Part 2

Making connections The cells that carry messages around your brain– making you think, feel, sense, and act– are called neurons. A human brain contains roughly 86 billion neurons and 84 billion other cells! Neurons The brain and nerves in your body contain neurons. Neurons connect to each other to create a network that information travels around. Each neuron collects incoming messages, then sends messages to other neurons or body parts. Dendrites Around their cell body, neurons have branches– called dendrites– that collect incoming messages. Axon Each neuron has a long, wiry axon, along which it sends outgoing messages. Myelin sheath Most axons are wrapped in a fatty substance called myelin, which makes messages travel faster. The longest axon in your body runs from your big toe to the base of your brain! Synapses Where the axon of one neuron meets the dendrite of another neuron is called a synapse. Connecting the brain The right and left sides, or hemispheres, of the brain are actually quite separate structures. Large bundles of axons connect one side to the other. The largest bundle is called the corpus callosum. Supporting cells Neurons might be the most important brain cells, but the brain couldn't function without other types of cell, too. Find out about some of these below. Oligodendrocytes These are fatty cells that wrap around axons in the brain to make the myelin sheaths. Microglia Microglia fight any germs that get into the brain and remove broken bits of cells. Astrocytes Astrocytes create the brain's structure, supply nutrients, and repair damage. Pericytes These cells control blood flow and decide what leaves the blood to enter the brain.

建立连接：大脑中帮你传递信息，让你产生思考、情绪、感知和行动的细胞，叫做神经元。 人类的大脑大约有860亿个神经元，还有840亿个其他细胞！ 神经元 你的大脑和身体神经里都有神经元。 神经元互相连接，形成一个信息可以在其中通行的网络。 每个神经元先收集传入的信息，再把信息传递给其他神经元或者身体部位。 树突 神经元的细胞体外有很多分支，叫做树突，负责收集传入的信息。 轴突 每个神经元都有一根细长的轴突，用来传出信息。 髓鞘 大多数轴突都包裹着一种叫做髓磷脂的脂肪物质，它能让信息传递得更快。 你体内最长的轴突从大脚趾一直延伸到脑底部！ 突触 一个神经元的轴突连接另一个神经元树突的位置，叫做突触。 连接大脑 大脑的左右两侧，也就是左右半球，其实是两个相对独立的结构。 大束的轴突把左右两个半球连接在一起。 最大的那束轴突叫做胼胝体。 支持细胞 神经元也许是大脑最重要的细胞，但大脑离开其他细胞也没法工作哦。 一起来认识下面几种其他细胞吧。 少突胶质细胞 这类富含脂肪的细胞会包裹大脑里的轴突，形成髓鞘。 小胶质细胞 小胶质细胞能消灭入侵大脑的病菌，还能清理坏死的细胞碎片。 星形胶质细胞 星形胶质细胞构成大脑的结构、供给营养，还能修复损伤。 周细胞 这类细胞控制血流量，还能决定血液里的哪些物质可以进入大脑。

Spikes Information travels through neurons by electrical impulses, called spikes. The number and pattern of spikes carries the information– about incoming sensory signals, memories and feelings, or outgoing instructions to control the body. 1 Spike starts When enough messages are received by a neuron's dendrites, a spike begins where the cell body meets the axon. 2 Spike travels The electrical spike then travels down the axon, away from the cell body toward the synapses. 3 On to the next neuron When the spike reaches a synapse, it causes the release of chemicals that pass the message to the next neuron. Sending impulses Neurons carry information around the brain and the rest of the nervous system using two things: tiny electrical and chemical messages. Even your thoughts are carried by electrical impulses! Spikes travel down axons without myelin a bit like a wave moving across the sea. WHITE AND GREY MATTER Some neurons are wrapped in a fatty myelin sheath. This helps speed up electrical spikes. Neurons with a myelin sheath look white, and neurons without one look grey. INCREASING THE SIGNAL Imagine a neuron that tracks how hungry you are– if you're full, it doesn't make any spikes, but as your stomach empties, the more spikes it creates. More spikes make you feel hungrier. Weak signal The fewer messages a neuron receives telling it to spike, the fewer spikes it sends. Strong signal When lots of messages arrive at a neuron's dendrites, it fires lots of spikes.

神经冲动 信息通过被称为神经冲动的电脉冲在神经元中传递。 神经冲动的数量和变化规律承载着不同信息：可能是传入的感知信号、记忆、情绪，也可能是传出的控制身体的指令。 1 冲动启动 当神经元的树突接收到足够多的信息后，就会在细胞体和轴突的连接处产生神经冲动。 2 冲动传递 电神经冲动接着会沿着轴突，从细胞体向突触方向传递。 3 传递给下一个神经元 当神经冲动到达突触后，会释放化学物质，把信息传递给下一个神经元。 传递冲动 神经元依靠微小的电信号和化学信号，在大脑和神经系统其他部分传递信息。 哪怕你的想法，也是靠电脉冲传递的哦！ 没有髓鞘的轴突上，神经冲动的传递就像海浪掠过海面一样。 白质和灰质 有些神经元外面包裹着脂肪性的髓鞘。 这能让电神经冲动传递得更快。 有髓鞘的神经元看起来是白色的，没有髓鞘的神经元看起来是灰色的。 增强信号 举个例子：有个神经元会感知你的饥饿程度——你吃饱的时候，它不产生神经冲动，胃越空，它产生的冲动就越多。 更多的冲动会让你感觉更饿。 弱信号 神经元接收到的触发冲动的信息越少，它发出的冲动就越少。 强信号 当大量信息到达神经元的树突，神经元就会产生大量神经冲动。

Crossing the divide When a spike reaches the end of an axon, it causes the release of chemical messengers called neurotransmitters at a tiny structure known as a synapse. The chemicals pass on the message to the next neuron. Synapses Synapses link neurons together. They contain the end of one neuron's axon, a tiny gap, and part of the dendrite of the next neuron. Chemical messages cross the gap in about one thousandth of a second! Most brain neurons form synapses with thousands of other neurons. Neurotransmitter packet Ready to go Neurotransmitters are stored in little round packets in an axon's end. When a neuron is quiet, they remain still but ready to move. Neurotransmitters cross the gap Neurotransmitter release An electrical spike causes some packets to move to the edge of the neuron and release the neurotransmitters inside into the gap. Neurotransmitters stick to receptors Passing on the message The neurotransmitters spread to the next neuron and stick to receptors there, which change that neuron's electrical activity and can cause it to spike. Stop or go? There are two types of synapse: inhibitory (STOP) and excitatory (GO). STOP synapses make it less likely that the next neuron will spike. GO synapses do the opposite. Neurons add up all the STOP and GO signals they receive to decide whether to spike. STOP synapses tell other neurons not to spike. They're important for making sure the brain doesn't get overexcited. GO synapses tell other neurons it's time for them to spike. Most neurons need to receive multiple GO signals before they spike.

信号传递 当电脉冲抵达轴突末端时，会在突触这个微小结构中，释放名为神经递质的化学信使 这些化学物质会把信息传递给下一个神经元 突触 突触把神经元连接在一起 它包含一个神经元的轴突末端、一道微小缝隙，还有下一个神经元的部分树突 化学信号穿过缝隙只需要大约千分之一秒！ 大脑里大多数神经元都会和数千个其他神经元形成突触 待命的神经递质小包 神经递质储存在轴突末端的小圆囊里 神经元安静的时候，它们就待着不动，随时准备出发 神经递质穿过缝隙 释放神经递质 电脉冲会让部分囊泡移动到神经元边缘，把里面的神经递质释放到缝隙中 神经递质结合受体 传递信息 神经递质扩散到下一个神经元，结合在那里的受体上，改变这个神经元的电活动，还能让它产生电脉冲 是停还是走？ 突触分为两种：抑制型（停）和兴奋型（走） "停"突触会降低下一个神经元产生电脉冲的可能性 "走"突触的作用正好相反 神经元会汇总收到的所有"停"和"走"信号，再决定要不要产生电脉冲 "停"突触告诉其他神经元不要产生电脉冲 它们对防止大脑过度兴奋非常重要 "走"突触告诉其他神经元，该产生电脉冲了 大多数神经元需要收到多个"走"信号，才会产生电脉冲

Reflexes Reflexes are automatic responses to a particular event– for example, sneezing when something irritates the nose. All animals have reflexes, and the brain isn't always involved in them. Spikes can travel directly from sensory organs to the spinal cord, then straight back to the muscles to cause a response. Sneezing clears the nose automatically– there's no need for the brain to think about it. Basic brains Human brains are very complicated, so many neuroscientists study animals with much simpler brains. This allows them to figure out the basic ways in which brains work, which helps us to understand all brains. Reflexes vs. thinking The brain and nervous system use information to help an animal survive. The simplest brains mainly use reflexes. Thinking is slower, as new sensory information is mixed with an animal's knowledge and thoughts about what it needs, before an action is chosen. The more complex a brain is, the more thinking it does. C. elegans uses mainly reflexes. If you touch either end of it, it automatically slithers away in the opposite direction. Not all animals have brains. PLANTS VS. ANIMALS Animals need brains because they move around. Plants live their lives in one place, so they don't need brains– although some, such as the Venus flytrap, have developed ways of moving quickly without neurons!

反射 反射是特定事件触发的自动反应——比如有东西刺激鼻子时就会打喷嚏 所有动物都有反射，而且反射不一定需要大脑参与 电脉冲可以直接从感觉器官传到脊髓，再直接传回肌肉触发反应 打喷嚏会自动清理鼻腔，根本不需要大脑思考指挥 基础大脑 人类的大脑非常复杂，所以许多神经科学家会研究大脑简单得多的动物 这样就能找出大脑运作的基本规律，帮助我们理解所有大脑的运作 反射vs思考 大脑和神经系统利用信息帮助动物生存 最简单的大脑主要依靠反射活动 思考的速度更慢，因为在选定行动前，大脑要把新的感官信息，和动物已有的知识、自身需求结合起来 大脑越复杂，能进行的思考就越多 秀丽隐杆线虫主要依靠反射活动 如果你碰它身体的任意一端，它就会自动朝反方向爬走 不是所有动物都有大脑 植物 VS 动物 动物会四处移动，所以需要大脑 植物一辈子都待在同一个地方，所以不需要大脑——不过有些植物比如捕蝇草，就算没有神经元也进化出了快速移动的能力！

Amazing animal brains There are millions of different species of animal, and they all have different brains. The structure of an animal's brain is closely linked to how that creature lives its life. Many brains Each of an octopus's eight arms contains its own "mini-brain", which is called a ganglion. Octopuses use their arms for catching prey, exploring, and tasting. Arms Each arm can move on its own, without the main brain telling it what to do. The octopus is the most intelligent animal without a backbone. BRAIN DIVERSITY These three creatures have brains that are very different from ours. Shrinking shrews In winter, food is scarce, so the common shrew shrinks its brain and skull to save energy. In spring, the brain grows back! Walking brains Tiny spiders have fairly big brains to help them build webs. In some species, the brain takes up 80 per cent of the body and even spills into the legs! No-brainers Sea squirts have disappearing brains! Baby sea squirts can swim, but adult sea squirts stick to rocks, so they don't need a brain. As they grow they absorb it.

奇妙的动物大脑 地球上有数百万种不同的动物，它们的大脑也各不相同 动物大脑的结构和它们的生活方式密切相关 多个大脑 章鱼的八只腕足，每一只都有自己的"迷你大脑"，叫做神经节 章鱼用腕足捕猎、探索环境、品尝味道 腕足 每只腕足都能自主行动，不需要主脑下达指令 章鱼是最聪明的无脊椎动物 大脑的多样性 这三种生物的大脑，和我们的大脑差异非常大 缩小的鼩鼱大脑 冬天食物匮乏，普通鼩鼱会缩小大脑和颅骨来节省能量 到了春天，大脑又会重新长回来！ 行走的大脑 小型蜘蛛拥有相对较大的大脑，帮它们织网 有些品种的蜘蛛，大脑占了身体的80%，甚至还延伸到了腿部！ 会消失的大脑 海鞘的大脑会消失！ 幼年期海鞘会游泳，成年海鞘吸附在岩石上，所以不再需要大脑 成长过程中它们会把大脑吸收掉

Evolution Humans belong to the ape family. By studying fossils of other species of human that no longer exist, we can see how modern humans evolved. Fossil skulls tell us how their brains changed in size and shape. Australopithecus africanus This ape lived in Africa and had certain features resembling a human. It could walk on two legs, had a more rounded skull, and probably used simple tools. Homo habilis The cranium of Homo habilis was expanded and its face and teeth were smaller than most apes. It used more complex stone tools. Homo erectus Homo erectus walked upright like modern humans and its brain was bigger than that of Homo habilis. It used stone axes and it may have made fires. Homo neanderthalensis Neanderthals were modern humans" closest relatives. They were shorter and more muscular, but their brains were the same size. They wore primitive clothes. Homo sapiens Our species evolved around 300,000 years ago. Our brains have not changed much since then, but human lifestyles are different because of what we've learned. Becoming human To understand where humans came from, scientists look at fossils and closely related animals. One of the most important parts of human evolution was the brain getting bigger. Family tree Evolution doesn't happen in a straight line. Many types of human species branched off from our ape ancestors, but only one branch led to modern humans. Close cousins Chimpanzees are our closest living relatives– 99% of their DNA is the same as ours! Their brain is three times smaller than a human's, but they make and use simple tools.

进化 人类属于猿科动物。 通过研究已经灭绝的其他古人类化石，我们能了解现代人是如何进化而来的。 头骨化石能告诉我们古人类大脑的体积和形状发生了怎样的变化。 南方古猿非洲种 这种猿生活在非洲，拥有某些和人类相似的特征。 它可以直立行走，头骨更圆润，大概率还会使用简单工具。 能人 能人的颅腔更大，脸部和牙齿都比大多数猿类更小。 它会使用更复杂的石器工具。 直立人 直立人和现代人一样直立行走，大脑体积比能人更大。 它会使用石斧，可能还已经掌握了生火的方法。 尼安德特人 尼安德特人是现代人的最近亲。 他们身材更矮、肌肉更发达，但大脑体积和我们一样大。 他们会穿原始的衣物。 智人 我们这个物种大约在30万年前演化形成。 从那之后我们的大脑没怎么变化，但因为积累了知识，人类的生活方式和从前大不相同了。 人类的起源 为了弄清人类从哪来，科学家会研究化石和与人类亲缘关系近的动物。 人类进化过程中最重要的变化之一就是大脑体积变大。 进化树 进化不是沿一条直线推进的。 我们的猿类祖先分化出了很多种古人类分支，只有一个分支最终演化成了现代人。 嫡亲近亲 黑猩猩是我们现存最近的亲戚——它们99%的DNA都和我们一模一样！ 它们的大脑只有人类的三分之一大，但也会制造和使用简单工具。