The study of the anatomy of the brain isneuroanatomy, while the study of its function isneuroscience. Numerous techniques are used to study the brain.Specimens from other animals, which may beexamined microscopically, have traditionally provided much information.Medical imaging technologies such asfunctional neuroimaging, andelectroencephalography (EEG) recordings are important in studying the brain. Themedical history of people withbrain injury has provided insight into the function of each part of the brain. Neuroscience research has expanded considerably, and research is ongoing.
The adult human brain weighs on average about 1.2–1.4 kg (2.6–3.1 lb) which is about 2% of the total body weight,[2][3] with a volume of around 1260 cm3 in men and 1130 cm3 in women.[4] There is substantial individual variation,[4] with the standardreference range for men being 1,180–1,620 g (2.60–3.57 lb)[5] and for women 1,030–1,400 g (2.27–3.09 lb).[6]
Thebrainstem, resembling a stalk, attaches to and leaves the cerebrum at the start of themidbrain area. The brainstem includes the midbrain, thepons, and themedulla oblongata. Behind the brainstem is thecerebellum (Latin:little brain).[7]
The cerebrum, brainstem, cerebellum, and spinal cord are covered by three membranes calledmeninges. The membranes are the toughdura mater; the middlearachnoid mater and the more delicate innerpia mater. Between the arachnoid mater and the pia mater is thesubarachnoid space andsubarachnoid cisterns, which contain thecerebrospinal fluid.[11] The outermost membrane of the cerebral cortex is the basement membrane of the pia mater called theglia limitans and is an important part of theblood–brain barrier.[12] In 2023 a fourth meningeal membrane has been proposed known as thesubarachnoid lymphatic-like membrane.[13][14] The living brain is very soft, having a gel-like consistency similar to soft tofu.[15] The cortical layers of neurons constitute much of the cerebralgrey matter, while the deeper subcortical regions ofmyelinatedaxons, make up thewhite matter.[7] The white matter of the brain makes up about half of the total brain volume.[16]
Structural and functional areas of the human brain
Human brain bisected in thesagittal plane, showing the white matter of the corpus callosum
Functional areas of the human brain. Dashed areas shown are commonly left hemisphere dominant.
Major gyri and sulci on the lateral surface of the cortexLobes of the brain
The cerebrum is the largest part of the brain and is divided into nearlysymmetrical left and righthemispheres by a deep groove, thelongitudinal fissure.[17] Asymmetry between the lobes is noted as apetalia.[18] The hemispheres are connected by fivecommissures that span the longitudinal fissure, the largest of these is thecorpus callosum.[7]Each hemisphere is conventionally divided into four mainlobes; thefrontal lobe,parietal lobe,temporal lobe, andoccipital lobe, named according to theskull bones that overlie them.[8] Each lobe is associated with one or two specialised functions though there is some functional overlap between them.[19] The surface of the brain isfolded into ridges (gyri) and grooves (sulci), many of which are named, usually according to their position, such as thefrontal gyrus of the frontal lobe or thecentral sulcus separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds.[20]
The outer part of the cerebrum is thecerebral cortex, made up ofgrey matter arranged in layers. It is 2 to 4 millimetres (0.079 to 0.157 in) thick, and deeply folded to give a convoluted appearance.[21] Beneath the cortex is the cerebralwhite matter. The largest part of the cerebral cortex is theneocortex, which has six neuronal layers. The rest of the cortex is ofallocortex, which has three or four layers.[7]
Cortical folds and white matter in horizontal bisection of head
The cerebrum contains theventricles where the cerebrospinal fluid is produced and circulated. Below the corpus callosum is theseptum pellucidum, a membrane that separates thelateral ventricles. Beneath the lateral ventricles is thethalamus and to the front and below is thehypothalamus. The hypothalamus leads on to thepituitary gland. At the back of the thalamus is the brainstem.[27]
Thebasal ganglia, also called basal nuclei, are a set of structures deep within the hemispheres involved in behaviour and movement regulation.[28] The largest component is thestriatum, others are theglobus pallidus, thesubstantia nigra and thesubthalamic nucleus.[28] The striatum is divided into a ventral striatum, and dorsal striatum, subdivisions that are based upon function and connections. The ventral striatum consists of thenucleus accumbens and theolfactory tubercle whereas the dorsal striatum consists of thecaudate nucleus and theputamen. The putamen and the globus pallidus lie separated from the lateral ventricles and thalamus by theinternal capsule, whereas the caudate nucleus stretches around and abuts the lateral ventricles on their outer sides.[29] At the deepest part of thelateral sulcus between theinsular cortex and the striatum is a thin neuronal sheet called theclaustrum.[30]
The cerebellum is divided into ananterior lobe, aposterior lobe, and theflocculonodular lobe.[32] The anterior and posterior lobes are connected in the middle by thevermis.[33] Compared to the cerebral cortex, the cerebellum has a much thinner outer cortex that is narrowly furrowed into numerous curved transverse fissures.[33]Viewed from underneath between the two lobes is the third lobe the flocculonodular lobe.[34] The cerebellum rests at the back of thecranial cavity, lying beneath the occipital lobes, and is separated from these by thecerebellar tentorium, a sheet of fibre.[35]
It is connected to the brainstem by three pairs ofnerve tracts calledcerebellar peduncles. Thesuperior pair connects to the midbrain; themiddle pair connects to the medulla, and theinferior pair connects to the pons.[33] The cerebellum consists of an inner medulla of white matter and an outer cortex of richly folded grey matter.[35] The cerebellum's anterior and posterior lobes appear to play a role in the coordination and smoothing of complex motor movements, and the flocculonodular lobe in the maintenance ofbalance[36] although debate exists as to its cognitive, behavioural and motor functions.[37]
Ten of the twelve pairs ofcranial nerves[a] emerge directly from the brainstem.[38] The brainstem also contains manycranial nerve nuclei andnuclei ofperipheral nerves, as well as nuclei involved in the regulation of many essential processes includingbreathing, control of eye movements and balance.[39][38] Thereticular formation, a network of nuclei of ill-defined formation, is present within and along the length of the brainstem.[38] Manynerve tracts, which transmit information to and from the cerebral cortex to the rest of the body, pass through the brainstem.[38]
Some 400genes are shown to be brain-specific. In all neurons,ELAVL3 is expressed, and in pyramidal cells,NRGN andREEP2 are also expressed.GAD1 – essential for the biosynthesis of the neurotransmitterGABA – is expressed in interneurons. Proteins expressed in glial cells include astrocyte markersGFAP andS100B whereasmyelin basic protein and the transcription factorOLIG2 are expressed in oligodendrocytes.[47]
Cerebrospinal fluid is a clear, colourlesstranscellular fluid that circulates around the brain in thesubarachnoid space, in theventricular system, and in thecentral canal of the spinal cord. It also fills some gaps in the subarachnoid space, known assubarachnoid cisterns.[48] The four ventricles, twolateral, athird, and afourth ventricle, all contain achoroid plexus that produces cerebrospinal fluid.[49] The third ventricle lies in the midline andis connected to the lateral ventricles.[48] A singleduct, thecerebral aqueduct between the pons and the cerebellum, connects the third ventricle to the fourth ventricle.[50] Three separate openings, themiddle and twolateral apertures, drain the cerebrospinal fluid from the fourth ventricle to thecisterna magna, one of the major cisterns. From here, cerebrospinal fluid circulates around the brain and spinal cord in the subarachnoid space, between the arachnoid mater and pia mater.[48]At any one time, there is about 150mL of cerebrospinal fluid – most within the subarachnoid space. It is constantly being regenerated and absorbed, and is replaced about once every 5–6 hours.[48]
Aglymphatic system has been described as the lymphatic drainage system of the brain.[51][52] The brain-wide glymphatic pathway includes drainage routes from the cerebrospinal fluid, and from themeningeal lymphatic vessels that are associated with thedural sinuses, and run alongside the cerebral blood vessels.[53][54] The pathway drainsinterstitial fluid from the tissue of the brain.[54]
The vertebral arteries emerge as branches of the left and rightsubclavian arteries. They travel upward throughtransverse foramina which are spaces in thecervical vertebrae. Each side enters the cranial cavity through the foramen magnum along the corresponding side of the medulla.[57] They give offone of the three cerebellar branches. The vertebral arteries join in front of the middle part of the medulla to form the largerbasilar artery, which sends multiple branches to supply the medulla and pons, and the two otheranterior andsuperior cerebellar branches.[59] Finally, the basilar artery divides into twoposterior cerebral arteries. These travel outwards, around the superior cerebellar peduncles, and along the top of the cerebellar tentorium, where it sends branches to supply the temporal and occipital lobes.[59] Each posterior cerebral artery sends a smallposterior communicating artery to join with the internal carotid arteries.
Cerebral veins draindeoxygenated blood from the brain. The brain has two main networks ofveins: an exterior orsuperficial network, on the surface of the cerebrum that has three branches, and aninterior network. These two networks communicate viaanastomosing (joining) veins.[60] The veins of the brain drain into larger cavities of thedural venous sinuses usually situated between the dura mater and the covering of the skull.[61] Blood from the cerebellum and midbrain drains into thegreat cerebral vein. Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into thespinal veins or into adjacent cerebral veins.[60]
The larger arteries throughout the brain supply blood to smallercapillaries. These smallest ofblood vessels in the brain, are lined with cells joined bytight junctions and so fluids do not seep in or leak out to the same degree as they do in other capillaries; this creates theblood–brain barrier.[44]Pericytes play a major role in the formation of the tight junctions.[62] The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (includinganaesthetics and alcohol).[44] The blood-brain barrier is not present in thecircumventricular organs—which are structures in the brain that may need to respond to changes in body fluids—such as thepineal gland,area postrema, and some areas of thehypothalamus.[44] There is a similarblood–cerebrospinal fluid barrier, which serves the same purpose as the blood–brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems.[44][63]
Neurulation and neural crest cellsPrimary and secondaryvesicle stages of development in the early embryo to the fifth weekBrain of a human embryo in the sixth week of development
At the beginning of the third week ofdevelopment, theembryonicectoderm forms a thickened strip called theneural plate.[64] By the fourth week of development the neural plate has widened to give a broadcephalic end, a less broad middle part and a narrow caudal end. These swellings are known as theprimary brain vesicles and represent the beginnings of theforebrain (prosencephalon),midbrain (mesencephalon), andhindbrain (rhombencephalon).[65][66]
Neural crest cells (derived from the ectoderm) populate the lateral edges of the plate at theneural folds. In the fourth week—during theneurulation stage—theneural folds close to form theneural tube, bringing together the neural crest cells at theneural crest.[67] The neural crest runs the length of the tube with cranial neural crest cells at the cephalic end and caudal neural crest cells at the tail. Cells detach from the crest andmigrate in a craniocaudal (head to tail) wave inside the tube.[67] Cells at the cephalic end give rise to the brain, and cells at the caudal end give rise to the spinal cord.[68]
The tubeflexes as it grows, forming the crescent-shaped cerebral hemispheres at the head. The cerebral hemispheres first appear on day 32.[69]Early in the fourth week, the cephalic part bends sharply forward in acephalic flexure.[67] This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure becomes the hindbrain (rhombencephalon). These areas are formed as swellings known as the threeprimary brain vesicles. In the fifth week of development fivesecondary brain vesicles have formed.[70] The forebrain separates into two vesicles – an anteriortelencephalon and a posteriordiencephalon. The telencephalon gives rise to the cerebral cortex, basal ganglia, and related structures. The diencephalon gives rise to the thalamus and hypothalamus. The hindbrain also splits into two areas – themetencephalon and themyelencephalon. The metencephalon gives rise to the cerebellum and pons. The myelencephalon gives rise to the medulla oblongata.[71] Also during the fifth week, the brain divides intorepeating segments calledneuromeres.[65][72] In thehindbrain these are known asrhombomeres.[73]
A characteristic of the brain is the cortical folding known asgyrification. For just over five months ofprenatal development the cortex is smooth. By the gestational age of 24 weeks, the wrinkled morphology showing the fissures that begin to mark out the lobes of the brain is evident.[74] Why the cortex wrinkles and folds is not well-understood, but gyrification has been linked to intelligence andneurological disorders, and anumber of gyrification theories have been proposed.[74] These theories include those based onmechanical buckling,[75][19]axonal tension,[76] anddifferential tangential expansion.[75] What is clear is that gyrification is not a random process, but rather a complex developmentally predetermined process which generates patterns of folds that are consistent between individuals and most species.[75][77]
The first groove to appear in the fourth month is the lateral cerebral fossa.[69] 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 thelateral sulcus and this marks out the temporal lobe.[69] By the sixth month other sulci have formed that demarcate the frontal, parietal, and occipital lobes.[69] A gene present in the human genome (ARHGAP11B) may play a major role in gyrification and encephalisation.[78]
Brain of human embryo at 4.5 weeks, showing interior of forebrain
The frontal lobe is involved in reasoning, motor control, emotion, and language. It contains themotor cortex, which is involved in planning and coordinating movement; theprefrontal cortex, which is responsible for higher-level cognitive functioning; andBroca's area, which is essential for language production.[79] Themotor system of the brain is responsible for thegeneration and control of movement.[80] Generated movements pass from the brain through nerves tomotor neurons in the body, which control the action ofmuscles. Thecorticospinal tract carries movements from the brain, through thespinal cord, to the torso and limbs.[81] Thecranial nerves carry movements related to the eyes, mouth and face.
Gross movement – such aslocomotion and the movement of arms and legs – is generated in themotor cortex, divided into three parts: theprimary motor cortex, found in theprecentral gyrus and has sections dedicated to the movement of different body parts. These movements are supported and regulated by two other areas, lyinganterior to the primary motor cortex: thepremotor area and thesupplementary motor area.[82] The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in amotor homunculus.[82] Impulses generated from the motor cortex travel along thecorticospinal tract along the front of the medulla and cross over (decussate) at themedullary pyramids. These then travel down thespinal cord, with most connecting tointerneurons, in turn connecting tolower motor neurons within thegrey matter that then transmit the impulse to move to muscles themselves.[81] The cerebellum andbasal ganglia, play a role in fine, complex and coordinated muscle movements.[83] Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as theextrapyramidal system.[84]
From the skin, the brain receives information aboutfine touch,pressure,pain,vibration andtemperature. From the joints, the brain receives information aboutjoint position.[86] Thesensory 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 asensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord. Thedorsal column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. The pathway fibres travel up the back part of the spinal cord to the back part of the medulla, where they connect withsecond-order neurons that immediatelysend fibres across the midline. These fibres then travel upwards into theventrobasal complex in the thalamus where they connect withthird-order neurons which send fibres up to the sensory cortex.[86] Thespinothalamic tract carries information about pain, temperature, and gross touch. The pathway fibres travel up the spinal cord and connect with second-order neurons in thereticular formation of the brainstem for pain and temperature, and also terminate at the ventrobasal complex of the thalamus for gross touch.[87]
Vision is generated by light that hits theretina of the eye.Photoreceptors in the retinatransduce the sensory stimulus oflight into an electricalnerve signal that is sent to thevisual cortex in the occipital lobe. Visual signals leave the retinas through theoptic nerves.Optic nerve fibres from the retinas' nasal halvescross to the opposite sides joining the fibres from the temporal halves of the opposite retinas to form theoptic tracts.The arrangements of the eyes' optics and the visual pathways mean vision from the leftvisual field is received by the right half of each retina, is processed by the right visual cortex, and vice versa. However, the left visual field of the left eye does not cross over to the right side. The right visual field of the right eye also does not cross over. Right of left and left of right cross at the optic chasm.[88]The optic tract fibres reach the brain at thelateral geniculate nucleus, and travel through theoptic radiation to reach the visual cortex.[89]
The brain controls therate of breathing, mainly byrespiratory centres in the medulla and pons.[96] The respiratory centres controlrespiration, by generating motor signals that are passed down the spinal cord, along thephrenic nerve to thediaphragm and othermuscles of respiration. This is amixed nerve that carries sensory information back to the centres. There are four respiratory centres, three with a more clearly defined function, and an apneustic centre with a less clear function. In the medulla a dorsal respiratory group causes the desire tobreathe in and receives sensory information directly from the body. Also in the medulla, the ventral respiratory group influencesbreathing out during exertion. In the pons thepneumotaxic centre influences the duration of each breath,[96] and theapneustic centre seems to have an influence on inhalation. The respiratory centres directly senses bloodcarbon dioxide andpH. Information about bloodoxygen,carbon dioxide and pH levels are also sensed on the walls of arteries in theperipheral chemoreceptors of the aortic and carotid bodies. This information is passed via the vagus and glossopharyngeal nerves to the respiratory centres. High carbon dioxide, an acidic pH, or low oxygen stimulate the respiratory centres.[96] The desire to breathe in is also affected bypulmonary stretch receptors in the lungs which, when activated, prevent the lungs from overinflating by transmitting information to the respiratory centres via the vagus nerve.[96]
Thehypothalamus in thediencephalon, is involved in regulating many functions of the body. Functions includeneuroendocrine regulation, regulation of thecircadian rhythm, control of theautonomic nervous system, and the regulation of fluid, and food intake. The circadian rhythm is controlled by two main cell groups in the hypothalamus. The anterior hypothalamus includes thesuprachiasmatic nucleus and theventrolateral preoptic nucleus which through gene expression cycles, generates a roughly 24 hourcircadian clock. In thecircadian day anultradian rhythm takes control of the sleeping pattern.Sleep is an essential requirement for the body and brain and allows the closing down and resting of the body's systems. There are also findings that suggest that the daily build-up of toxins in the brain are removed during sleep.[97] Whilst awake the brain consumes a fifth of the body's total energy needs.Sleep necessarily reduces this use and gives time for the restoration of energy-givingATP. The effects ofsleep deprivation show the absolute need for sleep.[98]
Thelateral hypothalamus containsorexinergic neurons that controlappetite andarousal through their projections to theascending reticular activating system.[99][100] The hypothalamus controls thepituitary gland through the release of peptides such asoxytocin, andvasopressin, as well asdopamine into themedian eminence. Through the autonomic projections, the hypothalamus is involved in regulating functions such as blood pressure, heart rate, breathing, sweating, and other homeostatic mechanisms.[101] The hypothalamus also plays a role in thermal regulation, and when stimulated by the immune system, is capable of generating afever. The hypothalamus is influenced by the kidneys: when blood pressure falls, therenin released by the kidneys stimulates a need to drink. The hypothalamus also regulates food intake through autonomic signals, and hormone release by the digestive system.[102]
While language functions were traditionally thought to be localised toWernicke's area andBroca's area,[103] it is now mostly accepted that a wider network ofcortical regions contributes to language functions.[104][105][106]
The cerebrum has acontralateral organisation with each hemisphere of the brain interacting primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa. This is theorized to be caused by a developmentalaxial twist.[108] Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, bothcross sides in the brainstem. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called theoptic chiasm, and half of the fibres from each nerve split off to join the other.[109] 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.[110] 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.[111] 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.[112][113]
The left and right sides of the brain appear symmetrical, but they function asymmetrically.[114] 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 important exceptions, involving language and spatial cognition. The left frontal lobe is dominant for language. If a key language area in the left hemisphere is damaged, it can leave the victim unable to speak or understand,[114] 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.[115] These patients do not show unusual behaviour 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.[115][116] 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.[116][117]
Emotions are generally defined as two-step multicomponent processes involvingelicitation, followed by psychological feelings, appraisal, expression, autonomic responses, and action tendencies.[118] Attempts to localise basic emotions to certain brain regions have been controversial; some research found no evidence for specific locations corresponding to emotions, but instead found circuitry involved in general emotional processes. Theamygdala,orbitofrontal cortex, mid and anteriorinsular cortex and lateralprefrontal cortex, appeared to be involved in generating the emotions, while weaker evidence was found for theventral tegmental area,ventral pallidum andnucleus accumbens inincentive salience.[119] Others, however, have found evidence of activation of specific regions, such as thebasal ganglia in happiness, thesubcallosalcingulate cortex in sadness, andamygdala in fear.[120]
Brain activity is made possible by the interconnections ofneurons that are linked together to reach their targets.[127] A neuron consists of acell body,axon, anddendrites. Dendrites are often extensive branches that receive information in the form of signals from the axon terminals of other neurons. The signals received may cause the neuron to initiate anaction potential (an electrochemical signal or nerve impulse) which is sent along its axon to the axon terminal, to connect with the dendrites or with the cell body of another neuron. An action potential is initiated at theinitial segment of an axon, which contains a specialised complex of proteins.[128] When an action potential reaches the axon terminal it triggers the release of aneurotransmitter at asynapse that propagates a signal that acts on the target cell.[129] These chemical neurotransmitters includedopamine,serotonin,GABA,glutamate, andacetylcholine.[130] GABA is the major inhibitory neurotransmitter in the brain, and glutamate is the major excitatory neurotransmitter.[131] Neurons link at synapses to formneural pathways,neural circuits, and large elaboratenetwork systems such as thesalience network and thedefault mode network, and the activity between them is driven by the process ofneurotransmission.
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 bodyglucose utilization.[139] The brain mostly uses glucose for energy, and deprivation of glucose, as can happen inhypoglycemia, can result in loss of consciousness.[140] 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, which forms the basis for thefunctional neuroimaging methods ofPET andfMRI.[141] These techniques provide a three-dimensional image of metabolic activity.[142] A preliminary study showed that brain metabolic requirements in humans peak at about five years old.[143]
The function ofsleep is not fully understood; however, there is evidence that sleep enhances the clearance of metabolic waste products, some of which are potentiallyneurotoxic, from the brain and may also permit repair.[52][144][145] Evidence suggests that the increased clearance of metabolic waste during sleep occurs via increased functioning of theglymphatic system.[52] Sleep may also have an effect on cognitive function by weakening unnecessary connections.[146]
The brain is not fully understood, and research is ongoing.[147]Neuroscientists, along with researchers from allied disciplines, study how the human brain works. The boundaries between the specialties ofneuroscience,neurology and other disciplines such aspsychiatry have faded as they are all influenced bybasic research in neuroscience.
Neuroscience research has expanded considerably. 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,[148] and was followed in 2013 by theBRAIN Initiative.[149] TheHuman Connectome Project was a five-year study launched in 2009 to analyse the anatomical and functional connections of parts of the brain, and has provided much data.[147]
An emerging phase in research may be that ofsimulating brain activity.[150]
Information about the structure and function of the human brain comes from a variety of experimental methods, including animals and humans. Information about brain trauma and stroke has provided information about the function of parts of the brain and the effects ofbrain damage.Neuroimaging is used to visualise the brain and record brain activity.Electrophysiology is used to measure, record and monitor the electrical activity of the cortex. Measurements may be oflocal field potentials of cortical areas, or of the activity of a single neuron. Anelectroencephalogram can record the electrical activity of the cortex usingelectrodes placed non-invasively on thescalp.[151][152]
The development ofcerebral organoids has opened ways for studying the growth of the brain, and of the cortex, and for understanding disease development, offering further implications for therapeutic applications.[155][156]
Advances inneuroimaging have enabled objective insights into mental disorders, leading to faster diagnosis, more accurate prognosis, and better monitoring.[163]
As of 2017[update], just under 20,000protein-coding genes are seen to be expressed in the human,[164] and some 400 of these genes are brain-specific.[167][168] The data that has been provided ongene expression in the brain has fuelled further research into a number of disorders. The long term use of alcohol for example, has shown altered gene expression in the brain, and cell-type specific changes that may relate toalcohol use disorder.[169] These changes have been noted in thesynaptictranscriptome in the prefrontal cortex, and are seen as a factor causing the drive to alcohol dependence, and also to othersubstance abuses.[170]
Other related studies have also shown evidence of synaptic alterations and their loss, in theageing brain. Changes in gene expression alter the levels of proteins in various neural pathways and this has been shown to be evident in synaptic contact dysfunction or loss. This dysfunction has been seen to affect many structures of the brain and has a marked effect on inhibitory neurons resulting in a decreased level of neurotransmission, and subsequent cognitive decline and disease.[171][172]
Cerebral atherosclerosis isatherosclerosis that affects the brain. It results from the build-up ofplaques formed ofcholesterol, in the large arteries of the brain, and can be mild to significant. When significant, arteries can become narrowed enough to reduce blood flow. It contributes to the development of dementia, and has protein similarities to those found in Alzheimer's disease.[177]
Brain tumours can be eitherbenign orcancerous. Most malignant tumoursarise from another part of the body, most commonly from thelung,breast andskin.[178] Cancers of brain tissue can also occur, and originate from any tissue in and around the brain.Meningioma, cancer of the meninges around the brain, is more common than cancers of brain tissue.[178] Cancers within the brain may cause symptoms related to their size or position, with symptoms including headache and nausea, or the gradual development of focal symptoms such as gradual difficulty seeing, swallowing, talking, or as a change of mood.[178] Cancers are in general investigated through the use of CT scans and MRI scans. A variety of other tests including blood tests and lumbar puncture may be used to investigate for the cause of the cancer and evaluate the type andstage of the cancer.[178] Thecorticosteroiddexamethasone is often given to decrease theswelling of brain tissue around a tumour. Surgery may be considered, however given the complex nature of many tumours or based on tumour stage or type,radiotherapy orchemotherapy may be considered more suitable.[178]
Epileptic seizures are thought to relate to abnormal electrical activity.[181] Seizure activity can manifest asabsence of consciousness,focal effects such as limb movement or impediments of speech, or begeneralized in nature.[181]Status epilepticus refers to a seizure or series of seizures that have not terminated within five minutes.[182] Seizures have a large number of causes, however many seizures occur without a definitive cause being found. In a person withepilepsy, risk factors for further seizures may include sleeplessness, drug and alcohol intake, and stress. Seizures may be assessed usingblood tests,EEG and variousmedical imaging techniques based on themedical history andmedical examination findings.[181] In addition to treating an underlying cause and reducing exposure to risk factors,anticonvulsant medications can play a role in preventing further seizures.[181]
Astroke is adecrease in blood supply to an area of the brain causingcell death andbrain injury. This can lead to a wide range ofsymptoms, including the "FAST" symptoms of facial droop, arm weakness, and speech difficulties (includingwith speaking andfinding words or forming sentences).[191] 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, whereasimbalance,double vision,vertigo and symptoms affecting more than one side of the body usually relate to the brainstem or cerebellum.[192]
Brain death refers to an irreversible total loss of brain function.[201][202] This is characterised bycoma, loss ofreflexes, andapnoea,[201] however, the declaration of brain death varies geographically and is not always accepted.[202] In some countries there is also a defined syndrome ofbrainstem death.[203] Declaration of brain death can have profound implications as the declaration, under the principle ofmedical futility, will be associated with the withdrawal of life support,[204] and as those with brain death often have organs suitable fororgan donation.[202][205] The process is often made more difficult by poor communication with patients' families.[206]
When brain death is suspected, reversibledifferential diagnoses such as, electrolyte, neurological and drug-related cognitive suppression need to be excluded.[201][204] Testing for reflexes[b] can be of help in the decision, as can the absence of response and breathing.[204] Clinical observations, including a total lack of responsiveness, a known diagnosis, andneural imaging evidence, may all play a role in the decision to pronounce brain death.[201]
Neuroanthropology is the study of the relationship between culture and the brain. It explores how the brain gives rise to culture, and how culture influences brain development.[207] Cultural differences and their relation to brain development and structure are researched in different fields.[208]
The skull ofPhineas Gage, with the path of the iron rod that passed through it without killing him, but altering his cognition. The case helped to convince people that mental functions were localised in the brain.[209]
Thephilosophy of the mind studies such issues as the problem of understandingconsciousness and themind–body problem. The relationship between the brain and themind is a significant challenge both philosophically and scientifically. This is because of the difficulty in explaining how mental activities, such as thoughts and emotions, can be implemented by physical structures such as neurons andsynapses, or by any other type of physical mechanism. This difficulty was expressed byGottfried Leibniz in the analogy known asLeibniz'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.
Doubt about the possibility of a mechanistic explanation of thought droveRené Descartes, and most other philosophers along with him, todualism: the belief that the mind is to some degree independent of the brain.[211] 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.[212][213] In the 19th century, the case ofPhineas Gage, a railway worker who was injured by a stout iron rod passing through his brain, convinced both researchers and the public that cognitive functions were localised in the brain.[209] 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 bematerialists, believing that mental phenomena are ultimately the result of, or reducible to, physical phenomena.[214]
The size of the brain and a person'sintelligence are not strongly related.[215] Studies tend to indicate small to moderatecorrelations (averaging around 0.3 to 0.4) between brain volume andIQ.[216] 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.[217][218]
Other animals, including whales and elephants, have larger brains than humans. However, when thebrain-to-body mass ratio is taken into account, the human brain is almost twice as large as that of abottlenose dolphin, and three times as large as that of achimpanzee. However, a high ratio does not of itself demonstrate intelligence: very small animals have high ratios and thetreeshrew has the largest quotient of any mammal.[219]
Earlier ideas about the relative importance of the differentorgans of the human body sometimes emphasised the heart.[220]Modern Western popular conceptions, in contrast, have placed increasing focus on thebrain.[221]
Research has disproved some commonmisconceptions about the brain. These include both ancient and modern myths. It is not true (for example) that neurons are not replaced after the age of two; nor that normal humans use onlyten per cent of the brain.[222] Popular culture has also oversimplified thelateralisation of the brain by suggesting that functions are completely specific to one side of the brain or the other.Akio Mori coined the term "game brain" for the unreliably supported theory that spending long periods playingvideo games harmed the brain's pre-frontal region, and impaired the expression of emotion and creativity.[223]
Historically, particularly in the early-19th century, the brain featured in popular culture throughphrenology, apseudoscience that assigned personality attributes to different regions of the cortex. The cortex remains important in popular culture as covered in books and satire.[224][225]
TheEdwin Smith Papyrus, anancient Egyptianmedical treatise written in the 17th century BC, contains the earliest recorded reference to the brain. Thehieroglyph for brain, occurring eight times in this papyrus, describes the symptoms, diagnosis, and prognosis of two traumatic injuries to the head. The papyrus mentions the external surface of the brain, the effects of injury (including seizures andaphasia), the meninges, and cerebrospinal fluid.[228][229]
In the fifth century BC,Alcmaeon of Croton inMagna Grecia, first considered the brain to be theseat of the mind.[229] Also in thefifth century BC in Athens, the unknown author ofOn the Sacred Disease, a medical treatise which is part of theHippocratic Corpus and traditionally attributed toHippocrates, believed the brain to be the seat of intelligence.Aristotle, in hisbiology initially believed the heart to be the seat ofintelligence, and saw the brain as a cooling mechanism for the blood. He reasoned that humans are more rational than the beasts because, among other reasons, they have a larger brain to cool their hot-bloodedness.[230] Aristotle did describe the meninges and distinguished between the cerebrum and cerebellum.[231]
Herophilus ofChalcedon in the fourth and third centuries BC distinguished the cerebrum and the cerebellum, and provided the first clear description of theventricles; and withErasistratus ofCeos experimented on living brains. Their works are now mostly lost, and we know about their achievements due mostly to secondary sources. Some of their discoveries had to be re-discovered a millennium after their deaths.[229] Anatomist physicianGalen in the second century AD, during the time of theRoman Empire, dissected the brains of sheep, monkeys, dogs, and pigs. He concluded that, as the cerebellum was denser than the brain, it must control themuscles, while as the cerebrum was soft, it must be where the senses were processed. Galen further theorised that the brain functioned by movement of animal spirits through the ventricles.[229][230]
In 2025, scientists reported the discovery of a preserved human brain from theeruption of Mount Vesuvius in 79 AD. A man inHerculaneum was caught in apyroclastic flow, and the extremely high temperature caused thevitrification of his brain, turning it intoglass and resulting in "a perfect state of preservation of the brain and its microstructures."[232] It appears to have been the only known case of a vitrified human brain.[232][233]
In 1316,Mondino de Luzzi'sAnathomia began the modern study of brain anatomy.[234]Niccolò Massa discovered in 1536 that the ventricles were filled with fluid.[235]Archangelo Piccolomini ofRome was the first to distinguish between the cerebrum and cerebral cortex.[236] In 1543Andreas Vesalius published his seven-volumeDe humani corporis fabrica.[236][237][238] The seventh book covered the brain and eye, with detailed images of the ventricles, cranial nerves,pituitary gland, meninges, structures of theeye, the vascular supply to the brain and spinal cord, and an image of the peripheral nerves.[239] Vesalius rejected the common belief that the ventricles were responsible for brain function, arguing that many animals have a similar ventricular system to humans, but no true intelligence.[236]
René Descartes proposed the theory ofdualism to tackle the issue of the brain's relation to the mind. He suggested that thepineal gland was where the mind interacted with the body, serving as the seat of the soul and as the connection through whichanimal spirits passed from the blood into the brain.[235] This dualism likely provided impetus for later anatomists to further explore the relationship between the anatomical and functional aspects of brain anatomy.[240]
Thomas Willis is considered a second pioneer in the study of neurology and brain science. He wroteCerebri Anatome (Latin:Anatomy of the brain)[c] in 1664, followed byCerebral Pathology in 1667. In these he described the structure of the cerebellum, the ventricles, the cerebral hemispheres, the brainstem, and the cranial nerves, studied its blood supply; and proposed functions associated with different areas of the brain.[236] The circle of Willis was named after his investigations into the blood supply of the brain, and he was the first to use the word "neurology".[241] Willis removed the brain from the body when examining it, and rejected the commonly held view that the cortex only consisted of blood vessels, and the view of the last two millennia that the cortex was only incidentally important.[236]
In the middle of 19th centuryEmil du Bois-Reymond andHermann von Helmholtz were able to use agalvanometer to show that electrical impulses passed at measurable speeds along nerves, refuting the view of their teacherJohannes Peter Müller that the nerve impulse was a vital function that could not be measured.[242][243][244]Richard Caton in 1875 demonstrated electrical impulses in the cerebral hemispheres of rabbits and monkeys.[245] In the 1820s,Jean Pierre Flourens pioneered the experimental method of damaging specific parts of animal brains describing the effects on movement and behavior.[246]
Drawing byCamillo Golgi of vertical section of rabbithippocampus, from his "Sulla fina anatomia degli organi centrali del sistema nervoso", 1885Drawing of cells in chickcerebellum bySantiago Ramón y Cajal, from "Estructura de los centros nerviosos de las aves", Madrid, 1905
Studies of the brain became more sophisticated with the use of themicroscope and the development of asilver stainingmethod byCamillo Golgi during the 1880s. This was able to show the intricate structures of single neurons.[247] This was used bySantiago Ramón y Cajal and led to the formation of theneuron doctrine, the then revolutionary hypothesis that the neuron is the functional unit of the brain. He used microscopy to uncover many cell types, and proposed functions for the cells he saw.[247] For this, Golgi and Cajal are considered the founders oftwentieth century neuroscience, both sharing theNobel prize in 1906 for their studies and discoveries in this field.[247]
Charles Sherrington published his influential 1906 workThe Integrative Action of the Nervous System examining the function of reflexes, evolutionary development of the nervous system, functional specialisation of the brain, and layout and cellular function of the central nervous system.[248] In 1942 he coined the termenchanted loom as a metaphor for the brain.John Farquhar Fulton, founded theJournal of Neurophysiology and published the first comprehensive textbook on the physiology of the nervous system during 1938.[249]Neuroscience during the twentieth century began to be recognised as a distinct unified academic discipline, withDavid Rioch,Francis O. Schmitt, andStephen Kuffler playing critical roles in establishing the field.[250] Rioch originated the integration of basic anatomical and physiological research with clinical psychiatry at theWalter Reed Army Institute of Research, starting in the 1950s.[251] During the same period, Schmitt established theNeuroscience Research Program, an inter-university and international organisation, bringing together biology, medicine, psychological and behavioural sciences. The word neuroscience itself arises from this program.[252]
Paul Broca associated regions of the brain with specific functions, in particular language inBroca's area, following work on brain-damaged patients.[253]John Hughlings Jackson described the function of themotor cortex by watching the progression ofepileptic seizures through the body.Carl Wernicke describeda region associated with language comprehension and production.Korbinian Brodmann divided regions of the brain based on the appearance of cells.[253] By 1950, Sherrington,Papez, andMacLean had identified many of the brainstem and limbic system functions.[254][255] The capacity of the brain to re-organise and change with age, and a recognised critical development period, were attributed toneuroplasticity, pioneered byMargaret Kennard, who experimented on monkeys during the 1930-40s.[256]
The human brain has many properties that are common to allvertebrate brains.[259] Many of its features are common to allmammalian brains,[260] most notably a six-layered cerebral cortex and a set of associated structures,[261] including the hippocampus andamygdala.[262] The cortex is proportionally larger in humans than in many other mammals.[263] Humans have more association cortex, sensory and motor parts than smaller mammals such as the rat and the cat.[264]
As aprimate brain, the human brain has a much larger cerebral cortex, in proportion to body size, than most mammals,[262] and a highly developed visual system.[265][266]
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^abCipolla, M.J. (January 1, 2009)."Anatomy and Ultrastructure".The Cerebral Circulation. Morgan & Claypool Life Sciences.Archived from the original on October 1, 2017 – via NCBI Bookshelf.
^Arsava, E. Y.; Arsava, E. M.; Oguz, K. K.; Topcuoglu, M. A. (2019). "Occipital petalia as a predictive imaging sign for transverse sinus dominance".Neurological Research.41 (4):306–311.doi:10.1080/01616412.2018.1560643.PMID30601110.S2CID58546404.
^Netter, F. (2014).Atlas of Human Anatomy Including Student Consult Interactive Ancillaries and Guides (6th ed.). Philadelphia, Penn.: W B Saunders Co. p. 114.ISBN978-1-4557-0418-7.
^abAzevedo, F.; et al. (April 10, 2009). "Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain".The Journal of Comparative Neurology.513 (5):532–541.doi:10.1002/cne.21974.PMID19226510.S2CID5200449.despite the widespread quotes that the human brain contains 100 billion neurons and ten times more glial cells, the absolute number of neurons and glial cells in the human brain remains unknown. Here we determine these numbers by using the isotropic fractionator and compare them with the expected values for a human-sized primate. We find that the adult male human brain contains on average 86.1 ± 8.1 billion NeuN-positive cells ("neurons") and 84.6 ± 9.8 billion NeuN-negative ("nonneuronal") cells.
^Yankova, Galina; Bogomyakova, Olga; Tulupov, Andrey (November 1, 2021). "The glymphatic system and meningeal lymphatics of the brain: new understanding of brain clearance".Reviews in the Neurosciences.32 (7):693–705.doi:10.1515/revneuro-2020-0106.PMID33618444.
^abcBacyinski A, Xu M, Wang W, Hu J (November 2017)."The Paravascular Pathway for Brain Waste Clearance: Current Understanding, Significance and Controversy".Frontiers in Neuroanatomy.11: 101.doi:10.3389/fnana.2017.00101.PMC5681909.PMID29163074.The paravascular pathway, also known as the "glymphatic" pathway, is a recently described system for waste clearance in the brain. According to this model, cerebrospinal fluid (CSF) enters the paravascular spaces surrounding penetrating arteries of the brain, mixes with interstitial fluid (ISF) and solutes in the parenchyma, and exits along paravascular spaces of draining veins. ... In addition to Aβ clearance, the glymphatic system may be involved in the removal of other interstitial solutes and metabolites. By measuring the lactate concentration in the brains and cervical lymph nodes of awake and sleeping mice, Lundgaard et al. (2017) demonstrated that lactate can exit the CNS via the paravascular pathway. Their analysis took advantage of the substantiated hypothesis that glymphatic function is promoted during sleep (Xie et al., 2013; Lee et al., 2015; Liu et al., 2017).
^Damasio, H. (2001). "Neural basis of language disorders". In Chapey, Roberta (ed.).Language intervention strategies in aphasia and related neurogenic communication disorders (4th ed.). Lippincott Williams & Wilkins. pp. 18–36.ISBN978-0-7817-2133-2.OCLC45952164.
^de Lussanet, M.H.E.; Osse, J.W.M. (2012). "An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates".Animal Biology.62 (2):193–216.arXiv:1003.1872.doi:10.1163/157075611X617102.S2CID7399128.
^Sander, David (2013). Armony, J.; Vuilleumier, Patrik (eds.).The Cambridge handbook of human affective neuroscience. Cambridge: Cambridge Univ. Press. p. 16.ISBN978-0-521-17155-7.
^Phan, KL; Wager, Tor; Taylor, SF.; Liberzon, l (June 1, 2002). "Functional Neuroanatomy of Emotion: A Meta-Analysis of Emotion Activation Studies in PET and fMRI".NeuroImage.16 (2):331–348.doi:10.1006/nimg.2002.1087.PMID12030820.S2CID7150871.
^Malenka, RC; Nestler, EJ; Hyman, SE (2009). "Preface". In Sydor, A; Brown, RY (eds.).Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. p. xiii.ISBN978-0-07-148127-4.
^abcdMalenka RC, Nestler EJ, Hyman SE, Holtzman DM (2015). "Chapter 14: Higher Cognitive Function and Behavioral Control".Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (3rd ed.). New York: McGraw-Hill Medical.ISBN978-0-07-182770-6.
^abMalenka RC, Nestler EJ, Hyman SE, Holtzman DM (2015). "Chapter 6: Widely Projecting Systems: Monoamines, Acetylcholine, and Orexin".Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (3rd ed.). New York: McGraw-Hill Medical.ISBN978-0-07-182770-6.
^abMalenka RC, Nestler EJ, Hyman SE, Holtzman DM (2015). "Chapter 14: Higher Cognitive Function and Behavioral Control".Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (3rd ed.). New York: McGraw-Hill Medical.ISBN978-0-07-182770-6.In conditions in which prepotent responses tend to dominate behavior, such as in drug addiction, where drug cues can elicit drug seeking (Chapter 16), or in attention deficit hyperactivity disorder (ADHD; described below), significant negative consequences can result. ... ADHD can be conceptualized as a disorder of executive function; specifically, ADHD is characterized by reduced ability to exert and maintain cognitive control of behavior. Compared with healthy individuals, those with ADHD have diminished ability to suppress inappropriate prepotent responses to stimuli (impaired response inhibition) and diminished ability to inhibit responses to irrelevant stimuli (impaired interference suppression). ... Functional neuroimaging in humans demonstrates activation of the prefrontal cortex and caudate nucleus (part of the dorsal striatum) in tasks that demand inhibitory control of behavior. ... Early results with structural MRI show a thinner cerebral cortex, across much of the cerebrum, in ADHD subjects compared with age-matched controls, including areas of [the] prefrontal cortex involved in working memory and attention.
^abWasserman DH (January 2009)."Four grams of glucose".American Journal of Physiology. Endocrinology and Metabolism.296 (1): E11–21.doi:10.1152/ajpendo.90563.2008.PMC2636990.PMID18840763.Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. ... The brain consumes ~60% of the blood glucose used in the sedentary, fasted person. ... The amount of glucose in the blood is preserved at the expense of glycogen reservoirs (Fig. 2). In postabsorptive humans, there are ~100 g of glycogen in the liver and ~400 g of glycogen in muscle. Carbohydrate oxidation by the working muscle can go up by ~10-fold with exercise, and yet after 1 h, blood glucose is maintained at ~4 g. ... It is now well established that both insulin and exercise cause translocation of GLUT4 to the plasma membrane. Except for the fundamental process of GLUT4 translocation, [muscle glucose uptake (MGU)] is controlled differently with exercise and insulin. Contraction-stimulated intracellular signaling (52, 80) and MGU (34, 75, 77, 88, 91, 98) are insulin independent. Moreover, the fate of glucose extracted from the blood is different in response to exercise and insulin (91, 105). For these reasons, barriers to glucose flux from blood to muscle must be defined independently for these two controllers of MGU.
^Tsuji, A. (2005)."Small molecular drug transfer across the blood-brain barrier via carrier-mediated transport systems".NeuroRx.2 (1):54–62.doi:10.1602/neurorx.2.1.54.PMC539320.PMID15717057.Uptake of valproic acid was reduced in the presence of medium-chain fatty acids such as hexanoate, octanoate, and decanoate, but not propionate or butyrate, indicating that valproic acid is taken up into the brain via a transport system for medium-chain fatty acids, not short-chain fatty acids. ... Based on these reports, valproic acid is thought to be transported bidirectionally between blood and brain across the BBB via two distinct mechanisms, monocarboxylic acid-sensitive and medium-chain fatty acid-sensitive transporters, for efflux and uptake, respectively.
^Vijay, N.; Morris, M.E. (2014)."Role of monocarboxylate transporters in drug delivery to the brain".Curr. Pharm. Des.20 (10):1487–98.doi:10.2174/13816128113199990462.PMC4084603.PMID23789956.Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate. ... MCT1 and MCT4 have also been associated with the transport of short chain fatty acids such as acetate and formate which are then metabolized in the astrocytes [78].
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