电位短暂反转极性，细胞内相对于外部变为正电。
Membrane potential during action potential: -70 mV → +40 mV → -70 mV
动作电位期间膜电位：-70 mV → +40 mV → -70 mV
3. Repolarisation and the Refractory Period | 复极化与不应期
Repolarisation restores the negative charge inside the axon. At the peak of the action potential, voltage-gated sodium channels become inactivated, while voltage-gated potassium channels open. K⁺ diffuses rapidly out of the axon, making the inside negative again.
复极化恢复轴突内部的负电状态。在动作电位峰值时，电压门控钠通道失活，而电压门控钾通道打开。K⁺ 快速扩散出轴突，使内部重新变负。
Hyperpolarisation: Because the potassium channels open slowly and remain open longer than necessary, more K⁺ leaves than is needed to restore -70 mV. The membrane potential temporarily falls below -70 mV, to about -80 mV, before the potassium channels close and the sodium-potassium pump restores the original resting potential.
超极化：由于钾通道打开较慢且关闭延迟，过多的 K⁺ 外流使膜电位暂时低于 -70 mV，降至约 -80 mV。随后钾通道关闭，钠钾泵使膜电位恢复正常静息值。
Refractory period follows depolarisation and has two phases:
不应期在去极化之后出现，包含两个阶段：
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Absolute refractory period: During depolarisation and most of repolarisation, the sodium channels are either open or inactivated. A second action potential cannot be triggered, no matter how strong the stimulus.
绝对不应期：在去极化和大部分复极化期间，钠通道要么开放，要么处于失活状态。无论刺激多强，都不能触发第二个动作电位。
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Relative refractory period: During the final phase of repolarisation, some sodium channels have recovered. A second action potential can be generated, but only by a stimulus stronger than normal because the membrane is hyperpolarised.
相对不应期：在复极化末期，部分钠通道已恢复。此时可以产生第二个动作电位，但需要比正常更强的刺激，因为膜处于超极化状态。
The refractory period ensures that action potentials are discrete, travel in one direction, and cannot fuse.
不应期确保动作电位是分立的、单向传导的，并且不能融合。
4. All-or-Nothing Principle | 全或无定律
Action potentials obey the all-or-nothing principle. If the stimulus is weaker than threshold, no action potential occurs. If the stimulus reaches threshold, a full-sized action potential is produced, with the same maximum amplitude regardless of stimulus strength.
动作电位遵循“全或无”定律。如果刺激低于阈值，则不产生动作电位；如果刺激达到阈值，则产生一个完整幅度的动作电位，其最大振幅与刺激强度无关。
This means that the strength of a stimulus is not encoded by the size of an individual action potential, but by the frequency of action potentials produced in a given time.
这意味着刺激强度并不通过单个动作电位的幅度来编码，而是通过一定时间内产生动作电位的频率来编码。
Stimulus below threshold → no response; stimulus at threshold → full action potential
低于阈值的刺激 → 无反应；达到阈值的刺激 → 产生完整动作电位
5. Propagation of an Action Potential | 动作电位的传导
An action potential is conducted along an axon by local currents. When depolarisation occurs at one region of the membrane, the inside of the axon becomes temporarily positive relative to the adjacent resting regions.
动作电位通过局部电流沿轴突传导。当膜某一区域发生去极化时，轴突内部相对于相邻静息区域暂时变正。
Positive charges (mainly Na⁺) spread along the inside of the axon by diffusion, creating local electrical circuits that depolarise the next region of membrane. This causes voltage-gated sodium channels in that adjacent region to open, triggering a new action potential.
正电荷（主要是 Na⁺）通过扩散沿轴突内部传播，形成局部电流回路，使相邻区域膜去极化。这导致该相邻区域的电压门控钠通道打开，触发新的动作电位。
The action potential therefore travels as a wave of depolarisation. The refractory period ensures that this wave cannot move backwards, because the membrane behind the active region is still in its refractory state and cannot respond.
因此，动作电位作为去极化波沿轴突传播。不应期确保该波不能反向传播，因为活动区域后方的膜仍处于不应状态而无法响应。
Conduction is also described as the movement of a "zone of depolarisation" along the axon, rather than the movement of ions over long distances.
传导也可描述为“去极化区”沿轴突移动，而不是离子长距离移动。
6. Saltatory Conduction | 跳跃式传导
In the mammalian nervous system, many axons are wrapped in myelin sheaths produced by Schwann cells. Myelin is a lipid-rich insulating layer that surrounds the axon in segments.
在哺乳动物神经系统中，许多轴突外包裹着由施万细胞产生的髓鞘。髓鞘是富含脂质的绝缘层，分段包绕轴突。
The myelin sheath prevents ion movement across the membrane except at unmyelinated gaps called nodes of Ranvier, where voltage-gated sodium channels are concentrated.
髓鞘阻止离子跨膜移动，只有郎飞结处的无髓鞘间隙例外，该处集中分布着电压门控钠通道。
As a result, depolarisation cannot spread through the myelinated regions. Instead, local currents jump from one node to the next, and action potentials are only regenerated at each node.
因此，去极化不能通过髓鞘区域传播。相反，局部电流从一个郎飞结跳到下一个郎飞结，动作电位只出现在每个结处。
This process is called saltatory conduction because the impulse appears to "jump" along the axon.
这个过程被称为跳跃式传导，因为冲动似乎在轴突上“跳跃”前进。
Saltatory conduction has two major advantages:
跳跃式传导有两个主要优势：
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Conduction is much faster than in unmyelinated axons of the same diameter.
在同一直径下，传导速度远快于无髓鞘轴突。
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It saves energy, because only the nodes need to restore ion gradients, so less Na⁺ and K⁺ are exchanged per action potential.
它节省能量，因为只有郎飞结需要恢复离子梯度，因此每次动作电位交换的 Na⁺ 和 K⁺ 更少。
7. Factors Affecting Conduction Speed | 影响传导速度的因素
Several structural and physiological factors determine how quickly an action potential is conducted along an axon.
多个结构和生理因素决定动作电位沿轴突传导的快慢。
| Factor | 因素 | Effect on conduction speed | 对传导速度的影响 |
| Myelination | 髓鞘 | Increases speed by saltatory conduction. | 通过跳跃式传导增加速度。 |
| Axon diameter | 轴突直径 | Larger diameter offers less resistance to local currents, so conduction is faster. | 直径越大，对局部电流的阻力越小，传导越快。 |
| Temperature | 温度 | Higher temperature increases rate of ion diffusion and ATPase activity, up to an optimum; beyond that, enzymes denature. | 温度升高增加离子扩散速率和 ATP 酶活性，但超过最适温度后，酶会变性。 |
8. Axon Diameter and Temperature | 轴突直径与温度
Axon diameter: In unmyelinated axons, a larger diameter reduces the electrical resistance of the cytoplasm, allowing local currents to spread further along the axon. This means the next region reaches threshold more quickly, increasing conduction velocity. Large, fast-conducting axons are found in reflex arcs, such as the squid giant axon used in early studies of the action potential.
轴突直径：在无髓鞘轴突中，直径越大，细胞质电阻越小，局部电流传播得更远。这样相邻区域能更快达到阈值，从而增加传导速度。大的、快速传导的轴突存在于反射弧中，例如早期动作电位研究中使用的乌贼巨大轴突。
Temperature: Nerve impulses depend on diffusion of ions and the activity of membrane pumps, both of which are temperature-sensitive. As temperature rises, ions move faster and the Na⁺/K⁺ pump works faster, so impulses are conducted more rapidly. However, if temperature becomes too high, membrane proteins and enzymes denature, and conduction stops. This is why nerve function is impaired during high fever.
温度：神经冲动依赖离子扩散和膜泵的活性，两者均对温度敏感。温度升高时，离子运动加快，Na⁺/K⁺ 泵工作加速，因此冲动传导更迅速。但如果温度过高，膜蛋白和酶会变性，传导停止。这就是高热时神经功能受损的原因。
In humans, normal body temperature is maintained near 37°C, which is optimal for neuronal function.
人类体温维持在 37°C 左右，这有利于神经元功能的最佳发挥。
9. Summary | 总结
To master this topic, remember the following key sequence:
为掌握本主题，请记住以下关键顺序：
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Resting potential is maintained by the Na⁺/K⁺ pump, K⁺ leak channels, and impermeable anions, giving a polarised membrane at -70 mV.
静息电位由钠钾泵、钾漏通道和不透膜的阴离子维持，使膜极化在 -70 mV。
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An action potential starts with depolarisation when voltage-gated Na⁺ channels open at threshold.
动作电位以去极化开始，在阈值处电压门控钠通道打开。
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Repolarisation follows as Na⁺ channels inactivate and voltage-gated K⁺ channels open, causing K⁺ efflux.
随后复极化发生，钠通道失活，电压门控钾通道打开，导致 K⁺ 外流。
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The refractory period prevents backward conduction and limits impulse frequency.
不应期防止逆向传导并限制冲动频率。
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Impulses are conducted by local currents; myelinated axons use saltatory conduction for faster, more energy-efficient transmission.
冲动通过局部电流传导；有髓鞘轴突利用跳跃式传导实现更快、更节能的传递。
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Conduction speed increases with myelination, larger axon diameter, and higher temperature up to an optimum.
传导速度随髓鞘、轴突直径增大以及温度升高（达到最适温度前）而增加。
Remember: Rest → Threshold → Depolarisation → Repolarisation → Refractory → Conduction
记住：静息 → 阈值 → 去极化 → 复极化 → 不应期 → 传导
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