How do nerves work?
神经是如何工作的？
Are nerves simply the wires in the body that conduct electricity, like the wires in the walls of your home or in your computer?
难道神经就只像是墙壁里的，或连接你计算机的电线一样传导电流吗？
This is an analogy often made, but the reality is that nerves have a much more complex job in the body.
通常我们会把神经比作电线，其实神经在人体内的功能复杂多了。
They are not just the wires, but the cells that are the sensors, detectors of the external and internal world,
它们不只是电线，还是具有感应功能的细胞，是体内和体外世界的探测器，
the transducers that convert information to electrical impulses, the wires that transmit these impulses,
是能够将信息转换为电冲动的传导器，是传递这些电冲动的电线，
the transistors that gate the information and turn up or down the volume
是阻截信息的晶体管，能上调或下调信息量，
and finally, the activators that take that information and cause it to have an effect on other organs.
最后，它是收取信息的启动装置，并对其他器官产生影响。
Consider this. Your mother gently strokes your forearm and you react with pleasure.
想想这个吧。当你妈妈抚摸你的手臂，你的反应是愉悦的。
Or a spider crawls on your forearm and you startle and slap it off.
或者当一只蜘蛛爬到你的手臂上，你受到惊吓，把它拍掉。
Or you brush your forearm against a hot rack while removing a cake from the oven and you immediately recoil.
或者，当你从烤箱取出蛋糕时，手臂碰到了炙热的烤架，你会马上收回来。
Light touch produced pleasure, fear, or pain.
轻触分别导致了愉悦、恐惧或疼痛。
How can one kind of cell have so many functions?
一类细胞怎么会具有这么多功能呢？
Nerves are in fact bundles of cells called neurons
事实上神经是由一大丛被称为神经元的细胞组成
and each of these neurons is highly specialized to carry nerve impulses,
而每一个神经元都会对应到专一的神经脉冲，
their form of electricity, in response to only one kind of stimulus, and in only one direction.
这种脉冲是以电的形式进行的，只针对一种刺激反应，并只能单向传递。
The nerve impulse starts with a receptor, a specialized part of each nerve, where the electrical impulse begins.
神经冲动启动从受体开始，受体是神经细胞一个专门部分，是电冲动的开始位置。
One nerve's receptor might be a thermal receptor, designed only to respond to a rapid increase in temperature.
有的神经受体可能是感受温度的，只在温度突然上升时反应。
Another receptor type is attached to the hairs of the forearm,
有的受体连接到前臂的毛发，
detecting movement of those hairs, such as when a spider crawls on your skin.
它们探测毛发的运动，比如蜘蛛在皮肤上的爬行时毛发的变化。
Yet another kind of neuron is low-threshold mechanoreceptor, activated by light touch.
还有一类神经细胞是很敏感的机械受体，通过轻触激活。
Each of these neurons then carry their specific information: pain, warning, pleasure.
每一个神经细胞传递他们接受的特定信息：疼痛，警告，乐趣。
And that information is projected to specific areas of the brain and that is the electrical impulse.
把信息投射到大脑的特定区域，这就是是电冲动。

The inside of a nerve is a fluid that is very rich in the ion potassium.
神经内部的液体含丰富的钾离子。
It is 20 times higher than in the fluid outside the nerve while that outside fluid has 10 times more sodium than the inside of a nerve.
在神经细胞内，钾离子浓度是胞外的20倍。而神经细胞外钠离子浓度是胞内的10倍。
This imbalance between sodium outside and potassium inside the cell
细胞内外之间钾钠不平衡，
results in the inside of the nerve having a negative electrical charge relative to the outside of the nerve, about equal to -70 or -80 millivolts.
导致神经细胞内相较于胞外带有约-70或-80毫伏的电位能。
This is called the nerve's resting potential.
这称为神经静息电位。
But in response to that stimulus the nerve is designed to detect, pores in the cell wall near the receptor of the cell open.
一旦神经探测到信息被激活，受体附近的细胞膜会打开小孔。
These pores are specialized protein channels that are designed to let sodium rush into the nerve.
这些小孔是特殊的蛋白通道，作用是让外部的钠离子进入神经内。
The sodium ions rush down their concentration gradient,
钠离子顺浓度梯度快速移动，
and when they do, the inside of the nerve becomes more positively charged-about +40 millivolts.
此时，神经内部就带正电荷，约是40毫伏。
While this happens, initially in the nerve right around the receptor,
一开始，受体附近的细胞会有这个变化，
if the change in the nerve's electrical charge is great enough,
如果神经的电荷的变化足够大，
if it reaches what is called threshold, the nearby sodium ion channels open,
达到了所谓“阈值”，会让周围细胞的钠离子通道打开，
and then the ones nearby those, and so on, and so forth,
以至于更大范围的钠离子的开启，如此传递下去。
so that the positivity spreads along the nerve's membrane to the nerve's cell body
这样一来，正电荷覆盖了整个神经，从膜到细胞体，
and then along the nerve's long, thread-like extension, the axon.
也覆盖了细长的、如同棉线一样的延伸部分，轴突。
Meanwhile, potassium ion channels open, potassium rushes out of the nerve, and the membrane voltage returns to normal.
同时，钾离子通道打开了，钾离子从神经内流出，膜电位从而恢复正常，
Actually, overshooting it a bit.
事实上，还矫枉过正。
And during this overshoot, the nerve is resistant to further depolarization
点位矫正过头时，神经不能再被去极化，
it is refractory, which prevents the nerve electrical impulse from traveling backwards.
此时神经是麻木的，防止了电冲动往回走。
Then, ion pumps pump the sodium back back out of the nerve,
然后，离子泵把神经内的钠离子送到外面，
and the potassium back into the nerve, restoring the nerve to its normal resting state.
同时，钾离子回到神经内，神经恢复到正常的静息电位。
The end of the nerve, the end of the axon, communicates with the nerve's target.
在神经细胞的末梢，也就是轴突的尾端，会负责传递信息给下一个目标。
This target will be other nerves in a specialized area of the spinal cord, to be processed and then transmitted up to the brain.
它的目标可能是一个位于脊椎的神经细胞，那里的神经细胞可以处理并传递信息给脑部。
Or the nerve's target may be another organ, such as a muscle.
神经的目标也有可能是某个组织，例如：肌肉组织。
When the electrical impulse reaches the end of the nerve,
当神经脉冲到达细胞末端时，
small vesicles, or packets, containing chemical neurotransmitters, are released by the nerve and rapidly interact with the nerve's target.
一些囊泡，也可说是包含化学传导物质的小包裹，会被神经细胞释放，快速作用于目标神经。
This process is called synaptic transmission,
此过程称为突触传递，
because the connection between the nerve and the next object in the chain is called a synapse.
因为在传输链中，神经与下一个神经连接的部位被称为突触。
And it is here, in this synapse, that the neuron's electrical information can be modulated, amplified,
就在这里，在这个突触中，神经元电信息可能会被调节，被放大，
blocked altogether or translated to another informational process.
被全部拦截，或者转换成另一种信息处理。