brain

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In vertebrates, including humans, and in some invertebrates , the part of the central nervous system located in the head is referred to as the brain or brain ( Old High German  hirni, hirne , Latin cerebrum , ancient Greek ἐγκέφαλος enképhalos ) . The brain, anatomically called encephalon or enkephalon (from ancient Greek ἐν en , German 'in' and κεφαλή kephalē , German 'head' ), lies protected in the cranial cavity , is surrounded by meninges and consists mainly of nerve tissue . At the level of the foramen magnum it merges into the spinal cord , both together form the central nervous system (CNS).

Prepared human brain
Rotationally animated model of a human brain (without right cerebrum ; frontal lobe marked in red)

Vertebrate brain

function

The vertebrate brain processes highly differentiated sensory perceptions and coordinates complex behaviors . It is therefore the repository for most of the complex information that the organism processes.

Not all information reaches the cerebral cortex and leads to consciousness . Peripheral nerve plexuses ( plexuses ) and, above all, centers in the brain stem process most of the excitations arriving from receptors unconsciously. Reflex arcs take on tasks that are carried out at the highest speed and without conscious processing and delaying influence. There is also such an autonomous nervous system in humans . It coordinates vegetative functions such as breathing, cardiovascular system, food intake, digestion and delivery, fluid intake and excretion, and reproduction.

Strongly networked neurons interact in the brain (see neural network and excitation conduction ). Its activity is examined in vivo by measuring brain waves using electroencephalography (EEG) and the electrical fields produced by the brain using magnetoencephalography (MEG).

evolution

In the course of evolution , the brain of "higher" animals has achieved a considerable degree of differentiation and internal organization ( cerebralization ). This is reflected in the psychological and physical development of the individual (see embryology ). The structure and, to a lesser extent, the volume of the brain correlate with learning and intelligence . The performance of the brain can only be understood in the hierarchy of the nervous system.

In addition to vertebrates, squids have highly complex brains that enable them to perform specific activities. In a broader sense, it is the central point of the nervous system of various invertebrates , such as annelids or insects . Depending on the type of brain, it is a cerebral ganglion or an upper pharyngeal ganglion . Two groups of invertebrates have particularly complex brains: arthropods (insects, crustaceans , and others), and cephalopods ( octopuses , squid, and similar mollusks). The brains of arthropods and cephalopods emerge from two adjacent nerve cords. Cephalopods like the octopus and squid have the largest brains of any invertebrate animal.

Freshly removed brain from a deer
Roebuck brain approx. Two hours after being killed

The highly developed vertebrate brain is very different from the rope ladder nervous system of arthropods . In insects, the digestive tract runs directly through the anterior nervous system (between the tritocerebrum and the subesophageal ganglion) so that the abdominal ganglia lie ventral (abdominal) of the intestinal tube, while in vertebrates the spinal cord lies dorsal (back) of the intestine.

structure

Different criteria can be decisive for a structure of the brain, so that different divisions into brain areas are possible, which do not have to be mutually exclusive. For an organization of the fully grown human brain, it can also be quite useful to take into account the knowledge gained from the investigation of its developmental steps.

For example, in ontogenetic brain development in humans after the neurulation of the central parts of the neural plate to the neural tube as the early embryonic attachment of the central nervous system, successive stages in the development of the brain can be seen in the further course. After the anterior neural tube opening is closed at the end of the fourth week of development, initially three so-called primary cerebral vesicles form from the anterior third of the neural tube, the structures of the prosencephalon , mesencephalon and rhombencephalon . They develop differently, so that five secondary cerebral vesicles can be distinguished in the five-week-old embryo - these lead to

4th week 5th week 6th week - end of life Ventricular system
brain anterior
neural tube 		Prosencephalon
forebrain		 Telencephalon
endbrain		 Lateral ventricle

Rhinencephalon , amygdala , hippocampus , neocortex , basal ganglia

Diencephalon
diencephalon		 Third ventricle

Thalamus dorsalis,
thalamus ventralis ( subthalamus ),
metathalamus (with knees ), hypothalamus with neurohypophysis , epithalamus with epiphysis

Mesencephalon
midbrain		 Mesencephalon
midbrain		 Aquaeductus mesencephali

Tectum (roof),
tegmentum (hood)

Rhombencephalon
hindbrain		 Metencephalon
hindbrain		 Fourth ventricle Pons (bridge),
cerebellum (cerebellum)
Myelencephalon
posterior brain		 Central channel Medulla oblongata
Extended Mark

The rough structure shown here follows the work of Pinel.

Human brain

MRI image of a human brain. Section sagittal , the nose is left. Click here for an animated sequence of cuts.

Rough subdivision of the human brain:

Side view

View of the cut surface of the halved brain (cut surfaces ocher)

View from below

The length of all nerve tracts in the brain of an adult is around 5.8 million kilometers, which is 145 times the circumference of the earth.

The volume of a human brain is around 1.27 liters for a man and around 1.13 liters for a woman.

construction

In simplified terms, four main areas can be distinguished.

Cerebrum

The cerebrum is divided into two hemispheres by an incision in the middle. Between these there is a broad connection of a thick nerve cord called the corpus callosum or bar, and other smaller connections.

Its 2–4 mm thick surface layer ( cerebral cortex , cortex ) is strongly folded and almost a quarter square meter in size. It contains around 16 billion nerve cells , which is about a fifth of the nerve cells in the entire brain. Nerve fibers run under the cortex . Aggregations of neurons are pink and the fibers containing myelin are white . In the dead brain, the neurons turn gray. Therefore, although they are pink during life, they are called gray matter .

The so-called bark fields can be localized on the bark , distinguishing between primary fields and association fields. The primary fields only process information of a certain quality, information about perceptions (sensation, e.g. sight, smell, touch) or about simple movements. The association fields coordinate various functions with one another. The assignment of a bark field to a specific function is repeatedly defined and relativized. Only the correct interaction of different fields enables a function.

The primary fields include, for example, the visual cortex , which is located at the rear pole of the brain and on which the projections of the visual pathway flow, and the auditory cortex , which is used to process acoustic stimuli and is located laterally in the temporal lobe .

Associative fields can be found in the front part of the brain, among other things. Their tasks are, for example, memory and higher thought processes.

The cortical fields and their functions can be separated from each other by examining the patient's activity after their failure (for example due to a stroke ) or the healthy brain using electrical stimulation, microscopic and other techniques. In addition to the cerebral cortex, other brain regions are usually involved in a specific function.

Diencephalon

To diencephalon includes four parts:

  1. Thalamus (upper part)
  2. Hypothalamus , which is connected to the pituitary gland (pituitary gland)
  3. Subthalamus
  4. Epithalamus

The thalamus is the mediator of sensory and motor signals to and from the cerebrum . With him all information of the sensory organs converges and is passed on. It consists mainly of gray matter . The hypothalamus controls numerous physical and psychological life processes and is itself partly neuronally controlled by the vegetative nervous system , partly hormonally via the bloodstream. The hypothalamus and pituitary gland (important endocrine gland in the body that is connected to the hypothalamus via the pituitary stalk) are the central link between the hormonal and nervous systems . The diencephalon is involved in sleep- wake control (see ARAS , pain perception, temperature regulation ).

Cerebellum

Two hemispheres can also be distinguished on the cerebellum . Additional parts are also delimited. For example, it is responsible for balance and movements and their coordination . In animals - compared to the cerebrum - it is often more developed than in humans, especially in species with flight ability or in fast predators .

In addition, the cerebellum is assigned a function in unconscious learning. Recent research (2005) suggests that it is involved in language acquisition and social learning.

Brain stem

The brain stem is the genetically oldest part of the brain. It forms the lowest part of the brain and consists of ascending and descending nerve fibers ( white matter ) and collections of neurons or somata (gray matter), morphologically from the midbrain , the bridge (pons) and the posterior brain (also called the elongated medulla oblongata) because it is located between the spinal cord and the bridge). The brainstem interconnects and processes incoming sensory impressions and outgoing motor information and is also responsible for elementary and reflex-like control mechanisms.

The nerve tracts of the two halves of the body cross in the posterior brain . In addition, many automatically running processes such as heartbeat , breathing or metabolism are controlled here. Important reflex centers are also located here, which trigger, for example, eyelid closure , swallowing , coughing and other reflexes . The lower end of the posterior brain connects to the spinal cord.

Brains of men and women

Magnetic resonance imaging images of a human brain

The brains of men and women differ in size and structure. On average, the brain of an adult man weighs around 1400 g, depending on the ethnic group . With the same stature of men and women, the brain is on average 100 g heavier in men. On the other hand, if you consider the weight of the brain in relation to body weight, the brain of women is heavier on average. The absolute brain weight is of little importance, as the example of the blue whale shows, whose brain weighs around 7 kg depending on its size. Not only does the total brain size differ between the sexes, but the relative size of different brain areas. The hippocampus and amygdala have been researched best.

The hippocampus , similar in shape and size to a seahorse , is responsible for learning and memories and has different anatomical structures and neurochemical compositions in men and women. In relation to the total brain, the hippocampus is larger in women. In men, however, the CA1 region is larger and the number of pyramidal cells is increased. Furthermore, there are different receptor affinities for different neurotransmitters and differences in long-term potentiation .
The amygdala plays a role in reproductive behavior and is the memory for emotional events. Studies have shown that there is a gender-specific hemispheric lateralization of the amygdala functions in relation to the memory of emotional moments, in the reaction to happy faces, in the interconnection of the amygdala with the rest of the brain as well as certain diseases such as depression. The left hemisphere is involved in women and the right hemisphere in men.
The two cerebral hemispheres also tend to be organized more asymmetrically in terms of language and spatial perception in men. H. the lateralization of the brain is more pronounced than in women, who in turn have larger frontal lobes.

There are various theories about the origin of this dimorphism . On the one hand, alternative splicing of mRNA is possible . For example, the splicing of channel proteins so that their permeability for ions is changed. On the other hand, epigenetic control mechanisms are relevant. These include, among other things, genomic imprinting and histone modification. In addition, the question is repeatedly asked to what extent the environment has an influence on dimorphism.

Another explanation is as follows: Sex hormones , such as testosterone and estrogens , not only have an effect on the gonads, but also in a variety of ways on the entire nervous system : on nerve cells , synapses , gene expression . This applies to the period of embryonic development and during childhood, puberty and adulthood. The sex hormones cause a typical male or female development of the nervous system. This can be seen, for example, in the praeoptica region in the hypothalamus, which is larger in young men than in women.

The Barr bodies are probably a decisive factor, as many X-linked genes are involved in the neuronal processes of brain development. Barr bodies result from the accidental inactivation of an X chromosome in women. As a result, the female tissue and organs, including the brain, represent a mosaic, as a different gene of the polymorphic X gene is expressed in each cell. Ian W. Craig and other scientists also suspect that the differences are largely due to X inactivation . Today it is mostly assumed that the different sex chromosomes are the most important reason for the dimorphism. These can influence development in two ways. On the one hand, the gene products of the chromosomes can act directly in the cells in which they are expressed. On the other hand, the gonosomes cause the development of the gonads , which form the sex hormones.

An imaging study on gender identity revealed striking differences between male, female and transsexual study participants with regard to the microstructure of the white matter in the brain. The fiber courses and thus the structure of the nerve connections showed clear differences, with the results of the transgender people being between those of men and women. The same study provided evidence of a close relationship between fiber flow and blood levels of sex hormones. These findings support the assumption that sex hormones have an influence on embryonic and early childhood brain development.

Brain power

The brain is a very active organ with a particularly high energy requirement. In adults it accounts for around 2% of the body mass, with around 20 watts it consumes around 20% of the basal metabolic rate , and in newborns 50%. It gains energy from the aerobic combustion of glucose , lactate and ketone bodies . Glucose cannot be completely replaced by the other energy sources. Immediately after birth, baby brains can use ketone bodies to a large extent for energy production. Some time after the toddler's diet has been changed to carbohydrate-rich food, the enzyme production required for this is reduced again or completely broken down and the ability to ketolysis (to use ketone bodies for energy) is lost again. The behavior of the blood glucose level in starvation metabolism suggests that a fully ketolysable brain processes ketone bodies (primarily over glucose, even if there is sufficient glucose supply via the blood).

The sodium pump needs 90% of the power , mostly in connection with action potentials . Since the brain has only small, area-dependent storage capacities for energy, a failure of the oxygen or glucose supply leads to functional failure ( syncope , fainting) after just ten seconds and to specific brain damage after a few minutes. The small, at first glance evolutionarily incomprehensible reservoirs are sometimes explained by a lack of space. According to another - evolutionary - explanation, the diet of people in the Paleolithic diverged very strongly from today's diets of civilization, whereby the ketolysis ability of the brains of that time was naturally preserved at all times. This is explained by the fact that the human organism does indeed store too much energy absorbed from food in the body fat stores - in a healthy, slim person weighing 70 kg, 85% of the usable body energies are present as body fat, 14.5% as proteins and only 0.5% as carbohydrates - but can hardly produce glucose from fat: Only 6% from the glycerine of the triglycerides, in the form of which fat is stored in the organism. Some scientists believe that the higher fat diets contributed to the growth of the human brain in the Paleolithic Age.

The effectiveness of the ketogenic diet in epilepsy , GLUT1 deficit syndrome and other cerebral diseases and starvation metabolism is based on the natural ability of human brains to perform ketolysis .

It has been known since 1994 that the nerve cells receive a precisely measured amount of energy from the blood via the astrocytes when required; it is the active process "Energy on Demand". The demand-dependent regulation of the blood supply to brain areas is called neurovascular coupling . From 1998 to 2004 Achim Peters developed the Selfish Brain Theory , according to which the human brain primarily covers its own, comparatively high demand when it comes to regulating the energy supply in the organism. According to another explanation, however, this only applies to brains that, due to long-term use of high-carbohydrate and high-calorie diets, can no longer use ketone bodies for energy production. So these are no longer capable of ketolysis. Such brains are no longer naturally connected to the fat metabolism and consequently have to cover their entire energy requirement via the much less powerful carbohydrate metabolism with its extremely low energy reserves.

Waste disposal of the brain

Due to the unusually high average metabolism in the brain, there is also an unusually high need for biochemical waste disposal. This is also of greater importance here because some substances, especially misfolded proteins , pose typical threats to the brain .

Waste disposal in the brain is made more difficult by the filter systems of the blood-brain barrier and the blood-liquor barrier and the lockout of the lymphatic system . The latter only extends into the meninges from the outside .

Astrocytes (star cells) of the glia and apposition of their processes on a vein. The space between the vein and these deposits is part of the glymphatic transport route.

Although there have been concrete signs of the existence of a special washout system in the brain since the 1980s, it was only discovered in 2012 with the help of novel detection methods as an independent internal circulatory system. Based on the lymphatic system and because of the crucial role of the glia (supporting cells), it was called the glymphatic system .

Through very narrow vascular spaces around the outer wall of veins, the so-called perivascular space (Spatium perivasculare) , a small part of the cerebrospinal fluid ( liquor cerebrospinalis ) from the space between the skull and the brain ( subarachnoid space or outer liquor space ) reaches all of them Areas of the brain, is distributed there with the help of the glia and ultimately flows back to the meninges and the lymphatic system outside the brain, taking along waste materials.

Comparison with computers

Often comparisons are made between the performance of a computer and that of the human brain. Ever since the brain was recognized as the seat of cognitive performance, it has always been compared in the literature with the most complex technical apparatus available (steam engine, telegraph). Attempts have been made to infer that of the brain from the functioning of computers. In the meantime there is an effort in computational neuroscience and bionic neuroinformatics to partially simulate the functioning of the brain on computers or to come up with new ideas for "intelligent" information processing (see Blue Brain ). The perspective arises that the brain, as a structure for the production of thought and knowledge, provides an architecture that is recommended for imitation. Artificial neural networks have already established themselves in the organization of artificial intelligence processes .

Computing power and power consumption

Comparisons with modern computers show the capabilities of the human brain. While the brain can handle about 10 13 analog arithmetic operations per second and needs about 15 to 20 watts of power, the supercomputer BlueGene / L from IBM can handle up to 3.6 · 10 14 floating point operations per second with double precision , but about 1.2 Megawatts are required. Intel's first teraflop chip prototype " Terascale " with 80 processor cores, however, manages around 10 12 floating point operations with single precision at 85 watts (or 2 · 10 12 floating point operations at 190 watts and 6.26  GHz ), which is still the 50 to 5000 times the energy requirement. Modern 3D graphics cards achieve comparable values ​​with a lower electrical power requirement, but graphics chips are more specialized in certain computing processes.

It should be noted, however, that the high computing power of the brain is mainly achieved through its many parallel connections (connectivity) and not through the high speed of the individual computing processes ( clock frequency ). Artificial neurons work 100,000 times faster than neurons in the human brain.

Storage

In addition to parallelization, a neural network simultaneously represents storage and processing logic, while these are separate in computers based on the Von Neumann architecture . This has the effect that in a simple neural network all memory is updated with every clock cycle, while a computer has to update the contents of the memory gradually.

Efficiency

Calculation processes that run efficiently on a computer can usually not be efficiently mapped in a neural network and vice versa. Due to the inefficiency of existing computer architectures for certain tasks, such as vision, neural networks, such as that of the neocortex , are simulated by neuromorphing .

In March 2009, as part of the FACETS project , artificial neural networks mapped 200,000 artificial neurons with 50 million artificial synapses on a single 8-  inch (20.32 cm diagonal) computer chip. In July 2014, IBM introduced TrueNorth , which integrates 1 million neurons and 256 million synapses on one chip with a TDP of 70 mW, or 16 million neurons with 4 billion synapses in a single rack .

The model of the hypothesis genius

Hermann von Helmholtz already had the view of seeing the brain as a “hypothesis genius” or a “prediction machine” , since other approaches to artificially simulate the brain led to previously unsolvable problems and failed. The approach assumes that the brain forms hypotheses and integrates and compares all impressions and perceptions in the stored patterns. If what is perceived no longer fits the individual hypothesis, it is discarded and a new one is created if necessary. This shows itself classically in the interpretation of tilt figures .

Number and network of nerve cells

While a rat's brain contains around 200 million neurons, according to recent studies, that of a human has an average of around 86  billion nerve cells. Of these, about 16 billion are neurons in the cerebral cortex (cerebral cortex) , about 69 billion in the cerebellum (cerebellum) and approximately 1 billion in the remaining regions of the brain (from brainstem , diencephalon , and basal ganglia ).

Neurons are connected to one another via synapses , an estimated 100 trillion in the human brain  , so that on average one nerve cell would be connected to 1000 others and could be reached from any other neuron in no more than four steps. But there are locally significant deviations from this mean, because it is not the density but the pattern of neural connections that is decisive for neural functions. A common organizational principle of the brain is the mapping of neighborhood relationships: what is next to each other in the body is often represented next to each other in brain areas ( somatotopia ).

Although only the nerve cells conduct excitations as neural impulses and transmit them as signals to synapses via neurotransmitters , the glial cells surrounding them play no insignificant role in this. The generally just as common, mostly smaller glial cells enable nerve cells to conduct excitation quickly and to transmit signals without interference , absorb released messenger substances, ensure the supply of nutrients and are involved in the physiological barriers of the blood-brain and blood-liquor barriers . In the developing brain, and in developing brain regions, they influence the formation, stability and weighting of the synaptic connections between neurons; if peripheral nerves are damaged, they form a lead structure necessary for restoration.

The aim of connectome research is to map all connections between neurons.

The twelve main pairs of nerves in the brain

  1. Olfactory nerve - enables smelling
  2. Optic nerve - conducts optical impulses
  3. Oculomotor nerve - supplies four of the six muscles that move the eye and other functions
  4. Trochlear nerve - supplies the upper oblique eye muscle
  5. Trigeminal nerve - provides information about touch from the facial area, enables chewing
  6. Abducens nerve - supplies the lateral eye muscle
  7. Facial nerve - enables facial movements and taste perception, among other things
  8. Vestibulocochlear nerve (N. statoacusticus) - carries information from the auditory and balance organs
  9. Glossopharyngeal nerve - carries information (such as taste) from the pharynx area and enables movement in this area
  10. Vagus nerve - essentially for perception, movement and vegetative functions - including glandular activity and hormone release
  11. Accessory nerve - allows movement through two large muscles of the neck and head
  12. Hypoglossal nerve - allows tongue movements

Research projects

At the beginning of his second term in office, the former US President Barack Obama announced plans for a very large research project called the Brain Activity Map Project , in the course of which the human brain is to be completely mapped. This would be the largest scientific undertaking in many years (the last being the Human Genome Project ). Experts hope for therapies against Alzheimer's disease and Parkinson's as well as knowledge about human thinking and feeling. The first approaches were published in July 2012 in the journal Neuron .

The US project should not be confused with the Human Brain Project , which was started by the EU in February 2013. A jury also selected brain research as a key future project; It is funded with one billion euros.

Others

In 2008, the remains of a 2500 year old human skull were found on the grounds of the University of York (England) , the brain of which has largely been preserved. Researchers suspect that the brain of the probably 26–45 year old man has been preserved so well to this day, partly because the head - a body was not found - was buried in wet clay immediately after death. A complete explanation of why the brain has not long since disintegrated has not yet been found.

Brain as a raw material is used in fat tanning .

The neurolinguistics examines how language is represented by the brain, is worked and learned.

See also

Portal: Mind and Brain  - Overview of Wikipedia content on Mind and Brain

literature

Web links

Commons : Brain  album with pictures, videos and audio files
Wikibooks: Neuroanatomy  - Learning and Teaching Materials
Wiktionary: Brain  - explanations of meanings, word origins, synonyms, translations
Wikiquote: Brain  Quotes

Individual evidence

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  16. ^ Herbert Lochs: Hunger metabolism . ( Memento of the original from October 21, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.5 MB). 2003, p. 23. @1@ 2Template: Webachiv / IABot / www.dgem.de
  17. ↑ Avital Schurr: Lactates: the ultimate cerebral oxidative energy substrate? In: Journal of Cerebral Blood Flow and Metabolism . Volume 26, 2006, pp. 142-152.
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  19. ^ Herbert Lochs: Hunger metabolism . ( Memento of the original from October 21, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.5 MB). 2003, p. 19. @1@ 2Template: Webachiv / IABot / www.dgem.de
  20. ^ A b Herbert Lochs: Hunger metabolism . ( Memento of the original from October 21, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.5 MB). 2003, p. 5. @1@ 2Template: Webachiv / IABot / www.dgem.de
  21. ^ Philip A. Wood: How Fat Works . Harvard University Press, Cambridge MA 2006.
  22. ^ Leslie C. Aiello, Peter Wheeler: The Expensive-Tissue Hypothesis. The Brain and the Digestive System in Human and Primate Evolution. In: Current Anthropology Volume 36, No. 2, 1995, pp. 199-221.
  23. ^ Herbert Lochs: Hunger metabolism . ( Memento of the original from October 21, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.5 MB). 2003. @1@ 2Template: Webachiv / IABot / www.dgem.de
  24. ↑ L. Pellerin, PJ Magistretti: Glutamate uptake into astrocytes stimulates aerobic glycolysis: a mechanism coupling neuronal activity to glucose utilization. In: Proc Natl Acad Sci USA . Volume 91, 1994, pp. 10625-10629.
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