How do nerves work? - Elliot Krane

神經是如何運作呢？ - Elliot Krane

譯者: Hao-Wei Chang 審譯者: Jephian Lin

How do nerves work?

神經是如何運作呢？

Are nerves simply the wires in the body

難道神經就只像是

that conduct electricity, like the wires in the walls of your home

牆壁裡的線路或 連接你電腦的電線一樣

or in your computer?

傳導電流嗎？

This is an analogy often made,

這是以個常用的比喻，

but the reality is that nerves have a much more complex job in the body.

但事實上人體內 神經的功能卻是複雜許多。

They are not just the wires, but the cells that are the sensors,

它們不只是傳遞線路， 有些細胞是感受器，

detectors of the external and internal world,

負責偵測身體內外的變化；

the transducers that convert information to electrical impulses,

有些像是轉換器， 接著將訊息轉化為電能脈衝；

the wires that transmit these impulses,

有些像電線， 傳遞這些脈衝；

the transistors that gate the information

有些像是電晶體般是訊號的閘道，

and turn up or down the volume-

並能調整其強度；

and finally, the activators that take that information

最後，動器接受到訊號

and cause it to have an effect on other organs.

並對其他器官產生影響。

Consider this. Your mother gently strokes your forearm

想想這個吧： 你媽媽輕拍你的手臂，

and you react with pleasure.

而你感到快樂。

Or a spider crawls on your forearm and you startle and slap it off.

或蜘蛛爬到你手臂上， 你嚇了一跳後把牠拍開。

Or you brush your forearm against a hot rack while removing a cake from the oven

或當你把蛋糕拿出烤爐時， 你的前臂擦過上面高溫的鐵架，

and you immediately recoil.

讓你立刻往後縮了一下。

Light touch produced pleasure, fear, or pain.

輕觸產生了愉快、害怕或是疼痛。

How can one kind of cell have so many functions?

一種細胞怎麼包含這麼多功能呢？

Nerves are in fact bundles of cells called neurons

事實上神經是由一大叢 被稱為神經元的細胞組成

and each of these neurons is highly specialized to carry nerve impulses,

而每一個神經元都會 對應到專一的神經脈衝。

their form of electricity,

這種脈衝是以電的形式進行。

in response to only one kind of stimulus, and in only one direction.

只針對一種刺激反應， 並只能將訊號傳往單一方向。

The nerve impulse starts with a receptor,

神經脈衝始於受器。

a specialized part of each nerve,

受器是神經細胞上特殊的部位，

where the electrical impulse begins.

也是電能脈衝起始的地方。

One nerve's receptor might be a thermal receptor,

神經細胞的受器可能是一個溫度感受器，

designed only to respond to a rapid increase in temperature.

專門負責感應快速升高的溫度。

Another receptor type is attached to the hairs of the forearm,

另外一種受器依附在前臂的毛髮上，

detecting movement of those hairs, such as when a spider crawls on your skin.

偵測毛髮的震動， 例如：蜘蛛在皮膚上的爬行。

Yet another kind of neuron is low-threshold mechanoreceptor,

有另外一種閾值較低的機械性受器， （譯注：閾值指是刺激受器、產生脈衝訊號的基準）

activated by light touch.

它們會被輕觸啓動。

Each of these neurons then carry their specific information:

每一種神經元傳遞特殊的訊息：

pain, warning, pleasure.

疼痛、警覺、愉悅。

And that information is projected to specific areas of the brain

而這些訊號藉由電能脈衝，

and that is the electrical impulse.

投射到腦部特殊的區域。

The inside of a nerve is a fluid that is very rich in the ion potassium.

在神經細胞中的液體含有高濃度的鉀離子。

It is 20 times higher than in the fluid outside the nerve

在神經細胞內 鉀離子濃度是胞外的 20 倍。

while that outside fluid has 10 times more sodium than the inside of a nerve.

而神經細胞外 鈉離子濃度是胞內的 10 倍

This imbalance between sodium outside and potassium inside the cell

神經細胞內外鈉、鉀離子的不平衡

results in the inside of the nerve having a negative electrical charge

導致神經細胞內相較於

relative to the outside of the nerve,

胞外帶有約

about equal to -70 or -80 millivolts.

-70 或 -80 毫伏特的電位能。

This is called the nerve's resting potential.

這稱為神經的靜止電位。

But in response to that stimulus the nerve is designed to detect,

在神經細胞接受到特定的刺激時，

pores in the cell wall near the receptor of the cell open.

受器附近細胞的 細胞膜上通道會開啓。

These pores are specialized protein channels

這些孔洞是特殊的蛋白質通道，

that are designed to let sodium rush into the nerve.

它們可以讓鈉離子進入神經細胞。

The sodium ions rush down their concentration gradient,

因為濃度差異，鈉離子會快速地湧入細胞，

and when they do, the inside of the nerve becomes more positively charged-

這會讓細胞內的電位能偏向正電 ——

about +40 millivolts.

大約 +40 毫伏特。

While this happens, initially in the nerve right around the receptor,

這會先發生在受器附近的神經細胞，

if the change in the nerve's electrical charge is great enough,

一但改變發生時， 如果神經細胞中電位能變化

if it reaches what is called threshold,

達到閾值，

the nearby sodium ion channels open, and then the ones nearby those,

鄰近的鈉離子通到就會開啓， 並促成下一組通道的開啓，

and so on, and so forth,

並依此類推。

so that the positivity spreads along the nerve's membrane

所以正電訊號就可以 在神經細胞膜上傳遞

to the nerve's cell body

到細胞本體，

and then along the nerve's long, thread-like extension, the axon.

然後傳向一個 長線狀的突出，這叫軸突。

Meanwhile, potassium ion channels open,

在此同時，鉀離子通到會開啓，

potassium rushes out of the nerve,

換鉀離子衝出神經細胞，

and the membrane voltage returns to normal.

並讓膜電位回覆正常。

Actually, overshooting it a bit.

事實上，會超過正常一點點。

And during this overshoot,

在電位能低於正常值時

the nerve is resistant to further depolarization-it is refractory,

神經較不容易被活化， 這被稱為「不反應期」

which prevents the nerve electrical impulse from traveling backwards.

這防止神經脈衝往回傳遞。

Then, ion pumps pump the sodium back back out of the nerve,

再來，離子幫浦將 鈉離子汲出神經細胞外

and the potassium back into the nerve,

再將鉀離子運回細胞，

restoring the nerve to its normal resting state.

使細胞回覆到正常的靜止電位。

The end of the nerve, the end of the axon,

在神經細胞的末梢， 也就是軸突的尾端，

communicates with the nerve's target.

會負責傳遞訊號給下一個標的。

This target will be other nerves in a specialized area of the spinal cord,

它的標的可能是 一個位於脊椎的神經細胞，

to be processed and then transmitted up to the brain.

那裡的神經細胞可以 處理並傳遞訊號給腦部。

Or the nerve's target may be another organ, such as a muscle.

神經的標的也有可能是某個組織， 例如：肌肉組織。

When the electrical impulse reaches the end of the nerve,

當神經脈衝到達細胞末端時，

small vesicles, or packets, containing chemical neurotransmitters,

一些的囊泡，也可說是 包含化學傳導物質小包裹，

are released by the nerve and rapidly interact with the nerve's target.

會被神經細胞釋放， 並快速的和其標的作用，

This process is called synaptic transmission,

這過程就稱為突觸傳導。

because the connection between the nerve and the next object in the chain

這是因為連接神經和標的

is called a synapse. And it is here, in this synapse,

被稱為突觸。而也是在突觸這裡，

that the neuron's electrical information can be modulated,

神經脈衝可以被調整、

amplified,

放大、

blocked altogether

阻斷

or translated to another informational process.