Because the real brain is ugly and not cute at all. Humans are disgusting.
But in the past month, I have been looking at sticky and bloodshot brain pictures in Google Pictures, which is simply hell. Now you have to go through the same experience, so please be prepared.
Now let's start with the outermost layer. I think one of the advantages of biology is that it is sometimes quite organized, and the brain also has some organized places. First, the human head structure is like a Russian doll.
The outermost layer of your head is hair, and the scalp is under it, and then you think it will reach the skull next-but in fact there are about 19 layers in the middle to reach the skull.
Between your skull and your brain [1], there is another pile of things like this:
Under the skull, the brain is wrapped in three layers of film.
The outermost layer is dura mater, a solid and uneven waterproof membrane. The dura mater will cling to the skull. I've heard that there is no pain perception area in the brain, but there is one on the dura mater-it's as sensitive as facial skin-and the pressure or impact on the dura mater often causes severe headaches.
The lower layer is called arachnoid mater, and you can see that the space under this film is covered with some elastic fibers. I always thought that my brain was just immersed in some kind of liquid, and then it just floated inside the skull. But in fact, the only gap between the outside of the brain and the inside of the skull is this arachnoid membrane. These fibrous substances can fix the position of the brain and prevent it from moving around. When the head is hit, they can also play a buffering role. This area is filled with spinal fluid with a density close to that of water, which can keep the brain buoyant.
The last layer is the pia mater, which is closely attached to the outer layer of the brain. Do you know why every time you see a picture of the brain, it is always covered with a sticky blood vessel? These blood vessels are not actually on the surface of the brain, but embedded in the pia mater. If you are not afraid of nausea, this video can see a professor peeling the pia mater from the human brain. )
The whole picture of the brain below, here should be a pig brain:
From left to right, you will see the skin (pink layer), then two layers of scalp, then the skull, dura mater and arachnoid membrane, and at the far right is the brain wrapped under the pia mater.
If we peel everything off the outer layer, we will see this pink thing:
This funny-looking thing is the most complex object known in the universe-although it weighs only three pounds, the neuroscientist Tim Hanson calls it "the thing with the highest information density, the highest degree of structure and the most complete self-organization known at present". With such a powerful brain, its running power is only 20 watts (an equally powerful computer will need 24 million watts to start).
Polina Anikeeva, a professor at MIT, described the brain as "a soft pudding that can be scooped with a spoon". The description given by brain surgeon Ben Rapoport feels more scientific: "the shape between pudding and jelly". He said that if you put a brain on the table, it will become flatter under the action of gravity, a bit like a jellyfish. We usually don't expect the brain to be so soft, because it is usually in a state of suspension in liquid.
But this is who we are. You see your body and face in the mirror, and you think it's you-but it's really just a skin. What you really look like is a grotesque jelly ball. I hope you don't mind saying that.
Although it sounds strange, you can't blame Aristotle or the ancient Egyptians, although they once concluded that the brain is just a meaningless "skull filler" (Aristotle thinks that the heart is the source of intelligence). [2]
Later, humans gradually learned more about the truth about the brain, but only partially.
Professor Krishna Shenoy made an analogy, saying that our understanding of the brain is just like that of the whole world map at the beginning of16th century.
Another professor, Jeff Lichtman, put it even more harshly. In the first class of his course, he always asks students a question: "If the knowledge contained in the brain is a mile, how far have we come on this journey?" He said that the students answered one third, half and a quarter-but the answer given by the professor was "about 3 inches".
The third professor is a neuroscientist Moran Cerf. He shared with me an old saying in neuroscience, which pointed out why trying to fully understand the brain is an unattainable paradox: "If the human brain is really that easy to understand, then we can't understand the brain with such a simple brain. 」
With the help of the great knowledge base that mankind is building, we may be able to do this one day in the future. Now, let's take a look at what humans know about the brain at present-starting from a macro perspective.
Let's first look at the main structure of the brain through the following hemispheric cross-sectional view:
Now let's take out the brain and remove the left hemisphere, so that we can see the structure clearly.
Neuroscientist Paul MacLean made a simple schematic diagram, which explained a basic concept we discussed before: in the process of evolution, the reptile brain first appeared, and later mammals developed a second brain structure based on it, and finally the appearance of human beings perfected the third brain structure.
Here are the corresponding positions of these structures on the real brain:
Next, let's take a look at each part here:
This is the oldest part of our brain:
This is the part of the brain section above where the frog boss is. In fact, the shape of frog's whole brain is very similar to this part of our brain:
After understanding the functions of these parts, you will understand why they are ancient-frogs and lizards can do what these parts can do. The following are the main parts (click on the animation to view the HD version):
Medulla oblongata only wants to keep you from dying. It is responsible for controlling some involuntary activities, such as heartbeat, breathing and blood pressure. In addition, if it thinks you are poisoned, it will make you vomit and do all kinds of thankless work.
The work of pons is very fragmentary. It is responsible for swallowing, bladder control, facial expression, chewing, saliva secretion, tears secretion and posture maintenance-basically doing everything according to mood.
The work of the midbrain is even more fragmentary than that of the pons. If what one part of the brain does is already taken care of by other parts, it will definitely not feel good. What we are talking about here is the midbrain, which is responsible for vision, hearing, movement control, alertness, body temperature control and other things that other brain parts are doing. Other parts of the brain don't seem to like the midbrain very much, because you can see how different the proportions of "forebrain, midbrain and hindbrain" are, so it seems that the midbrain is rejected by other parts.
However, the pons and midbrain also have a worthy work. They are also responsible for controlling the autonomic movement of the eyes, which is a serious matter. So if you are rolling your eyes, it means that your pons and midbrain are doing one of their full-time jobs.
This looks a bit strange. What looks like the scrotum of the brain is your cerebellum. The cerebellum is responsible for keeping your balance, coordinating your hands and feet and walking normally. Here is a video of the calm professor who showed the anatomical structure of the cerebellum.
Above the brain stem is the limbic system-the part of the brain that makes people so crazy.
The marginal system is a survival system. A general statement is that when you are doing something that your dog will do-eating, drinking, mating, fighting, avoiding or escaping from terrible things-this is the limbic system in pulling strings. Whether you want to admit it or not, as long as you are doing any of the above things, you are in the survival mode of primitive people.
The limbic system also controls your emotions, and emotions are also the needs of survival in the final analysis-they are more advanced survival mechanisms and are essential for animals living in complex social structures.
In my previous article, I mentioned instant reward monkeys, social survival mammoths, and other animals-they all refer to the limbic system. Whenever there is an internal struggle in your brain, the job of the limbic system may be to encourage you to do something you will regret later.
I firmly believe that learning to control the limbic system is a symbol of human maturity and also the core struggle of human beings. This is not to say that we will live better without the marginal system-the marginal system is half the reason why we are human, and most of the fun in our life is related to the satisfaction of emotions or animal instinctive desires-but the marginal system doesn't know that you live in a civilized society, and if you let it go too far, it will soon ruin your life.
Ok, let's take a closer look. The edge system consists of many small parts, but we only introduce some of the most important parts:
The amygdala can be said to be the concentration of negative emotions in the brain. It is responsible for anxiety, sadness, and our response to fear. There are two amygdalas in the brain. Strangely, the amygdala on the left is more optimistic. In addition to the usual negative emotions, it sometimes produces happy emotions, while the one on the right has been in a bad mood.
Hippocampus (as the name implies, because it looks like a hippocampus) is like a sketchpad of memory. If you put a mouse into a maze, it will slowly remember the maze path, because the memory of the maze path will be encoded into the hippocampus of the mouse-indeed. When the mouse walks to different positions in the maze, different parts of its two hippocampus will be awakened, because each part of the maze corresponds to a certain part of the hippocampus. But if the mouse does other tasks after remembering a maze and is put back into the original maze after one year, it will be difficult for it to recall how to get to the maze. Because at this time, most of the contents on the sketch board of hippocampus have been cleared, so as to make room for memorizing new things.
The disease described in the movie Memento is real-anterograde amnesia is caused by the damage of hippocampus. The onset of Az Harmo's disease begins in the hippocampus, and then slowly spreads to other parts of the brain, which is why patients with Az Harmo's disease first become forgetful, and then a series of other serious symptoms appear.
Thalamus is located in the center of the brain. It is like a middleman in the sensory system. It is responsible for receiving information from the sensory organs and then transmitting it to the cerebral cortex for processing. When you are sleeping, the thalamus also sleeps with you, which means that the middleman responsible for transmitting the senses is off duty. So during deep sleep, you usually don't wake up because of a slight sound, light or touch. If you want to wake someone who is in a deep sleep, you must be loud enough to wake up the thalamus.
The only exception is smell, which is the only sense that can bypass the thalamus. This is why smelling salts can be used to wake the comatose. Now that we are here, let's add a cold knowledge: the sense of smell is the function of the olfactory bulb, and it is the oldest sense. Unlike other senses, the sense of smell is located in the depths of the limbic system, and it is closely related to both the amygdala and the hippocampus-that is why the sense of smell can evoke specific memories and emotions.
At last, we talked about cortex, which is also called "cortical cortex", "neocortex", "cerebrum" and "pallium".
As the most important part of the whole brain, it can't even figure out its own name. So what's going on?
The cortex is almost omnipotent-it is responsible for processing auditory, visual and sensory information, as well as language, movement, thinking, planning, personality and many other aspects.
The cortex can be divided into four leaves:
The responsibilities of these parts are really not coherent to describe, because each part has done a lot of things and there are a lot of overlapping functions between them, but we can briefly summarize them:
The frontal lobe is responsible for your personality and a series of things that we think are related to "thinking", including functions such as reasoning, planning and execution. Among them, most of your thinking behavior takes place in the front part of the frontal lobe called the prefrontal cortex-the wise man in your brain. In the internal struggle of the brain mentioned before, the prefrontal cortex is the opposite side to the limbic system. It is a rational decision-maker who urges you to finish your work; Tell you not to worry about the internal voice of other people's opinions; I hope you don't haggle over trifles.
If you don't think these tasks are troublesome enough, the frontal lobe is also responsible for the movement of your body. The anterior gyrus at the top of the frontal lobe is your "primary motor cortex".
One of the functions of parietal lobe is tactile control, and the most important one is the "primary somatosensory cortex", which is just behind the main motor cortex.
The main motor cortex and the main somatosensory cortex, which are next to each other, are particularly interesting because neuroscientists have found that each of their positions corresponds to a certain body part. This leads to one of the most frightening pictures in this paper-"homuculus".
Dwarf figure was put forward by the pioneer of neurosurgery, Wilder Penfield, which vividly shows the body parts corresponding to the main motor cortex and the main somatosensory cortex. The larger the proportion of body parts in the picture, the larger the area they occupy in the corresponding cortex. Here are some interesting findings:
First of all, the brain is responsible for facial and hand movements and touch more than all other body parts combined. Although this sounds a bit unbelievable, it is actually understandable, because we need to make very subtle facial expressions, and our hands need to be extremely dexterous, but other parts of the body, such as shoulders, knees and back, do not need to be so meticulous in their movements and feelings. This is why humans can play the piano with their fingers, but not with their toes.
Secondly, the proportion of body parts corresponding to these two kinds of cortex is also highly similar. I can understand this, but it never occurred to me that the part of the body that needs motion control most is also the most sensitive part.
Finally, I came across the following picture, and it has been lingering in my mind ever since, so now I will let you experience this feeling-a 3D version of a dwarf.
Let's continue—
The temporal lobe is responsible for storing most of your memories. In addition, because it is next to your ear, it is also the location of the auditory cortex.
Finally, located in the back of your head is the occipital lobe, which is almost completely used to process visual information.
For a long time, I thought that these large brain lobes are the parts that make up the brain-just like the partitions we see in the 3D model. But in fact, the cortex only takes up 2 mm of the outermost layer of the brain-which is equivalent to the thickness of a coin-and the space below the surface layer is basically a complex connection of various nerve tissues.
If you peel off the cortex from the brain, you can get a 2 mm thick area of 2,000 to 2,400 square centimeters, [4] which is equivalent to a square napkin of 48 cm x 48 cm (19 x 19 inch).
This napkin is where most of your brain behavior takes place-it is the reason why you can think, move, feel, see, hear, remember, speak and understand language. This is simply the best napkin ever.
Remember when I said "You're just a jelly ball"? Well, actually, what you think of yourself is mainly your cortex. In other words, you are actually a napkin.
When the whole brain and the stripped cortex are put together, we can clearly see the napkin area increased by these folds:
Although it is not perfect, modern science has basically grasped the whole picture of the brain. In addition, we have a certain understanding of the details of the brain. Next is an introduction to the details of the brain:
Although we have long understood that the brain is the source of human intelligence, it was not long ago that the scientific community figured out the structure of the brain. Scientists have known that the human body is made up of cells, but it was not until the end of19th that Italian surgeon Camillo Golgi discovered a staining method to reveal the true face of brain cells. The result he finally found was surprising:
This doesn't look like what cells should look like. Gorky didn't realize that what he found was actually a "neuron".
Scientists later realized that for almost all animals, neurons are the core units that constitute the brain and nervous system, as well as the huge communication network within them.
But it was not until the 1950 s that scientists further discovered the way of communication between neurons.
Axon, that is, the slender protrusion on a neuron that carries information, is usually very small in diameter-because it is so small, scientists can't use it for experiments until recently. In the1930s, J·Z· Young, a British zoologist, accidentally made a discovery that subverted the traditional cognition-squid has extremely huge nerve axons, which can be used for experiments. Decades later, scientists Alan Hodgkin and Andrew Huxley used the giant axon of squid to finally find out the way that neurons transmit information-action potential. Its principle is this:
First of all, there are many types of neurons:
For the sake of simplicity, we only discuss a simple and common neuron-vertebral cell, which you can find in the motor cortex. If we want to draw a neuron icon, we can draw a villain first:
Then add some more legs and hair to him, take off his arm, and finally lengthen him-so we draw a neuron.
Then we add a few more neurons.
I'm not going to explain the detailed principle of action potential here, because it will involve many unnecessary and boring professional contents, which you should have learned in junior high school biology class. If you want to fully understand the relevant information, I suggest you read this high-quality popular science article of Khan Academy. Next, we will only understand some basic concepts related to the theme of this article.
Now, the tail of our neuron villain-the axon-has a negative "resting potential", which means it will be slightly negatively charged when it is at rest. Our neuron villain's hair (dendrites) will always be touched by other villain's feet [5], although he may be reluctant. Other people's feet will drop a chemical called neurotransmitter on his hair, which will pass through his head (cell body, or "soma"), and depending on the nature of the chemical, he will slightly change the charge carried by his body. Although this will make our neuron villain a little uncomfortable, it is not a big problem-nothing will happen except that.
But if enough chemicals touch his hair to make his charge rise to a certain value, that is, the "threshold potential" of neurons, then the villain will be in action potential, that is to say, he will be shocked.
This is an either-or state: our villain is either completely unchanged or completely shocked. He won't be partially or excessively shocked-either he won't be shocked at all or he will be shocked to the same extent every time.
When this happens, a current will flow from his body (axon) to his foot (axon terminal), and the latter will touch the hair of other villains (this contact point is called synapse). In this process, the charge of the villain's body will temporarily change from negative to positive, and then quickly return to his normal negative potential state. When this action potential reaches the little man's foot, the axon tip will release chemicals to the hair it is touching, which may cause the touched little man to get an electric shock, just as he did before.
This is the way information is transmitted in the nervous system-chemical information is transmitted through tiny gaps between neurons, and current information is triggered through neurons-but when the body needs to transmit a signal quickly, neurons can also transmit information through current.
The transmission speed of action potential is between 1 and 100 meters per second. Part of the reason why there is such a large range of changes is that another cell in the nervous system-Schwann cell-is like an old woman who always thinks that her grandson doesn't have enough clothes, and she has been covering her axon with a thick blanket-myelin sheath. The whole process is like this:
In addition to protection and insulation, myelin sheath is the main reason that affects the information transmission speed of neurons-when the axon is wrapped by myelin sheath, the transmission speed of action potential will be much faster. [7]
Let's take an example to illustrate the influence of myelin sheath on the speed of information transmission: for example, when your toe kicks something, you will immediately realize what you just did, but it may take you a second or two to start to feel the dull pain in your toe. You can immediately feel something kicked and a sharp pain, because the pain information is transmitted to the brain through the axon wrapped by myelin sheath, and you begin to feel dull pain later because this pain is transmitted through the "class C nerve fiber" without myelin sheath protection, and its transmission speed is about 1 m per second.
In a sense, neurons are very similar to transistors in computers-they all transmit information in binary languages of "1" (activated by action potential) and "0" (not activated by action potential). But unlike computer transistors, neurons in the brain are always changing.
You must have had such an experience. You learned a new skill and mastered it well, but the next day you found that you couldn't. The reason why you can learn this skill on the first day is that the amount or concentration of chemicals that transmit signals between neurons has changed. Repeated behavior will lead to changes in these chemicals, so that you can make progress, but the next day, the chemicals that have been adjusted before will return to the normal level, and your previous progress will disappear.
But if you keep practicing, you will eventually master this skill for a long time. In this process, you are actually telling the brain that "this is not a one-time job", and then the neural network of the brain will make structural adjustments that can last for a long time. Neurons will change their shape and position, strengthen or weaken different connections, and build a fixed path according to the skills they need to learn.
Neurons can change themselves chemically, structurally and even functionally, and constantly optimize the neural network of the brain according to the external world. This phenomenon is called neuroplasticity. The baby's brain has the highest neuroplasticity. After the baby was born, his brain had no idea what kind of life he would have in the future: a medieval warrior with first-class fencing? A 17th century musician who was good at playing harpsichord? Or a modern scholar who should not only remember and organize massive information, but also manage complex interpersonal relationships? In any case, the baby's brain is ready to constantly adjust itself and can cope with any form of life in the future.
Although babies have the strongest neuroplasticity, this ability will accompany us all our lives, so human beings can grow, change and learn new knowledge, and this is also the reason why we can form new habits and change old ones-habits are actually a reflection of the existing neural structure of the brain. If you want to change your habits, you need to exert great willpower to overthrow the neural path established by your brain before, but if you can persist long enough, your brain will eventually be instructed to change the previous path, and new behavior habits will no longer need the support of willpower. The brain has made corresponding physiological changes for new habits.
This unimaginable huge neural network is composed of about 1000 billion neurons in the brain-this number is similar to the number of stars in the Milky Way, or more than ten times the global population. Among them, 150 to 20 billion neurons are located in the cortex, and the rest are located in the lower parts of the brain (surprisingly, the number of neurons in the cerebellum is more than three times that of the cortex).
Now let's take a closer look at another cross-sectional view of the brain-but this time, instead of cutting the brain into two hemispheres, we cut it from the middle:
The internal substances of the brain can be divided into gray matter and white matter. Gray matter looks darker and consists of cell bodies, dendrites and axons of brain neurons. The main component of white matter is the axon responsible for transmitting information between nerve cell bodies or other parts of the body. The reason why white matter is white is that these axons are usually wrapped by myelin sheath, which is some white adipose tissue.
Gray matter mainly exists in two areas of the brain-the limbic system and the inside of the brain stem as mentioned above, and the cerebral cortex as thick as coins. The large white matter between them is mainly composed of axons of cortical neuron. The cortex is like the command center of the brain, and it transmits its instructions through a large number of axons existing in the underlying white matter.
The following is the most beautiful conceptual diagram of gray matter and white matter that I have ever seen, which was produced by Dr. Greg. A. Dunn and Dr. Brian Edwards. You can clearly see the structural difference between the outer gray matter and the lower white matter (click on the picture to view the HD version):
These cortical axons may transmit information to other parts of the cortex, to the brain below the cortex, or to other parts of the body through the spinal cord (the telling function of the nervous system). [8]
Let's take a look at what a complete nervous system looks like: