Human brain

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This article is about the human brain. For information about brains in other animals, see Brain.
Human brain
Human brain and skull
Cerebral lobes: the frontal lobe (pink), parietal lobe (green) and occipital lobe (blue)
Details
Precursor Neural tube
System Central nervous system
Neuroimmune system
Artery Internal carotid arteries, vertebral arteries
Vein Internal jugular vein, internal cerebral veins, external veins: (superior and inferior cerebral veins, and middle cerebral veins), basal vein, terminal vein, choroid vein, cerebellar veins
Identifiers
Latin Cerebrum[1]
Greek ἐγκέφαλος (enképhalos)[2]
MeSH D001921
TA A14.1.03.001
FMA 50801
Anatomical terminology

The human brain is the centre of the human central nervous system, located within the head, and consisting of the cerebrum, brainstem, and cerebellum.[3] Much of the size of the human brain comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought.

The human cerebral cortex is a thick layer of neural tissue that covers the two cerebral hemispheres that make up most of the brain. This layer is folded in a way that increases the amount of surface area that can fit into the volume available. The pattern of folds is similar across individuals but shows many small variations. The cortex is divided into four lobes – the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. (Some classification systems also include a limbic lobe and treat the insular cortex as a lobe.) Within each lobe are numerous cortical areas, each associated with a particular function, including vision, motor control, and language. The left and right hemispheres are broadly similar in shape, and most cortical areas are replicated on both sides. Some areas, though, show strong lateralization, particularly areas that are involved in language. In most people, the left hemisphere is dominant for language, with the right hemisphere playing only a minor role. There are other functions, such as visual-spatial ability, for which the right hemisphere is usually dominant.

Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the human brain is susceptible to damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head or other trauma, a stroke, or poisoning by a number of chemicals that can act as neurotoxins, such as alcohol. Infection of the brain, though serious, is rare because of the protective blood-to brain and blood-to cerebral fluid barriers. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, forms of dementia including Alzheimer's disease, (mostly as the result of aging) and multiple sclerosis. A number of psychiatric conditions, such as schizophrenia and clinical depression, are thought to be associated with brain dysfunctions, although the nature of these is not well understood. The brain can also be the site of brain tumors and these can be benign or malignant.

The study of the anatomy of the brain is neuroanatomy and the study of the function is neuroscience. A number of different techniques are used to study the brain. Animals, where it is easier to obtain brain specimens, provide one such source. The brain or specimens may be examined microscopically. Medical imaging technologies such as functional neuroimaging and EEG recordings are important techniques in studying the brain. Examining the history of people with brain injury has also provided great insight into the function of each part of the brain.

Structure

Anatomy

Human brain viewed through a mid-line incision showing the white matter of the corpus callosum
Diagram showing features of cerebral outer membranes and supply of blood vessels

The adult human brain weighs on average about 1.2–1.4 kg (2.6–3.1 lb), or about 2% of total body weight,[4][5] with a volume of around 1260 cm3 in men and 1130 cm3 in women, although there is substantial individual variation.[6] Neurological differences between the sexes have not been shown to correlate in any simple way with IQ or other measures of cognitive performance.[7]

The cerebrum consisting of the cerebral hemispheres forms the largest part of the brain and is situated above other brain structures. The hemispheres have an outer covering of neural tisssue - the cerebral cortex which has a convoluted appearance.[8]

The brainstem, resembling a stalk, attaches to and leaves the cerebrum at the start of the midbrain area. The brainstem includes the midbrain, pons, and medulla oblongata. Behind the brainstem the cerebellum (little brain) is located. This structure also has a (much thinner) outer cortex and is also separated into two lobes by the cerebellar vermis. Viewed from underneath between the two lobes is a third lobe known as the flocculonodular lobe.[9] The cerebellar cortex is narrowly furrowed horizontally and this distinguishes the cerebellum from any other brain structure.

The cerebrum, brainstem, cerebellum and spinal cord are covered by three membranes called meninges. The membranes are the tough dura mater; the middle arachnoid mater and the delicate inner pia mater. Between the arachnoid mater and the pia mater is the subarachnoid space which contains the cerebrospinal fluid.[10] In the cerebral cortex close to the pial basement membrane is a limiting membrane called the glia limitans and this is the outermost layer of the cortex.[11] The living brain is very soft, having a gel-like consistency similar to soft tofu. Although referred to as gray matter, the live cortex is pinkish-beige in color and slightly off-white in the interior.

The hemispheres are connected by a very large bundle of nerves (the largest white matter structure in the brain) called the corpus callosum. This tract crosses the midline above the level of the thalamus. There are also two much smaller connections, the anterior commissure and hippocampal commissure, as well as many subcortical connections that cross the midline. The corpus callosum is the main avenue of communication between the two hemispheres, though. It connects each point on the cortex to the mirror-image point in the opposite hemisphere, and also connects to functionally related points in different cortical areas.

Microanatomy

The human brain is composed of neurons, glial cells, neural stem cells and blood vessels. The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells.[12] Out of these neurons, 16 billion (19%) are located in the cerebral cortex (including subcortical white matter), 69 billion (80%) are in the cerebellum.[5][12] The glia limitans of the cortex is made up of astrocyte foot processes which serves in part to contain the cells of the brain.[11]

Cerebrum

Main articles: Cerebrum and Cerebral cortex
Human brain viewed from below
Bisection of head, showing the cortical folds, and the underlying white matter[13]

The cerebrum is the largest part of the human brain, and is divided by a deep groove – the longitudinal fissure into left and right hemispheres.[14][15] The whole of the cerebrum is covered by a layer of neural tissue called the cerebral cortex. The cerebral cortex is also divided along the division of the hemispheres (the longitudinal fissure), and is divided into three regions based on function[citation needed] primary sensory areas, which receive signals from the sensory nerves and tracts by way of relay nuclei in the thalamus. Primary sensory areas include the visual area of the occipital lobe, the auditory area in parts of the temporal lobe and insular cortex, and the somatosensory cortex in the parietal lobe. A second category is the primary motor cortex, which sends axons down to motor neurons in the brainstem and spinal cord. This area occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The third category consists of the remaining parts of the cortex, which are called the association areas. These areas receive input from the sensory areas and lower parts of the brain and are involved in the complex processes of perception, thought, and decision-making.[16] It has been estimated that if the cerebral cortex could be completely unfolded it would give a total surface area of about 2000 square cm.[17]

The cerebral cortex is folded in a way that allows a large surface area to fit within the confines of the skull. When unfolded, each cerebral hemisphere cortex has a total surface area of about 1.3 square feet (0.12 m2).[18] Each cortical ridge is called a gyrus, and each groove or depression separating one gyrus from another is called a sulcus.

Lobes

Main article: Lobes of the brain
Lobes of the brain

The cerebral cortex is nearly symmetrical with left and right hemispheres.[19] Each hemisphere is conventionally divided into four lobes, the frontal lobe, parietal lobe, temporal lobe, and occipital lobe.[19] Sometimes two smaller regions are included as lobes – the limbic lobe and the insular cortex (previously known as the insular lobe). The frontal lobe is the largest. A region of the cortex which is made up of part of the frontal lobe and part of the parietal lobe is called the paracentral lobule.

With the exception of the occipital lobe, that is entirely dedicated to vision, each of the lobes contains a variety of brain areas that have minimal functional relationship. The parietal lobe, for example, contains areas involved in somatosensation, hearing, language, attention, and spatial cognition. The main functions of the frontal lobe are to control attention, abstract thinking, behavior, problem solving tasks, and physical reactions and personality.[20][21] The occipital lobe is the smallest lobe; its main functions are visual reception, visual-spatial processing, movement, and color recognition.[20][21] The temporal lobe controls auditory and visual memories, language, and some hearing and speech.[20]

Anatomical features

Major gyri and sulci on the lateral surface of the cortex

Although there are enough variations in the shape and placement of gyri and sulci (cortical folds) to make every brain unique, most human brains show sufficiently consistent patterns of folding that allow them to be named. Many of the gyri and sulci are named according to the location on the lobes or other major folds on the cortex. These include:

Brainstem

Main article: Brainstem

The brainstem lies beneath the cerebrum and consists of the midbrain, pons and medulla. It lies in the back part of the skull, resting on the part of the base of the skull known as the clivus, and ends at the foramen magnum, a large hole (foramen) at the bottom of the skull. The brainstem continues below this as the spinal cord.[22]

Many nerve tracts, which transmit information to and from the cerebral cortex to the rest of the body, pass through the brainstem.[22] Many of the cranial nerves[a] emerge from the brainstem.[22] The brainstem also contains nuclei of many cranial and peripheral nerves, as well as nuclei involved in the regulation of many essential processes including breathing, control of eye movements and balance.[23][22] The reticular formation, a network of nerves of ill-defined function, is present within and along the length of the brainstem.[22]

Cerebellum

Main article: Cerebellum

The cerebellum (Latin: little brain) is a two-lobed structure, connected in the middle by the vermis.[24] It rests at the backmost and lowest part of the posterior cranial fossa. It lies beneath the occipital lobes of the cerebrum, and is separated from these by the cerebellar tentorium, a sheet of fibre.[25]

The cerebellum is divided into an anterior lobe, a posterior lobe, and the flocculonodular lobe.[26] It is connected to the midbrain of the brainstem by the superior cerebellar peduncle, the pons middle cerebellar peduncle and the medulla inferior cerebellar peduncles.[24] Enclosed between the cerebellum and the brainstem lies a space and a tube containing cerebrospinal fluid - the cerebral aqueduct and fourth ventricle.[27] The cerebellum consists of an inner medulla of white matter and an outer cortex of grey matter. The surface of the cerebellum is richly folded.[25] The cerebellum's anterior and posterior lobes appears to play a role in the coordination and smoothing of complex motor movements, and the flocculonodular lobe in the maintenance of balance[28] although debate exists as to its cognitive, behavioral and motor functions.[29]

Blood supply

Main article: Cerebral circulation
Blood supply

Arteries bring oxygenated blood to the brain. The brain receives blood from two major sources: at the front, from the left and right internal carotid arteries, and from the back, the vertebral arteries.[30] These two circulations join together (anastamose) in the Circle of Willis, a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons.[31]

Blood drainage

Veins drain deoxygenated blood from the brain. The brain has two main networks of veins: an exterior network, which sits on the cerebrum and has three braches, and an interior network. These two networks communicate via joining (anastamosing) veins.[35] The veins of the brain drain into larger cavities called sinuses usually situated between the dura mater and the covering of the skull.[36] Blood from the cerebellum and midbrain drains into the great cerebral vein, whereas from the medulla and pons of the brainstem have a variable pattern of drainage, either into the spinal veins or into adjacent cerebral veins.[35]

The blood in the deep part of the brain drains, through veins and networks of veins (plexi) into the cavernous sinus at the front, and the superior and inferior petrosal sinuses at the sides, and the inferior sagittal sinus at the back.[36] Blood drains from the outer brain into the large superior saggital sinus, which rests in the midline on top of the brain. Blood from here joins with blood from the straight sinus at confluence of sinuses.[36]

Blood from here drains into the left and right transverse sinuses.[36] These then drain into the sigmoid sinuses, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large internal jugular veins.[36][35]

The blood-brain barrier

The larger arteries throughout the brain supply blood to smaller capillaries. These blood vessels are joined by tight junctions and so do not let fluids seep in and leak out to the same degree as other parts of the body, creating the blood-brain barrier.[37] The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including anaesthetics and alcohol).[37] A similar barrier exists between blood and cerebrospinal fluid.[37] The barrier is not present in areas of the brain that may respond to changes in body fluids, such as the pineal gland, area postrema, and some areas of the hypothalamus.[37]

Cerebrospinal fluid

Main article: Cerebrospinal fluid

Cerebrospinal fluid is clear, colourless fluid that circulates around the brain and in the central canal of the spinal cord in the subarachnoid space, as well as within some spaces within the brain.[38] Most cerebrospinal fluid is created in the choroid plexus of the lateral ventricles, the two large cavities inside the cerebrum, and in the walls of the third ventricle. The third ventricle lies in the midline and is connnected to the two ventricles.[38] A single tube, the cerebral aqueduct connects this ventricle to the fourth ventricle, found between the pons and cerebrum, and three separate tubes (one in the middle and two at the sides). From here, cerebrospinal fluid circulates around the brain and spinal cord in the subarachnoid space, between the arachnoid mater and pia mater.[38]

At any one time, there is about 150mL of cerebrospinal fluid - most within the subarachnoid space. It is constantly being created and reabsorbed, and replaces about once every 5-6 hours.[38] In other parts of the body, circulation in the lymphatic vasculature clears extracellular waste products from the cell tissue.[39] For the tissue of the brain, such a system has not yet been identified.[39] However, the presence of a glymphatic pathway has been proposed.[39]

Development

Brain of a human embryo in the sixth week of development

During the first three weeks of development, the embryonic ectoderm forms a thickened strip called the neural plate. By the fourth week of development the neural plate has widened to give a broad cephalic end, a less broad middle part and a narrow caudal end. These swellings represent the beginnings of the forebrain, midbrain and hindbrain.[40] In the fourth week in the neurulation stage the neural plate folds and closes (at the neural crest) to form the neural tube.[41] Cells inside the tube at the cephalic end will give rise to the brain and cells at the caudal end will give rise to the spinal cord.[42]

The tube flexes as it grows, forming the crescent-shaped cerebral hemispheres at the head. The cerebral hemispheres first appear on day 32.[43] Early in the fourth week the cephalic part bends sharply forward in a cephalic flexure.[41] This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure will become the hindbrain (rhombencephalon). In the fifth week of developmement five brain vesicles have formed. The forebrain separates into two vesicles an anterior telencephalon and a posterior diencephalon. The telencephalon will give rise to the cerebral cortex, basal ganglia, and related structures. The diencephalon will give rise to the thalamus and hypothalamus. The hindbrain also splits into two areas - the metencephalon and the mylencephalon. The metencephalon will give rise to the cerebellum and pons. The myelencephalon will give rise to the medulla oblongata.[41]

Also during the fifth week the brain is dividing into repeating segments called neuromeres. [40][42] The neuromeres are an example of segmentation.[42]

A characteristic of the brain is gyrification, or wrinkling of the cortex. In the womb, the cortex starts off as smooth but starts to form fissures that begin to mark out the different lobes of the brain. These fissures form as a result of the growing hemispheres that increase in size due to a sudden growth in cells of the gray matter. The underlying white matter does not grow at the same rate and the hemispheres are crowded into the small cranial vault.[44] The first cleft to appear in the fourth month is the lateral cerebral fossa. The expanding caudal end of the hemisphere has to curve over in a forward direction to fit into the restricted space. This covers the fossa and turns it into a much deeper ridge known as the lateral sulcus and this marks out the temporal lobe.[43] By the sixth month other sulci have formed which demarcate the frontal, parietal and occipital lobes.[43] A gene present in the human genome (ArhGAP11B) may play a major role in gyrification and encephalisation.[45]

Brain of human embryo at 4.5 weeks, showing interior of forebrain 
Brain interior at 5 weeks 
Brain viewed at midline at 3 months 

Physiology

Neurotransmission

Metabolism

PET image of the human brain showing energy consumption

The brain consumes up to twenty percent of the energy used by the human body, more than any other organ.[46] Brain metabolism normally relies upon blood glucose as an energy source, but during times of low glucose (such as fasting, exercise, or limited carbohydrate intake), the brain will use ketone bodies for fuel with a smaller need for glucose. The brain can also utilize lactate during exercise.[47] Long-chain fatty acids cannot cross the blood–brain barrier, but the liver can break these down to produce ketones. However, the medium-chain fatty acids, octanoic and heptanoic acids, can cross the barrier and be used by the brain.[48] The brain stores glucose in the form of glycogen, albeit in significantly smaller amounts than that found in the liver or skeletal muscle.[49]

Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization.[50] The brain mostly uses glucose for energy, and deprivation of glucose, as can happen in hypoglycemia, can result in loss of consciousness. The energy consumption of the brain does not vary greatly over time, but active regions of the cortex consume somewhat more energy than inactive regions: this fact forms the basis for the functional brain imaging methods PET and fMRI.[51] These functional imaging techniques produce a three-dimensional image of metabolic activity.

Function

Motor control

Locations of two brain areas historically associated with language processing, Broca's area and Wernicke's area, and associated regions of sound processing and speech.
(Associated cortical regions involved in vision, touch sensation, and non-speech movement are also shown.)

The motor system of the brain is responsible for the generation and control of movement.[52] Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles. The corticospinal tract carries movements from the brain, through the spinal cord, to the torso and limbs.[53] The cranial nerves carry movements related to the eyes, mouth and face.

Gross movement - such as locomotion and the movement of arms and legs - is generated in the motor cortex, divided into three parts: the primary motor cortex, found in the prefrontal gyrus and has sections dedicated to the movement of different body parts. These movements are supported and regulated by other two areas, lying anterior to the primary motor cortex: the premotor area and the supplementary motor area.[54] The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in a motor cortical homunculus.[54] Impulses generated from the motor cortex travel along the corticospinal tract along the front side of the medulla, with crossing over (decussating) at the pyramids of the medulla. These then travel down the spinal cord, with most connecting to interneurons, in turn connecting to lower motor neurons within the grey matter that then transmit the impulse to move to muscles themselves.[53] The cerebellum and basal ganglia, a series of structures including the putamen, caudate nucleus, substantia nigra, globus pallidus and areas of the thalamus play a role in fine, complex and coordinated muscle movements.[55] Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as the extrapyramidal system.[56]

Sensory

The brain receives and processes sensory information from around the body. This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centers of the brain exposed to the blood. The brain also receives and interprets the special senses (vision, smell, hearing, and taste).

The brain receives sensorimotor information through the spinal cord. From the skin, the brain receives information about fine and gross touch, pain, vibration and temperature. From the joints, the brain receives information about joint position. The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord.

Vision is generated by light that hits the retina of the eye, which is transformed into a nerve signal by receptors and sent ultimately to the visual cortex in the occipital lobe. Vision from the left visual field lands on the right side of each retina (and visa-versa) and passes through the optic nerve until some information changes sides, so that all information about one side of the visual field passes through tracts in the opposite side of the brain. The nerves reach the brain at the lateral geniculate nucleus, and travel through the optic radiation to reach the optic cortex.[57]

Hearing and balance are both generated in the inner ear. The movement of liquids within the inner ear is generated by motion (for balance) and transmitted vibrations generated by the ossicle bones (for sound). This creates a nerve signal that passes through the vestibulocochlear nerve. From here, it passes through to the cochlear nuclei, the superior olivary nucleus, the medial geniculate nucleus, and finally the auditory radiation to the auditory cortex.[58]

Smell is generated by receptor cells situated in the nose. This information passes through a relatively permeable part of the skull to the olfactory nerve. The nerve joins the brain where the midbrain meets the cerebrum. From here, information passes through to an evolutionarily "old" area and a [[lateral olfactory area|"newer" area].[59] Taste is generated from receptors on the tongue and passed along the facial and glossopharyngeal nerves into the tractus solitarius in the brainstem. Some taste information is also passed from the pharynx into this area via the vagus nerve. Information is then passed from here through the thalamus into the gustatory cortex.[60]

The brain receives information from the blood vessels, the brain receives information about blood oxygen and carbon dioxide levels, blood temperature and blood pH.

Language

Image showing Broca's area and Wernicke's area, and their connection via the arcuate fasciculus

Language functions are generally localized to Wernicke's area and Broca's area.[61][62] Wernicke's area is at the posterior part of the superior temporal gyrus of the dominant half of the brain, and seems to be responsible for creation and interpretation of spoken thought.[62] Broca's area is located in the prefrontal cortex and prefrontal cortex, most commonly on the left side of the brain, and is responsible for the creation of motor activity responsible for speaking.[62] These two areas are connected by the arcuate fasciculus.[62] Areas of the cerebellum, basal ganglia and areas of the motor cortex related to the face and larynx also play a role in coordinating and regulating muscle movements during speech.[62] There has been substantial debate over these pathways.[63][64]

The study of how language is represented, processed, and acquired by the brain is neurolinguistics, which is a large multidisciplinary field drawing from cognitive neuroscience, cognitive linguistics, and psycholinguistics. This field originated from the 19th-century discovery that damage to different parts of the brain appeared to cause different symptoms: physicians noticed that individuals with damage to a portion of the left inferior frontal gyrus now known as Broca's area had difficulty in producing language (Broca's aphasia) whereas those with damage to a region in the left superior temporal gyrus, now known as Wernicke's area, had difficulty in understanding it.[65]

Lateralization

Each hemisphere of the brain interacts primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa. The developmental cause for this is uncertain.[66] Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross sides in the brainstem. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve split off to join the other.[67] The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain.[68] Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa.[66] Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field.[69][70]

In most respects, the left and right sides of the brain are symmetrical in terms of function. For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several very important exceptions, involving language and spatial cognition. In most people, the left hemisphere is "dominant" for language: a stroke that damages a key language area in the left hemisphere can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.

A substantial part of current understanding of the interactions between the two hemispheres has come from the study of "split-brain patients"—people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures. These patients do not show unusual behavior that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it. Most of these patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown.[citation needed]

Emotion

Main article: Emotion

Emotions are generally defined as two step multicomponent processes involving elicitation, followed by psychological feelings, appraisal, expression, autonomic responses, and action tendencies. [71] Attempts to localize basic emotions to certain brain regions have been controversial, with some research finding no evidence for specific locations corresponding to emotions, and instead circuitry involved in general emotional processes. The amygdala, orbitofrontal cortex, mid and anterior insula cortex and lateral prefrontal cortex, appeared to be involved approach related emotions, while weaker evidence was found for the ventral tegmental area, ventral pallidum and nucleus accumbens in incentive salience.[72] Others, however, have found evidence of activation of specific regions, such as the basal ganglia in happiness, the subcallosal cingulate cortex in sadness, and amygdala in fear.[73]

Executive functions

Executive functions is an umbrella term for various cognitive processes and sub-processes,[74] that allow for the control of thought and behavior. These functions include the ability to filter information, or attention, the ability to manipulate working memory, the ability to switch tasks, response inhibition, and the ability to determine the relevance of information.[75] The prefrontal cortex plays a significant role in executive functions.[76] Neuroimaging during tasks testing executive function, such as stroop task and memory tasks, have found that cortical maturation of the prefrontal cortex correlates with executive function in children.[77] Future planning involves activation of the dorsolateral prefrontal cortex, anterior cingulate cortex, angular prefrontal cortex, right prefrontal cortex, and supramarginal gyrus. Working memory manipulation involves the DLPFC, inferior frontal gyrus, and areas of the parietal cortex. Response inhibition involves multiple areas of the cortex, including the inferior frontal gyrus, and ventrolateral prefrontal cortex. Task shifting doesn't involve specific regions of the brain, but instead involves multiple regions of the prefrontal cortex and parietal lobe.[78]

The study of executive function in Parkinson's disease suggests subcortical areas such as the amygdala, hippocampus and basal ganglia and important in these processes. Dopamine modulation of the prefrontal cortex is responsible for the efficacy of dopaminergic drugs on executive function, and gives rise to the Yerkes Dodson Curve.[79] The the inverted U represents decreased executive functioning with excessive arousal(or increased catecholamine release during stress), and decreased executive functioning with insufficient arousal.[80] The low activity polymorphism of Catechol-O-methyltransferase is associated with slight increase in performance on executive function tasks in healthy persons.[81] Executive functions are impaired in multiple disorders include anxiety disorder, major depressive disorder, bipolar disorder, attention deficit hyperactivity disorder, schizophrenia and autism.[82] Lesions to the prefrontal cortex, such as in the case of Phineas Gage, may also result in deficits of executive function. Damage to these areas may also manifest in deficits of other areas of function, such as motivation, and social functioning.[83]

Regulation

The brain, especially the hypothalamus, is heavily involved in regulating multiple bodily functions. The diencephalon include the neuroendocrine regulation, regulation of the circadian rhythm, control of the autonomic nervous system, regulation of fluid homeostasis, and food intake. The circadian rhythm is controlled by two main cell groups in the rostral and caudal hypothalamus. The rostral hypothalamus includes the suprachiasmatic nucleus, which through gene expression cycles generates roughly 24 long clock, and Ventrolateral preoptic nucleus. The caudal hypothalamus contains orexinergic neurons that control arousal through their projections to the ascending reticular activating system.[84] The hypothalamus controls the pituitary gland through the release of peptides such as oxytocin, and vasopressin, as well as dopamine into the median eminence. The hypothalamus influences the autonomic nervous system, through ascending projections into autonomic cell groups in the brain stem. Through the autonomic projections, the hypothalamus is involved in regulating functions such as blood pressure, heart rate, breathing, sweating, and other homeostatic mechanisms.[85] The hypothalamus also plays a role in thermal regulation, and when stimulated by the immune system, is capable of generating a fever. The hypothalamus is influenced by the kidneys. When blood pressure falls, the renin released by the kidneys stimulate the hypothalamus to elicit drinking behavior. The hypothalamus also regulated food intake through autonomic signals, and hormone release by the digestive system.[86]

Society and culture

The mind

Main articles: Cognition and Mind

Understanding the mind–body problem, which is the relationship between the brain and the mind, is a significant challenge both philosophically and scientifically. This is because of the difficulty reconciling how mental activities, such as thoughts and emotions, can be implemented by physical structures such as neurons and synapses, or by any other type of physical mechanism. This difficulty was expressed by Gottfried Leibniz in an analogy known as Leibniz's Mill:

One is obliged to admit that perception and what depends upon it is inexplicable on mechanical principles, that is, by figures and motions. In imagining that there is a machine whose construction would enable it to think, to sense, and to have perception, one could conceive it enlarged while retaining the same proportions, so that one could enter into it, just like into a windmill. Supposing this, one should, when visiting within it, find only parts pushing one another, and never anything by which to explain a perception.

— Leibniz, Monadology[87]

Doubt about the possibility of a mechanistic explanation of thought drove René Descartes, and most of humankind along with him, to dualism: the belief that the mind is to some degree independent of the brain.[88] There has always, however, been a strong argument in the opposite direction. There is clear empirical evidence that physical manipulations of, or injuries to, the brain (for example by drugs or by lesions, respectively) can affect the mind in potent and intimate ways.[89] For example, a person suffering from Alzheimer's disease – a condition that causes physical damage to the brain – also experiences a compromised mind. Similarly, someone who has taken a psychedelic drug may temporarily lose their sense of personal identity (ego death) or experience profound changes to their perception and thought processes. Likewise, a patient with epilepsy who undergoes cortical stimulation mapping with electrical brain stimulation would also, upon stimulation of his or her brain, experience various complex feelings, hallucinations, memory flashbacks, and other complex cognitive, emotional, or behavioral phenomena.[90] Following this line of thinking, a large body of empirical evidence for a close relationship between brain activity and mental activity has led most neuroscientists and contemporary philosophers to be materialists, believing that mental phenomena are ultimately the result of, or reducible to, physical phenomena.[91]

Routing of neural signals from the two eyes to the brain

Research

The boundaries between the specialties of neuroscience, neurology and other disciplines such as psychiatry have faded as they are all influenced by basic research in neuroscience. Neuroscientists, along with researchers from allied disciplines, study how the human brain works. Such research has expanded considerably in recent decades. The "Decade of the Brain", an initiative of the United States Government in the 1990s, is considered to have marked much of this increase in research,[92] and was followed in 2013 by the BRAIN Initiative.

Information about the structure and function of the human brain comes from a variety of experimental methods. Most information about the cellular components of the brain and how they work has come from studies of animal subjects, using a variety of techniques. The advances made in neuroimaging have enabled objective and biological insights to be made into mental disorders which can lead to a faster diagnosis, a more accurate prognosis and help to monitor progress of the disorders.[93] There have been reported differences in brain area volumes seen in some disorders, notably schizophrenia and dementia. This area of research is known as brain morphometry. Different biological approaches using imaging have given more insight for example into the disorders of depression and obsessive-compulsive disorder.

A key source of information about the function of brain regions is the effects of damage to them.[94] In humans, strokes have long provided a "natural laboratory"[clarification needed] for studying the effects of brain damage.

Research has disproved some common misconceptions. These include the belief that neurons are not replaced after the age of two – neurogenesis has been shown to take place in some areas of the brain; the belief that vaccines cause autism, and that only ten per cent of the brain is used.

Electrodes and magnetic fields

By placing electrodes on the scalp, it is possible to record the summed electrical activity of the cortex using a methodology known as electroencephalography (EEG).[95] EEG records average neuronal activity from the cerebral cortex and can detect changes in activity over large areas but with low sensitivity for sub-cortical activity. EEG recordings are sensitive enough to detect tiny electrical impulses lasting only a few milliseconds. Most EEG devices have good temporal resolution, but low spatial resolution.

Electrodes can also be placed directly on the surface of the brain (usually during surgical procedures that require removal of part of the skull). This technique, called electrocorticography (ECoG), offers finer spatial resolution than electroencephalography, but is very invasive. In addition to measuring the electric field directly via electrodes placed over the skull, it is possible to measure the magnetic field that the brain generates using a method known as magnetoencephalography (MEG).[96] This technique also has good temporal resolution like EEG but with much better spatial resolution. The greatest disadvantage of MEG is that, because the magnetic fields generated by neural activity are very subtle, the neural activity must be relatively close to the surface of the brain to detect its magnetic field. MEGs can only detect the magnetic signatures of neurons located in the depths of cortical folds (sulci) that have dendrites oriented in a way that produces a field.

Imaging

Computed tomography of human brain, from base of the skull to top, taken with intravenous contrast medium
A scan of the brain using fMRI
Main article: Neuroimaging
Further information: Brain mapping and Outline of brain mapping

There are several methods for detecting brain activity changes using three-dimensional imaging of local changes in blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the bloodstream. A newer method, functional magnetic resonance imaging (fMRI), has considerably better spatial resolution and involves no radioactivity.[97] Using the most powerful magnets currently available, fMRI can localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal resolution is poor: when brain activity increases, the blood flow response is delayed by 1–5 seconds and lasts for at least 10 seconds. Thus, fMRI is a very useful tool for learning which brain regions are involved in a given behavior, but gives little information about the temporal dynamics of their responses. A major advantage for fMRI is that, because it is non-invasive, it can readily be used on human subjects. Another functional imaging method is functional near-infrared spectroscopy.

Clinical significance

Damage or disease of the brain can manifest in a wide variety of ways. When the head undergoes trauma, for example in contact sport, traffic, after a fall fall or in work accidents, the brain may be affected, particularly if there is bleeding within the skull that compresses the brain tissue or damages its blood supply. Concussion, characterised by a confused or dazed state, may result from a traumatic injury. In addition to the site of injury, the opposite side of the brain may be affected, termed a contrecoup injury.

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease and motor neuron diseases are caused by the gradual death of individual neurons, leading to diminution in movement control, memory, and cognition.

The brain, although protected by the blood-brain barrier, can be affected by infections including viruses, bacteria and fungi. Infection of the meninges, the membranes that cover the brain, can lead to meningitis. Creutzfeldt–Jakob disease and its variant (vCJD) (sometimes called human mad cow disease) are linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans, (endemic only to Papua New Guinea tribes). Both are linked to the ingestion of neural tissue, and may explain the tendency in human and some non-human species to avoid cannibalism. There are also demyelinating and dysmyelinating diseases that damage the myelin sheath of neurons, that can have an infectious cause. Viral or bacterial infections can also cause other encephalopathies, and encephalomyelitis.

Brain tumors both benign and malignant can form. These can either originate from any tissue in or around the brain. The most common are those growths that affect the glial cells known as gliomas. (This term has been extended to include all primary brain tumors.)[98] Secondary cancers can form in the brain as a result of brain metastasis.

Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder, may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic function. These disorders may be treated by psychotherapy, psychiatry, social intervention and personal recovery work or cognitive behavioural therapy; the underlying issues and associated prognoses vary significantly between individuals.

Seizures are thought to relate to excess and uncontrolled activity in one or many parts of the brain. Seizure activity can manifest as absence, focal effects such as limb movement or impediments of speech, or be generalized in nature. Status epilepticus refers to a seizure that has not terminated in a short timespan. A large number of factors can cause seizures, however many seizures occur without a definitive cause being found. In a person with epilepsy, risk factors for further seizures may include sleeplessness, drug and alcohol intake, and stress. Seizures may be assessed using EEG and various medical imaging techniques. Seizures can sometimes be managed with anticonvulsant drugs in addition to other treatments.

Many brain disorders are congenital. Tay-Sachs disease, fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors,. Normal development of the brain can be altered by genetic factors, drug use, nutritional deficiencies, the presence of teratogenic drugs, and infectious diseases during pregnancy. Age is a major risk factor for many neurological and neurodegenerative disorders.

Stroke

Main article: Stroke

A stroke is a decrease in blood supply to an area of the brain causing cell death and brain injury. This can lead to a wide range of stroke symptoms, including the "FAST" symptoms of facial droop, arm weakness, and speech difficulties (including difficulties speaking and difficulties finding words or forming sentences).[99] Symptoms relate to the function of the affected area of the brain and can point to the likely site and cause of the stroke. Difficulties with movement, speech, or sight usually relate to the cerebrum, whereas imbalance, double vision, vertigo and symptoms affecting more than one side of the body usually relate to the brainstem or cerebellum.[100]

Most strokes result from loss of blood supply, typically because of an embolus, rupture of a fatty plaque or narrowing of small arteries. Strokes can also result from bleeding within the brain.[101] Transient ischemic attacks (TIAs) are strokes in which symptoms resolve within 24 hours.[101] Investigation into the stroke will involve a medical examination (including a neurological examination) and the taking of a medical history, focusing on the duration of the symptoms and risk factors (including hypertension, atrial fibrillation, and smoking).[102][103] Further investigation is needed in younger patients.[102] An ECG and telemetry may be conducted to identify atrial fibrillation, carotid ultrasound to investigate for narrowing of the carotid arteries, and an echocardiogram to investigate for clots within the heart, diseases of the heart valves or a patent foramen ovale that may point to a cardiac cause.[102] Blood tests are routinely done as part of the workup including diabetes tests and a lipid profile.[102]

Some treatments for storke are time-critical. These include clot dissolution or surgical removal of a clot for ischaemic strokes, and decompression for haemorrhagic strokes.[104][105] As stroke is time critical,[106] hospitals and even pre-hospital care of stroke involves expedited investigations - usually a CT scan to investigate for a haemorrhagic stroke and an CT or MR angiogram to evaluate arteries that supply the brain.[102] MRI scans, not as widely available, may be able to demonstrate the affected area of the brain more accurately, particularly with ischaemic stroke.[102]

Having experienced a stroke, a person may be admitted to a stroke unit, and treatments may be directed as preventing future strokes, including ongoing anticoagulation (such as aspirin or clopidogrel), antihypertensives, and lipid-lowering drugs.[104] A multidisciplinary team consisting of speech pathologists, physiotherapists, occupational therapists, psychologists and others plays a large role in supporting a person affected by a stroke and rehabilitation.[107][102]

Brain death

Main article: Brain death

Brain death refers to an irreversible total loss of brain function.[108][109][110] This is characterised by coma, loss of reflexes, and apnoea,[109][108] however, the declaration of brain death varies geographically and is not always accepted.[110] Declaration of brain death can have profound implications as the declaration, under the principle of medical futility, will be associated with the withdrawal of life support,[111] and as those with brain death often have organs suitable for organ donation.[110][112] The process is often complicated by problems, including those related to poor communication and preparation of families.[108]

When brain death is suspected, reversible differential diagnosis such as hypothermia-induced coma, electrolyte, neurological and drug-related cognitive suppression are first excluded.[109][111] Testing for reflexes[c] can be of help in the decision, as can the absence of response and breathing.[111] Clinical observations, including a total lack of responsiveness, a known diagnosis, and neural imaging evidence, may all play a role in the decision to pronounce brain death.[109]

Comparative anatomy

The human brain has many properties that are common to all vertebrate brains, including a basic division into three parts called the forebrain, midbrain, and hindbrain, with interconnected fluid-filled ventricles, and a set of generic vertebrate brain structures including the medulla oblongata and pons of the brainstem, the cerebellum, optic tectum, thalamus, hypothalamus, basal ganglia, olfactory bulb, and many others.

It has the same general structure as the brains of other mammals. Large animals such as whales and elephants have larger brains in absolute terms, but when measured using a measure of relative brain size, which compensates for body size, the quotient for the human brain is almost twice as large as that of a bottlenose dolphin, and three times as large as that of a chimpanzee, though the quotient for a treeshrew's brain is larger than that of a human's.[113]

As a mammalian brain, the human brain has special features that are common to all mammalian brains,[114] most notably a six-layered cerebral cortex and a set of associated structures,[115] including the hippocampus and amygdala.[116] The upper surface of the forebrain of other vertebrates is covered in a layer of neural tissue called the pallium. The pallium is a relatively simple three-layered cell structure. The hippocampus and the amygdala originate from the pallium but in mammals they are much more complex.

As a primate brain, the human brain has a much larger cerebral cortex, in proportion to body size, than most mammals,[116] and a very highly developed visual system.[117][118]

As a hominid brain, the human brain is substantially enlarged even in comparison to the brain of a typical monkey. The sequence of evolution from Australopithecus (four million years ago) to Homo sapiens (modern man) was marked by a steady increase in brain size, particularly in the frontal lobes, which are associated with a variety of high-level cognitive functions.

The field of genomics has shown that humans and other primates have some differences in DNA sequencing, and genes are differentially expressed in many brain regions. The functional differences between the human brain and the brains of other animals also arise from many gene–environment interactions.[119]

The neuroimmune system of the brain is structurally distinct from the peripheral immune system, which protects the rest of the body. In particular, the immune system is composed primarily of hematopoietic cells and anatomical barriers, while the neuroimmune system is composed of glia, mast cells, and various brain barriers (e.g., blood–brain barrier and blood-cerebrospinal fluid barrier).

The same structures are present in other mammals, although they vary considerably in relative size. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost perfectly smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.

Corticalization is reflected in function as well as structure. In a rat, surgical removal of the entire cerebral cortex leaves an animal that is still capable of walking around and interacting with the environment.[120] In a human, comparable cerebral cortex damage produces a permanent state of coma. The amount of association cortex, relative to the other two categories of sensory and motor, increases dramatically as one goes from simpler mammals, such as the rat and the cat, to more complex ones, such as the chimpanzee and the human.[121]

Evolution

A reconstruction of Homo habilis

The Gale Encyclopedia of Science states, "As human's position changed and the manner in which his or her skull balanced on the spinal column pivoted, the brain expanded, altering the shape of the cranium."[122] In the course of evolution of the Homininae, the human brain has grown in volume from about 600 cm3 in Homo habilis to about 1500 cm3 in Homo neanderthalensis.[citation needed] Subsequently, there has been a shrinking over the past 28,000 years.[citation needed] The male brain has decreased from 1,500 cm3 to 1,350 cm3 while the female brain has shrunk by the same relative proportion.[123] For comparison, Homo erectus, a relative of humans, had a brain size of 1,100 cm3. However, the little Homo floresiensis, with a brain size of 380 cm3, a third of that of their proposed ancestor Homo erectus, used fire, hunted, and made stone tools at least as sophisticated as those of Homo erectus.[124] There has been very little change in brain size from Neanderthals to modern humans, however it is estimated that neanderthals had larger visual systems.[125] The notion "As large as you need and as small as you can" has been used to summarize the opposite evolutionary constraints on human brain size.[126][127] Changes in the size of the human brain during evolution have been reflected in changes in the ASPM and microcephalin genes.[128]

Studies tend to indicate small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and IQ.[129] The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampi, and the cerebellum, but these only account for a relatively small amount of variance in IQ, which itself has only a partial relationship to general intelligence and real-world performance.[130][131]

History

See also

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Books
  • Ort, Bruce Ian Bogart, Victoria (2007). Elsevier's integrated anatomy and embryology. Philadelphia, Pa.: Elsevier Saunders. ISBN 978-1-4160-3165-9. 
  • Hall, John (2011). Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, Pa.: Saunders/Elsevier. ISBN 978-1-4160-4574-8. 
  • Longo, Dan; Fauci, Anthony; Kasper, Dennis; Hauser, Stephen; Jameson, J.; Loscalzo, Joseph (August 11, 2011). Harrison's Principles of Internal Medicine (18 ed.). McGraw-Hill Professional. ISBN 978-0-07-174889-6. 
  • editor-in-chief, Susan Standring ; section editors, Neil R. Borley; et al. (2008). Gray's anatomy : the anatomical basis of clinical practice (40th ed.). London: Churchill Livingstone. ISBN 978-0-8089-2371-8. 
  • Britton, the editors Nicki R. Colledge, Brian R. Walker, Stuart H. Ralston ; illustated by Robert (2010). Davidson's principles and practice of medicine. (21st ed. ed.). Edinburgh: Churchill Livingstone/Elsevier. ISBN 978-0-7020-3085-7. 
  • Larsen, William J. (2001). Human embryology (3. ed. ed.). Philadelphia, Pa.: Churchill Livingstone. ISBN 0-443-06583-7. 

Notes

General references

  • Campbell, Neil A. and Jane B. Reece. (2005). Biology. Benjamin Cummings. ISBN 0-8053-7171-0
  • McGilchrist, Iain (2009). The Master and His Emissary: The Divided Brain and the Making of the Western World. USA: Yale University Press. ISBN 0-300-14878-X. 
  • Ramachanandran, V S (2011), The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human. W. W. Norton & Company.
  • Simon, Seymour (1999). The Brain. HarperTrophy. ISBN 0-688-17060-9
  • Thompson, Richard F. (2000). The Brain: An Introduction to Neuroscience. Worth Publishers. ISBN 0-7167-3226-2

External links