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In all vertebrate animals, as well as the majority of invertebrate ones, the brain functions as the hub of the nervous system. It is situated inside the head, typically close to the sensory structures that support senses like vision. It is the most intricate organ in the body of a vertebrate. A human's cerebral cortex is thought to have 14-16 billion neurons, and the cerebellum is thought to have 55-70 billion neurons. Each neuron has thousands of synapses that link them together. These neurons normally communicate with one another through the use of long fibers called axons, which send potential action trains to distant regions of the brain or body, where they target certain recipient cells.


Physiologically, a body's other organs are concentrated under the supervision of the brain. By causing patterns of muscle activity and the secretion of hormone-like substances, they influence the rest of the body. This concentrated control makes rapid and well-organized responses to environmental changes possible.

Although the functioning of each brain cell is now very well understood, the mystery surrounding how millions of them work together remains. Modern neuroscience's most recent models consider the brain as a biological computer, quite different from an electronic computer in terms of its workings but comparable in that it gathers information from the environment, stores it, and processes it in various ways.


Anatomically, it looks like a glob encircled by a thin strip of dark-colored material and has a blue patch in the center. The rat olfactory bulb cross section is simultaneously stained in two separate ways; one stain reveals the cell bodies of neurons, the other the receptors for the neurotransmitter GABA. It might be challenging to pinpoint common traits because the size and form of the brain vary widely between species.

Yet, several brain architecture principles hold for a diverse variety of animals. Nearly all animal species share certain features of brain structure; others set "advanced" brains apart from more primitive ones or set vertebrates apart from invertebrates.

Visual inspection is the easiest method for learning brain anatomy, but several more complex methods have been created. Although brain tissue is too delicate to work within its normal form, it can be made workable by soaking in alcohol or other fixatives before being cut open for internal inspection. Visually, the brain's interior is divided into sections of "grey matter," which has a dark color, and "white matter," which has a lighter color. Slices of brain tissue can be stained with several substances to highlight regions with concentrated quantities of particular molecules, providing further information. A microscope can also be used to look at the microstructure of brain tissue. With a microscope, it can also look at the microstructure of brain tissue and follow the network of connections between different parts of the brain.

Cell Structure

Neurons and glial cells comprise most of the cells in the brains of all species. Many glial cells, also known as glia or neuroglia, carry out various vital tasks, such as providing structural support, metabolic support, insulation, and developmental guidance. Nonetheless, neurons are typically regarded as the most significant brain cells. The ability of neurons to transmit messages across great distances to particular target cells distinguishes them from other types of cells. The axon, a thin protoplasmic fiber that extends from the cell body and projects, typically with many branches, to other locations, sometimes close, sometimes in distant portions of the brain or body, is how they transmit these messages. An axon can be incredibly long; for instance, if a pyramidal cell (an excitatory neuron) of the cerebral cortex were magnified until its cell body reached the size of an adult human, its axon would be similarly magnified until it reached the length of a cable with a few centimeters in diameter and more than a kilometer in length. Action potentials, which are electrochemical pulses that last less than a billionth of a second and move along these axons at speeds of 1 to 100 meters per second, are the signals transmitted by these axons. Some neurons continuously release action potentials at rates of 10-100 per second, typically in erratic patterns, whereas other neurons are generally silent but periodically release a burst of action potentials.

Axons use specialized connectors called synapses to communicate with neighboring neurons. Up to thousands of synaptic connections can form between one axon and neighboring cells. A neurotransmitter is produced at a synapse in response to an action potential that has traveled down an axon. The target cell's membrane contains receptor molecules that the neurotransmitter binds to.

The essential components of the brain's functionality are synapses. Cell-to-cell communication is the brain's fundamental function, and synapses are the places where this communication takes place. Even the brain of a fruit fly has several million synapses, compared to the estimated 100 trillion synapses in the human brain. These synapses serve a wide range of purposes. Some are excitatory, which stimulates the target cell; others are inhibitory; yet others function by activating second messenger systems, which alter the target cells' internal chemistry in intricate ways. Many synapses are dynamically adjustable, which means that their strength can change under the control of the signaling patterns that flow through them. It is generally accepted that the brain's basic mechanism for learning and remembering is an activity-dependent alteration of synapses.

Axons occupy the majority of the brain's volume and are frequently grouped in structures known as nerve fiber tracts. The fatty insulating layer of myelin that surrounds a myelinated axon serves to considerably speed up the signal transmission. Some axons are not myelinated. In contrast to the darker-colored grey matter that denotes regions with dense concentrations of neuron cell bodies, myelin is white, giving sections of the brain that are exclusively made up of nerve fibers the appearance of white matter.

Function of the Brain

  1. The brain stores information obtained from the senses. It is then utilised to decide what steps the organism should take.
  2. The brain processes the raw data to derive knowledge about the environment's structure. The processed data is then combined with knowledge of the animal's current demands and memories of earlier events.
  3. Ultimately, it creates motor response patterns based on the findings. These signal-processing activities necessitate complex interactions among numerous functional subsystems.
  4. The brain's job is to give coherent animal control over its behaviour.
  5. A centralised brain allows for the co-activation of groups of muscles in intricate patterns, the ability for stimuli to affect one area of the body to cause responses in other sections and the ability to prevent separate parts of the body from acting in opposition to one another.


Generic bilaterian nervous system

The digestive system of a rod-shaped body extends from the mouth to the anus on either end. A nerve cord with a brain at its end is located close to the mouth and runs alongside the digestive system.

A bilaterian animal's nervous system consists of a nerve cord with segmental enlargements and a "brain" in the front. All living multicellular animals, except a few basic ones like sponges (which lack a nervous system) and cnidarians (which have a nervous system made up of a diffuse nerve net), are bilaterians, which are defined as having a bilaterally symmetric body shape (that is, left and right sides that are approximate mirror images of each other). It has been proposed that the common ancestor of all bilaterians had the form of a straightforward tubeworm with a segmented body and first appeared late in the Cryogenian period, 700-650 million years ago. All contemporary bilaterians, including vertebrates, have that fundamental worm shape in their body and nervous system architecture. The basic bilateral body structure consists of a tube with a hollow stomach cavity that extends from the mouth to the anus, a nerve cord with an enlargement (a ganglion) for each body segment, and a particularly large ganglion at the front known as the brain. In some species, like nematode worms, the brain is small and straightforward; in other species, like vertebrates, it is the most complicated organ in the body. Leeches and certain other worm species have what is referred to as a "tail brain," an enlarged ganglion at the back end of the nerve cord.

Echinoderms and tunicates are two living bilaterian species that lack an identifiable brain. Whether the occurrence of these brainless species suggests that the first bilaterians lacked a brain or whether their predecessors underwent an evolutionary process that caused a brain structure to vanish in the past has not been determined with certainty.


A fly lying still on a shiny surface. The camera is faced with a big red eye. Except for the black pigment at the end of its abdomen, the body seems transparent.

Drosophila fruit flies have been the subject of in-depth research to understand the function of genes in brain development. This group comprises mollusks, arthropods, tardigrades, and many worm species. Invertebrate body designs vary widely, and the same is true of their brain architecture.

Arthropods (insects, crustaceans, arachnids, and other invertebrates) and cephalopods (octopuses, squids, and similar mollusks) are two categories of invertebrates with particularly complex brains. Arthropods and cephalopods get their brains from a pair of parallel nerve cords that run through their bodies.

The brains of various invertebrate species have been extensive study because of characteristics that make them useful for experiments, including:

  • Fruit flies (Drosophila) have been a natural subject for studying the role of genes in brain development because of the wide range of techniques available for studying their genetics. Despite the huge evolutionary gap between insects and mammals, several Drosophila neurogenetic characteristics apply to people. The first biological, for example, was identified by examining Drosophila mutants that showed disrupted daily activity cycles. A search in the genomes of vertebrates revealed a set of analogous genes, which were found to play similar roles in the mouse biological clock-and, therefore, almost certainly in the human biological clock. Studies on Drosophila also show that most brain regions are continuously reorganized in response to specific living conditions.
  • Like Drosophila, the nematode worm Caenorhabditis elegans has been extensively investigated due to its significance in genetics. Sydney Brenner chose it as a model organism in the early 1970s to investigate how genes regulate development. Working with this worm benefits from its highly stereotyped body plan: Each worm's hermaphrodite nervous system comprises exactly 302 neurons that are consistently located in the same locations and form consistent synaptic connections. To map out every neuron and synapse in the body, Brenner's team dissected worms into countless ultrathin sections, photographed each under an electron microscope, and then visually matched fibres from section to section. The connectome of C.elegans, representing the entire neural wiring diagram, was attained. No other organism possesses information that comes close to this level of specificity, and the knowledge amassed has made numerous investigations possible that would not have been feasible otherwise.
  • Because of the ease and accessibility of its neural system, Nobel Prize-winning neurophysiologist Eric Kandel chose the sea slug Aplysia California as a model for researching the cellular basis of learning and memory. This sea slug has been the subject of numerous experiments.


The first vertebrates may have resembled the contemporary hagfish in morphology when they emerged more than 500 million years ago (Mya), during the Cambrian epoch. About 445 million years ago, jawed fish, amphibians, reptiles, and mammals first appeared (approximately). Although modern sharks, amphibians, reptiles, hagfishes, lampreys, and mammals exhibit a gradient of size and complexity closely following the evolutionary sequence, each species has an equally extensive evolutionary history. The basic anatomical elements of each of these brains are the same; however, in hagfish, many are very basic. The most important component (telencephalon) in mammals is substantially extended and elaborated.

The most frequent way to compare brains is by their size. Several species of vertebrates have been researched to determine the relationship between the size of the brain, the size of the body, and other factors. The average relationship between brain and body size is a complex linear one. When compared to their physical size, smaller animals typically have brains that are larger overall. With an exponent of roughly 0.75, the connection between brain volume and mammal body mass follows a power law. This formula captures the main tendency, but every mammal family deviates from it to some extent, partly due to the intricacy of the behaviour. Primate brains, for instance, are 5 to 10 times larger than the model predicts. Compared to their prey, predators often have greater brains.

All vertebrate brains share a basic structure that becomes most obvious during the earliest stages of embryonic development. The forebrain, midbrain, and hindbrain develop from three swellings (the prosencephalon, mesencephalon, and rhombencephalon, respectively) that initially appear at the front end of the neural tube as the brain. The three brain regions are about similar in size during the initial stages of development. The size of the three components is similar in many vertebrate species, including fish and amphibians. Still, the forebrain grows far more in mammals than the other two portions, and the midbrain shrinks dramatically. Vertebrate brains are composed of incredibly delicate tissue. The hue of living brain tissue is generally white on the inside and reddish on the outside. Meninges, a network of connective tissue membranes that divide the skull from the brain, are present around vertebrate brains. The blood-brain barrier, which prevents the passage of many poisons and pathogens, is formed by the cells that line blood vessel walls joining closely to one another.

The telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata are the six primary divisions into which neuroanatomists typically split the vertebrate brain. These regions all have intricate interior structures. To fit into the available space, some components, like the cerebral cortex and the cerebellar cortex, are made up of folded or convoluted layers. The thalamus and hypothalamus, for example, are made up of clusters of numerous tiny nuclei. The vertebrate brain can be divided into thousands of different areas based on subtle neuronal structure, chemistry, and connectivity variations.

All vertebrate brains have the same fundamental elements, although certain branches of vertebrate evolution have resulted in significant aberrations of brain geometry, particularly in the forebrain region. The main parts of a shark's brain are visible, but in teleost fishes (the vast majority of fish species now exist), the forebrain has become "everted," much like a sock turned inside out. The anatomy of the forebrain is likewise significantly altered in birds. It may be challenging to match the brain parts of one species with those of another due to these abnormalities.

Below is a list of some of the most significant vertebrate brain parts, followed by a quick explanation of how they currently work:

  • Many tiny nuclei in the medulla and spinal cord are engaged in various sensory and involuntary motor tasks, including heart rate, digestion, and vomiting.
  • Just above the medulla in the brainstem is the pons. It has nuclei that regulate functions like sleep, breathing, swallowing, bladder control, equilibrium, eye movement, facial expressions, and posture, among other frequently voluntary yet basic actions.
  • The hypothalamus is a little area at the base of the forebrain, yet despite its diminutive size, it is surprisingly complex and significant. Countless tiny nuclei make it, and each one has unique connections and neurochemistry. Other automatic or partially voluntary activities the hypothalamus performs include eating, drinking, sleeping, and releasing certain hormones.
  • The thalamus is made up of several nuclei, some of which are involved in transmitting information to and from the cerebral hemispheres and others of which are engaged in motivation. Action-generating systems for several "consummatory" activities, including eating, drinking, defecating, and copulation, appear in the subthalamic area (zona incerta). The cerebellum modifies other brain systems' motor or cognitive outputs to make them accurate and certain. Animals without the cerebellum can still perform specific tasks, but their movements become halting and awkward. Its accuracy is not innate; rather, it is developed through trial and error. The cerebellum may play a significant role in some types of brain plasticity, such as the muscle coordination gained while riding a bicycle. The cerebellum makes up about 10% of the brain's total volume and contains 50% of all neurons.
  • Actions can be directed towards locations in space thanks to the optic tectum, most frequently in reaction to visual information. The superior colliculus is the best-studied aspect of its role in controlling eye movements in animals. Moreover, it controls reaching motions and other object-focused movements. Strong visual inputs are received, but it also receives information from other senses that can guide behaviour, such as auditory input in owls and information from thermosensitive pit organs in snakes. This area is the biggest portion of the brain in some primitive fish, such as lampreys. The midbrain includes the superior colliculus.
  • The most intricate and most recent evolutionary development of the brain as an organ is the pallium, a layer of grey matter located on the forebrain's surface. The cerebral cortex is what it is known as in mammals and reptiles. The pallium is involved in various processes, including smell and spatial memory. It replaces several other brain regions' tasks in mammals where it grows to such a size as to dominate the brain. In the cerebral cortex of many mammals, folds or bulges known as gyri form deep ridges or fissures known as sulci. The cortex's surface area is increased by the folds, increasing the quantity of grey matter and the amount of information that can be processed and stored.
  • In a strict sense, only mammals have a hippocampus. Yet, all vertebrates have an equivalent of the region it arises from, the medial pallium. There is proof that this area of the brain plays a role in complex behaviours in fish, birds, reptiles, and mammals, including spatial memory and navigation.
  • The forebrain's basal ganglia are a collection of related structures. The basal ganglia appear primarily involved in action selection because they give inhibitory signals to all brain regions that can produce motor behaviours and, under the right conditions, can remove the inhibition to allow the action-generating systems to carry out their activities. The basal ganglia's most significant brain changes caused by reward and punishment occur.
  • Olfactory sensory signals are processed by the olfactory bulb, a unique structure, which then transmits its output to the olfactory section of the pallium. It is a significant part of the brain in many animals. Still, it is significantly diminished in humans and other primates (whose senses are dominated by information acquired by sight rather than smell).


The biggest distinction between the brains of mammals and those of other vertebrates is their size. An average mammal has a brain around twice the size of a bird of the same size and ten times the size of a reptile's brain. Yet there are significant variances in shape as well, not only in terms of size. Mammals have hindbrains and midbrains that are largely similar to those of other vertebrates, but the forebrain, which is dramatically larger and structurally changed, shows remarkable changes.

The area of the brain that most clearly separates mammals is the cerebral cortex. In non-mammalian vertebrates, the pallium, a relatively straightforward three-layered structure, lines the surface of the cerebrum. Neocortex, also known as the isocortex, is a complex six-layered structure that develops from animal pallium. The hippocampus and amygdala are two regions at the margin of the neocortex that are significantly more developed in mammals than in other vertebrates.

Several brain regions undergo modifications as a result of the cerebral cortex's development. In mammals, the superior colliculus, which in most vertebrates plays a significant role in the visual control of behaviour, decreases to a tiny size. Many of its tasks are assumed by visual regions of the cerebral cortex. There is no analogue in other vertebrates to the mammalian cerebellum's considerable section (the neocerebellum) devoted to supporting the cerebral cortex.


Though often larger in proportion to body size than the brains of other mammals, humans and other primates have the same basic brain architecture as other mammals. The encephalization quotient (EQ) is employed when comparing brain sizes between species. It considers the nonlinearity of the brain-to-body connection. The majority of other primates have an EQ in the 2- to 3-range, but humans have an average EQ in the 7- to 8-range. Dolphins have EQ levels greater than non-human primates, while almost all mammals have noticeably lower values.

The cerebral cortex, particularly the prefrontal cortex and the areas of the cortex involved in vision, has grown significantly, accounting for the majority of the monkey brain's enlargement. With an intricate web of connections, the visual processing network of primates has at least 30 distinct brain regions. More than half of the monkey neocortex's entire surface is thought to be taken up by visual processing sections. Planning, working memory, motivation, attention, and executive control are all responsibilities of the prefrontal cortex. For primates compared to other species, and particularly for humans, it occupies a sizeable amount of the brain.


The stages of the brain's development are precisely planned. At the early embryonic stages, it takes the form of a straightforward swelling at the front of the nerve cord, but as it develops, it takes on a complex array of areas and connections. Neurons develop in distinct regions that also include stem cells, travel through the tissue to their final sites, and ultimately die. Once positioned, neurons' axons sprout and travel through the brain, branching and extending until the tips reach their targets and create synaptic connections. In the early stages of development, neurons and synapses are produced in excess in a number of regions of the nervous system, and subsequently, the extra ones are removed.

All vertebrate species share the same early phases of brain development. The neural plate, the forerunner of the nervous system, develops from a thin strip of ectoderm running down the midline of the back of the embryo as it changes from a round blob of cells into a wormlike shape. The neural tube, a hollow lead of cells with a fluid-filled ventricle at its core, is enclosed by the lips that line the neural groove when the neural plate folds inward to create it. Three cysts, which are the predecessors to the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain), are formed at the front end of the brain by the ventricles and cord swelling. The cerebral cortex, basal ganglia, and other associated structures are located in the telencephalon, which forms as the forebrain divides into two compartments at the following stage (which will contain the thalamus and hypothalamus). The cerebellum and pons are located in the metencephalon, which separates from the hindbrain roughly at the same time as the myelencephalon (which will contain the medulla oblongata). These regions each contain proliferative zones where neurons and glial cells are produced; the resulting cells subsequently travel, occasionally over great distances, to their final placements.

When a neuron is established, it surrounds itself with dendrites and an axon. Axons grow very sophisticatedly because they frequently protrude far from the cell body and must reach specific targets. The growth cone, a glob of protoplasm at the end of a developing axon, is covered in chemical sensors. These sensors pick up on the immediate environment, leading the growth cone to be drawn in one direction or another along its route, depending on which cellular components are attracting or repelling it. The growth cone travels through the brain using this pathfinding process until it reaches its target region, prompted by additional chemical cues to start creating synapses. Thousands of genes produce proteins that have an impact on axonal pathfinding throughout the entire brain. Yet, only a portion of the final synaptic network is influenced by genes. Axons "overgrow" in various areas of the brain before being "pruned" by neuronal activity-dependent mechanisms. Each location on the surface of the retina is connected to a matching position in a midbrain layer in the projection from the eye to the midbrain, for instance, in the adult structure. Chemical cues initially direct each retinal axon to the appropriate general area in the midbrain. Still, after that, it branches out widely and establishes initial contact with a broad range of midbrain neurons. Similar processes take place in other brain regions: an initial synaptic matrix is created as a result of chemical guidance set by genetics, but it is gradually modified by activity-dependent mechanisms, which are partially fueled by internal dynamics and partly by external sensory inputs. In some situations, such as the retina-midbrain system, activity patterns are dependent on brain-specific mechanisms that seem to exist primarily to direct growth.

The creation of new neurons occurs mostly before birth in humans and many other species, and the baby brain has a significantly higher neuron density than the adult brain. Yet, there are a few regions where the growth of new neurons continues throughout life. The olfactory bulb, which is involved in smell, and the dentate gyrus of the hippocampus, where there is evidence that the new neurons play a role in storing recently acquired memories, are the two regions for which adult neurogenesis is well established. With these two exceptions, the set of neurons present in early childhood remains present throughout life. Unlike other types of cells in the body, glial cells are produced continuously throughout life.

The nature vs. nurture dispute concerns whether traits like the mind, personality, and intelligence may be linked to a person's upbringing or genes. Neuroscience research has amply demonstrated the significance of these aspects, despite the fact that many details are still up in the air. Both the general shape of the brain and how the brain responds to experience are governed by genes. The matrix of synaptic connections, which in its mature form includes significantly more information than the genome, must be refined via experience. In other ways, the presence or lack of experience throughout crucial stages of growth is all that matters. As an example, there is strong evidence that animals raised in richer environments have thicker cerebral cortices, which indicate a higher density of synaptic connections, than animals whose levels of stimulation are restricted. In other ways, the quantity and quality of experience are essential.

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