Brain

(CC) Image: Jeibmann and Paulus, 2009
Schematic of the brain of a larva from Drosophila melanogaster, a popular model organism. Abbreviations: (b), eye imaginal discs (ed), wing discs (wd), leg discs (ld), mouth hooks (mh) and gonads (gd). Magnification: Schematic view of brain hemispheres (b), eye imaginal discs (ed) as well as antennal discs (ad). The eye imaginal disc will form the adult compound eye whereas the antennal disc will develop into the antenna, the adult olfactory organ. The optic stalk (os) connects the brain hemispheres with the eye imaginal discs, whereas Bolwig nerve (Bn) constitutes the link of the larval brain to the larval photo receptors.

Comparison of rodent and primate brains (from Herculano-Houzel, 2009). Brain size is not a reliable indicator of number of neurons across orders. Because of the different cellular scaling rules that apply to rodent and primate brains, primates always concentrate larger numbers of neurons in the brain than rodents of a similar, or even larger, brain size. Data from Herculano-Houzel et al. (2006, 2007). Illustration by Lorena Kaz.

The brain, or encephalon (Greek for 'in the head'), is the control center of the central nervous system. In vertebrates, the brain is in the head, protected by the skull and is close to the primary sensory apparatus of vision, hearing, taste and olfaction. Whereas all vertebrates have a brain, invertebrates have either a centralized brain or collections of individual ganglia. Brains can be extremely complex; the human brain typically contains 80 to 90 billion neurons, depending on the size, with each neuron linked to as many as 10,000 others.

Overview

Most vertebrate brains exhibit a visible distinction between grey matter, which consists of the cell bodies of the nerve cells (neurons), and white matter, which consists of the fibers (axons) that connect them. The axons are surrounded by a fatty insulating sheath called myelin, giving the white matter its color. The topmost outer layer of the mammalian brain is grey matter called cerebral cortex. Deep in the brain, compartments of white matter (fasciculi, fiber tracts), grey matter (nuclei) and spaces filled with cerebrospinal fluid (ventricles) are found. The outer layer of the brain is known as the cortical surface, a concept used to describe the boundaries — in segmented images of the brain — of either white matter or cerebrospinal fluid with grey matter, or representations computed on the basis of these two surfaces.

The brain innervates the head through cranial nerves, and is continuous with the spinal cord, which innervates the body through spinal nerves. Fibers that transmit signals from the brain are called efferent fibers, and fibers that transmit signals to the brain are called afferent (or sensory) fibers. The brain controls a wide variety of functions. It is the site of reason and intelligence, which include such components as cognition, perception, attention, memory and emotion. It controls posture and movements. It makes possible cognitive, motor and other forms of learning. The brain performs many functions automatically, without the need for conscious awareness, such as controlling hormone secretion and homeostatic functions such as control of heart rate, blood pressure, respiration (breathing), fluid balance, and body temperature.

Neurons are the electrically active brain cells that process information, but are outnumbered in the brain by Glial cells which perform important supporting functions. Neurons use considerable amounts of energy; as well as being electrically active, neurons must constantly synthesise neurotransmitters - the chemical messengers by which neurons 'talk' to each other. The human brain contains about 100 billion neurons, each of which talks directly to about 10,000 other neurons at specialised junctions called synapses. This communication is partly electrical and partly chemical; neurons are electrically excitable cells, meaning that they can generate electrical signals that travel along their axons, coding information. At the ends of the axons there are nerve terminals where chemicals are released in response to these electrical signals. These chemicals, called neurotransmitters, activate (or inhibit) the neurons adjacent to the nerve endings.

The human brain uses a huge number of different chemical signals for this communication. The most abundant neurotransmitters are the excitatory neurotransmitter glutamate and the inhibitory transmitter GABA, but different neurons may use acetylcholine, histamine or catecholamines such as noradrenaline and dopamine, or any of more than 100 different neuropeptides, such as the opioid peptides, neurosteroids, endocannabinoids and even gases such as nitric oxide. Some neurons also communicate directly with each other by direct electrical coupling (via gap junctions). Neurons differ from each other in what substances they use to communicate to other neurons, in what substances they are sensitive to, in which parts of the brain or body they project to, and in many features of their electrical properties that determine exactly how they process information. Exactly how many different types of neuron there are is not clear, but some very important populations of neurons have only a few hundred members.

The study of the brain is known as neuroscience. The term "brain map" is used to describe a planar representation of a brain or part thereof.

History

The Edwin Smith Surgical Papyrus, written in the 17th century BCE [1], contains the earliest recorded reference to the brain. The papyrus describes the symptoms, diagnosis, and prognosis of two patients, wounded in the head, who had compound fractures of the skull. It contains descriptions of the cranial sutures, the meninges, the external surface of the brain, the CSF, and the intracranial pulsations.

Early human views on the function of the brain regarded it to be a form of “cranial stuffing”. In Egypt, from the late Middle Kingdom onwards, in preparation for mummification, the brain was regularly removed, for it was the heart that was assumed to be the seat of intelligence. This view was challenged by Hippocrates (Hippocrates of Cos, 460–370 BCE, often referred to as 'The Father of Medicine'), who believed that the brain must be involved with sensation as the eyes, ears, nose and tongue were all located in the head, and was also likely to be the seat of intelligence. However, Aristotle (384–322 BCE) continued to maintain that the heart was the center of intelligence and the brain was a mere "cooling device". This view was accepted until the time of Galen(129-200 CE). Galen, as a physician to Roman gladiators, had ample opportunity to observe the consequences of injuries to the brain and spinal cord; he also performed numerous public dissections of animals for teaching the principles of medicine, and he described the cerebrum, cerebellum and ventricles. When he observed the fluid inside the ventricles his observations seemed to confirm the prevalent notion that the body functioned according to the balance of the four 'vital fluids' or bodily humors. He proposed that the brain contained a pneuma physicon (animal spirit) that controlled movement, perception, and the senses.

Galen's authority dominated medicine until the 16th century, partly because his detailed anatomical texts survived the Fall of Rome. Most of Galen's writings were first translated to the Syriac language by Nestorian monks in Persia. Muslim scholars (mainly in Baghdad) subsequently translated these into Arabic, and they became important sources for Arabian scholars such as Avicenna, Rhazes, and Maimonides.

Mind and brain

A distinction is often made in the philosophy of mind between the mind and the brain, and there is some controversy as to their exact relationship, consideration of which is called the mind-body problem.

Neuroscientists have found that some patterns of neural network activities correlate with some mental states, and have identified regions of the brain that participate in various personality disorders, mood disorders, addictions, and dementias, such as Alzheimer's disease. It should be noted, however, that the activities of these regions are not entirely localized, but are coupled by the brain's circuitry, not in a static, fixed manner, but in an ever-changing dynamic manner. Brain "disorder" is, technically, an umbrella term for medical conditions describing a significant deviation, in brain structure or function, from what is typical among humans.

Comparative anatomy

Three groups of animals have notably complex brains: the arthropods (insects and crustaceans), the cephalopods (octopuses, squids, and similar mollusks), and the craniates (vertebrates). The brain of arthropods and cephalopods arises 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.

The brain of craniates develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord. In craniates, the brain is protected by the bones of the skull. In vertebrates, increasing complexity in the cerebral cortex correlates with height on the phylogenetic and evolutionary tree. Primitive vertebrates such as fish, reptiles, and amphibians have fewer than six layers of neurons in the outer layer of their brains. This cortical configuration is called the allocortex (or heterotypic cortex).

More complex vertebrates including mammals have a six-layered neocortex (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex. In mammals, increasing convolutions of the brain are characteristic of animals with more advanced brains. The folds are called gyri, while the spaces between the folds are called sulci.

Although the general histology of the brain is similar from person to person, the structural anatomy can differ. Apart from the gross embryological divisions of the brain, the location of specific gyri and sulci, primary sensory regions, and other structures differs between species.

Invertebrates

In insects, the brain comprises two optical lobes, the protocerebrum, the deutocerebrum, and the tritocerebrum. The optical lobes are behind each eye and process visual stimuli. The protocerebrum contains the mushroom bodies, which respond to smell, and the central body complex. In some species such as bees, the mushroom body receives input from the visual pathway as well. The deutocerebrum includes the antennal lobes, which are similar to the mammalian olfactory bulb, and the mechanosensory neuropils which receive information from touch receptors on the head and antennae. The antennal lobes of flies and moths are quite complex.

In cephalopods, the brain has a 'supraesophageal mass' and a 'subesophageal mass', separated by the esophagus. These masses are connected to each other on either side of the esophagus by the basal lobes and the dorsal magnocellular lobes. The large optic lobes are anatomically separate from the rest of the brain, and are joined to it by optic stalks. However, the optic lobes perform much visual processing, and so functionally are part of the brain.

Vertebrates

The telencephalon (cerebrum) is the largest region of the mammalian brain; and is the structure that most people identify as 'the brain'. In humans and several other animals, the fissures (sulci) and convolutions (gyri) give the cerebrum a wrinkled appearance. In non-mammalian vertebrates with no cerebrum, the metencephalon is the highest center in the brain. Because humans walk upright, there is a flexure, or bend, in the brain between the brain stem and the cerebrum. Other vertebrates do not have this flexure.

Behind (or in humans, below) the cerebrum is the cerebellum. The cerebellum is involved in the control of movement, and is connected by thick white matter fibers (cerebellar peduncles) to the pons. The cerebrum and the cerebellum each have two hemispheres. The telencephalic hemispheres are connected by the corpus callosum, another large white matter tract which is the largest commissure in the mammalian brain, connecting the two brain hemispheres. An outgrowth of the telencephalon called the olfactory bulb is a major structure in many animals, but in humans and other primates it is relatively small, this structure processes information arising from the sense of smell.

Vertebrate nervous systems are distinguished by bilaterally symmetrical encephalization. Encephalization refers to the tendency for more complex organisms to gain larger brains through evolutionary time. Larger vertebrates have a complex, layered and interconnected neuronal circuitry. In modern species most closely related to the first vertebrates, brains are covered with grey matter that has a three-layer structure (allocortex). Their brains also contain deep brain nuclei and fiber tracts forming the white matter. Most regions of the human cerebral cortex have six layers of neurons (neocortex).

An important corollary is the study of Hemispheric_lateralization.

Humans

The structure of the human brain differs from that of other animals in several important ways. These differences allow for many abilities over and above those of other animals, such as the ability to use language to express thoughts and communicate with others. In humans, the neocortex, the most complex part of the cerebral cortex, is particularly large compared with other animals. especially to the prefrontal cortex. Although humans have some unique neural capacities, much of their brain structure is similar to that of other mammals. Basic systems that alert the nervous system to stimuli, that sense events in the environment, and monitor the condition of the body are similar to those of even non-mammalian vertebrates.

Neurobiology

The brain contains two broad classes of cells, neurons and glia, both of which contain several different cell types which perform different functions. Interconnected neurons form neural networks (or neural ensembles). These networks are like man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically neurons are connected to at least a thousand other neurons.

Histology

(CC) Image: Rönnbäck and Hansson, 2004
Schematic drawing of cellular regulation of extracellular glutamate concentrations ([Glu]_ec) in normal brain function. Two neuronal cell bodies with processes (white) make contact with each other through a synapse (center). Astrocytic (pink) processes encapsulate the synapse and cover also the abluminal side of the blood vessel wall (right). The endothelial cells covering the luminal (blood) side of the vessel wall and the astrocytic processes make up the blood brain barrier (BBB). An oligodendroglial cell (bluish), with its myelin encapsulating the axon, and a microglial cell (yellow) are seen. The astrocytes, with their high-affinity glutamate transporters, are the main site for keeping [Glu]_ec low. Even neurons express glutamate transporters, as do oligodendroglial cells, and endothelial cells at their abluminal side.

Neurons are the cells that generate action potentials and convey information to other cells; these constitute the essential class of brain cells. As well as neurons, the brain contains glial cells, in fact there are about ten times as many glial cells as there are neurons. Glial cells ("glia" is Greek for “glue”) form a complex and sophisticted support system for the neurons. They make the insulating myelin, provide structure to the neuronal network, guide axons to their targets, manage waste, and clean up neurotransmitters. Most types of glia in the brain are present in the entire nervous system. Exceptions include the oligodendrocytes which myelinate neural axons (a role performed by Schwann cells in the peripheral nervous system). The myelin in the oligodendrocytes insulates the axons of some neurons. White matter in the brain is myelinated neurons, while grey matter contains mostly cell bodies, dendrites, and unmyelinated portions of axons and glia. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the neuropil.

In mammals, the brain is surrounded by connective tissues called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is composed of (from the outside in) the dura mater, arachnoid mater, and pia mater. 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, which provides nutrients to the cells of the brain, and is a 'sink' for diluting and disposing of waste products. 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. However, there are some specialised regions of the brain where the blood brain barrier is 'leaky', in some cases in order to allow substances released by neurons to enter the blood, and in some cases to allow specialised neurons to 'sample' the chemical composition of the blood. These sites are collectively known as "circumventricular organs". The structures are adjacent to one of the cerebral ventricles, and all are very densely vascularised. Technically, the blood-brain barrier (BBB) consists of specialized non-fenestrated tightly-joined endothelial cells with tight junctions that form a transport barrier for certain substances between the cerebral capillaries and the brain tissue.

The brain is bathed in cerebrospinal fluid (CSF), produced by the vascular choroid plexuses. CSF circulates between layers of the meninges and through cavities in the brain called ventricles, which are continuous with the spinal cord's central canal. The spaces between neurons are filled with extracellular fluid, which is continuously exchanged with CSF. It is important both chemically for metabolism and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5kg; its mass and density are such that it will begin to collapse under its own weight. The CSF allows the brain to 'float', easing the physical stress caused by its mass.

Function

Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then processed throughout the central nervous system; reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to control important muscles throughout the body. Anatomically, most afferent and efferent nerves (with the exception of the cranial nerves) are connected to the spinal cord, which carries signals to and from the brain and the rest of the body.

Sensory input is processed by the brain to recognize danger, find food, identify potential mates, and perform more sophisticated functions. Visual, touch, and auditory sensory pathways of vertebrates are routed to specific nuclei of the thalamus and then to regions of the cerebral cortex that are specific to each sensory system. Olfactory pathways are routed to the olfactory bulb, then to various parts of the olfactory system. Taste is routed through the brainstem and then to other portions of the gustatory system.

To control movement, the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brainstem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone.

It is hypothesized that in the brains of humans, consciousness emerges from the massively complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and limbic functions as well. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.

Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimuli from any source can trigger a general arousal process that focuses cortical operations to processing the new information. This focusing of cognition is known as attention. Cognitive priorities are constantly changed by factors such as hunger, fatigue, belief, unfamiliar information, or threat. The simplest dichotomy related to the processing of threats is the fight-or-flight response mediated by the amygdala and other limbic structures.

Brain pathology

Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory, and movement. Head trauma caused, for example, by vehicle and industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant swelling (edema) than by the impact itself. Stroke, caused by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.

Other problems in the brain can be more accurately classified as diseases rather than injuries. Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease are caused by the gradual death of individual neurons, leading to decrements in movement control, memory, and cognition. Currently only the symptoms of these diseases can be treated. Mental illnesses, such as clinical depression, schizophrenia, bipolar disorder, and post-traumatic stress disorder are brain diseases that impact personality and, typically, other aspects of mental and somatic function. These disorders may be treated by psychiatric therapy, pharmaceutical intervention, or through a combination of treatments; therapeutic effectiveness varies significantly among individuals.

Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as mad cow disease), is deadly in cattle and humans and is linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans. Both are linked to the ingestion of neural tissue, and may explain the tendency in some species to avoid cannibalism. Viral or bacterial causes have been substantiated in multiple sclerosis, Parkinson's disease, Lyme disease, encephalopathy, and encephalomyelitis.

Some brain disorders are congenital. Tay-Sachs disease, Fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Malfunctions in the embryonic development of the brain can be caused by genetic factors, by drug use, and disease during a mother's pregnancy.

Study of the brain

Other matters

Computer scientists have produced simulated neural networks loosely based on the structure of neuron connections in the brain. Artificial intelligence seeks to replicate brain function—although not necessarily brain mechanisms—but as yet has been met with limited success. Creating algorithms to mimic a biological brain is very difficult because the brain is not a static arrangement of circuits, but a network of vastly interconnected neurons that are constantly changing their connectivity and sensitivity. More recent work in both neuroscience and artificial intelligence models the brain using the mathematical tools of chaos theory and dynamical systems. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and artificial intelligence.