📚 A-Level CIE Biology: Nervous System Key Points | 神经系统考点精讲
The nervous system enables rapid communication between different parts of the body, coordinating responses to internal and external changes. For CIE A-Level Biology, understanding the electrical and chemical nature of nerve impulses, synaptic transmission, and the organisation of reflex pathways is essential.
神经系统使身体各部分之间得以快速交流，协调对内外部变化的响应。对于CIE A-Level生物学，理解神经冲动的电化学本质、突触传递以及反射通路的组织至关重要。
1. Introduction to the Nervous System | 神经系统简介
The nervous system is a highly specialised network of neurones and supporting cells that transmits signals in the form of electrical impulses. These impulses travel along neurones and are passed between cells at specialised junctions called synapses, allowing for rapid and coordinated responses.
神经系统是由神经元和支持细胞构成的高度特化的网络，以电冲动的形式传递信号。这些冲动沿神经元传导，并在称为突触的特化连接处传递至其他细胞，从而实现快速而协调的反应。
In vertebrates, the system is divided into the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), consisting of nerves connecting the CNS to the rest of the body. The PNS is further subdivided into sensory and motor pathways, with the motor system including the somatic and autonomic divisions.
在脊椎动物中，该系统分为中枢神经系统（CNS），包括脑和脊髓，以及外周神经系统（PNS），由连接中枢神经系统与身体其余部分的神经组成。外周神经系统进一步细分为感觉和运动通路，运动系统又包含躯体与自主神经分部。
2. Types of Neurones | 神经元类型
Sensory neurones carry impulses from receptors towards the CNS. They have a long dendrite and a shorter axon, with the cell body located in a ganglion outside the spinal cord. Their structure facilitates the transmission of information about stimuli such as temperature, pressure, or pain.
感觉神经元将冲动从感受器传向中枢神经系统。它们具有长树突和短轴突，细胞体位于脊髓外的神经节中。其结构有利于传导关于温度、压力或疼痛等刺激的信息。
Motor neurones transmit impulses from the CNS to effectors such as muscles and glands. These neurones have a large cell body within the CNS and a long axon that extends to the effector organ, allowing rapid activation of responses.
运动神经元将冲动从中枢神经系统传递至肌肉和腺体等效应器。这些神经元在CNS内部具有较大的细胞体，并有一长轴突延伸到效应器官，可快速激活反应。
Relay neurones (interneurones) are found entirely within the CNS and connect sensory and motor neurones. They integrate information and are essential in reflex arcs and complex neural pathways, often having many dendrites to receive multiple inputs.
中间神经元（联络神经元）完全存在于CNS内部，连接感觉与运动神经元。它们负责信息整合，是反射弧和复杂神经通路的关键，通常具有许多树突以接收多重输入。
3. Resting Potential | 静息电位
The resting potential of a neurone is approximately -70 mV (inside negative relative to the outside). This potential difference across the membrane is maintained even when the neurone is not transmitting a signal, due to the uneven distribution of ions and selective permeability of the membrane.
神经元的静息电位约为-70 mV（膜内相对于膜外为负）。即使在神经元不传递信号时，这种跨膜电位差依然存在，这源自离子的不均匀分布和膜的选择性通透。
The sodium–potassium pump actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell per ATP molecule hydrolysed. This creates concentration gradients: a high Na⁺ concentration outside and a high K⁺ concentration inside. The membrane also contains many non-gated potassium leak channels, allowing K⁺ to diffuse out, leaving behind negatively charged proteins and organic anions, which generates the negative internal potential.
钠钾泵每水解一分子ATP，主动转运3个Na⁺出细胞，并转运2个K⁺入细胞。这形成了浓度梯度：膜外Na⁺浓度高，膜内K⁺浓度高。膜上还存在许多非门控的钾漏通道，允许K⁺外流，留下带负电的蛋白质和有机阴离子，从而产生内负的电位。
Although the resting membrane is largely impermeable to Na⁺, the slight leakage and the strong electrochemical gradient for Na⁺ mean that the pump must continuously work to maintain the resting potential. The electrochemical equilibrium for K⁺ primarily determines the value near -70 mV.
尽管静息膜对Na⁺基本不通透，但由于少量的漏出和强烈的Na⁺电化学梯度，泵必须持续工作以维持静息电位。钾离子的电化学平衡主要决定了约-70 mV的数值。
4. Action Potential | 动作电位
An action potential is a rapid, transient reversal of membrane potential that propagates along an axon. It follows an all-or-nothing principle: once the threshold potential (around -55 mV) is reached, a full action potential is generated without variation in amplitude.
动作电位是沿轴突传播的膜电位的快速、短暂反转。它遵循“全或无”原则：一旦达到阈电位（约-55 mV），便会触发一个完整的动作电位，其幅度不变。
Depolarisation occurs when voltage-gated Na⁺ channels open, allowing Na⁺ to rush into the cell. This influx causes the membrane potential to become positive, peaking around +30 mV to +40 mV. Shortly after, the Na⁺ channels inactivate and voltage-gated K⁺ channels open, leading to repolarisation as K⁺ efflux restores the negative interior.
去极化发生时，电压门控Na⁺通道开放，Na⁺大量内流。这一内流使膜电位变正，峰值约为+30 mV至+40 mV。随后，Na⁺通道失活，电压门控K⁺通道开放，K⁺外流导致复极化，使膜内恢复负值。
Often there is a brief hyperpolarisation phase where the potential becomes more negative than the resting value because the K⁺ channels close slowly. The sodium–potassium pump and leak channels then restore the resting ionic distribution, preparing the neurone for the next stimulus.
通常会出现短暂的超极化阶段，膜电位比静息值更负，因K⁺通道关闭缓慢。之后，钠钾泵和漏通道恢复静息的离子分布，使神经元为下一个刺激做好准备。
5. Propagation of Action Potentials | 动作电位的传播
Once an action potential is generated at the axon hillock, local currents depolarise the adjacent membrane, triggering a new action potential further along the axon. This sequential depolarisation ensures unidirectional movement from the cell body to the axon terminal.
一旦在轴丘处产生动作电位，局部电流会使邻近膜去极化，从而在轴突更远处触发新的动作电位。这种顺序去极化确保了从细胞体到轴突末梢的单向传导。
In unmyelinated neurones, the impulse travels relatively slowly because each patch of membrane must undergo depolarisation and repolarisation. The wave of Na⁺ and K⁺ channel openings occurs continuously along the entire length of the axon, a process called continuous conduction.
在无髓鞘神经元中，冲动传导相对较慢，因为每一段膜都必须经历去极化和复极化。Na⁺和K⁺通道的开放波沿整个轴突长度连续发生，这一过程称为连续传导。
Myelinated neurones exhibit saltatory conduction. The myelin sheath, formed by Schwann cells or oligodendrocytes, insulates the axon except at nodes of Ranvier, where ion channels are concentrated. The action potential appears to jump from node to node, greatly increasing conduction speed and conserving energy.
有髓鞘神经元表现为跳跃传导。由施万细胞或少突胶质细胞形成的髓鞘对轴突起绝缘作用，只在郎飞结处聚集有离子通道。动作电位似乎从一个结“跳跃”到下一个结，从而大幅提高传导速度并节省能量。
6. Synaptic Transmission | 突触传递
Synapses are the junctions where a neurone communicates with another cell, usually a neurone, muscle fibre, or gland cell. The classic cholinergic synapse uses the neurotransmitter acetylcholine (ACh). When an action potential arrives at the presynaptic terminal, voltage-gated Ca²⁺ channels open, allowing Ca²⁺ ions to flood into the cytoplasm.
突触是神经元与另一细胞（通常是另一神经元、肌纤维或腺细胞）之间通信的连接点。典型的胆碱能突触使用神经递质乙酰胆碱（ACh）。当动作电位到达突触前末梢时，电压门控Ca²⁺通道开放，Ca²⁺离子大量涌入胞质。
The rise in Ca²⁺ triggers synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft by exocytosis. ACh then diffuses across the narrow cleft and binds to specific ligand-gated Na⁺ channels (nicotinic receptors) on the postsynaptic membrane, causing them to open.
Ca²⁺浓度的升高触发突触小泡与突触前膜融合，通过胞吐作用将乙酰胆碱释放到突触间隙中。随后ACh扩散通过狭窄的间隙，并与突触后膜上的特异性配体门控Na⁺通道（烟碱型受体）结合，使通道开放。
Na⁺ influx through these receptors generates an excitatory postsynaptic potential (EPSP). If the EPSP is large enough to bring the membrane to threshold, an action potential will be initiated in the postsynaptic cell. The acetylcholinesterase enzyme in the cleft rapidly hydrolyses ACh into choline and acetate, terminating the signal and preventing sustained stimulation.
通过这些受体的Na⁺内流产生兴奋性突触后电位（EPSP）。如果EPSP足够大，使膜达到阈值，就会在突触后细胞中引发动作电位。间隙中的乙酰胆碱酯酶迅速将ACh水解为胆碱和乙酸，从而终止信号并防止持续刺激。
7. Neuromuscular Junction | 神经肌肉接头
The neuromuscular junction (NMJ) is a specialised synapse between a motor neurone and a skeletal muscle fibre. Its structure is similar to a cholinergic synapse, but the postsynaptic membrane is highly folded (junctional folds) and contains a vast number of ACh receptors to ensure reliable muscle contraction.
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突触。其结构类似胆碱能突触，但突触后膜高度折叠（接头褶），并含有大量的ACh受体，以确保肌肉可靠收缩。
When an action potential reaches the motor end plate, Ca²⁺-dependent exocytosis releases ACh into the synaptic cleft. ACh binds to nicotinic receptors on the sarcolemma, causing Na⁺ influx and a local depolarisation called the end-plate potential (EPP). The EPP is always large enough to reach threshold, triggering a muscle action potential that sweeps along the sarcolemma and into T-tubules, leading to contraction.
当动作电位到达运动终板时，依赖Ca²⁺的胞吐作用将ACh释放入突触间隙。ACh与肌膜上的烟碱型受体结合，引起Na⁺内流和局部去极化，称为终板电位（EPP）。EPP总是大到足以达到阈值，从而触发一个沿肌膜传播并进入T管的肌肉动作电位，最终导致收缩。
Drugs and toxins can affect the NMJ. For example, curare blocks ACh receptors, causing paralysis, while organophosphates inhibit acetylcholinesterase, leading to sustained muscle contraction due to the accumulation of ACh.
药物和毒素可影响NMJ。例如，箭毒阻断ACh受体导致麻痹；而有机磷化合物抑制乙酰胆碱酯酶，使ACh积聚，引起持续肌肉收缩。
8. The Reflex Arc | 反射弧
A reflex arc is the simplest neural pathway mediating an involuntary response to a stimulus, bypassing conscious thought. The components of a typical spinal reflex arc are: receptor, sensory neurone, relay neurone (in the spinal cord), motor neurone, and effector (muscle or gland).
反射弧是介导对刺激做出不随意反应的最简单神经通路，绕过了意识参与。典型脊髓反射弧的组成包括：感受器、感觉神经元、中间神经元（位于脊髓）、运动神经元和效应器（肌肉或腺体）。
For instance, the knee-jerk reflex involves a stretch receptor in the quadriceps muscle. When the tendon is tapped, the receptor sends an impulse along a sensory neurone into the spinal cord, which directly synapses with a motor neurone that innervates the same muscle, causing contraction. There is no relay neurone in this monosynaptic reflex, but most reflexes involve at least one relay neurone.
例如，膝跳反射涉及股四头肌内的牵张感受器。敲击肌腱时，感受器沿感觉神经元将冲动传入脊髓，直接与支配同一肌肉的运动神经元形成突触，引起收缩。在这种单突触反射中没有中间神经元，但大多数反射至少包含一个中间神经元。
In a withdrawal reflex (e.g., touching a hot object), interneurones in the spinal cord integrate signals and simultaneously stimulate flexor muscles and inhibit antagonistic extensor muscles via inhibitory interneurones, ensuring a coordinated response. The pathway also sends signals to the brain to allow conscious awareness of pain.
在屈曲反射（如触摸烫物）中，脊髓中的中间神经元整合信号，并同时通过抑制性中间神经元刺激屈肌、抑制拮抗伸肌，确保协调反应。该通路也将信号传入大脑以产生痛觉意识。
9. Organisation of the Nervous System | 神经系统组织
The vertebrate nervous system is organised hierarchically. The CNS, consisting of the brain and spinal cord, is the integrating centre. The PNS carries all sensory and motor information. The motor division of the PNS is divided into the somatic nervous system, which controls voluntary skeletal muscle movements, and the autonomic nervous system, which regulates involuntary functions of internal organs.
脊椎动物的神经系统分级组织。由脑和脊髓组成的中枢神经系统是整合中心。外周神经系统负责传递所有感觉和运动信息。外周神经的运动分部再分为躯体神经系统（控制随意骨骼肌运动）和自主神经系统（调节内脏的不随意功能）。
The autonomic nervous system is further divided into the sympathetic and parasympathetic divisions. The sympathetic system prepares the body for ‘fight or flight’ – increasing heart rate, dilating pupils, and redirecting blood to muscles. The parasympathetic system dominates during ‘rest and digest’, slowing heart rate and stimulating digestion.
自主神经系统进一步分为交感和副交感分部。交感系统使身体为“战斗或逃跑”做好准备——加快心率、扩瞳、将血液重新分配至肌肉。副交感系统在“休息和消化”时占主导，使心率减慢并促进消化。
Many autonomic pathways involve two neurones in series: a preganglionic neurone with its cell body in the CNS and a postganglionic neurone in an autonomic ganglion. The sympathetic ganglia often lie close to the spinal cord, while parasympathetic ganglia are located near or within the target organ, giving rise to different neurotransmitter usage (e.g., noradrenaline for sympathetic postganglionic fibres, acetylcholine for parasympathetic).
许多自主神经通路包含两个串联神经元：节前神经元（胞体在中枢神经系统）和节后神经元（胞体在自主神经节）。交感神经节通常靠近脊髓，而副交感神经节位于靶器官附近或内部，导致神经递质使用不同（如交感节后纤维使用去甲肾上腺素，副交感纤维使用乙酰胆碱）。
10. Myelinated and Unmyelinated Neurones | 有髓鞘与无髓鞘神经元
A key distinction in nerve fibre structure is the presence or absence of a myelin sheath, which dramatically influences impulse propagation speed and energy efficiency. The table below summarises the main differences.
神经纤维结构的一个关键区别在于有无髓鞘，这显著影响冲动传播速度和能量效率。下表总结了主要差异。
| Feature | Myelinated Neurone | Unmyelinated Neurone |
|---|---|---|
| Myelin sheath | Present, formed by Schwann cells (PNS) or oligodendrocytes (CNS) | Absent; axons are simply embedded in glial cell cytoplasm or naked |
| Nodes of Ranvier | Regular gaps where ion channels are concentrated | None |
| Conduction mode | Saltatory conduction – impulse jumps from node to node | Continuous conduction – depolarisation spreads along entire membrane |
| Speed | Fast, up to 120 m/s | Slow, typically 0.5–10 m/s |
| Energy demand | Lower – fewer ions move across the membrane because depolarisation is restricted to nodes | Higher – the entire membrane must be depolarised and repolarised, requiring more active pumping |
| Axon diameter relation | Conduction speed increases with larger diameter and thicker myelin | Speed increases with larger diameter only |
The reflective white appearance of myelinated tracts in the CNS (white matter) contrasts with the grey colour of unmyelinated regions (grey matter), which contain cell bodies and synapses. In the PNS, myelin also assists axonal regeneration after injury.
中枢神经系统中髓鞘化通路（白质）的白色反光外观与含有细胞体和突触的无髓鞘区域（灰质）的灰色形成对比。在外周神经中，髓鞘还有助于损伤后轴突的再生。
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