A-Level Biology: The Nervous System & Nerve Impulses

Introduction to the Nervous System · 神经系统简介

The nervous system is the body’s rapid-response communication network, allowing organisms to detect changes in their internal and external environments and coordinate appropriate responses. Unlike the hormonal system which relies on chemical messengers travelling through the bloodstream, the nervous system uses electrical impulses transmitted along specialised cells called neurones, enabling reaction times measured in milliseconds rather than seconds. This speed is essential for survival behaviours such as withdrawing from a painful stimulus, maintaining balance, and coordinating complex movements. In A-Level Biology, understanding the nervous system spans multiple topics including cell membranes, protein channels, and electrochemical gradients.

神经系统是人体快速响应的通信网络，使生物体能够感知内外环境的变化并协调适当的反应。与依赖化学信使通过血液运输的内分泌系统不同，神经系统使用沿称为神经元的特化细胞传递的电脉冲，反应时间以毫秒而非秒为单位。这种速度对于生存行为至关重要，例如从疼痛刺激中撤回、维持平衡以及协调复杂的运动。在A-Level生物学中，理解神经系统涉及多个主题，包括细胞膜、蛋白质通道和电化学梯度。

Structure of Neurones · 神经元的结构

Neurones are the functional units of the nervous system, specialised for the rapid transmission of electrical signals. A typical motor neurone consists of a cell body containing the nucleus and most organelles, multiple short dendrites that receive signals from other neurones, and a single long axon that transmits impulses away from the cell body. The axon is insulated by a myelin sheath formed by Schwann cells, which wrap around the axon multiple times. Gaps in the myelin sheath called nodes of Ranvier are critical for the rapid propagation of action potentials through a mechanism known as saltatory conduction.

神经元是神经系统的功能单位，专门用于快速传递电信号。典型的运动神经元由包含细胞核和大多数细胞器的细胞体、多个接收其他神经元信号的短树突以及一个将脉冲从细胞体传出的长轴突组成。轴突由施万细胞形成的髓鞘绝缘，施万细胞多次缠绕轴突。髓鞘中的间隙称为朗飞结，对于通过称为跳跃传导的机制快速传播动作电位至关重要。

There are three main types of neurones, each with a distinct structure adapted to its function. Sensory neurones carry impulses from receptors to the central nervous system (CNS) and have a long dendron carrying the impulse toward the cell body. Relay neurones (also called interneurones) are found entirely within the CNS and connect sensory neurones to motor neurones, with numerous short dendrites and a short axon. Motor neurones carry impulses from the CNS to effectors such as muscles and glands, with a long axon and a cell body located within the CNS or in ganglia just outside it.

神经元有三种主要类型，每种都具有适应其功能的独特结构。感觉神经元将脉冲从感受器传递到中枢神经系统（CNS），并具有一个将脉冲传向细胞体的长树突。中间神经元完全位于CNS内，连接感觉神经元和运动神经元，具有许多短树突和短轴突。运动神经元将脉冲从CNS传递到效应器如肌肉和腺体，具有长轴突和位于CNS内或紧邻其外的神经节中的细胞体。

The 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, typically around -70 mV. This resting potential is established and maintained by the unequal distribution of sodium (Na⁺) and potassium (K⁺) ions across the axon membrane. The sodium-potassium pump (Na⁺/K⁺-ATPase) actively transports three Na⁺ ions out of the axon and two K⁺ ions into the axon for every molecule of ATP hydrolysed. This creates concentration gradients, with Na⁺ more concentrated outside and K⁺ more concentrated inside. Additionally, the membrane is more permeable to K⁺ than Na⁺ at rest because more potassium ion channels are open, allowing K⁺ to diffuse out down its concentration gradient, making the inside negative.

当神经元不传递脉冲时，轴突内部相对于外部带负电，通常约为-70 mV。这种静息电位由钠（Na⁺）和钾（K⁺）离子在轴突膜上的不均匀分布建立和维持。钠钾泵（Na⁺/K⁺-ATP酶）每水解一个ATP分子，主动将三个Na⁺离子运出轴突，并将两个K⁺离子运入轴突。这产生浓度梯度，Na⁺在外部浓度更高，K⁺在内部浓度更高。此外，膜在静息时对K⁺的通透性大于Na⁺，因为更多的钾离子通道处于开放状态，允许K⁺沿其浓度梯度扩散出去，使内部带负电。

The Action Potential · 动作电位

An action potential is a rapid, temporary reversal of the membrane potential that travels along the axon. It is triggered when the membrane is depolarised to a threshold potential of approximately -55 mV, which causes voltage-gated sodium ion channels to open. Na⁺ ions rush into the axon down both their concentration and electrical gradients, causing the membrane potential to rise rapidly to around +40 mV during the depolarisation phase. This influx of positive charge is the rising phase of the action potential and follows the all-or-nothing principle: once threshold is reached, an action potential of fixed magnitude will always fire.

动作电位是沿轴突传播的膜电位的快速、暂时性反转。当膜去极化至约-55 mV的阈电位时触发，导致电压门控钠离子通道打开。Na⁺离子沿其浓度梯度和电梯度涌入轴突，在去极化阶段使膜电位迅速上升至约+40 mV。这种正电荷的内流是动作电位的上升阶段，遵循全或无原则：一旦达到阈值，固定大小的动作电位将始终触发。

After approximately 0.5 ms, the voltage-gated sodium channels inactivate and voltage-gated potassium channels open. K⁺ ions diffuse out of the axon, carrying positive charge out and causing repolarisation as the membrane potential falls back toward the resting level. There is typically a brief period of hyperpolarisation where the potential becomes more negative than the resting potential because the potassium channels are slow to close. The refractory period that follows ensures that action potentials travel in one direction and limits the maximum frequency of impulse transmission.

大约0.5毫秒后，电压门控钠通道失活，电压门控钾通道打开。K⁺离子扩散出轴突，带出正电荷，导致复极化，膜电位回落至静息水平。通常会出现短暂超极化阶段，电位变得比静息电位更负，因为钾通道关闭较慢。随后的不应期确保动作电位沿一个方向传播，并限制脉冲传递的最大频率。

Saltatory Conduction · 跳跃传导

In myelinated neurones, action potentials do not travel continuously along the entire length of the axon. Instead, they jump from one node of Ranvier to the next in a process called saltatory conduction (from Latin saltare, meaning “to leap”). The myelin sheath acts as an electrical insulator, preventing ion movement across the membrane in the internodal regions. Voltage-gated sodium and potassium channels are concentrated at the nodes of Ranvier, where depolarisation occurs. This saltatory mechanism dramatically increases the speed of conduction, with myelinated axons transmitting impulses at up to 120 m/s compared to approximately 2 m/s in unmyelinated axons.

在有髓神经元中，动作电位不会沿轴突的整个长度连续传播。相反，它们通过称为跳跃传导的过程从一个朗飞结跳到下一个朗飞结。髓鞘充当电绝缘体，防止离子在结间区域的跨膜运动。电压门控钠和钾通道集中在朗飞结处，去极化在此发生。这种跳跃机制极大地提高了传导速度，有髓轴突以高达120 m/s的速度传递脉冲，而无髓轴突约为2 m/s。

Several factors affect the speed of conduction. A larger axon diameter reduces internal resistance, allowing faster ion flow. A thicker myelin sheath provides better insulation and longer internodal distances, so the action potential jumps further between nodes. Temperature also affects speed because ion diffusion and enzyme activity in the sodium-potassium pump are temperature-dependent ： within physiological limits, warmer temperatures increase conduction velocity. Damage to the myelin sheath, as seen in multiple sclerosis, slows or blocks impulse transmission, demonstrating the critical importance of myelination.

多个因素影响传导速度。更大的轴突直径减少内部电阻，允许更快的离子流动。更厚的髓鞘提供更好的绝缘和更长的结间距离，因此动作电位在节点之间跳跃更远。温度也影响速度，因为离子扩散和钠钾泵中的酶活性依赖于温度：在生理限度内，较高温度增加传导速度。髓鞘损伤（如多发性硬化症中所见）会减慢或阻断脉冲传递，证明髓鞘形成的关键重要性。

Synaptic Transmission · 突触传递

When an action potential reaches the presynaptic terminal of an axon, it triggers the opening of voltage-gated calcium ion channels. Ca²⁺ ions diffuse into the presynaptic knob, causing synaptic vesicles containing neurotransmitter (such as acetylcholine) to fuse with the presynaptic membrane and release their contents into the synaptic cleft by exocytosis. The neurotransmitter then diffuses across the narrow synaptic cleft (approximately 20 nm wide) and binds to specific receptor proteins on the postsynaptic membrane. This binding is highly specific ： each receptor has a complementary shape to its neurotransmitter, analogous to enzyme-substrate specificity.

当动作电位到达轴突的突触前末梢时，它触发电圧门控钙离子通道的开放。Ca²⁺离子扩散进入突触前小结，导致含有神经递质（如乙酰胆碱）的突触囊泡与突触前膜融合，通过胞吐作用将其内容物释放到突触间隙中。神经递质随后扩散穿过狭窄的突触间隙（约20 nm宽），并与突触后膜上的特定受体蛋白结合。这种结合高度特异：每个受体与其神经递质具有互补形状，类似于酶-底物特异性。

The binding of neurotransmitter to postsynaptic receptors opens ligand-gated sodium channels, allowing Na⁺ ions to enter the postsynaptic neurone and causing depolarisation. If the neurotransmitter is excitatory (e.g., acetylcholine at neuromuscular junctions), this generates an excitatory postsynaptic potential (EPSP) that brings the membrane closer to threshold. However, some neurotransmitters are inhibitory (e.g., GABA), opening chloride or potassium channels that cause hyperpolarisation and produce inhibitory postsynaptic potentials (IPSPs). The neurotransmitter in the synaptic cleft must be rapidly removed to prevent continuous stimulation ： acetylcholine is hydrolysed by the enzyme acetylcholinesterase, and the breakdown products are reabsorbed by the presynaptic neurone for recycling.

神经递质与突触后受体的结合打开配体门控钠通道，允许Na⁺离子进入突触后神经元，导致去极化。如果神经递质是兴奋性的（例如神经肌肉接头处的乙酰胆碱），则产生兴奋性突触后电位（EPSP），使膜更接近阈值。然而，一些神经递质是抑制性的（例如GABA），打开氯或钾通道导致超极化，产生抑制性突触后电位（IPSP）。突触间隙中的神经递质必须被迅速清除以防止持续刺激：乙酰胆碱被酶乙酰胆碱酯酶水解，分解产物被突触前神经元重新吸收以供循环利用。

Summation & Integration · 总和与整合

A single EPSP is typically too small (around 0.5-1 mV) to reach the threshold for an action potential. Postsynaptic neurones integrate multiple synaptic inputs through summation. Spatial summation occurs when several presynaptic neurones release neurotransmitter simultaneously at different synapses on the same postsynaptic neurone, and their EPSPs add together. Temporal summation occurs when a single presynaptic neurone releases neurotransmitter in quick succession, and the EPSPs overlap and add up because each EPSP lasts a few milliseconds. The net effect of all EPSPs and IPSPs at any moment determines whether the postsynaptic neurone fires an action potential.

单个EPSP通常太小（约0.5-1 mV），无法达到动作电位的阈值。突触后神经元通过总和整合多个突触输入。空间总和发生在几个突触前神经元在同一突触后神经元上不同突触处同时释放神经递质时，它们的EPSP相加。时间总和发生在单个突触前神经元快速连续释放神经递质时，由于每个EPSP持续几毫秒，EPSP重叠并相加。所有EPSP和IPSP在任何时刻的净效应决定突触后神经元是否触发动作电位。

Drugs & Synaptic Function · 药物与突触功能

Many drugs exert their effects by interfering with synaptic transmission. They can act as agonists that mimic neurotransmitters by binding to and activating receptors, or as antagonists that block receptors without activating them. Drugs can also inhibit the enzymes that break down neurotransmitters (prolonging their effect in the cleft), stimulate or inhibit neurotransmitter release, or block the reuptake of neurotransmitters from the synaptic cleft. Understanding these mechanisms is a common A-Level exam question that links synaptic physiology to pharmacology and real-world applications, such as the action of organophosphate pesticides that inhibit acetylcholinesterase.

许多药物通过干扰突触传递发挥作用。它们可以作为激动剂，通过结合并激活受体来模拟神经递质，或作为拮抗剂，阻断受体而不激活它们。药物还可以抑制分解神经递质的酶（延长其在间隙中的作用）、刺激或抑制神经递质释放，或阻断突触间隙中神经递质的再摄取。理解这些机制是A-Level考试中的常见问题，将突触生理学与药理学和实际应用联系起来，例如抑制乙酰胆碱酯酶的有机磷农药的作用。

Exam Tips & Common Pitfalls · 考试技巧与常见误区

When answering exam questions on the nervous system, precision in terminology is essential. Students frequently confuse “resting potential” (a state) with “action potential” (an event), or fail to specify that it is voltage-gated sodium channels ： not all sodium channels ： that open during depolarisation. Always state the direction of ion movement clearly, e.g., “Na⁺ ions diffuse into the axon” rather than simply “sodium moves.” When describing the refractory period, explicitly link it to the inactivation of sodium channels and explain its functional significance: ensuring unidirectional propagation and limiting impulse frequency. In synaptic transmission questions, marks are routinely allocated for mentioning Ca²⁺ entry, exocytosis of vesicles, diffusion across the cleft, and the role of acetylcholinesterase ： missing any of these steps typically costs marks.

在回答关于神经系统的考试问题时，术语的精确性至关重要。学生经常混淆”静息电位”（一种状态）和”动作电位”（一个事件），或未能明确指出在去极化过程中开放的是电压门控钠通道：而非所有钠通道。请始终清晰地说明离子运动的方向，例如”Na⁺离子扩散进入轴突”而非简单地说”钠离子移动”。在描述不应期时，明确将其与钠通道的失活联系起来，并解释其功能意义：确保单向传播和限制脉冲频率。在突触传递问题中，通常会为提到Ca²⁺进入、囊泡胞吐、间隙扩散以及乙酰胆碱酯酶的作用分配分数：缺少这些步骤中的任何一步通常都会丢分。

Diagrams are heavily rewarded in nervous system questions. Be prepared to sketch and label a motor neurone (cell body, dendrites, axon, myelin sheath, nodes of Ranvier), an action potential graph (resting potential, threshold, depolarisation, repolarisation, hyperpolarisation), and a synapse (presynaptic knob, vesicles, synaptic cleft, postsynaptic membrane with receptors). Pay special attention to units: resting potential is in millivolts and negative, conduction velocity is in metres per second, and the synaptic cleft width is approximately 20 nm. Comparative questions ： myelinated vs. unmyelinated, excitatory vs. inhibitory synapses, spatial vs. temporal summation ： are high-mark opportunities if you structure your answer with clear contrasting points.

图表在神经系统问题中获得高度认可。请做好绘制和标注运动神经元（细胞体、树突、轴突、髓鞘、朗飞结）、动作电位图（静息电位、阈值、去极化、复极化、超极化）以及突触（突触前小结、囊泡、突触间隙、带有受体的突触后膜）的准备。特别注意单位：静息电位以毫伏为单位且为负值，传导速度以米每秒为单位，突触间隙宽度约为20 nm。比较性问题：有髓与无髓、兴奋性与抑制性突触、空间与时间总和：如果你用清晰的对比点构建答案，这些是高分的得分机会。

Key Bilingual Terms · 核心双语术语

Neurone · 神经元 | Resting Potential · 静息电位 | Action Potential · 动作电位 | Depolarisation · 去极化 | Repolarisation · 复极化 | Hyperpolarisation · 超极化 | Threshold Potential · 阈电位 | Myelin Sheath · 髓鞘 | Node of Ranvier · 朗飞结 | Saltatory Conduction · 跳跃传导 | Sodium-Potassium Pump · 钠钾泵 | Voltage-Gated Channel · 电压门控通道 | Synapse · 突触 | Neurotransmitter · 神经递质 | Acetylcholine · 乙酰胆碱 | Acetylcholinesterase · 乙酰胆碱酯酶 | EPSP · 兴奋性突触后电位 | IPSP · 抑制性突触后电位 | Spatial Summation · 空间总和 | Temporal Summation · 时间总和 | Refractory Period · 不应期 | All-or-Nothing Principle · 全或无原则