Nerves and Muscles
A-Level Biology Topic 15 9:38 English narration · English + 中文 subtitles burned in
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Touch something hot and your hand is already moving before you feel the pain.
碰到很烫的东西，你的手在你感到疼之前就已经在动了。
The signal never went to your brain to ask permission.
这个信号根本没有上到大脑去请示。
It ran into the spinal cord, turned straight around, and came back out to the muscle.
它跑进脊髓，直接掉头，又回到肌肉。
Your brain finds out afterwards.
你的大脑是事后才知道的。
Impulses like that travel at up to about a hundred and twenty metres a second.
这样的冲动传播速度可达每秒约一百二十米。
In this lesson we look at what is actually travelling, how it crosses the gaps between cells, and how it makes a muscle pull.
这节课我们来看究竟是什么在传播、它怎样跨过细胞之间的缝隙，以及它怎样让肌肉收缩。
The body has two coordination systems, and you should be able to compare them.
身体有两套协调系统，你应该能够比较它们。
The nervous system sends fast electrical impulses along nerve cells.
神经系统沿神经细胞发送快速的电冲动。
The endocrine system sends chemical hormones — things like ADH, insulin and glucagon — in the blood.
内分泌系统在血液中发送化学激素——比如抗利尿激素、胰岛素和胰高血糖素。
Compare them properly.
要好好比较它们。
First the signal itself: electrical against chemical.
先看信号本身：电的对化学的。
Then how it travels: along nerve cells, or dissolved in the blood.
再看它怎样传播： 沿着神经细胞，还是溶在血液里。
Then the speed: the nervous system is very fast, the endocrine system slower.
再看速度：神经系统非常快，内分泌系统慢一些。
And finally how long the effect lasts: nervous effects are short, hormonal effects last longer.
最后看效果持续多久：神经的效果短，激素的效果持续更久。
A neurone is a nerve cell, and there are three kinds you must know.
神经元就是神经细胞，你必须知道三种。
A sensory neurone carries impulses from a receptor towards the brain or the spinal cord.
感觉神经元把冲动从感受器带向大脑或脊髓。
An intermediate neurone, sometimes called a relay neurone, connects a sensory neurone to a motor neurone.
中间神经元，有时叫中继神经元，把感觉神经元和运动神经元连起来。
And a motor neurone carries impulses out to an effector, such as a muscle.
运动神经元把冲动送出去给效应器，比如肌肉。
Every neurone has a long fibre called the axon, and that is what the impulse runs along.
每个神经元都有一条叫轴突的长纤维，冲动就沿着它跑。
Many axons are wrapped in a fatty sheath, with small gaps left between the segments.
许多轴突外面裹着一层脂肪鞘，各段之间留有小小的缝隙。
Put those neurones together and you get a reflex arc: a stimulus is detected by a receptor — a chemoreceptor in a taste bud detects chemicals in food, for instance — and passed along a sensory neurone to a relay, then to a motor neurone and an effector, which produces the response.
把这些神经元连起来，就得到反射弧：刺激被感受器察觉， 沿感觉神经元传到中继神经元，再到运动神经元和效应器，由效应器做出反应。
Here is the impulse itself.
这就是冲动本身。
What is travelling is not a substance but a change in the voltage across the membrane.
传播的不是某种物质，而是跨膜电压的变化。
Watch the inside of the cell flip from negative to positive and back again, and watch that flip move along.
看着细胞内部从负变正又变回来，再看着这个翻转沿着膜移动。
Each patch of membrane sets off the patch next to it, so the change travels without weakening.
每一小片膜都引发它旁边的那一片，所以这个变化能一路传下去而不减弱。
Now the detail.
现在看细节。
When no impulse is passing, the neurone sits at its resting potential: the inside is about seventy millivolts more negative than the outside.
没有冲动通过时，神经元处于静息电位：内部比外部大约负七十毫伏。
That is held by a pump which moves sodium ions out and potassium ions in, and by the membrane letting potassium leak out more easily.
这靠一个把钠离子泵出、把钾离子泵入的泵来维持，也靠膜让钾更容易漏出去。
Then a stimulus arrives.
然后刺激来了。
Sodium channels open and sodium rushes in, so the inside briefly becomes positive.
钠通道打开，钠涌进来，内部短暂地变成正的。
That is depolarisation.
这叫去极化。
A moment later the potassium channels open, potassium leaves, and the potential swings back to negative.
片刻之后钾通道打开，钾离开，电位又摆回负值。
That is repolarisation.
这叫复极化。
And immediately afterwards comes the refractory period, when the sodium channels cannot open again.
紧接着是不应期，这时钠通道无法再次打开。
That does two useful things: it restores the resting state, and it stops the impulse travelling backwards.
这做了两件有用的事：恢复静息状态，并且阻止冲动往回传。
Some neurones are much faster than others, and the reason is insulation.
有些神经元比别的快得多，原因在于绝缘。
Those axons are wrapped in a fatty sheath called myelin.
那些轴突外面裹着一层叫髓鞘的脂肪鞘。
The impulse cannot cross the myelinated stretches at all, because ions cannot get through.
冲动根本无法穿过被髓鞘包住的那些段，因为离子过不去。
They can only cross at the small gaps between the segments, called the nodes of Ranvier.
它们只能在各段之间的小缝隙处穿过，这些缝隙叫郎飞结。
So instead of depolarising every part of the membrane in turn, the impulse jumps from one node to the next.
所以冲动不是挨个把每一小片膜去极化，而是从一个结跳到下一个结。
That jumping has a name: saltatory conduction.
这种跳跃有个名字：跳跃式传导。
It is much faster, and it is also cheaper, because far less membrane has to be pumped back to resting afterwards.
它快得多，也更省， 因为事后需要泵回静息状态的膜少得多。
Now watch what happens where two neurones meet.
现在看两个神经元相遇的地方发生了什么。
They do not actually touch.
它们其实并不接触。
There is a tiny gap, and the electrical signal cannot jump it.
中间有一条极小的缝隙，电信号跨不过去。
So the signal is converted into a chemical one, carried across by molecules that drift over the gap and are caught on the other side.
所以信号被转换成化学信号，由一些分子飘过缝隙，在另一边被接住。
That gap is a synapse, and you should learn a cholinergic one in four steps.
那条缝隙就是突触，胆碱能突触你应该分四步来记。
One: the impulse arrives at the end of the first neurone, and calcium ions enter it.
第一步：冲动到达第一个神经元的末端，钙离子进入它。
Two: that makes vesicles fuse with the membrane and release a neurotransmitter called acetylcholine.
第二步：这使小泡与膜融合，释放出一种叫乙酰胆碱的神经递质。
Three: the acetylcholine diffuses across the gap and binds to receptors on the next neurone.
第三步：乙酰胆碱扩散过缝隙，结合到下一个神经元的受体上。
Four: that starts a new impulse in the next neurone.
第四步：这在下一个神经元里引发新的冲动。
And here is why a synapse works in one direction only. The vesicles are found only in the neurone before the gap, and the receptors only on the membrane after it.
而突触之所以只朝一个方向工作，原因在这里： 小泡只存在于缝隙之前的那个神经元里，受体只在缝隙之后的膜上。
So the transmitter can only ever cross one way.
所以递质永远只能朝一个方向过去。
When a motor neurone reaches a muscle, the junction between them is called a neuromuscular junction, and it works just like a synapse.
当运动神经元到达肌肉时，它们之间的接头叫神经肌肉接头，工作方式就像突触一样。
Now look inside the muscle itself.
现在看肌肉内部。
Striated muscle is made of long fibres, and each fibre is divided into repeating units called sarcomeres, marked off by lines called Z-lines.
横纹肌由长纤维组成，每条纤维被分成一个个重复的单位， 叫肌节，由叫 Z 线的线分隔开。
That repeating pattern is exactly why the muscle looks striped.
正是这种重复的图案让肌肉看起来有条纹。
Inside each sarcomere are two kinds of filament: thick ones made of myosin, and thin ones made of actin.
每个肌节里有两种肌丝：由肌球蛋白构成的粗肌丝，和由肌动蛋白构成的细肌丝。
Two more structures matter.
还有两个结构很重要。
The T-tubule system carries the impulse deep into the fibre, and the sarcoplasmic reticulum stores calcium ions and releases them when the impulse arrives.
T 小管系统把冲动带到纤维深处， 肌质网储存钙离子，并在冲动到达时把它们释放出来。
Contraction itself is the sliding filament model, in four steps.
收缩本身就是肌丝滑动模型，分四步。
First, the impulse makes the sarcoplasmic reticulum release calcium ions.
第一，冲动让肌质网释放钙离子。
Second, those calcium ions bind to troponin, which makes tropomyosin move and uncover the binding sites on the actin.
第二，这些钙离子结合到肌钙蛋白上，使原肌球蛋白移开，露出肌动蛋白上的结合位点。
Third, the myosin heads can now attach to the actin and pull it inwards, using energy from ATP.
第三，肌球蛋白头部现在可以附着到肌动蛋白上，利用 ATP 的能量把它往里拉。
Fourth, the thin filaments slide over the thick ones.
第四，细肌丝滑过粗肌丝。
And notice the word. The filaments slide — they do not get shorter themselves.
注意用词：肌丝是滑动——它们自己并没有变短。
What happens is that each sarcomere gets shorter, because its Z-lines are pulled closer together, and all those small shortenings add up to a muscle that contracts.
发生的是每个肌节变短了，因为它的 Z 线被拉得更近， 而所有这些小小的缩短加起来，就是一块收缩的肌肉。
Plants coordinate too, without a single nerve.
植物也会协调，而且一根神经都没有。
The Venus fly trap has tiny stiff hairs on its trap.
捕蝇草的夹子上有细小而硬的毛。
Touch them and a fast electrical signal spreads through the leaf; the cells quickly lose water and change shape, and the trap snaps shut.
碰到它们，一个快速的电信号就在叶片里扩散；细胞迅速失水、改变形状，夹子啪地合上。
Auxin is a plant hormone that drives elongation growth — it makes cells grow longer.
生长素是一种让细胞伸长的植物激素。
It does that by making the cell pump protons into its own cell wall.
它的做法是让细胞把质子泵进自己的细胞壁。
That acidifies the wall and loosens it, so the cell can stretch as water enters.
酸使细胞壁松弛，于是水进来时细胞就能伸长。
And gibberellin controls germination in a barley seed: it switches on the genes for amylase, which breaks stored starch down into sugars to feed the growing seedling.
赤霉素控制大麦种子的萌发：它打开产生淀粉酶的基因， 淀粉酶把储存的淀粉分解成糖，供正在生长的幼苗使用。
Here is a question that asks two things at once. Why does an impulse cross a synapse in one direction only, and why does myelination speed it up? Take the direction first.
这道题一次问两件事：冲动为什么只朝一个方向跨过突触，髓鞘化又为什么让它更快？
The vesicles of neurotransmitter are found only in the presynaptic neurone, and the receptors only on the postsynaptic membrane.
先说方向。 装神经递质的小泡只存在于突触前神经元里，受体只在突触后膜上。
So the transmitter can only ever cross one way, which makes the synapse a one-way valve.
所以递质永远只能朝一个方向过去，这就让突触成了一个单向阀。
Now the speed.
再说速度。
The myelin sheath insulates the membrane, so ions can only cross at the nodes of Ranvier.
髓鞘让膜绝缘，所以离子只能在郎飞结处穿过。
The impulse therefore jumps from node to node instead of depolarising every part of the membrane in turn.
于是冲动从一个结跳到下一个结，而不是挨个把每一小片膜去极化。
Notice the method in both halves: locate the structure responsible for each effect.
注意两半用的是同一个方法：为每个效应找出负责的结构。
Four things the examiner keeps asking for.
考官反复要的四点。
First, whenever you describe the action potential, give the ion and the direction: sodium in, then potassium out.
第一，描述动作电位时，要给出离子和方向：钠进去，然后钾出来。
Second, when myelin speeds an impulse up, call it saltatory conduction by name, and mention the nodes of Ranvier.
第二，髓鞘让冲动变快时，要点名说出跳跃式传导，并提到郎飞结。
Third, give the synapse steps in order: calcium in, vesicles fuse, neurotransmitter across, receptors, new impulse.
第三，突触的步骤要按顺序给出：钙进入、小泡融合、神经递质过去、受体、新的冲动。
Fourth, remember the filaments slide — they do not shorten.
第四，记住肌丝是滑动——它们自己不变短。
It is the sarcomere that gets shorter.
变短的是肌节。
Five leftover facts from the handout.
讲义里还剩五件事实。
First, detecting a stimulus starts at a receptor: a chemoreceptor in a taste bud detects chemicals in food, and that triggers an action potential in the sensory neurone.
第一，检测刺激从感受器开始： 味蕾里的化学感受器检测到食物中的化学物质， 就会在感觉神经元里触发动作电位。
Second, resting potential is not only the sodium potassium pump — the membrane is also more permeable to potassium, so the inside stays more negative.
第二，静息电位不只靠钠钾泵—— 膜对钾也更通透，所以内侧保持更负。
Third, the refractory period restores rest and limits frequency: how often impulses can be sent.
第三，不应期恢复静息，并限制频率： 冲动能发送得多勤。
Fourth, a neuromuscular junction is like a synapse, but between a motor neurone and a muscle.
第四，神经肌肉接头像突触， 但在运动神经元和肌肉之间。
Fifth, auxin acidifies the cell wall by pumping protons; gibberellin switches on the amylase genes in a barley seed.
第五，生长素靠泵入质子酸化细胞壁； 赤霉素在大麦种子里打开淀粉酶基因。
So there it is.
就是这样了。
A voltage that travels, a gap it has to be carried across, and filaments that slide past each other to pull.
一个会传播的电压，一条必须被载着跨过的缝隙， 还有互相滑过以产生拉力的肌丝。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。