Neuronal Communication: Action Potentials and Synaptic Transmission — 神经元通讯：动作电位与突触传递

一、神经元的结构与功能分工：树突、轴突与髓鞘 | Neuron Structure and Functional Specialisation: Dendrites, Axon and Myelin Sheath

神经元是神经系统的基本功能单位，专门负责接收、整合和传递电信号。一个典型的运动神经元由三个主要部分组成：细胞体（cell body）、树突（dendrites）和轴突（axon）。细胞体中含有细胞核和大部分细胞器，是整个神经元的代谢中心；树突是从细胞体伸出的许多短而分支的突起，负责接收来自其他神经元或感受器的信号；轴突则是一条细长的单一突起，负责把动作电位从细胞体快速传导到轴突末梢。

A neuron is the basic functional unit of the nervous system, specialised to receive, integrate and transmit electrical signals. A typical motor neuron is made up of three main parts: the cell body, the dendrites and the axon. The cell body contains the nucleus and most of the organelles, and acts as the metabolic centre of the neuron. The dendrites are many short, branching extensions that receive signals from other neurons or from receptors. The axon is a single, long thin extension that carries action potentials rapidly from the cell body to the axon terminal.

在脊椎动物体内，许多轴突外面包被着一层由施万细胞（Schwann cells，周围神经系统）或少突胶质细胞（oligodendrocytes，中枢神经系统）形成的髓鞘（myelin sheath）。髓鞘的作用类似电线外层的绝缘层，能显著提高动作电位的传导速度。相邻两段髓鞘之间的裸露区域称为郎飞结（node of Ranvier），是动作电位得以”跳跃式”再生的关键位置，我们将在第五节详细讨论。

In vertebrates, many axons are wrapped in a myelin sheath formed by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system). The myelin sheath acts like the insulating layer around an electrical wire, dramatically increasing the speed of action potential conduction. The exposed gaps between adjacent segments of myelin are called the nodes of Ranvier, and they are the key locations where the action potential is regenerated in a “jumping” manner, which we will discuss in detail in section five.

二、静息电位的形成：钠钾泵与离子浓度梯度 | The Resting Potential: The Sodium-Potassium Pump and Ion Concentration Gradients

当神经元没有受到刺激、处于安静状态时，膜内外存在一个稳定的电位差，称为静息电位（resting potential），通常约为 -70 mV，即膜内比膜外低约 70 毫伏。这个负值意味着细胞膜发生了极化（polarised）：膜内侧聚集了较多负电荷，膜外侧聚集了较多正电荷。

When a neuron is not being stimulated and is at rest, there is a stable potential difference across its membrane called the resting potential, which is normally about -70 mV. This means that the inside of the membrane is about 70 millivolts more negative than the outside. The negative value indicates that the membrane is polarised: more negative charge accumulates on the inside and more positive charge on the outside.

静息电位的形成主要依赖两个因素。第一，钠钾泵（sodium-potassium pump）通过主动运输，每消耗一分子 ATP 就向膜外泵出 3 个钠离子（Na⁺），同时向膜内泵入 2 个钾离子（K⁺），从而在膜内外建立起离子浓度梯度：膜外 Na⁺ 浓度高，膜内 K⁺ 浓度高。第二，细胞膜对 K⁺ 的通透性远高于对 Na⁺ 的通透性，钾离子沿着浓度梯度通过泄漏通道（leak channels）大量外流，把正电荷带出膜外，使膜内相对变负。因此，静息电位本质上是由 K⁺ 外流主导、并由钠钾泵维持的一种平衡状态。

Two factors are mainly responsible for the resting potential. First, the sodium-potassium pump uses active transport to pump three sodium ions (Na⁺) out of the cell and two potassium ions (K⁺) into the cell for every molecule of ATP consumed, establishing an ion concentration gradient: Na⁺ is more concentrated outside and K⁺ is more concentrated inside. Second, the membrane is much more permeable to K⁺ than to Na⁺, so potassium ions flow out of the cell down their concentration gradient through leak channels, carrying positive charge out and making the inside relatively negative. The resting potential is therefore a balance dominated by the outward movement of K⁺ and maintained by the sodium-potassium pump.

三、动作电位的四个阶段：去极化、复极化、超极化与不应期 | The Four Phases of the Action Potential: Depolarisation, Repolarisation, Hyperpolarisation and the Refractory Period

当神经元受到足够强的刺激时，膜电位会发生一次快速而短暂的变化，这个变化过程称为动作电位（action potential）。动作电位可以清晰地划分为四个阶段：去极化、复极化、超极化和不应期。

When a neuron receives a sufficiently strong stimulus, the membrane potential undergoes a rapid, brief change called the action potential. It can be clearly divided into four phases: depolarisation, repolarisation, hyperpolarisation and the refractory period.

去极化阶段（depolarisation）：刺激使膜电位从 -70 mV 上升到阈电位（约 -55 mV），一旦达到阈值，大量电压门控钠离子通道（voltage-gated Na⁺ channels）打开，Na⁺ 顺着浓度梯度快速涌入膜内，膜电位迅速上升并向 0 靠拢，最终反转为正值，达到约 +30 至 +40 mV 的峰值。复极化阶段（repolarisation）：钠离子通道随即失活关闭，同时电压门控钾离子通道打开，K⁺ 大量外流，把正电荷带出膜外，膜电位迅速回落。超极化阶段（hyperpolarisation）：由于 K⁺ 通道关闭相对缓慢，K⁺ 外流会短暂过度，使膜电位一度低于静息电位（例如降到约 -80 mV），随后钠钾泵把离子浓度逐步恢复，膜电位回到 -70 mV。

During depolarisation, the stimulus raises the membrane potential from -70 mV to the threshold potential of about -55 mV. Once threshold is reached, many voltage-gated sodium channels open and Na⁺ rushes into the cell down its concentration gradient, so the membrane potential rises rapidly, crosses zero and reverses to a peak of about +30 to +40 mV. During repolarisation, the sodium channels inactivate and close, while voltage-gated potassium channels open, allowing K⁺ to flow out and carry positive charge away, so the membrane potential falls rapidly again. During hyperpolarisation, because the potassium channels close relatively slowly, the outward flow of K⁺ overshoots, briefly driving the membrane potential below the resting level (for example to about -80 mV). The sodium-potassium pump then gradually restores the ion concentrations and the membrane potential returns to -70 mV.

不应期（refractory period）分为绝对不应期和相对不应期。在绝对不应期内，钠离子通道处于失活状态，无论刺激多强都无法引发新的动作电位；在相对不应期内，膜仍处于超极化状态，只有更强的刺激才能引发下一次动作电位。不应期的存在保证了动作电位只能单向传导，并且限制了神经元的最高发放频率。

The refractory period is divided into an absolute and a relative phase. During the absolute refractory period, the sodium channels are inactivated and no new action potential can be triggered however strong the stimulus. During the relative refractory period, the membrane is still hyperpolarised and only a stronger-than-normal stimulus can trigger the next action potential. The refractory period ensures that action potentials travel in one direction only and limits the maximum firing frequency of a neuron.

四、动作电位的全或无定律与阈电位 | The All-or-Nothing Law and the Threshold Potential of Action Potentials

动作电位遵循”全或无定律”（all-or-nothing law）：一旦刺激强度达到阈电位，就会产生一个完整的、固定幅度的动作电位；如果刺激没有达到阈值，则完全不产生动作电位。换句话说，动作电位的大小不会随着刺激强度的增加而变大，任何一次动作电位在幅度和形状上都是基本相同的。

The action potential obeys the all-or-nothing law: once the stimulus reaches the threshold potential, a full action potential of fixed amplitude is produced; if the stimulus does not reach threshold, no action potential is produced at all. In other words, the size of an action potential does not increase with the strength of the stimulus, and every action potential is essentially the same in amplitude and shape.

那么，神经系统是如何传递”刺激强弱”这一信息的呢？答案在于发放频率而不是幅度。更强的刺激会使神经元在单位时间内产生更多次动作电位，即发放频率更高；较弱的刺激则产生较低频率的动作电位。这一原则对理解感觉系统的编码方式至关重要：例如皮肤感受器就是通过改变动作电位的频率来编码压力的强弱。

So how does the nervous system convey information about the strength of a stimulus? The answer lies in the frequency of firing rather than the amplitude. A stronger stimulus causes the neuron to produce more action potentials per unit time, that is, a higher firing frequency, while a weaker stimulus produces a lower frequency. This principle is crucial for understanding how sensory systems encode information: for example, skin receptors encode the intensity of pressure by changing the frequency of action potentials.

五、动作电位沿轴突的传导：跳跃传导与髓鞘的作用 | Propagation of Action Potentials Along the Axon: Saltatory Conduction and the Role of Myelin

动作电位一旦在轴突起始段产生，就会沿着轴突向末梢传导。传导的基本机制是局部电流（local current）：动作电位处的膜内带正电，会通过轴浆向邻近的静息区域流动，使邻近区域的膜去极化并达到阈值，从而在那里触发新的动作电位。由于刚发生过动作电位的区域处于不应期，动作电位只能向一个方向推进。

Once an action potential is generated at the axon hillock, it travels along the axon toward the terminal. The basic mechanism of conduction is the local current: the inside of the membrane at the site of an action potential is positively charged, and this charge flows through the axoplasm to the adjacent resting region, depolarising it to threshold and triggering a new action potential there. Because the region that has just fired is in its refractory period, the action potential can only advance in one direction.

在无髓鞘的轴突中，动作电位沿着轴突连续地逐点传导，速度较慢且耗能较多。而在有髓鞘的轴突中，髓鞘起到绝缘作用，局部电流只能在郎飞结之间”跳跃”，动作电位只在郎飞结处再生，这种传导方式称为跳跃传导（saltatory conduction）。跳跃传导有两个显著优点：一是传导速度大幅提高，二是钠钾泵只需在郎飞结处恢复离子浓度，大大节省了能量。临床上，髓鞘受损的疾病（如多发性硬化症）会导致动作电位传导减慢甚至中断，从而出现运动和感觉障碍。

In an unmyelinated axon, the action potential travels continuously point by point, which is slow and energy-consuming. In a myelinated axon, the myelin sheath insulates the membrane, so the local current jumps between the nodes of Ranvier and the action potential is regenerated only at the nodes. This mode of conduction is called saltatory conduction. It has two major advantages: conduction speed is greatly increased, and the sodium-potassium pump only needs to restore ion concentrations at the nodes, saving a great deal of energy. Clinically, diseases in which the myelin sheath is damaged, such as multiple sclerosis, slow or block action potential conduction and cause motor and sensory problems.

六、突触的结构与化学传递 | Synapse Structure and Chemical Transmission

两个神经元之间或神经元与效应器之间的连接处称为突触（synapse）。化学突触由三部分组成：突触前膜（presynaptic membrane）、突触间隙（synaptic cleft，宽约 20 至 30 纳米）和突触后膜（postsynaptic membrane）。突触前末梢内含有大量装着神经递质的突触小泡（synaptic vesicles），突触后膜上则分布着与递质特异性结合的受体蛋白。

The junction between two neurons, or between a neuron and an effector, is called a synapse. A chemical synapse consists of three parts: the presynaptic membrane, the synaptic cleft (about 20 to 30 nanometres wide) and the postsynaptic membrane. The presynaptic terminal contains many synaptic vesicles filled with neurotransmitter, and the postsynaptic membrane carries receptor proteins that bind specifically to the transmitter.

突触传递是单向的，只能从突触前神经元传向突触后神经元，这与动作电位在轴突上的单向传导方向保持一致。突触的存在也解释了信号传递为何比单纯的轴突传导更慢：跨越突触间隙、递质扩散以及受体结合都需要时间，这段时间称为突触延搁（synaptic delay）。

Synaptic transmission is unidirectional, passing only from the presynaptic to the postsynaptic neuron, which keeps the overall direction of signal flow consistent with the one-way conduction along the axon. The presence of a synapse also explains why transmission is slower than conduction along an axon alone: crossing the cleft, diffusion of the transmitter and receptor binding all take time, a period known as the synaptic delay.

七、神经递质的释放与突触后电位 | Neurotransmitter Release and Postsynaptic Potentials

当一个动作电位到达突触前末梢时，末梢膜上的电压门控钙离子通道打开，Ca²⁺ 从突触间隙涌入末梢内部。钙离子的进入促使突触小泡移向突触前膜并与膜融合，通过胞吐作用（exocytosis）把神经递质释放到突触间隙中。递质随后扩散穿过间隙，与突触后膜上的特异性受体结合。

When an action potential arrives at the presynaptic terminal, voltage-gated calcium channels in the terminal membrane open and Ca²⁺ flows in from the synaptic cleft. The influx of calcium causes the synaptic vesicles to move toward and fuse with the presynaptic membrane, releasing their neurotransmitter into the cleft by exocytosis. The transmitter then diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.

递质与受体结合后，会改变突触后膜对某些离子的通透性，从而引起突触后膜电位的变化，这种局部电位变化称为突触后电位。如果递质使突触后膜对 Na⁺ 的通透性增加，Na⁺ 内流会使膜电位上升（去极化），产生兴奋性突触后电位（EPSP）；如果递质使膜对 Cl⁻ 或 K⁺ 的通透性增加，则会使膜电位下降（超极化），产生抑制性突触后电位（IPSP）。

Binding of the transmitter to its receptors changes the permeability of the postsynaptic membrane to certain ions, producing a local change in membrane potential known as a postsynaptic potential. If the transmitter increases the permeability of the postsynaptic membrane to Na⁺, the resulting inward flow of Na⁺ raises the membrane potential (depolarisation) and produces an excitatory postsynaptic potential (EPSP). If the transmitter increases membrane permeability to Cl⁻ or K⁺, the membrane potential falls (hyperpolarisation) and an inhibitory postsynaptic potential (IPSP) is produced.

八、兴奋性与抑制性突触：EPSP 与 IPSP 的整合 | Excitatory and Inhibitory Synapses: Integrating EPSPs and IPSPs

兴奋性突触和抑制性突触是神经系统中最基本的两种突触类型。兴奋性突触释放兴奋性递质，使突触后膜去极化，产生 EPSP，使突触后神经元更容易达到阈电位；抑制性突触释放抑制性递质，使突触后膜超极化，产生 IPSP，使突触后神经元更难被激发。一个神经元通常同时接收来自成千上万个突触的输入，其中既有兴奋性的也有抑制性的。

Excitatory and inhibitory synapses are the two most fundamental types of synapse in the nervous system. An excitatory synapse releases an excitatory transmitter that depolarises the postsynaptic membrane and produces an EPSP, making the postsynaptic neuron more likely to reach threshold. An inhibitory synapse releases an inhibitory transmitter that hyperpolarises the postsynaptic membrane and produces an IPSP, making the postsynaptic neuron harder to excite. A single neuron typically receives inputs from thousands of synapses, some excitatory and some inhibitory.

突触后神经元是否产生动作电位，取决于这些输入的总和，这一过程称为突触整合（summation）。空间总和（spatial summation）指来自多个不同突触的电位在同一时刻叠加；时间总和（temporal summation）指来自同一个突触的快速连续多次电位在时间上累积。只有当总的去极化达到阈电位时，突触后神经元才会在轴突起始段产生动作电位。这种”整合再决定”的机制赋予了神经系统强大的信息处理能力。

Whether the postsynaptic neuron fires an action potential depends on the total of these inputs, a process called summation. Spatial summation refers to potentials arriving at the same time from several different synapses adding together; temporal summation refers to the accumulation of rapidly repeated potentials from a single synapse. Only when the overall depolarisation reaches the threshold potential does the postsynaptic neuron generate an action potential at the axon hillock. This “integrate-then-decide” mechanism gives the nervous system its powerful information-processing ability.

九、神经递质的类型与作用机制：乙酰胆碱与多巴胺 | Types of Neurotransmitters and Their Mechanisms: Acetylcholine and Dopamine

神经递质种类繁多，常见的有乙酰胆碱（acetylcholine, ACh）、多巴胺（dopamine）、去甲肾上腺素（noradrenaline）、血清素（serotonin）和 γ-氨基丁酸（GABA）等。乙酰胆碱是运动神经末梢与骨骼肌之间神经肌肉接头处的兴奋性递质，它结合到肌细胞膜上的受体后打开 Na⁺ 通道，使肌细胞膜去极化并最终引发肌肉收缩。乙酰胆碱发挥作用后会被突触间隙中的乙酰胆碱酯酶（acetylcholinesterase）迅速分解，从而终止信号，保证肌肉能够及时放松。

There are many kinds of neurotransmitter, including acetylcholine (ACh), dopamine, noradrenaline, serotonin and gamma-aminobutyric acid (GABA). Acetylcholine is the excitatory transmitter at the neuromuscular junction between a motor nerve ending and skeletal muscle. It binds to receptors on the muscle cell membrane and opens Na⁺ channels, depolarising the muscle membrane and ultimately triggering contraction. After acting, acetylcholine is rapidly broken down by acetylcholinesterase in the cleft, terminating the signal and allowing the muscle to relax in time.

多巴胺是中枢神经系统中一种重要的神经递质，参与运动控制、奖赏和情绪调节。多巴胺释放后通过再摄取（reuptake）机制被突触前末梢回收。许多精神药物正是通过影响神经递质的释放、再摄取或受体结合来发挥作用，这也是下一节要讨论的重点内容。

Dopamine is an important neurotransmitter in the central nervous system, involved in motor control, reward and mood regulation. After release, dopamine is taken back up by the presynaptic terminal through a reuptake mechanism. Many psychoactive drugs act by affecting the release, reuptake or receptor binding of neurotransmitters, which is the focus of the next section.

十、突触传递的调控与药物影响 | Modulation of Synaptic Transmission and the Effects of Drugs

突触传递是许多药物作用的靶点，理解这些作用机制有助于解释药物的疗效和副作用。例如，有机磷农药和一些神经毒气通过抑制乙酰胆碱酯酶，使乙酰胆碱无法被分解而在突触间隙持续累积，导致肌肉持续收缩、痉挛甚至呼吸肌麻痹。相反，箭毒（curare）等药物则阻断乙酰胆碱受体，使神经信号无法传递到肌肉，导致肌肉松弛麻痹。

Synaptic transmission is the target of many drugs, and understanding these mechanisms helps explain both their therapeutic effects and their side effects. For example, organophosphate pesticides and some nerve gases inhibit acetylcholinesterase so that acetylcholine is not broken down and accumulates in the cleft, causing sustained muscle contraction, spasms and even paralysis of the respiratory muscles. By contrast, drugs such as curare block acetylcholine receptors so that signals cannot reach the muscle, causing muscular relaxation and paralysis.

在兴奋剂与成瘾药物方面，可卡因阻断多巴胺的再摄取，使突触间隙中的多巴胺浓度升高，从而产生强烈的愉悦感；长期使用会改变突触的可塑性，这正是成瘾的神经生物学基础之一。抗抑郁药物中的选择性血清素再摄取抑制剂（SSRI）则通过延长血清素在突触间隙中的作用时间来改善情绪。这些例子都说明，神经递质的正常代谢对健康至关重要。

Among stimulants and addictive drugs, cocaine blocks the reuptake of dopamine, raising its concentration in the cleft and producing intense pleasure; long-term use alters synaptic plasticity, which is one neurobiological basis of addiction. Selective serotonin reuptake inhibitors (SSRIs), a class of antidepressant, improve mood by prolonging the action of serotonin in the cleft. All of these examples show that the normal metabolism of neurotransmitters is essential for health.

十一、常见考点与答题技巧：神经元通讯的计算题与图表题 | Common Exam Questions and Answer Techniques: Calculations and Graph Questions on Neuronal Communication

在 A-Level 生物学考试中，神经元通讯经常以图表题、计算题和实验题的形式出现。常见的图表题要求考生识别动作电位曲线上的各个阶段，标注静息电位、阈电位、去极化、复极化和超极化的位置，并解释每一阶段对应的离子通道状态变化。答题时要注意：去极化对应 Na⁺ 内流和 Na⁺ 通道打开；复极化对应 Na⁺ 通道失活和 K⁺ 通道打开、K⁺ 外流；超极化则对应 K⁺ 通道关闭滞后。

In A-Level Biology exams, neuronal communication frequently appears as graph, calculation and experiment questions. A common graph question asks you to identify the phases of an action potential curve, label the resting potential, threshold potential, depolarisation, repolarisation and hyperpolarisation, and explain the channel state changes behind each phase. When answering, note that depolarisation corresponds to Na⁺ influx and open Na⁺ channels; repolarisation corresponds to inactivation of Na⁺ channels and opening of K⁺ channels with K⁺ efflux; and hyperpolarisation corresponds to the delayed closing of K⁺ channels.

关于传导速度的计算，考生应掌握公式：传导速度 = 传导距离 ÷ 传导时间。例如，若测得神经冲动沿一段长 0.3 米的轴突传导耗时 0.006 秒，则传导速度约为 50 米每秒。题目还可能要求比较有髓鞘与无髓鞘轴突的传导速度差异，此时应联系跳跃传导与局部电流逐点传导的区别作答。此外，涉及突触的实验题常要求解释为什么跨越突触的传递是单向的，答案要点是神经递质只存在于突触前末梢的小泡中，受体只分布在突触后膜上。

For conduction-speed calculations, you should be familiar with the formula: conduction speed equals conduction distance divided by conduction time. For example, if an impulse travels along an axon 0.3 metres long in 0.006 seconds, the conduction speed is about 50 metres per second. Questions may also ask you to compare the conduction speeds of myelinated and unmyelinated axons, in which case you should refer to the difference between saltatory conduction and continuous point-by-point conduction by local currents. Experiment questions about the synapse often ask why transmission across a synapse is one-way; the key points are that neurotransmitter is found only in the vesicles of the presynaptic terminal and that receptors are located only on the postsynaptic membrane.

Summary | 总结

本文系统梳理了神经元通讯的核心内容。静息电位由钠钾泵和 K⁺ 外流共同维持，约为 -70 mV；当刺激达到阈电位时，电压门控钠离子通道打开引发去极化，随后钠通道失活、钾通道打开完成复极化，并经历短暂超极化后恢复到静息状态。动作电位遵循全或无定律，刺激强度以发放频率编码，并借助髓鞘和郎飞结实现高速的跳跃传导。

This article has systematically covered the core content of neuronal communication. The resting potential of about -70 mV is maintained by the sodium-potassium pump together with the outward flow of K⁺. When a stimulus reaches the threshold potential, voltage-gated sodium channels open to trigger depolarisation; the sodium channels then inactivate and potassium channels open to complete repolarisation, followed by a brief hyperpolarisation before returning to rest. The action potential obeys the all-or-nothing law, stimulus strength is encoded as firing frequency, and myelination with its nodes of Ranvier enables fast saltatory conduction.

在突触层面，动作电位通过钙离子内流触发神经递质的胞吐释放，递质与突触后膜受体结合后产生 EPSP 或 IPSP，经空间与时间总和决定突触后神经元是否发放。神经递质如乙酰胆碱和多巴胺在信号终止、药物作用与成瘾机制中扮演关键角色。掌握离子通道状态变化与动作电位各阶段的对应关系，是解答考试中图表题与计算题的基础。

At the synapse, the action potential triggers the exocytotic release of neurotransmitter via calcium influx; the transmitter binds to postsynaptic receptors to produce an EPSP or IPSP, and spatial and temporal summation decide whether the postsynaptic neuron fires. Neurotransmitters such as acetylcholine and dopamine play key roles in signal termination, drug action and the mechanisms of addiction. Mastering the correspondence between ion-channel states and the phases of the action potential is the foundation for answering graph and calculation questions in the exam.

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