📚 A-Level Biology: Nervous System Key Points | A-Level 生物：神经系统考点精讲
The nervous system is a rapid communication network that uses electrical impulses and chemical signals to coordinate the activities of an organism. It allows organisms to detect changes in their environment and respond appropriately. In A-Level Biology, understanding the structure and function of neurones, the mechanisms of impulse transmission, and the organisation of the nervous system is essential.
神经系统是一个利用电脉冲和化学信号来协调生物体活动的快速通讯网络。它使生物体能够感知环境变化并作出适当反应。在A-Level生物学科中，理解神经元的结构与功能、冲动传递机制以及神经系统的组织是至关重要的。
1. Overview of the Nervous System | 神经系统概述
The nervous system can be divided into the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all nerves outside the CNS. Neurones are the basic functional units that transmit information in the form of nerve impulses.
神经系统可分为中枢神经系统（CNS，包括脑和脊髓）和外周神经系统（PNS，包括CNS以外的所有神经）。神经元是传递神经冲动信息的基本功能单位。
Three main types of neurones are sensory neurones (carry impulses from receptors to the CNS), relay neurones (intermediate neurones within the CNS that connect sensory to motor neurones), and motor neurones (carry impulses from the CNS to effectors such as muscles and glands).
神经元的三种主要类型是感觉神经元（将冲动从感受器传至中枢神经系统）、中间神经元（中枢神经系统内连接感觉和运动神经元的中间神经元）和运动神经元（将冲动从中枢神经系统传到效应器，如肌肉和腺体）。
2. Structure of a Motor Neurone | 运动神经元的结构
A typical motor neurone has a cell body containing the nucleus and many organelles, dendrites that receive incoming impulses, and a long axon that conducts impulses away from the cell body. The axon is often insulated by a myelin sheath, which is formed by Schwann cells wrapping around the axon. Gaps between adjacent Schwann cells are called nodes of Ranvier.
典型的运动神经元有一个包含细胞核和众多细胞器的细胞体、接收冲动的树突以及将冲动传离细胞体的长轴突。轴突常由髓鞘绝缘，髓鞘由施万细胞包裹轴突形成。相邻施万细胞之间的间隙称为郎飞结。
Sensory neurones have a different structure: they are pseudounipolar, with a single axon that splits into two branches – one running to a receptor and the other into the CNS. The cell body is located in a ganglion just outside the spinal cord.
感觉神经元结构不同：它们是假单极的，具有一条轴突，该轴突分成两支——一支连向感受器，另一支进入中枢神经系统。细胞体位于脊髓外的一个神经节内。
Relay neurones are multipolar, with many dendrites and a short axon, and are entirely within the CNS.
中间神经元是多极的，有许多树突和短轴突，完全位于中枢神经系统内。
3. 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. This resting potential is typically around -70 mV. The membrane is said to be polarised.
神经元未传递冲动时，轴突内部相对于外部带负电。这一静息电位通常约为-70 mV，此时膜处于极化状态。
The resting potential is established and maintained by the sodium-potassium pump, which actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell, using ATP. This creates concentration gradients for these ions.
静息电位由钠钾泵建立和维持。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，从而形成这些离子的浓度梯度。
In addition, the axon membrane contains many potassium leak channels that are permanently open. At rest, the membrane is more permeable to K⁺ than to Na⁺, so K⁺ diffuses out down its concentration gradient, leaving the inside more negative.
此外，轴突膜上有许多始终开放的钾泄漏通道。静息时，膜对K⁺的通透性大于Na⁺，因此K⁺沿浓度梯度向外扩散，使膜内更负。
4. Generation of Action Potential | 动作电位的产生
If a stimulus depolarises the membrane to the threshold potential (approximately -55 mV), voltage-gated sodium channels open. Na⁺ ions rush into the cell down their electrochemical gradient, causing a rapid rise in membrane potential to about +40 mV – this is depolarisation.
若刺激使膜电位去极化达到阈电位（约-55 mV），电压门控钠通道开放。Na⁺沿电化学梯度涌入细胞，使膜电位迅速上升至约+40 mV，这就是去极化。
Shortly after opening, the sodium channels become inactivated. At the same time, voltage-gated potassium channels open, allowing K⁺ to flow out of the cell. This repolarises the membrane, bringing the potential back down toward the resting level.
钠通道开放后很快会失活。同时，电压门控钾通道开放，使K⁺外流。这使膜复极化，电位回到接近静息水平。
Because the potassium channels close relatively slowly, there is a temporary overshoot known as hyperpolarisation (or undershoot). The sodium-potassium pump then restores the original ion distribution, and the resting potential is re-established. During repolarisation and hyperpolarisation, the neurone is in a refractory period and cannot fire another action potential.
由于钾通道关闭相对缓慢，会出现一过性的超极化（也称下冲）。随后钠钾泵恢复原有的离子分布，静息电位得以重建。在复极化和超极化期间，神经元处于不应期，无法产生另一个动作电位。
5. Propagation of Action Potentials | 动作电位的传导
When an action potential occurs in one region of the axon, local electrical currents flow to adjacent regions. This depolarises the neighbouring membrane to threshold, triggering a new action potential. In this way, the impulse moves along the axon without loss of strength, and it travels in one direction because the preceding region is refractory.
当轴突某一区域产生动作电位时，局部电流流向邻近区域，使相邻膜去极化达到阈值，触发出新的动作电位。这样，冲动沿轴突传导而不会减弱，且因前面区域处于不应期而单向传导。
In unmyelinated neurones, this process occurs continuously along the whole length of the axon membrane, which is relatively slow. In myelinated neurones, the myelin sheath acts as an insulator, preventing ion exchange. Action potentials can only occur at the nodes of Ranvier where the axon membrane is exposed. The impulse appears to jump from node to node – this is called saltatory conduction, and it is much faster and more energy-efficient than continuous conduction.
在无髓鞘神经元中，这一过程沿轴突膜全程连续发生，速度相对较慢。在有髓鞘神经元中，髓鞘起到绝缘作用，阻止离子交换。动作电位只能在膜暴露的郎飞结处产生。冲动似乎从一个结跳到下一个结——这称为跳跃传导，它比连续传导快得多，且更节能。
6. Synaptic Structure | 突触的结构
A synapse is a junction between two neurones or between a neurone and an effector cell. The presynaptic terminal (synaptic knob) contains many mitochondria and numerous synaptic vesicles filled with neurotransmitter. The synaptic cleft is the narrow gap separating the presynaptic and postsynaptic membranes. The postsynaptic membrane bears specific receptor proteins for the neurotransmitter.
突触是两个神经元之间或神经元与效应细胞之间的接头。突触前末梢（突触小体）含有许多线粒体和大量充满神经递质的突触小泡。突触间隙是将突触前膜和突触后膜分隔开的窄隙。突触后膜上带有针对神经递质的特异性受体蛋白。
7. Synaptic Transmission Mechanism | 突触传递机制
When an action potential arrives at the presynaptic knob, it depolarises the membrane, causing voltage-gated calcium ion channels to open. Ca²⁺ ions diffuse into the presynaptic knob from the synaptic cleft.
动作电位到达突触前末梢时，使膜去极化，导致电压门控钙离子通道开放。Ca²⁺从突触间隙扩散进入突触前末梢。
The influx of Ca²⁺ causes synaptic vesicles to move, fuse with the presynaptic membrane, and release neurotransmitter into the synaptic cleft by exocytosis.
Ca²⁺内流使突触小泡移动，与突触前膜融合，并通过胞吐作用将神经递质释放到突触间隙中。
The neurotransmitter diffuses across the cleft and binds specifically to receptors on the postsynaptic membrane. This binding opens ligand-gated ion channels, generating a postsynaptic potential. Depending on the neurotransmitter and receptor, this can be an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).
神经递质扩散通过间隙，并与突触后膜上的受体特异性结合。这种结合会打开配体门控离子通道，产生突触后电位。根据神经递质和受体的不同，这可以是兴奋性突触后电位（EPSP）或抑制性突触后电位（IPSP）。
8. Neurotransmitters and Cholinergic Synapses | 神经递质与胆碱能突触
Acetylcholine (ACh) is a common excitatory neurotransmitter in vertebrates. A synapse that uses ACh is called a cholinergic synapse. An important example is the neuromuscular junction (NMJ), where a motor neurone synapses with a skeletal muscle fibre.
乙酰胆碱（ACh）是脊椎动物体内一种常见的兴奋性神经递质。使用ACh的突触称为胆碱能突触。一个重要的例子是神经肌肉接头（NMJ），即运动神经元与骨骼肌纤维形成突触的部位。
At the postsynaptic membrane of the NMJ, ACh binds to receptors that act as sodium ion channels, causing Na⁺ influx and depolarisation. If sufficient depolarisation reaches the threshold, an action potential is triggered in the muscle fibre, leading to contraction.
在神经肌肉接头的突触后膜上，ACh与充当钠离子通道的受体结合，引起Na⁺内流和去极化。如果足够的去极化达到阈值，就会在肌纤维中触发动作电位，导致肌肉收缩。
To prevent continuous stimulation, the enzyme acetylcholinesterase, located in the synaptic cleft, rapidly hydrolyses ACh into acetate and choline. Choline is taken back up into the presynaptic neurone and reused to synthesise more ACh.
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。胆碱被重新吸收回突触前神经元，用于重新合成ACh。
Some neurotransmitters are inhibitory; for example, GABA opens chloride ion channels, causing hyperpolarisation and making it less likely that the postsynaptic neurone will fire (IPSP).
有些神经递质是抑制性的；例如，GABA打开氯离子通道，引起超极化，使突触后神经元更难产生冲动（产生抑制性突触后电位）。
9. Reflex Arc and Its Components | 反射弧及其组成
A reflex is a rapid, automatic, involuntary response to a stimulus that follows a specific neural pathway called a reflex arc. The basic components of a reflex arc are: receptor → sensory neurone → integration centre (often the spinal cord) → motor neurone → effector.
反射是对刺激的快速、自动、非随意反应，遵循一条特定的神经通路，即反射弧。反射弧的基本组成部分为：感受器 → 感觉神经元 → 整合中心（常为脊髓） → 运动神经元 → 效应器。
The knee-jerk reflex is a well-known example. Tapping the patellar tendon stretches muscle spindles (receptors) in the quadriceps muscle. Sensory neurones transmit impulses into the spinal cord, where they make direct excitatory synapses with motor neurones supplying the same muscle. Simultaneously, they send branches to inhibitory interneurones that suppress motor neurones of the antagonist (hamstring) muscle, allowing the lower leg to extend smoothly.
膝跳反射是一个众所周知的例子。敲击髌韧带会拉伸股四头肌内的肌梭（感受器）。感觉神经元将冲动传入脊髓，在那里与支配同一肌肉的运动神经元形成直接的兴奋性突触。同时，感觉神经元发出分支到抑制性中间神经元，这些中间神经元抑制拮抗肌（腘绳肌）的运动神经元，使小腿顺畅伸展。
10. Central and Peripheral Nervous Systems | 中枢与外周神经系统
The CNS processes and integrates information. The brain is the main control centre for complex behaviours, while the spinal cord coordinates fast reflexes and conveys ascending (sensory) and descending (motor) information.
中枢神经系统负责处理和整合信息。脑是复杂行为的主要控制中心，而脊髓协调快速反射并传导上行（感觉）和
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