Chapter 9 Chapter 7 An Overview of the Human Brain

"The more they watched, the more amazed they were. He knew so much, how could his little brain hold it." —Oliver Goldsmith, The Deserted Village, an idyll The nervous systems of all mammals, from the mouse to the human, seem to be constructed according to the same blueprint, although they differ greatly in size, e.g., mice and elephants have different brain sizes, proportions of various parts Not all are the same.The brains of reptiles, birds, amphibians, and fish are strikingly different from those of mammals, but they are still related.I won't discuss it much here.Nor am I going to describe the development of the brain during fetal and early childhood.These are, of course, important topics that contribute to our understanding of the mature brain.Generally speaking, genes (and the acquired processes controlled by genes in development) seem to specify the main structure of the nervous system, but many components of this structure need to be constantly adjusted and refined by experience throughout the entire life process.

How the rest of the body attaches to and communicates with the brain is an all too obvious fact, but little thought is given to it.The nervous system is what receives information from various sensors in the body.The so-called sensor is to convert chemical or physical influences, such as light, sound or pressure, into electrical signals. Some sensors respond to a wealth of information from outside the body, like the eye, which acts as a photoreceptor that responds to light.They monitor the external environment.There are also sensors that respond to activity in the body, such as whether you have a stomachache or changes in the acidity of your blood.Therefore, they also monitor changes in the body. The motor output of the nervous system controls the muscles of the body, and the brain also affects the release of various chemical substances in the body, such as certain hormones.Peripheral cells directly related to all input and output only account for a small part of the total number of nerve cells, so a large number of nerve cells only participate in the information processing inside the system.

There are various ways of partitioning the central nervous system. One simple way is to divide it into three parts: the spinal cord, the brainstem (at the top of the spinal cord), and the forebrain above it.The spinal cord receives sensory information from the body and transmits commands to the muscles.Since we are concerned with vision, we will not discuss the spinal cord and below the brainstem further.Our main interest is the forebrain, specifically the neocortex, which is the largest part of the cerebral cortex. The cerebral cortex (often referred to simply as the cortex) is divided into two separate layers of cells, one on each side of the brain. For the human brain, the total area of ​​these two layers of nerve cells is slightly larger than a handkerchief, so they need to be fully folded The thickness of the nerve cell layer varies slightly, generally 2-5 mm thick, and it constitutes the gray matter of the cortex.Gray matter is mainly composed of neurons, cell bodies and branches, and also includes many auxiliary cells called "glial cells".There are about 10 neurons per square millimeter in the cortex. ①Therefore, there are tens of billions of neurons in the neocortex of the human brain, which can be compared with the number of stars in the Milky Way.

Some connections between neurons are local, generally extending less than a millimeter, and at most only a few millimeters; but some connections can leave a certain area of ​​​​the cortex, extend a certain distance, and reach other areas of the cortex or outside the cortex. place.These long-distance connections are covered with a fatty sheath made of a substance called myelin.The fat sheath speeds up the transmission of signals, and it also has a white, shiny surface, hence the name white matter.About 40 percent of the brain is white matter, which is these long-range connections, which vividly and concisely illustrates how much interconnection and communication there is in the brain.

The neocortex is the most complex part of the cortex.The old cortex (paleocorex) is a thin sheet mainly related to olfactory function.The hippocampus (sometimes called the paleocortex) is an interesting high-level structure (meaning it is further away from the input of the sensory system).The memory encoding for one event in some new, long-range series of events is stored in the hippocampus for about several weeks before the information is transmitted to the neocortex. There are also several subcortical structures in the front of the brain that are connected to the cortex, as shown in Figure 23.The most important part of this is called the thalamus, which is sometimes called the entrance to the cortex.Because the main input to the cortex must pass through here, ③ as shown in Figure 24.The thalamus is generally divided into twenty-four regions, each associated with specific subregions of the neocortex.Each area of ​​the thalamus has numerous connections to cortical areas and receives information from there.The true purpose of this feedback connection has not been clarified.Many other connections from the neocortex don't go through the thalamus, and these connections go directly to other parts of the brain.The thalamus straddles the important entrance to the cortex, but not the main exit.

There is a well-developed structure not far from the thalamus, commonly referred to as the striatum, as shown in Figure 25.Although their exact function is unknown, these regions play an important role in motor control.A few specialized regions of the thalamus (collectively called the laminar nuclei) project primarily to the striatum and, more broadly, to the neocortex. The localization of different mental functions in the neocortex has been debated for over a century.One extreme view is that of holism, in which all areas of the cortex function roughly the same, while the opposite view holds that each small area of ​​the cortex performs quite different tasks.

In the early 19th century, the Viennese anatomist Franz Joseph Gall believed in the localization of brain functions, labeling parts of the skull with various exotic attributes (e.g. reverence, benevolence, Respect, etc.), and these attributes are considered to be localized in the cortex, as shown in Figure 26, the pottery-like human brain model with these marks still exists.Gall believed that by studying the bumps of the skull, many characteristics of a person could be deduced.When I was a kid, a local fortune teller asked for the bump on my skull in order to defraud my mother of money.He declared my skull bumps to be very interesting, but for an extra fee he could study them in more detail.But I never found the relevant properties he deduced.

Although Gall was the first advocate of important brain function localization, his specific ideas were completely wrong, and as a result, cortical localization has given the medical community a very bad name.Now, through the detailed study of the macaque cortex and the support of human brain data, we believe that there is a certain degree of functional localization in the cortex, but cortical regions with obviously different properties participate in most mental activities. Therefore, we cannot Take the idea of ​​​​positioning to extremes. As a perhaps useful analogy, use the properties of a small organic molecule, such as sugar or vitamin C.Each atom is positioned in relation to other atoms, and each different atom has its own properties—for example, an oxygen atom is very different from a hydrogen atom.Although some atoms are usually more important than others, and the overall properties of a molecule depend on the interactions between those atoms that make up the molecule, sometimes the electrons linking the atoms are completely localized.In some cases, such as aromatic compounds like benzene, some electrons are distributed over many, many atoms.

We can therefore draw a rough map of the neocortex and label different regions according to their main functions, as shown in Figure 27.The visual area is located on the back of the head, see Figure 23, the auditory area is located on the sides of the head, and the tactile area is located on the top of the head.Just in front of the somatosensory areas are the areas that control the output of voluntary movements, that is to say the volitional commands from these areas control the movement of the muscles.The exact function of the forebrain areas has not been determined, but they may be responsible for planning, especially long-term planning and high-level cognitive tasks.A small area in the forebrain may be involved in voluntary eye movement.

It is well known, but also very strange, that the left side of the cortex is mostly directly related to the right side of the body. ① A bundle of nerve fibers called the "corpus sacrum" connects the two regions of the cortex.In the human brain, there are about 500 million nerve fibers in the corpus callosum, and they transmit in both directions. Humans have a unique language function.In all right-handed and most left-handed people, language areas are predominantly located on the left side of the brain.There are at least two main areas related to language.One area is located in the back of the brain, called "Wernicke's area", and the other, when it was first discovered, was called "Broca's area", which is near the front side of the brain and away from the main motor area. not far.So far, none of them has been understood in detail, mainly because no animal has such a highly developed language, and animals are our main experimental materials for understanding the brain.Near these two areas lie other areas, notably the frontal cortex, which is also involved in language processing (see Chapter 9).I am sure to be able to show that each of these large areas, including Bullock's and Winico's areas, is made up of many unique small cortical areas connected together in complex ways.

A blow to the left side of the head can cause partial paralysis of the right side of the body and interfere with speech, although the undamaged right hemisphere may still be able to swear and even sing, and besides, such a person may still Can distinguish between male and female voices.This latter function may be lost if the right brain is damaged.Although the ability to sing music is lost, the ability to speak remains intact. These examples illustrate two points: There are indeed some functional divisions in the brain, but exactly which ones are localized is not as one might suspect. Outside the cortex there is an area called the hypothalamus, shown in Figure 23, which is critical to many functions of the body and has many small subregions whose main function is to respond to hunger, Thirst, temperature, sex, and similar bodily processes play a regulating role.The hypothalamus is closely connected to the pituitary gland.The pituitary gland is a tiny organ that secretes various hormones into the blood. The cerebellum is a large, conspicuous but not important brain region located at the back of the head.In some fish, such as: electric fish, sharks, etc., the cerebellum is highly developed.It may be involved in the control of movement, especially some tricky movements.However, it is possible for people born without a cerebellum to live normally.Another important area located in the brainstem is the reticular formation.They have many tightly interacting domains whose functions are only partially understood.Neurons in this area control the various stages of wakefulness and sleep.Clusters of these nerve cells send signals to various parts of the forebrain, including the neocortex, for example, a small cluster of neurons called the locus coeruleus sends signals to various places including the cortex.These nerve fibers can extend from the anterior to posterior regions of the cortex.On this pathway, tens of thousands of connections are formed with other nerve cells.The exact function of the coeruleus is unclear.During the rapid eye movement (REM) phase of sleep, when most of our dreams occur, the nerve cells in the coeruleus become largely inactive.This inactivity has the potential to place a memory in long-term memory and may also help explain why we cannot recall most dreams we have. At the top of the brainstem is a pair of structures that are important to the visual system.In lower vertebrates like frogs, this pair of structures is called the optic tectum, and in mammals the superior colliculus, and they probably make up a major part of the frog's visual system.But in mammals (especially primates), this role is filled by the neocortex.In mammals, the superior colliculus is primarily associated with eye movements, especially spontaneous eye movements. Compared with other organs in our body, the human brain is not a single structure.Just as the heart, liver, kidneys, and pancreas have very different functions, each area of ​​the brain has a specific function.However, the different organs in the body interact very closely, with the liver being the blood-making organ and the heart pumping the blood.There are also many interactions in the brain, not only the spinal cord, but also the areas above it, such as the motor cortex, the striate cortex, and the cerebellum, that are involved in motor control.Involved in vision are the superior colliculus, the visual part of the thalamus, and the visual cortex, each of which has to do its job. Broadly speaking, we have a fairly good understanding of the major functions of most of the body's organs and how each organ performs its function.One or two examples will illustrate that this knowledge is still fairly new.When I started studying biology in the late 40s, the function of the thymus was unknown, and no one would even guess that it played a key role in our immune system.I first learned about it because the calf thymus is a convenient source of DNA.Unfortunately our understanding of the different parts of the brain is still at a fairly early stage.What are the exact functions of the thalamus, striate cortex, and cerebellum?We can only give a general overview of their behaviour.A detailed understanding will serve for further research.We also only have a rough understanding of the function of the hippocampus, but no unified understanding of its exact function.All this awaits further discovery. After describing what the brain is at the highest level, let's go down to the lower level and look at the main components and individual nerve cells in the visual system. ① The first visual area of ​​primates is an exception, which has more than twice this number of neurons. ②The word thalamus comes from Greek, and it means the inner chamber, that is, the meaning of the bridal chamber.A large part of the visual thalamus is called the occipital lobe, a word that means pillow. ③ This is not the case for the brainstem and other slightly diffuse systems. (1) Smell is an exception, the right side of the nose is connected to the right side of the brain.