The Nervous System and Neurons: A Summary 神经系统与神经元总结
The nervous system is responsible for processing and transmitting information throughout the body. It controls everything from sensory perception and movement to cognition and emotion.
神经系统负责处理和传递全身的信息。它控制着从感觉和运动到认知和情绪的所有功能。
It has two main divisions:
它分为两个主要部分：
- Central Nervous System (CNS) – composed of the brain and spinal cord.
中枢神经系统（CNS）：由大脑和脊髓组成。 - Peripheral Nervous System (PNS) – includes all the nerves that branch out from the CNS to the rest of the body.
外周神经系统（PNS）：包括从中枢神经系统延伸至全身的神经。
The basic functional unit of the nervous system is the neuron, a specialized cell designed to transmit electrical and chemical signals.
神经系统的基本功能单位是神经元，一种专门用于传递电信号和化学信号的细胞。
Structure of a Neuron 神经元的结构
A typical neuron has several main parts:
一个典型的神经元由以下几个主要部分组成：
- Dendrites 树突: Branch-like structures that receive signals from other neurons.
类似树枝的结构，接收来自其他神经元的信号。 - Cell body (soma 细胞体/胞体): Contains the nucleus and organelles; processes incoming signals.
含有细胞核和细胞器；负责处理输入信号。 - Axon 轴突: A long projection that sends electrical impulses away from the cell body.
长突起，将电信号传导离开细胞体。 - Myelin sheath 髓鞘: An insulating layer (often from Schwann cells in PNS) that speeds up signal transmission.
绝缘层（在PNS中通常由施旺细胞形成），加快信号传导速度。 - Axon terminals 轴突末端: The endpoints where neurotransmitters (e.g. acetylcholine) are released to communicate with the next cell.
末端释放神经递质（如乙酰胆碱），与下一个细胞交流。
Types of Glial Cells 神经胶质细胞的类型
Glial cells support and protect neurons. Their types differ in CNS and PNS.
神经胶质细胞负责支持和保护神经元。它们在中枢和外周有所不同。
- CNS 中枢神经系统:
- Astrocytes 星形胶质细胞
- Microglia 小胶质细胞
- Oligodendrocytes 少突胶质细胞
- Ependymal cells 室管膜细胞
- PNS 外周神经系统:
- Schwann cells 施旺细胞（形成髓鞘）
- Satellite glial cells 卫星细胞（支持神经元胞体）
Classification of Neurons (by structure) 神经元的分类（按结构）
Neurons are categorized by the number and type of extensions from the cell body.
神经元根据从胞体延伸出的突起的数量和类型进行分类。
Multipolar neurons 多极神经元
- One axon, many dendrites 一条轴突，多条树突
- Most common type 最常见的类型
- Integrate signals from other neurons 整合来自其他神经元的信号
Bipolar neurons 双极神经元
- One axon, one dendrite 一条轴突，一条树突
- Found in sensory organs (e.g., retina) 存在于感觉器官（如视网膜）
Unipolar neurons 单极神经元
- One single process that splits into two branches 单一突起分为两支
- Transmit sensory info from body to CNS 将感觉信息从身体传递至CNS
- Original structure is truly a single extension 原始结构是真正的单一突起
Pseudounipolar neurons 假单极神经元
- Start as bipolar in development (one axon + one dendrite) 发育时为双极（一个轴突 + 一个树突）
- Later fuse into a single process that splits into two 后来融合成一个突起并分成两支
- Despite having two branches, structurally different from unipolar 尽管有两支，但结构上不同于单极
- Common in sensory pathways 常见于感觉通路