the brain is an organ that acts as a central nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the sensory organs for the senses like sight. The brain is the most complex organ in the vertebrate body. In humans, the cerebral cortex contains about 15-33 billion neurons, each connected by synapses to thousands of other neurons. These neurons communicate with each other by using long protoplasmic fibers called axons, which carry a signal train called the action potential to a remote part of the brain or body that targets a particular receiving cell.
Physiologically, the function of the brain is to exert central control over other organs. The brain acts throughout the body either by producing a pattern of muscle activity and by moving the secretion of a chemical called a hormone. This centralized control allows a fast and coordinated response to environmental change. Some basic types of responses such as reflexes may be mediated by the spinal cord or peripheral ganglia, but behavioral controls aimed specifically at complex sensory input require information that integrates central brainpower.
The operation of individual brain cells is now understood in great detail but the way they work together in groups of millions has not been solved. The latest model in modern neuroscience treats the brain as a biological computer, very different in the mechanics of electronic computers, but is similar in the sense that it gets information from the surrounding world, stores it, and processes it in various ways.
This article compares the properties of the brain across the range of animal species, with the greatest attention to vertebrates. It deals with the human brain as far as it shares other brain properties. The ways in which the human brain is different from other brains are covered in human brain articles. Some of the topics that may be discussed here are covered there because more can be said about them in the human context. The most important are brain diseases and the effects of brain damage, which are covered in human brain articles.
Video Brain
Anatomy
The shape and size of the brain vary widely among species, and identifying common traits is often difficult. Nevertheless, there are a number of principles of brain architecture prevailing in various species. Some aspects of brain structure are common to almost all ranges of animal species; others distinguish the "advanced" brain from the more primitive, or distinguish vertebrates from invertebrates.
The simplest way to obtain information about the anatomy of the brain is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its original state is too soft to work on, but can be hardened by immersion in alcohol or other fixative, and then sliced ​​apart for interior inspection. Visually, the interior of the brain consists of an area called gray, with a dark color, separated by a white matter area, with a brighter color. More information can be obtained by staining the sliced ​​brain tissue with various chemicals that produce areas where certain types of molecules are present in high concentrations. It is also possible to examine the microstructure of brain tissue using a microscope, and to track patterns of connection from one region of the brain to another.
Mobile structure
The brain of all species consists mainly of two broad classes of cells: neurons and glial cells. Glial cells (also known as glia or neuroglia ) come in several types, and perform a number of important functions, including structural support, metabolic support, insulation, and development guidance.. Neurons, however, are usually considered the most important cells in the brain. Properties that make unique neurons are their ability to send signals to specific target cells over long distances. They send these signals using axons, which are thin protoplasmic fibers that extend from the cell body and the project, usually with many branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body. The length of the axon can be remarkable: for example, if the pyramidal cell, (excitatory neuron) of the cerebral cortex is enlarged so that its cell body becomes the size of the human body, its axon, equally enlarged, will be a few centimeters in diameter, extending over one kilometer. These axons send signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and run along the axons at speeds of 1-100 meters per second. Some neurons emit the potential of action constantly, at a rate of 10-100 per second, usually in an irregular pattern; the other neurons are mostly calm, but occasionally issue an explosion of action potential.
Axons send signals to other neurons through a special connection called a synapses. One axon can make several thousand synaptic connections with other cells. When the action potential, traveling along the axon, arrives at the synapse, it causes a chemical called a neurotransmitter to be released. Neurotransmitters bind to receptor molecules in the target cell membrane.
Synapses are the main functional elements of the brain. An important function of the brain is cell-to-cell communication, and synapses are the point at which communication occurs. The human brain is thought to contain about 100 trillion synapses; even the fruit fly brain contains several million. The functions of these synapses are very diverse: some are excitatory (pulling target cells); the other is an inhibitor; others work by enabling a second messenger system that alters the internal chemistry of their target cells in a complicated way. A large number of synapses can be dynamically modified; that is, they are able to change the power in a way that is controlled by the pattern of signals passing through them. It is widely believed that activity-dependent modification of synapses is the brain's primary mechanism for learning and remembering.
Most of the space in the brain is taken by the axon, which is often incorporated in so-called nerve fiber channels . A myelin axon is enclosed in a fatty insulation sheath, which serves to increase the speed of signal propagation. (There are also unmyelinated axons). Myelin is white, making part of the brain filled exclusively with nerve fibers appearing as light-colored white matter, in contrast to darker gray matter that marks the area with high density of neuron cell cells.
Evolution
The generic bilateral nervous system
Except for some primitive organisms such as sponges (which have no nervous system) and cnidaria (which have a nervous system consisting of spreading nerve webs), all multicellular animals live are bilaterian, meaning animals with bilateral symmetrical body shapes (ie, left side and right which is the approximate mirror image of each other). All bilaterians are thought to have come from a common ancestor that emerged at the beginning of the Cambrian period, 485-540 million years ago, and it has been hypothesized that this common ancestor had a simple tubular worm shape with a segmented body. At the schematic level, the basic shape of the worm continues to be reflected in the body and architecture of the nervous system of all modern bilateral people, including vertebrates. The basic bilateral body shape is a tube with a hollow intestinal cavity that flows from the mouth to the anus, and the nerve with enlargement (ganglion) for each body segment, with a very large ganglion on the front, called the brain. Small and simple brain in some species, such as nematode worms; in other species, including vertebrates, it is the most complex organ in the body. Some types of worms, such as leeches, also have an enlarged ganglion on the back end of a neural cable, known as the "brain of the tail".
There are several types of bilateral that have no recognizable brain, including echinoderms and tunicata. It is not yet definitively determined whether the existence of this brainless species shows that the earliest bilaterians had no brains, or whether their ancestors evolved in a way that led to the loss of existing brain structures.
Invertebrates
This category includes tardigrades, arthropods, mollusks, and various types of worms. The diversity of invertebrate body plans is matched by the same diversity in brain structure.
Two groups of invertebrates have a very complex brain: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopus, squid, and similar molluscs). Arthropods and cephalopods arise from parallel twinning wires that extend through the animal's body. Arthropods have a central brain, a supraesophageal ganglion, with three divisions and a large optical lobe behind each eye for visual processing. Cephalopods such as octopuses and squid have the largest brains of any invertebrates.
There are several invertebrate species whose brain has been studied intensively because they have properties that make them comfortable for experimental work:
- Fruit flies ( Drosophila ), as many of the techniques available to study their genetics, have been a natural subject for studying the role of genes in brain development. Regardless of the great evolutionary distance between insects and mammals, many neurogenetic aspects of Drosophila have proven relevant to humans. The first biological clock gene, for example, is identified by examining the Drosophila mutant showing the cycle of impaired daily activity. The search in the vertebrate genome reveals a set of analog genes, which are found to play a similar role in the biological clocks of mice - and therefore almost certainly within the human biological clock as well. Studies conducted on Drosophila, also show that most areas of brain neuropil are constantly rearranged throughout life in response to certain living conditions.
- Nematode worms Caenorhabditis elegans , such as Drosophila , have been studied primarily because of the importance of genetics. In the early 1970s, Sydney Brenner chose him as a model organism to learn how genes control development. One of the advantages of working with this worm is that the body plan is highly stereotyped: the hermaphrodite nervous system contains exactly 302 neurons, always in the same place, making identical synaptic connections in each worm. Brenner's team sliced ​​the worm into thousands of parts of the ultrathin and photographed each one under an electron microscope, then visually matching the fibers from part to part, to map every neuron and synapse throughout the body. The complete neuronal wiring diagram of C.elegans - the connectome achieved. Nothing approaches this level of detail is available to other organisms, and the information gained has enabled a lot of research that is not possible.
- Sea snail Aplysia californica was selected by Nobel neurophysiologist Eric Kandel as a model for studying the cellular basis of learning and memory, due to the simplicity and accessibility of the nervous system, and has been examined in hundreds of experiments.
Vertebrata
The first vertebrates appeared more than 500 million years ago (Mya), during the Cambrian period, and may resemble modern ornamental fish in shape. Sharks appear around 450 Mya, amphibians around 400 Mya, reptiles around 350 Mya, and about 200 Mya mammals. Each species has an equally long history of evolution, but modern hagfish brains, lampreys, sharks, amphibians, reptiles, and mammals show a gradient of size and complexity that roughly follows the sequence of evolution. All of these brains contain the same basic anatomical component set, but many are not perfect in hagfish, whereas in the most important mammals (telencephalon) are highly elaborated and expanded.
Brain is easiest compared to their size. The relationship between brain size, body size and other variables has been studied in various vertebrate species. As a rule, brain size increases with body size, but not in simple linear proportions. In general, smaller animals tend to have larger brains, which are measured as fractions of body size. For mammals, the relationship between brain volume and body mass basically follows the law of force with an exponent of about 0.75. This formula illustrates a central tendency, but every mammalian family departs from it to some degree, in a way that reflects some of the complexity of their behavior. For example, primates have a brain 5 to 10 times larger than the formula predictions. Predators tend to have larger brains than their prey, relative to body size.
All vertebrate brains have the same common form, which is most evident during the early stages of embryonic development. In its earliest form, the brain appears as three swelling at the front end of the neural tube; This swelling eventually becomes the forebrain, midbrain, and back brain (prosencephalon, mesencephalon, and rhombencephalon, respectively). In the early stages of brain development, the three areas are almost the same size. In many vertebrate classes, such as fish and amphibians, the three parts remain the same in size in adults, but in the mammals the forebrain becomes much larger than the other, and the midbrain becomes very small.
The vertebrate brain is made up of very soft tissues. Live brain tissue is pink on the outside and mostly white on the inside, with subtle color variations. The vertebrate brain is surrounded by a connective tissue membrane system called the meninges that separate the skull from the brain. The blood vessels enter the central nervous system through the hole in the meningeal layer. The cells in the blood vessel wall join tightly to one another, forming a blood-brain barrier, which blocks the path of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thus posing a special challenge in the treatment of brain diseases).
Neuroanatomists typically divide vertebrate brains into six major areas: telencephalon, diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata. Each of these areas has a complex internal structure. Some parts, such as the cerebral cortex and cerebellar cortex, consist of a folded or convoluted layer to fit within the available space. Other parts, such as the thalamus and hypothalamus, consist of many small nuclei groups. Thousands of distinguishable regions can be identified in vertebrate brains based on differences in neural, chemical, and connectivity structures.
Although the same basic components are present in all vertebrate brains, some branches of vertebrate evolution have caused substantial distortions of brain geometry, especially in the frontal brain region. The shark's brain shows the basic components in a direct way, but in teleost fish (the majority of fish species present), the forebrain has become "evert", like an upside-down sock. In birds, there are also major changes in the structure of the forebrain. This distortion can make it difficult to match brain components of one species to another.
Here is a list of some of the most important vertebrate brain components, along with a brief description of their function as understood today:
- The medulla , along with the spinal cord, contains many small nuclei involved in various sensory and involuntary motor functions such as vomiting, heart rate and digestive processes.
- The pons is located in the brainstem just above the medulla. Among other things, it contains nuclei that control often voluntary but simple actions such as sleep, breathing, swallowing, bladder function, balance, eye movement, facial expression, and posture.
- The hypothalamus is a small area at the base of the forebrain, whose complexity and importance deny its size. It consists of many small nuclei, each with different connections and neurochemicals. The hypothalamus is involved in voluntary additional or partial voluntary actions such as sleep and wake cycles, eating and drinking, and releasing some hormones.
- The thalamus is a collection of nuclei with multiple functions: some are involved in conveying information to and from the brain hemisphere, while others are involved in motivation. The subthalamic region (incerta zone) appears to contain an action-producing system for some types of "fulfillment" behaviors such as eating, drinking, defecating, and copulation.
- cerebellum modulates the output of other brain systems, whether motors are related or related to the mind, to make them sure and precise. Cerebellum removal does not prevent animals from doing anything in particular, but it makes the action hesitant and awkward. This precision is not built-in, but it is learned by trial and error. Muscle coordination learned while riding a bicycle is an example of the type of nerve plasticity that may occur mostly in the cerebellum. 10% of the total brain volume consists of the cerebellum and 50% of all neurons held in its structure.
- optical tektum allows actions to be directed to a point in space, most often in response to visual input. In mammals it is usually referred to as a superior colliculus, and its best function is to direct eye movement. It also directs the reach of motions and actions directed towards other objects. It receives strong visual input, but also other sensory inputs that are useful in directing action, such as auditory input in owls and input from the pit thermosensitive organ on the snake. In some primitive fish, such as lampreys, this region is the largest part of the brain. The superior colliculus is part of the midbrain.
- The pallium is a layer of gray matter located on the surface of the forebrain and is the most complex and most recent evolutionary development of the brain as an organ. In reptiles and mammals, it is called cerebral cortex . Some functions involve pallium, including odor and spatial memory. In a mammal, where it becomes so large that it dominates the brain, it takes over the function of many other areas of the brain. In many mammals, the cerebrum cortex consists of a folded protuberance called gyri that creates deep wrinkles or cracks called sulci. The folds increase the surface area of ​​the cortex and therefore increase the amount of gray matter and the amount of information that can be stored and processed.
- The hippocampus , strictly speaking, is found only in mammals. However, the area comes from the medallion, pallium, has colleagues in all vertebrates. There is evidence that this part of the brain is involved in complex events such as spatial memory and navigation in fish, birds, reptiles, and mammals.
- basal ganglia is a group of interconnected structures in the forebrain. The main function of the basal ganglia appears to be the selection of action: they send signals of inhibition to all parts of the brain that can produce motor behavior, and in the right circumstances can release the inhibition, so that the action-generating system can execute their actions. Gifts and punishments use the most important nerve effects by altering connections within the basal ganglia.
- The olfactory bulb is a special structure that processes the olfactory sensory signal and sends its output to the olfactory part of the pallium. It is a major brain component in many vertebrates, but greatly reduced in humans and other primates (whose senses are more dominated by information obtained from sight and not odor).
Mammals
The most obvious difference between the brains of mammals and other vertebrates is in size. On average, mammals have about twice as many brains as the size of the same bird body, and are ten times larger than the size of the same reptile body.
Size, however, is not the only difference: there is also a substantial difference in form. The midbrain and midbrain of a mammal are generally similar to other vertebrates, but dramatic differences appear in the forebrain, which is very enlarged and also changes in structure. The cerebral cortex is part of the brain that greatly differentiates mammals. In non-mammalian vertebrates, the cerebrum surface is coated with a relatively simple three-tier structure called pallium. In mammals, the pallium evolves into a six-ply complex structure called neocortex or isocortex . Some areas at the edge of the neocortex, including the hippocampus and amygdala, are also much more developed in mammals than other vertebrates.
Elaborations of the cerebral cortex bring changes to other areas of the brain. The superior colliculus, which plays a major role in behavioral visual control in most vertebrates, shrinks to small size in mammals, and many of its functions are taken over by the visual area of ​​the cerebral cortex. Most cerebellum mammals (neocerebellum) are dedicated to supporting the cerebral cortex, which has no pairs in other vertebrates.
Primates
The human brain and other primates contain the same structure as other mammalian brains, but are generally larger in body size proportions. The encephalization quotient (EQ) is used to compare brain size across species. This takes into account the non-linear relationship of brain-to-body. Humans have an average EQ in the range of 7 to 8, while most other primates have EQs in the 2-to-3 range. Dolphins have a higher value than primates other than humans, but almost all other mammals have a much lower EQ value.
Most of the primate brain enlargement comes from the massive expansion of the cerebral cortex, especially the prefrontal cortex and the parts of the cortex involved in vision. The primate visual processing network includes at least 30 distinct areas of the brain, with a complex interconnect network. It is estimated that the visual processing area occupies more than half of the total primocal neocortex surface. The prefrontal cortex performs functions that include planning, working memory, motivation, attention, and executive control. This requires a much larger proportion of the brain for primates than other species, and most of the human brain.
Maps Brain
Development
The brain develops in a complicated sequence of stages. It changes shape from simple swelling on the front of the nerve strap at the early embryonic stage, to complex arrangement of areas and connections. Neurons are created in special zones containing stem cells, and then migrate through the network to reach their final location. Once the neurons have positioned themselves, their axons sprout and navigate in the brain, branched and elongated as they go, until the tip reaches their target and forms a synaptic connection. In some parts of the nervous system, neurons and synapses are produced in excess amounts during the initial stages, and then the unneeded will be trimmed.
For vertebrates, the early stages of neural development are similar across species. When the embryo changes from a round cell clump to a worm-like structure, the ectoderm's narrow path that runs along the midline of the back is induced into a neural plate, a precursor of the nervous system. The neural plate folds inward to form a nerve groove, and then the lining lining the groove merges to encase the neural tube, the hollow casing of the cell with the liquid-filled ventricle at the center. At the front end, the ventricles and umbilical cord swell into three vesicles which are the precursors of the forebrain, midbrain, and cerebellum. In the next stage, the forebrain is divided into two vesicles called telencephalon (which will contain the cerebral cortex, basal ganglia, and related structures) and diencephalon (which will contain the thalamus and hypothalamus). At about the same time, the back of the brain is divided into metencephalon (which will contain the cerebellum and pons) and myelencephalon (which will contain medulla oblongata). Each of these areas contains a proliferative zone in which neurons and glial cells are produced; the resulting cells then migrate, sometimes for long distance, to their last position.
Once the neuron is in place, it extends the dendrite and axons to the surrounding area. Axons, as they generally extend great distances from cell bodies and need to achieve certain targets, grow in a very complex way. The axon tip grows composed of a protoplasmic blob called a growth cone, encrusted with a chemical receptor. These receptors sense the local environment, causing growth cones to be attracted or rejected by various cellular elements, and are thus drawn in a certain direction at each point along its path. The result of this trace search process is the growth cones navigating through the brain until it reaches the destination, where other chemical cues cause it to begin producing synapses. Considering the entire brain, thousands of genes create products that affect the making of axonal pathways.
The synaptic tissue that eventually arises is only partially determined by the genes. In many parts of the brain, axons initially "grow too fast", and then are "trimmed" by mechanisms that depend on neural activity. In the projection of the eye to the midbrain, for example, the structure in adults contains a very precise mapping, connecting each point on the surface of the retina to the corresponding point of the midbrain layer. In the first stage of development, each axon of the retina is guided rightwards in the center of the midbrain by chemical cues, but then its branches are very heavy and make initial contact with various midbrain stems. The retina, before birth, contains a special mechanism that causes it to generate a spontaneously derived activity wave at a random point and then spread slowly across the retinal lining. This wave is useful because it causes neighboring neurons to be active at the same time; that is, they produce a pattern of neural activity that contains information about the spatial arrangement of neurons. This information is exploited in the midbrain by a mechanism that causes the synapses to weaken, and eventually disappears, if activity in the axon is not followed by the activity of the target cell. The result of this advanced process is the gradual adjustment and tightening of the map, so that finally in the proper adult form.
The same thing happens in other areas of the brain: an early synaptic matrix is ​​produced as a result of genetically determined chemical guidance, but then gradually perfected by an activity-dependent mechanism, partially driven by internal dynamics, in part by external sensory input. In some cases, such as the mid-retinal system, the pattern of activity depends on a mechanism that operates only in the developing brain, and appears to exist only to guide its development.
In humans and many other mammals, new neurons are created primarily before birth, and the baby's brain contains far more neurons than the adult brain. However, there are some areas where new neurons continue to be produced throughout life. The two areas in which adult neurogenesis is well-formed are the olfactory bulb, involved in the sense of smell, and the dentate gyrus of the hippocampus, where there is evidence that new neurons play a role in storing newly acquired memories. With these exceptions, however, the set of neurons present in early childhood is the set that is present for life. Glial cells are different: like most cell types in the body, they are produced over a lifetime.
There is a long debate about whether the quality of thought, personality, and intelligence can be attributed to heredity or upbringing - this is nature and nurture controversy. Although much detail remains to be resolved, neuroscience research has clearly shown that both factors are important. The genes determine the general shape of the brain, and genes determine how the brain reacts to the experience. Experience, however, is necessary to improve the synaptic connection matrix, which in its developed form contains much more information than the genome does. In some ways, what matters is the presence or absence of experience during a critical development period. In other respects, the quantity and quality of experience is important; for example, there is substantial evidence that animals raised in enriched environments have a thicker cerebral cortex, exhibiting higher synaptic connection density, than animals whose stimulated levels are limited.
Physiology
Brain function depends on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells. The electrical properties of neurons are controlled by various biochemical and metabolic processes, especially the interactions between neurotransmitters and receptors that occur in synapses.
Neurotransmitters and receptors
Neurotransmitters are chemicals released in synapses when the action potential activates them - neurotransmitters attach themselves to receptor molecules on the target cell membrane of the synapse, and thus alter the electrical or chemical properties of the receptor molecule. With few exceptions, every neuron in the brain releases the same neurotransmitter chemistry, or a combination of neurotransmitters, on all the synaptic connections it makes to other neurons; This rule is known as the Dale principle. Thus, a neuron can be characterized by a neurotransmitter it releases. Most psychoactive drugs use their effects by altering certain neurotransmitter systems. This applies to drugs such as cannabinoids, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others.
The two most widely used neurotransmitters in vertebrate brains are glutamate, which almost always gives stimulatory effects on target neurons, and gamma-aminobutyric acid (GABA), which is almost always inhibitory. Neurons that use these transmitters can be found in almost every part of the brain. Because of their ubiquity, drugs acting on glutamate or GABA tend to have broad and powerful effects. Some general anesthesia works by reducing the effects of glutamate; most of the tranquilizers exert their sedative effects by increasing the GABA effect.
There are dozens of other chemical neurotransmitters used in the more limited area of ​​the brain, often dedicated to certain functions. Serotonin, for example - a major target of antidepressant drugs and a lot of dietary help - comes exclusively from a small brainstem region called the raphe nucleus. Norepinephrine, which is involved in arousal, comes exclusively from a small nearby area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have many sources in the brain, but are not scattered everywhere such as glutamate and GABA.
Electrical activity
As a side effect of the electrochemical process used by neurons for signaling, brain tissue produces an electric field when active. When large numbers of neurons show synchronized activity, the electric fields they produce can be large enough to detect beyond the skull, using electroencephalography (EEG) or magnetoencephalography (MEG). EEG recordings, along with recordings made from electrodes grown in the brains of animals such as rats, show that the brains of living animals continue to be active, even during sleep. Each part of the brain exhibits a mixture of rhythmic and non-rhythmic activity, which may vary according to the state of behavior. In mammals, the cerebral cortex tends to exhibit large slow delta waves during sleep, faster alpha waves when the animals are awake but neglectful, and disorganized activity that appears chaotic when the animal is actively involved in the task. During epileptic seizures, the mechanism of inhibition of the brain fails to function and electrical activity rises to the pathological level, resulting in an EEG trace showing large waves and invisible spikes in healthy brains. Relating to this population-level pattern with the computational function of individual neurons is the main focus of current research in neurophysiology.
Metabolism
All vertebrates have a blood-brain barrier that allows metabolism in the brain to operate differently from metabolism in other parts of the body. Glial cells play a major role in brain metabolism by controlling the chemical composition of the fluid surrounding the neurons, including ion levels and nutrients.
Brain tissue consumes large amounts of energy in proportion to its volume, so the big brain places heavy metabolic demands on animals. The need to limit weight in order, for example, to fly, appears to have led to selection for the reduction of brain size in some species, such as bats. Most brain energy consumption goes into maintaining the electrical charge (membrane potential) of neurons. Most vertebrate species devote between 2% to 8% of basal metabolism to the brain. However, in primates, the percentage is much higher - in humans it increases to 20-25%. Brain energy consumption does not vary over time, but the active regions of the cerebral cortex consume more energy than inactive regions; this forms the basis for functional brain imaging methods of PET, fMRI, and NIRS. The brain typically derives most of its energy from oxygen-dependent glucose metabolism (ie blood sugar), but ketones provide a major alternative source, together with the contribution of medium chain fatty acids (caprylic and heptanoic acids), lactate, acetate, and possibly amino acids.
Function
Information from sensory organs is collected in the brain. It is used to determine what action the organism must take. The brain processes raw data to extract information about the environmental structure. Next it combines information that is processed with information about the current needs of the animals and with the memory of the past. Finally, based on the results, it produces a motor response pattern. These signal processing tasks require a complex interaction between the various functional subsystems.
The function of the brain is to provide coherent control over the actions of animals. The central brain allows the muscle groups to be activated together in a complex pattern; it also allows the stimulation to overwrite one part of the body to evoke the response in another, and it can prevent different body parts from acting cross each other.
Perception
The human brain is equipped with information about light, sound, atmospheric chemical composition, temperature, head orientation, limb position, chemical composition of the bloodstream, and more. In other animals additional senses are present, such as the infrared heat of the snake, the magnetic field flavor of some birds, or the sense of the electric field of some species of fish.
Each sensory system starts with specialized receptor cells, such as neurons that receive light in the retina of the eye, or vibration-sensitive neurons in the ear cochlea. Sensory cell axons receptors move into the spinal cord or brain, where they transmit their signals to a first-order sensory nucleus dedicated to one specific sensory modality. This primary sensory nucleus sends information to a high sensory area dedicated to the same modality. Finally, through the road station in the thalamus, signals are sent to the cerebral cortex, where they are processed to extract the relevant features, and integrate with signals originating from other sensory systems.
Motor control
The motor system is the area of ​​the brain involved in initiating body movement, that is, in activating the muscles. Except for the muscles that control the eye, which are moved by the nucleus in the midbrain, all the voluntary muscles in the body are directly innervated by motor neurons in the spinal cord and the back of the brain. Spinal motor neurons are controlled either by the intrinsic neural circuitry to the spinal cord, and by the descending input of the brain. The intrinsic spinal circuit implements many reflex responses, and contains pattern generators for rhythmic movements such as walking or swimming. Connection down from the brain allows for more sophisticated control.
The brain contains several areas of the motor that project directly into the spinal cord. At the lowest level are the motor areas in the medulla and the pons, which control stereotypical movements such as walking, breathing, or swallowing. At higher levels are areas in the midbrain, such as the red nucleus, which is responsible for coordinating arm and leg movement. At higher levels is the main motor cortex, a piece of tissue located on the posterior edge of the frontal lobe. The main motor cortex sends projections into the subcortical motor area, but also sends large projections directly to the spinal cord, via a pyramidal channel. This direct corticospinal projection allows for proper voluntary control of fine motion detail. Other brain areas associated with the motor exert a secondary effect by projecting to the main motor area. Among the most important secondary areas are the premotor cortex, the basal ganglia, and the cerebellum.
In addition to all of the above, the brain and spinal cord contain extensive circuits to control the autonomic nervous system, which works by secreting hormones and by modulating "smooth" bowel muscles.
Arousal
Many animals alternate between sleep and wake in the daily cycle. Passion and awareness are also modulated on a better time scale by the brain area network.
A key component of the passion system is the suprachiasmatic nucleus (SCN), a small part of the hypothalamus located just above the point where the optic nerves of the two eyes cross each other. SCN contains the body's central biological clock. The neurons there show a rise in activity level with a period of about 24 hours, circadian rhythms: the fluctuations of this activity are driven by rhythmic changes in the expression of a series of "clock genes". SCN continues to keep time even if it is removed from the brain and placed on a warm nutrient solution plate, but usually receives input from the optic nerve, via the retinohypothalamic tract (RHT), allowing the daily dark-light cycle to clock calibration.
SCN projects into a set of areas in the hypothalamus, brainstem, and midbrain involved in carrying out the sleep-wake cycle. An important component of this system is the reticular formation, a cluster of diffused neurons dispersed through the lower brain nucleus. The reticular neuron sends a signal to the thalamus, which in turn sends an activity-level control signal to each section of the cortex. Damage to the reticular formation may result in a permanent coma.
Sleep involves major changes in brain activity. Until the 1950s it was generally believed that the brain basically died during sleep, but it is now far from being true; activity continues, but the pattern becomes very different. There are two types of sleep: REM sleep (dreaming) and NREM (non-REM, usually without dreaming), which repeat in slightly varying patterns throughout the sleep episode. Three broad types of different brain activity patterns can be measured: REM, light NREM and deep NREM. During deep NREM sleep, also called slow-wave sleep, activity in the cortex takes on a synchronized large waveform, while in awake it is noisy and out of sync. Neurotransmitter levels of norepinephrine and serotonin decrease during slow-wave sleep, and fall almost to zero during REM sleep; acetylcholine levels show a reverse pattern.
Homeostasis
For any animal, survival requires maintaining various body state parameters within a limited range of variations: these include temperature, moisture content, salt concentration in the bloodstream, blood glucose levels, blood oxygen levels, and so on. The ability of an animal to regulate its internal environment - the environment, as the pioneer of the physiologist Claude Bernard calls it - is known as homeostasis (Greek for "standing still"). Maintaining homeostasis is an important function of the brain. The basic principle underlying homeostasis is negative feedback: each time the parameter is different from the set, the sensor generates an error signal that generates a response that causes the parameter to shift back to its optimum value. (This principle is widely used in engineering, for example in temperature control using a thermostat.)
In vertebrates, the part of the brain that plays the largest role is the hypothalamus, a small area at the base of the forebrain whose size does not reflect the complexity or importance of its function. The hypothalamus is a collection of small nuclei, most of which are involved in basic biological functions. Some of these functions relate to passion or social interaction such as sexuality, aggression, or mother behavior; but many of them are related to homeostasis. Some hypothalamic nuclei receive input from sensors located in the lining of blood vessels, conveying information about temperature, sodium levels, glucose levels, blood oxygen levels, and other parameters. This hypothalamus nucleus sends an output signal to the motor area that can produce action to correct deficiencies. Some output also goes to the pituitary gland, a small gland attached to the brain just below the hypothalamus. The pituitary gland secretes hormones into the bloodstream, where they circulate throughout the body and cause changes in cellular activity.
Motivation
Individual animals need to express behaviors that support survival, such as finding food, water, shelter, and couples. The motivational system in the brain monitors the state of satisfaction of these goals, and activates the behavior to meet every emerging need. The motivation system works primarily with reward-punishment mechanisms. When certain behaviors are followed by beneficial consequences, the reward mechanism in the brain is activated, which induces structural changes in the brain that cause the same behavior to be repeated later, whenever the same situation arises. Conversely, when behavior is followed by unfortunate consequences, the mechanism of brain punishment is activated, encouraging structural changes that cause behavior to be suppressed when a similar situation arises in the future.
Most organisms studied to date use a reward-punishment mechanism: for example, worms and insects can change their behavior to find food sources or avoid danger. In vertebrates, a reward-punishment system is applied by a particular set of brain structures, at the heart of the basal ganglia, a set of interconnected areas at the base of the forebrain. Basal ganglia are the central sites in which decisions are made: basal ganglia exerting ongoing inhibitory control over most motor systems in the brain; when this inhibition is released, the motor system is allowed to execute the action programmed to run. Rewards and punishments function by altering the relationship between the input received by the basal ganglia and the emitted decision signals. Reward mechanisms are better understood than punitive mechanisms, because their role in drug abuse causes them to be studied intensively. Research has shown that dopamine neurotransmitters play a central role: addictive drugs such as cocaine, amphetamines, and nicotine either cause increased levels of dopamine or cause dopamine effects in the brain to be increased.
Learning and memory
Almost all animals are able to modify their behavior as a result of experience - even the most primitive types of worms. Because behavior is driven by brain activity, behavioral changes must correspond to changes in the brain. Already in the late 19th century theorists such as Santiago Ramón Cajal argue that the most plausible explanation is that learning and memory are expressed as a change in the synaptic relationship between neurons. Until 1970, however, experimental evidence to support the synaptic plasticity hypothesis was lacking. In 1971 Tim Bliss and Terje LÃÆ'¸mo published a paper on the phenomenon now called long-term potentiation: the paper shows concrete evidence of synaptic changes induced by activity that lasted for at least several days. Since then technical progress has made such experiments easier to perform, and thousands of studies have been made that have clarified the synaptic mechanism of change, and revealed other types of activity-driven synaptic changes in different areas of the brain, including cerebral cortex, hippocampus, basal ganglia, and cerebellum. The brain-derived neurotrophic factor (BDNF) and physical activity seem to play a beneficial role in the process.
Neuroscientists today distinguish some types of learning and memory that the brain implements in different ways:
- Working memory is the brain's ability to maintain a temporary representation of information about the tasks that animals do today. This type of dynamic memory is thought to be mediated by the formation of cell assemblies - groups of active neurons that retain their activity by continually stimulating each other.
- Episodic memory is the ability to remember details of a particular event. This kind of memory can last a lifetime. Much of the evidence implies the hippocampus plays an important role: people with severe damage to the hippocampus sometimes show amnesia, the inability to form long-lasting episodic memories.
- Semantic memory is the ability to learn facts and relationships. This kind of memory may be stored mostly in the cerebral cortex, mediated by changes in the relationship between cells representing certain types of information.
- Instrumental learning is the ability for rewards and punishments to modify behavior. It is implemented by a brain area network centered on basal ganglia.
- Motor learning is the ability to improve body movement patterns by practicing, or more generally by repetition. A number of brain areas are involved, including premotor cortex, basal ganglia, and especially cerebellum, which function as a large memory bank for micro adjustment of movement parameters.
Research
The field of neuroscience includes all approaches that seek to understand the brain and the rest of the nervous system. Psychology seeks to understand thoughts and behavior, and neurology is a medical discipline that diagnoses and treats diseases of the nervous system. The brain is also the most important organ studied in psychiatry, a branch of medicine that serves to study, prevent, and treat mental disorders. Cognitive science seeks to unite neuroscience and psychology with other fields that occupy themselves with the brain, such as computer science (artificial intelligence and similar fields) and philosophy.
The oldest method of studying the brain is anatomical, and until the mid-20th century, much of the progress in neuroscience came from the development of better cell stains and better microscopy. Neuroanatomists study large-scale brain structures as well as microscopic structures of neurons and their components, especially synapses. Among other tools, they use a large number of stains that reveal neural, chemical, and connectivity structures. In recent years, the development of immunostaining techniques has allowed the investigation of neurons that express a particular set of genes. Also, functional neuroanatomy uses medical imaging techniques to relate variations in the structure of the human brain to differences in cognition or behavior.
Neurophysiologists study the chemical, pharmacological, and electrical properties of the brain: their main tool is drugs and recording devices. Thousands of experimentally developed drugs affect the nervous system, some in a very specific way. Records of brain activity can be performed using electrodes, either embedded in the scalp as in the EEG study, or implanted in the animal's brain for extracellular recordings, which can detect the potential action produced by individual neurons. Because the brain does not contain pain receptors, it is possible to use these techniques to record the brain activity of animals that are awake and behave without causing adversity. The same technique has sometimes been used to study brain activity in human patients suffering from stubborn epilepsy, in cases where there is a medical need to implant an electrode to localize the area of ​​the brain responsible for epileptic seizures. Functional imaging techniques such as functional magnetic resonance imaging are also used to study brain activity; these techniques are mainly used with human subjects, since they require conscious subjects to remain immobile for long periods of time, but they have a great advantage as non-invasive.
Another approach to brain function is to examine the consequences of damage to certain areas of the brain. Although protected by skulls and meninges, surrounded by cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the delicate nature of the brain makes it susceptible to various diseases and some types of damage. In humans, the effects of stroke and other types of brain damage have been a major source of information about brain function. Since there is no ability to experimentally control the nature of the damage, however, this information is often difficult to interpret. In animal studies, most commonly involving rats, it is possible to use locally injected electrodes or chemicals to produce appropriate patterns of damage and then examine the consequences for behavior.
Computational neuroscience includes two approaches: first, the use of computers to study the brain; second, the study of how the brain performs computation. On the one hand, it is possible to write computer programs to simulate the operation of a group of neurons by utilizing a system of equations depicting their electrochemical activity; Such simulations are known as biologically realistic neural networks . On the other hand, it is possible to study algorithms for neural computing by simulating, or analyzing mathematically, the operation of a simplified "unit" that has some properties of neurons but abstracts out much of their biological complexity. The function of brain computing is studied by computer scientists and neurologists.
Computational neurogenetic modeling is concerned with the study and development of dynamic neuronal models for modeling brain function with respect to genes and dynamic interactions between genes.
Recent years have increased the application of genomics and genetic techniques to brain studies and focus on the role of neurotrophic factors and physical activity in neuroplasticity. The most common subjects are mice, due to the availability of technical tools. It may now be relatively easy to "paralyze" or mutate various genes, and then examine its effect on brain function. A more sophisticated approach is also used: for example, using Cre-Lox recombination it is possible to activate or deactivate genes in certain parts of the brain, at any given time.
History
The oldest brain found in Armenia in the Areni-1 cave complex. The brain, estimated to be over 5,000 years old, is found in the skull of a girl aged 12 to 14 years. Although their brains are wrinkled, they are well preserved because of the climate found inside the cave.
Early philosophers were divided on whether the place of the soul lies in the brain or heart. Aristotle liked the heart, and thought that brain function was just to cool the blood. Democritus, the inventor of the atomic theory of matter, argues for three parts of the soul, with head intelligence, emotion in the heart, and lust of the heart. Hippocrates, "father of medicine", came down firmly in favor of the brain. In his treatise on epilepsy he writes:
Men should know that from anything other than the brain comes pleasure, fun, laughter and sport, and sorrow, sadness, sadness, and lamentation.... And by the same organs we go crazy and delirious, and fear and terror attack us, some at night, and some during the day, and untimely dreams and odyssey, and caring unsuitable ones, and ignorance of circumstances this, desuetude, and unskilled. All these things we bear from the brain, when it's not healthy...
- Hippocrates, On the Sick Disease
The Roman physician, Galen, also debated the importance of the brain, and theorized deeply about how it worked. Galen traces the anatomical relationship between the brain, nerves, and muscles, indicating that all the muscles in the body are connected to the brain through a branched nerve network. He postulates that the nerves activate the muscles mechanically by carrying a mysterious substance called pneumata psychic, usually translated as "animal spirit". Galen's ideas were widely known during the Middle Ages, but little progress was made until the Renaissance, when detailed anatomical studies continued, combined with theoretical speculations of Renà © Descartes and those who followed. Descartes, like Galen, thinks of the nervous system in hydraulic terms. He believes that the highest cognitive function is performed by non-physical res cogitans, but the majority of human behavior, and all animal behavior, can be mechanically explained.
The first real advance towards modern understanding of nerve function, though, stems from the investigation of Luigi Galvani, who discovered that a static electric shock applied to the open nerves of a dead frog could cause his legs to contract. Since then, every major advance in understanding has followed more directly than the development of new inquiry techniques. Until the early years of the 20th century, the most important advancement came from a new method for cell staining. The most critical is the discovery of Golgi dyes, which (when used correctly) gives only a fraction of neurons, but puts them entirely, including cell bodies, dendrites, and axons. Without such stains, brain tissue under a microscope emerges as an impenetrable fabric of protoplasmic fibers, where it is impossible to determine any structure. In the hands of Camillo Golgi, and especially of the Spanish neuroanatomist Santiago RamÃÆ'³n y Cajal, this new stain reveals hundreds of different types of neurons, each with its own unique dendritic structure and connectivity pattern.
In the first half of the 20th century, advances in electronics enabled the investigation of the electrical properties of nerve cells, culminating in work by Alan Hodgkin, Andrew Huxley, and others on the biophysics of potential action, and the work of Bernard Katz and others. on electrochemical synapses. These studies complement the anatomical picture with the conception of the brain as a dynamic entity. Reflecting on a new understanding, in 1942 Charles Sherrington visualized the workings of the waking brain from sleep:
The largest sheet of mass, which is where there is almost no flickering or moving light, becomes a rhythmic sphere of rhythm with rapidly winding darts leading to it and to the right. The brain wakes up and with that thought back. As if Milky Way enters a cosmic dance. Quickly, the mass of heads into enchanted looms where millions of flashing shuttles embrace the pattern of dissolution, always a meaningful pattern, though nothing ever lasts; harmony subpattern shift.
- - Sherrington, 1942, Man on his Nature
The invention of electronic computers in the 1940s, along with the development of mathematical information theory, led to the awareness that the brain could potentially be understood as an information processing system. This concept forms the basis of the field of cybernetics, and ultimately spawns a field now known as computational neuroscience. The earliest attempts at cybernetics are rather rough because they treat the brain as essentially a disguised digital computer, as in John Von Neumann's 1958 book, The Computer and the Brain . Over the years, though, gathering information about the electrical responses of brain cells recorded from behaving animals has continued to transfer theoretical concepts toward increased realism.
One of the most influential early contributions was a 1959 paper entitled What the frog's eye sees in the frog's brain: the paper examines the visual responses of neurons in the retina and the optical toad of frogs, and comes to the conclusion that some neurons in the frog's tektum are connected to combine the basic responses in ways that make them function as "bug bugs". A few years later, David Hubel and Torsten Wiesel discovered cells in the main visual cortex of monkeys that became active when sharp edges moved across certain points in the field of view - the invention they won the Nobel Prize. Follow-up studies in high-level visual areas find cells that detect binocular differences, colors, movements, and shapes, with areas that lie in increasing distance from the main visual cortex that shows an increasingly complex response. Other investigations of unrelated areas of the brain have revealed cells with correlated responses, some related to memory, some to an abstract type of cognition such as space.
Theorists have worked to understand these response patterns by building mathematical models of neurons and neural networks, which can be simulated using computers. Some useful models are abstract, focusing on the conceptual structure of nerve algorithms rather than details of how they are implemented in the brain; Other models try to include data about the biophysical properties of real neurons. No model at any level is considered a fully valid description of brain function. The underlying difficulty is that advanced computation by neural networks requires distributed processing where hundreds or thousands of neurons work cooperatively - the method of recording current brain activity is only able to isolate the action potential of several dozen neurons at a time.
Furthermore, even single neurons appear complex and capable of performing calculations. Thus, a model of the brain that does not reflect this is too abstract to be representative of brain surgery; models that try to capture these are very expensive and can be debated with these computing resources. However, the Human Brain Project tries to build a realistic and detailed computational model of the entire human brain. The wisdom of this approach has been contested in public, with high-profile scientists on both sides of the argument.
In the second half of the 20th century, the development of chemistry, electron microscopy, genetics, computer science, functional brain imaging, and other fields increasingly opened new windows into the structure and function of the brain. In the United States, the 1990s was officially designated as the "Brain Decade" to commemorate the progress made in brain research, and to promote funding for such research.
In the 21st century, this trend continues, and several new approaches have become famous, including multi-electrode recording, allowing the activity of many brain cells to be recorded all at the same time; genetic engineering, which allows the molecular components of the brain to be altered experimentally; genomics, allowing variation in brain structure to be correlated with variations in DNA properties and neuroimaging.
Other uses
As food
Animal brains are used as food in various cuisines.
In ritual
Some archaeological evidence suggests that European Neanderthal mourning rituals also involve brain consumption.
The people of Fore Papua New Guinea are known to eat the human brain. In burial rituals, those close to the dead will eat the brains of the dead to create a sense of immortality. A prion disease called kuru has been traced to this.
For tanning
The brain can be useful for hunters: most animals have enough brain material to use in their own tannery.
See also
References
External links
- Brain from Top to Bottom, at McGill University
Source of the article : Wikipedia
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