The Human Brain-I

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, balance, taste, and smell.
Brains can be extremely complex. The human brain contains roughly 100 billion neurons, linked with up to 10,000 synaptic connections each. Each cubic millimeter of cerebral cortex contains roughly one billion synapses.[1] These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells.

From a philosophical point of view, it might be said that the most important function of the brain is to serve as the physical structure underlying the mind. From a biological point of view, though, the most important function is to generate behaviors that promote the welfare of an animal. Brains control behavior either by activating muscles, or by causing secretion of chemicals such as hormones. Even single-celled organisms may be capable of extracting information from the environment and acting in response to it.

[2] Sponges, which lack a central nervous system, are capable of coordinated body contractions and even locomotion.[3] In vertebrates, the spinal cord by itself contains neural circuitry capable of generating reflex responses as well as simple motor patterns such as swimming or walking.[4] However, sophisticated control of behavior on the basis of complex sensory input requires the information-integrating capabilities of a centralized brain.

Despite rapid scientific progress, much about how brains work remains a mystery. The operations of individual neurons and synapses are now understood in considerable detail, but the way they cooperate in ensembles of thousands or millions has been very difficult to decipher. Methods of observation such as

EEG recording and functional brain imaging tell us that brain operations are highly organized, but these methods do not have the resolution to reveal the activity of individual neurons.

Macroscopic structure

The brain is the most complex biological structure known,

[6] and comparing the brains of different species on the basis of appearance is often difficult. Nevertheless, there are common principles of brain architecture that apply across a wide range of species. These are revealed mainly by three approaches. The evolutionary approach means comparing brain structures of different species, and using the principle that features found in all branches that have descended from a given ancient form were probably present in the ancestor as well. The developmental approach means examining how the form of the brain changes during the progression from embyronic to adult stages. The genetic approach means analyzing gene expression in various parts of the brain across a range of species. Each approach complements and informs the other two.
The cerebral cortex is a part of the brain that most strongly distinguishes mammals from other vertebrates, primates from other mammals, and humans from other primates. In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple layered structure called the pallium.[7] In mammals, the pallium evolves into a complex 6-layered structure called neocortex. In primates, the neocortex is greatly enlarged in comparison to its size in non-primates, especially the part called the frontal lobes. In humans, this enlargement of the frontal lobes is taken to an extreme, and other parts of the cortex also become quite large and complex.
The relationship between brain size, body size and other variables has been studied across a wide range of species. Brain size increases with body size but not proportionally. Averaging across all orders of mammals, it follows a power law, with an exponent of about 0.75.[8] This formula applies to the average brain of mammals but each family departs from it, reflecting their sophistication of behavior.[9] For example, primates have brains 5 to 10 times as large as the formula predicts. Predators tend to have larger brains. When the mammalian brain increases in size, not all parts increase at the same rate. The larger the brain of a species, the greater the fraction taken up by the cortex.[10]

Bilaterians

Body plan of a generic bilaterian animal. The nervous system has the form of a nerve cord with segmental enlargements, and a "brain" at the front.
With the exception of a few primitive forms such as sponges and jellyfish, all living animals are bilaterians, meaning animals with a bilaterally symmetric body shape (that is, left and right sides that are approximate mirror images of each other).
All bilaterians are thought to have descended from a common ancestor that appeared early in the Cambrian period, 550-600 million years ago.[11] This ancestor had the shape of a simple tube worm with a segmented body, and at an abstract level, that worm-shape continues to be reflected in the body and nervous system plans of all modern bilaterians, including humans.[12] The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".

Invertebrates

Drosophila

For invertebrates—insects, molluscs, worms, etc.—the components of the brain differ so greatly from the vertebrate pattern that it is hard to make meaningful comparisons except on the basis of genetics. Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs).[13] The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing.[13] Cephalopods have the largest brains of any invertebrates. The brain of the octopus in particular is highly developed, comparable in complexity to the brains of some vertebrates.
There are a few invertebrates whose brains have been studied intensively. The large sea slug Aplysia was chosen by Nobel Prize-winning neurophysiologist Eric Kandel, because of the simplicity and accessibility of its nervous system, as a model for studying the cellular basis of learning and memory, and subjected to hundreds of experiments.[14] The most thoroughly studied invertebrate brains, however, belong to the fruit fly Drosophila and the tiny roundworm Caenorhabditis elegans.
Because of the large array of techniques available for studying their genetics, fruit flies have been a natural subject for studying the role of genes in brain development.[15] Remarkably, many aspects of Drosophila neurogenetics have turned out to be relevant to humans. The first biological clock genes, for example, were identified by examining Drosophila mutants that showed disrupted daily activity cycles.[16] A search in the genomes of vertebrates turned up 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 as well.[17]
Like Drosophila, C. elegans has been studied largely because of its importance in genetics.[18] In the early 1970s, Sydney Brenner chose it as a model system for studying the way that genes control development. One of the advantages of working with this worm is that the body plan is very stereotyped: the nervous system of the hermaphrodite morph contains exactly 302 neurons, always in the same places, making identical synaptic connections in every worm.[19] In a heroic project, Brenner's team sliced worms into thousands of ultrathin sections and photographed every section under an electron microscope, then visually matched fibers from section to section, in order to map out every neuron and synapse in the entire body.[20] Nothing approaching this level of detail is available for any other organism, and the information has been used to enable a multitude of studies that would not have been possible without it.

Vertebrates

The brain of a shark.

The brains of vertebrates are made of very soft tissue, with a texture that has been compared to Jello.[21] Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle variations in color. Vertebrate brains are surrounded by a system of connective tissue membranes called meninges that separate the skull from the brain.[22] This three-layered covering is composed of (from the outside in) the dura mater ("hard mother"), arachnoid mater ("spidery mother"), and pia mater ("soft mother"). The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid (CSF), which circulates in the narrow spaces between cells and through cavities called ventricles, and serves to nourish, support, and protect the brain tissue. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood.
The first vertebrates appeared over 500 million years ago (mya), during the Cambrian period, and may have somewhat resembled the modern hagfish in form.[23] Sharks appeared about 450 mya, amphibians about 400 mya, reptiles about 350 mya, and mammals about 200 mya. No modern species should be described as more "primitive" than others, since all have an equally long evolutionary history, but the brains of modern hagfishes, lampreys, sharks, amphibians, reptiles, and mammals show a gradient of size and complexity that roughly follows the evolutionary sequence.[24] All of these brains contain the same set of basic anatomical components, but many are rudimentary in hagfishes, whereas in mammals the foremost parts are greatly elaborated and expanded.
All vertebrate brains share a common underlying form, which can most easily be appreciated by examining how they develop.[25] The first appearance of the nervous system is as a thin strip of tissue running along the back of the embryo. This strip thickens and then folds up to form a hollow tube. The front end of the tube develops into the brain. In its earliest form, the brain appears as three swellings, which eventually become the forebrain, midbrain, and hindbrain. In many classes of vertebrates these three parts remain similar in size in the adult, but in mammals the forebrain becomes much larger than the other parts, and the midbrain quite small.
Neuroanatomists usually consider the brain to consist of six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla.[26] Each of these areas in turn has a complex internal structure. Some areas, such as the cortex and cerebellum, consist of layers, folded or convoluted to fit within the available space. Other areas consist of clusters of many small nuclei. If fine distinctions are made on the basis of neural structure, chemistry, and connectivity, thousands of distinguishable areas can be identified within the vertebrate brain.
Some branches of vertebrate evolution have led to substantial changes in brain shape, especially in the forebrain. The brain of a shark shows the basic components in a straighforward way, but in teleost fishes (the great majority of modern species), the forebrain has become "everted", like a sock turned inside out. In birds, also, there are major changes in shape.[27] One of the main structures in the avian forebrain, the dorsal ventricular ridge, was long thought to correspond to the basal ganglia of mammals, but is now thought to be more closely related to the neocortex.[28]

Main anatomical regions of the vertebrate brain.

Several brain areas have maintained their identities across the whole range of vertebrates, from hagfishes to humans. Here is a list of some of the most important areas, along with a very brief description of their functions as currently understood (but note that the functions of most of them are still disputed to some degree):

The

medulla, along with the spinal cord, contains many small nuclei involved in a wide variety of sensory and motor functions.

The

hypothalamus is a small region at the base of the forebrain, whose complexity and importance belies its size. It is composed of numerous small nuclei, each with distinct connections and distinct neurochemistry. The hypothalamus is the central control station for sleep/wake cycles, control of eating and drinking, control of hormone release, and many other critical biological functions.[29]

The

thalamus is a collection of nuclei with diverse functions. Some of them are involved in relaying information to and from the cerebral hemispheres. Others are involved in motivation. The subthalamic area (zona incerta) seems to contain action-generating systems for several types of "consummatory" behaviors, including eating, drinking, defecation, and copulation.[30]

The

cerebellum modulates the outputs of other brain systems to make them more precise. Removal of the cerebellum does not prevent an animal from doing anything in particular, but it makes actions hesitant and clumsy. This precision is not built-in, but learned by trial and error. Learning how to ride a bicycle is an example of a type of neural plasticity that may take place largely within the cerebellum.[31]

The

tectum, often called "optic tectum", allows actions to be directed toward points in space. In mammals it is called the "superior colliculus", and its best studied function is to direct eye movements. It also directs reaching movements, though. It gets strong visual inputs, but also inputs from other senses that are useful in directing actions, such as auditory input in owls, input from the thermosensitive pit organs in snakes, etc. In some[which?] fishes, it is the largest part of the brain.[32]

The

pallium is a layer of gray matter that lies on the surface of the forebrain. In reptiles and mammals it is called cortex instead. The pallium is involved in multiple functions, including olfaction and spatial memory. In mammals, where it comes to dominate the brain, it subsumes functions from many subcortical areas.[33]

The

hippocampus, strictly speaking, is found only in mammals. However, the area it derives from, the medial pallium, has counterparts in all vertebrates. There is evidence that this part of the brain is involved in spatial memory and navigation in fishes, birds, reptiles, and mammals.[34]

The

basal ganglia are a group of interconnected structures in the forebrain, of which our understanding has increased enormously over the last few years. The primary function of the basal ganglia seems to be action selection. They send inhibitory signals to all parts of the brain that can generate actions, and in the right circumstances can release the inhbition, so that the action-generating systems are able to execute their actions. Rewards and punishments exert their most important neural effects within the basal ganglia.[35]

The

olfactory bulb is a special structure that processes olfactory sensory signals, and sends its output to the olfactory part of the pallium. It is a major brain component in many vertebrates, but much reduced in primates.[36]

Dear friends,this is basics of our brain which I discuss today. In next session,We will discuss another aspects of brain. Till then,have a healthy life...............