of neuron structure.

神經元的結構

Neurons in adults have a soma.

成年人的神經元細胞具有一個細胞本體

It's also called a cell body-- soma.

也稱作細胞體

And they have processes called neurites,

神經元有神經突

which are divided into dendrites and axons.

會分化成為軸突和樹突

Dendrites are usually short, branched processes

樹突比較短、分支較多

that are often covered in small spines that

會有許多小分支

increase their surface area and perform some other functions.

以增加它們的表面積且也執行其他的一些功能

So these are dendrites.

這些就是樹突

And then the other neurite they have

而另一種神經突

is called an axon, which is usually

叫做軸突

long and unbranched until it reaches its end.

通常較長且無分支，一直延伸到底端

So this is the axon.

這就是軸突

The area where the axon leaves the soma

這就是軸突離開胞體的地方

is called the axon hillock.

叫做軸丘

The axon may be short or it may be very long, up to one meter

軸突或短或長，長的可以到一公尺

or more.

或更長

And it usually is unbranched for most or all of that length,

而且常常是沒有分支的

until it gets to the end, in these structures,

在這些結構中

which are called axon terminals.

最末多是軸突的終端

And at this point, it will often branch and create

而這個地方可能會有些許分岔

multiple axon terminals.

形成多個軸突終端

The first part of the axon is called

軸突的第一個部分

the axon initial segment.

叫做軸突初始片段

Or it's also called the trigger zone.

或者叫做 trigger zone

And we'll get into the reason for that in the next video.

我們在下一部影片中會解釋這是為什麼

Axons can be so long that they are

軸突能夠長的很長

dependent on systems that transport substances

但依賴從細胞體運來的

from the soma, which contains most of the organelles,

物質維持運作，這些物質大多由胞器運送至末端

to the axon terminals, and vice versa.

且反之亦然

Things have to be transported both directions.

物質的運送是雙向的

And the axon is dependent on those systems.

且軸突依賴這些系統維持運作

Large axons are usually wrapped in a sheath

較大的軸突通常外圍

of a material called myelin.

有髓鞘包覆

And axons that have a myelin sheath

而這些具有髓鞘包覆的軸突

have little gaps between these segments

通常在這些片段之間有些空隙

of myelin call nodes of Ranvier.

稱為蘭氏結

So the sheath I've drawn in yellow

我把髓鞘用黃色畫

is the myelin, each of these little segments of sheath here.

每個小的髓鞘片段都是

And these gaps that regularly interrupt the sheath

而這些間隙通常會出現在髓鞘間

are called nodes of Ranvier, these little gaps in the myelin

叫做蘭氏結，氏在髓鞘間的

sheath.

小間隙

The axon terminals will come very

軸突終端會與目標身神經元

close to the target cells of the neuron.

細胞距離非常接近

And I'll just draw it here.

我會畫成這樣

So these are the target cells.

這些就是目標細胞

And these targets cells may be another neuron,

這些目標細胞可能是另一個神經元

they may be a muscle cell, or they may be a gland cell.

也可能是肌肉細胞，或者腺體細胞

A few neurons even have axons that terminate on capillaries,

有些神經元甚至會將軸突延伸到微血管中

to secrete substances called hormones into the bloodstream.

釋放賀爾蒙激素進入血管當中

The place where an axon terminal comes

軸突終端

close to touching the target cell is called a synapse.

與目標細胞之間的交接處稱為突觸

This is a pretty typical structure for a neuron.

這是神經元中一個十分典型的構造

But there are multiple structural types

但有許多種結構類型

of neurons, each of which can be further divided into subtypes.

的神經元，每種神經元都會分出亞型

So let's go over some of the big categories

我們來看看幾個比較大的

of structural types of neurons.

神經元結構的種類

In the central nervous system, neurons

在中樞神經系統中，神經元們

start as neural stem cells, which

從神經幹細胞開始生成

turn into most of the cell types of the central nervous system.

最後變成中樞神經系統中的大部分的細胞類型

And these neural stem cells then differentiate

而這些神經幹細胞之後

into cells called neuroblasts.

會分化成神經母細胞

And don't worry about the details here.

我們現在先停在這邊，細節的部份

Because we'll go into a lot more detail

我們會在之後介紹神經系統發展的

in other videos on development of the nervous system.

其他的影片中提到

But neural stem cells and neuroblasts

但是神經幹細胞和神經母細胞

look pretty similar.

看起來相當類似

They're basically just shapeless cells without processes.

基本上沒有形狀和動作

Neural stem cells can become almost any neural cell

神經幹細胞能轉化成幾乎任何一種

of the central nervous system, while neuroblasts can only

中樞神經系統的神經細胞，而神經母細胞

become neurons.

就只能轉化成神經元而已

Neuroblasts will then migrate away

神經母細胞會從

from the neural stem cells to the location

神經幹細胞旁遷離

that their somas will have after development.

至特定地區然後成長發展出細胞體

Neuroblasts then extend a process, which is an axon,

神經母細胞會變成神經元然後長出軸突

toward their target cell.

與目標細胞靠近

And that axon is tipped with this structure called a growth

而在軸突尖端會有個叫做生長板的

cone-- growth cone.

東西

The axon growth cone follows guidance cues

軸突生長板跟著環境中的指令

in the environment until it reaches

帶領軸突生長直到抵達

the target cell of the neuron.

目標神經元細胞

A similar process occurs for neurons

在周圍神經系統的神經元

in the peripheral nervous system.

也經歷相似的過程

But the original and the migrating cells

但是引導那些神經元

for those neurons are neural crest cells, instead of

開始進入生長初始階段和細胞遷移的

neural stem cells and neuroblasts.

是神經脊細胞而不是神經幹細胞和神經母細胞

Neurons at this stage have only one process, which is an axon.

在這個階段的神經元只有一個分支，就是軸突

So they are now called unipolar neurons-- unipolar.

所以叫做單極神經元

That's the structural type of this neuron

這是這種類型神經元的結構

because there's one pole to the cell, one process giving

因為只有單極延伸至細胞

a sense of direction on this otherwise shapeless cell.

給予似乎形狀上沒有規則的細胞一個方向性

Unipolar neurons are present in humans,

單極細胞主要在人類發育過程當中

mainly during development.

被找到

The next structural type of neuron has a soma.

而下個介紹的神經元結構種類是具有一個細胞體

And it has one axon.

和一個軸突

But it also has one dendrite.

以及包含一個樹突

So since this structural type of neuron

因為有兩個分化部分

has two processes, or two poles, it's

或是兩個端點

called a bipolar neuron-- bipolar.

就被稱作雙極神經元

The next structural type of neuron

下一個神經元結構

has a soma, just like the others, and one axon.

和其他種神經元一樣具有一個細胞體和一個軸突

But it has multiple dendrites.

但它有多個樹突

And so since it's going to have multiple poles,

所有會有多個端點

it's called a multipolar neuron-- multipolar.

就被稱作多極神經元

And this is the most common structural type

而這是在成人身上

of neuron in adult humans.

最常被發現的神經元類型

The last big category of structural types of neurons

最後一個數量上比較多的一種結構的神經元

is a little different.

有點不同

It has a soma, like all the rest.

跟其他種類一樣有個細胞體

And then it has one a short process

然後有個端分支延伸出去

coming out of the soma, that then divides

然後迅速分岔成

into two long processes going in different directions.

兩個往不同方向延伸的

And these are both axons.

很長的軸突

The axon bringing information in from the periphery

其中一個將資訊從周圍帶進細胞體的

is called the peripheral axon.

稱為周圍軸突

And the axon bringing information

另一個將資訊

into the central nervous system is called the central axon.

但近中樞神經系統的軸突稱為中樞軸突

The very end of the peripheral axon acts a lot like dendrites

周圍軸突的末端功能與

do on the other structural types of neurons.

其他結構的神經元中的樹突類似

And we'll start to go over the function of dendrites and axons

我們會開始在接下來的影片中

in the next video.

介紹軸突和樹突的功能

And then this part of the peripheral axon near the end

然後周圍軸突尾端的部份

is the axon initial segment, where the trigger zone, just

是軸突的起始片段，也就是 trigger zone

like this part is on a multipolar neuron

就像在多極神經元中

close to the soma.

靠近細胞體的部份一樣

And just like in these neurons where this is the trigger zone,

而就像這種神經元的這裡，是它的 trigger zone

and then the end of the axon has the axon terminals,

且最後有個軸突終端

in this type of neuron this is the trigger zone of the axon.

而在這個種類的神經元當中

And then the axon terminals are all the way

而軸突終端一直延伸到

at this end of the central axon.

這裡的中樞軸突末端

So this type of neuron has a big, long, funny name.

所以這類的神經元有個很長很好笑的名字

It's called a pseudounipolar neuron-- pseudounipolar.

叫做偽單極神經元

And the reason is that it's kind of, sort

是因為它有點像

of like a unipolar neuron, with only one process

單極神經元，只有一端從細胞體

coming out of the soma.

分支出來

But that little short process immediately

而這個短小的分支立刻

splits into these two long axons.

分岔朝兩個方向延伸形成很長的軸突

So it's really a different shape than the unipolar neurons.

所以實際上它與單極神經元形狀也不同