神经元是如何工作的

关于我在一本心理学复习了生物学，我将它作为笔记放在这里......

神经元的结构是从树突(dendrites)开始的（见图2-5）​，树突作为接收者可以接收大部分的输入信息。就像一棵树的树枝向外伸展来获取阳光，这些树突纤维从细胞体向外伸展，它们像一张网一样收集信息，这些信息来自其他神经元或者受到刺激的感觉器官（例如，眼睛、耳朵或皮肤）​。

树突将信息传递给神经元的中间部分，这个部分被称为细胞体(cell body)或胞体(soma)，它是神经元的指令中心。除了容纳细胞的染色体外，胞体还会对细胞接收到的成百上千（有时，成千上万）的信息进行实时评估。这些评估非常复杂，因为神经元收到的信息有些是兴奋的(excitatory)（如“放电”​）​，有些是抑制的(inhibitory)（如“不要放电”​）​。胞体做出的这个决定取决于它的总体唤醒水平，激活水平取决于输入信息的总和。

当兴奋压倒抑制时，神经元发出信息，信息沿着单一的“发射器”纤维轴突(axon)传递出去。轴突就像是一棵树的树干。就像树木的高度各不相同一样，这些轴突的长度差异也很大。大学的篮球运动员身上从脊髓到脚趾的轴突长度可能会超过91厘米，而大脑内的中间神经元的轴突可能只有几分之一厘米。当信息到达轴突末端时，它会传递到另一个神经元。在我们给你详细展示这个过程之前，请先了解一下轴突中发生的事情。

动作电位(the action potential)

当细胞体内的唤醒程度达到临界水平时，它会在轴突内触发一个电子脉冲——类似于相机的闪光灯开启——我们称之为神经元“激发”​。这是什么意思呢？就像电池，轴突从化学物质中获得电能。这种化学物质就是离子(ions)。在它正常的静息状态——也就是静息电位(resting potential)——轴突内的离子带有负电荷。但是这个负电荷状态很容易打破。当细胞体开始兴奋时，它会触发一系列事件，即动作电位(action potential)。这会暂时性反转电位，并使得电信号沿着轴突向前（见图2-5）​，从而让神经元放电。

电荷如何从负转为正？

在动作电位期间，与胞体相邻的小块轴突膜中的小孔打开，正离子快速流入。轴突那块区域内的内部电位即刻从负转为正（这发生在千分之一秒的时间内）​。然后，就像一排多米诺骨牌倒下一样，细胞膜内的这些电荷沿着轴突前进。最终，电信号从胞体到达轴突末梢。动作电位没有半途而废的：轴突要么“激发”​，要么“不激发”​。神经科学家把这称之为“全或无定律”(all-or-none principle)。顺便提一下，当这个过程失去控制，大量神经元变得高度敏感，过于容易放电时，就可能导致癫痫发作。

在放电后，细胞的“离子泵”即刻放出正电位离子并将神经元恢复至静息电位，准备下次放电。令人难以置信的是，整个复杂的过程持续时间可能不到百分之一秒。这是多么惊人的表现！一旦完成，动作电位所携带的信号就预备传递到下一个神经元。下面，让我们来了解一下接下来的过程。

突触传递(Synaptic Transmission)

那么，当电脉冲到达轴突时会发生什么呢？它会自动传递给下一个神经元吗？很不幸，它不会。它还有一个挑战需要完成！原因如下：尽管神经细胞彼此靠近，但是它们实际上并没有接触。它们之间有一个微小的间隙，被称为突触(synapse)（见图2-5）​。这个间隙作用相当于一个电绝缘体。这个突触间隙阻止了电荷直接从轴突跳入下一个细胞的树突中(Dermietzel,2006)。相反，它必须将自己从电信号转化为化学信号才能跨越这个间隙。这就是神经递质（neurotransmitters，一种你可能听说过的物质）发挥作用的地方。

神经递质

当电脉冲到达轴突末梢（或末梢分支）时，突触小体(terminal buttons)中的微小气泡状囊泡爆裂，将其中的化学物质释放到突触中。这种化学物质就是神经递质。然后，这些神经递质尝试带着神经信号跨过间隙进入下一个神经元（见图2-5）​。

About I reviewed biology in a psychology book that I put here as notes ......

The structure of a neuron begins with dendrites (see Figure 2-5), which act as receivers for most of the input information. Like the branches of a tree that stretch outward to get sunlight, these dendritic fibers stretch outward from the cell body, and they act as a web to collect information that comes from other neurons or from stimulated sensory organs (e.g., the eyes, ears, or skin).
Dendrites transmit information to the middle part of the neuron, called the cell body or soma, which is the neuron's command center. In addition to housing the cell's chromosomes, the soma makes real-time evaluations of the hundreds (and sometimes, thousands) of messages the cell receives. These evaluations are complex because some of the information received by the neuron is excitatory (e.g., “discharge”) and some is inhibitory (e.g., “don't discharge").

Translated with DeepL.com (free version)
When excitation overwhelms inhibition, the neuron sends a message, which travels along a single “transmitter” fiber, the axon. An axon is like the trunk of a tree. Just as trees vary in height, these axons vary greatly in length. The axon from the spinal cord to the toes in a college basketball player may be more than 91 centimeters long, while the axon of an interneuron in the brain may be only a fraction of a centimeter. When a message reaches the end of an axon, it is passed on to another neuron. Before we show you this process in detail, understand what happens in an axon.

动作电位(the action potential)

When a critical level of arousal is reached within the cell, it triggers an electrical pulse within the axon - similar to a camera's flash turning on - that we call neuronal “excitation “. What does this mean? Like a battery, the axon receives electrical energy from a chemical. This chemical is ion(ions). In its normal resting state - the resting potential - the ions in the axon have a negative charge. But this negative state of charge is easily broken. When the cell body becomes excited, it triggers a series of events known as the action potential. This temporarily reverses the potential and causes an electrical signal to travel forward along the axon (see Figure 2-5), thereby allowing the neuron to discharge.

How does charge change from negative to positive?

During an action potential, small pores in the membrane of the axon adjacent to the cytosol open and positive ions flow in rapidly. The internal potential within that area of the axon instantly shifts from negative to positive (this happens in a thousandth of a second). Then, like a row of dominoes falling, these charges within the cell membrane advance along the axon. Eventually, the electrical signals travel from the cytosol to the end of the axon. There is no halfway point in an action potential: the axon is either “excited” or “unexcited”. Neuroscientists call this the “all-or-none principle”. Incidentally, when this process gets out of control and large numbers of neurons become highly sensitive and discharge too easily, seizures can result.

Immediately after the discharge, the cell's “ion pump” releases positively charged ions and restores the neuron to its resting potential, ready for the next discharge. Incredibly, this complex process can take less than a hundredth of a second. What an amazing performance! Once completed, the signal carried by the action potential is ready to be transmitted to the next neuron. Here, let's take a look at the process that follows.

Synaptic transmission

So what happens when an electrical impulse reaches an axon? Is it automatically passed on to the next neuron? Unfortunately, it doesn't. It has another challenge to accomplish! Here's why: even though the nerve cells are close to each other, they don't actually touch. There is a tiny gap between them, called a synapse (see Figure 2-5). This gap acts as the equivalent of an electrical insulator. This synaptic gap prevents the charge from jumping directly from the axon into the dendrite of the next cell (Dermietzel, 2006). Instead, it must convert itself from an electrical signal to a chemical signal in order to cross this gap. This is where neurotransmitters, a substance you may have heard of, come into play.

Neurotransmitters

When an electrical impulse reaches an axon terminal (or terminal branch), tiny bubble-like vesicles in synaptic vesicles (terminal buttons) burst, releasing chemicals from them into the synapse. This chemical is the neurotransmitter. These neurotransmitters then try to carry the nerve signal across the gap to the next neuron (see Figure 2-5).