Editing Human brain (section)

==Structure==
[[File:Human brain.jpg|thumb|Human brain (sagittal section)]]
{{See also|List of regions in the human brain}}

=== Gross anatomy ===
{{See also|Evolution of the brain#Evolution of the human brain|Neuroscience of sex differences}}
The adult human brain weighs on average about {{convert|1.2-1.4|kg|abbr=on}} which is about 2% of the total body weight,<ref name=CarpenterCh1>{{cite book |title=Carpenter's Human Neuroanatomy |last1=Parent |first1=A. |last2=Carpenter |first2=M.B. |publisher=Williams & Wilkins |year=1995 |isbn=978-0-683-06752-1 |chapter=Ch. 1}}</ref><ref name="Bigos">{{cite book |last1=Bigos |first1=K.L. |last2=Hariri |first2=A. |last3=Weinberger |first3=D. |title=Neuroimaging Genetics: Principles and Practices |publisher=[[Oxford University Press]] |isbn=978-0-19-992022-8 |year=2015 |page=157 |url=https://books.google.com/books?id=TF_iCgAAQBAJ&pg=PA157}}</ref> with a volume of around 1260&nbsp;[[cubic centimetre|cm<sup>3</sup>]] in men and 1130&nbsp;cm<sup>3</sup> in women.<ref name=Cosgrove>{{cite journal |last1=Cosgrove |first1=K.P. |last2=Mazure |first2=C.M. |last3=Staley |first3=J.K. |title=Evolving knowledge of sex differences in brain structure, function, and chemistry |year=2007 |journal=Biol Psychiatry |volume=62 |pages=847–855 |pmid=17544382 |pmc=2711771 |doi=10.1016/j.biopsych.2007.03.001 |issue=8}}</ref> There is substantial individual variation,<ref name=Cosgrove/> with the standard [[reference range]] for men being {{convert|1180-1620|g|lb|abbr=on}}<ref name="MolinaDiMaio2012">{{cite journal|last1=Molina|first1=D. Kimberley|last2=DiMaio|first2=Vincent J.M.|title=Normal Organ Weights in Men|journal=The American Journal of Forensic Medicine and Pathology|volume=33|issue=4|year=2012|pages=368–372|issn=0195-7910|doi=10.1097/PAF.0b013e31823d29ad|pmid=22182984|s2cid=32174574}}</ref> and for women {{convert|1030-1400|g|lb|abbr=on}}.<ref name="MolinaDiMaio2015">{{cite journal|last1=Molina|first1=D. Kimberley|last2=DiMaio|first2=Vincent J. M.|title=Normal Organ Weights in Women|journal=The American Journal of Forensic Medicine and Pathology|volume=36|issue=3|year=2015|pages=182–187|issn=0195-7910|doi=10.1097/PAF.0000000000000175|pmid=26108038|s2cid=25319215}}</ref>

The [[cerebrum]], consisting of the [[cerebral hemisphere]]s, forms the largest part of the brain and overlies the other brain structures.{{sfn|Gray's Anatomy|2008|pp=227-9}} The outer region of the hemispheres, the [[cerebral cortex]], is [[grey matter]], consisting of [[Cerebral cortex#Layers|cortical layers]] of [[neuron]]s. Each hemisphere is divided into four main [[lobes of the brain|lobes]] – the [[frontal lobe]], [[parietal lobe]], [[temporal lobe]], and [[occipital lobe]].{{sfn|Gray's Anatomy|2008|pp=335-7}} Three other lobes are included by some sources which are a ''central lobe'', a [[limbic lobe]], and an [[Insular cortex|insular lobe]].<ref name="Ribas">{{cite journal |page=7 |pmid=20121437|year=2010|last1=Ribas|first1=G. C.|title=The cerebral sulci and gyri|journal=Neurosurgical Focus|volume=28|issue=2|doi=10.3171/2009.11.FOCUS09245|doi-access=free}}</ref> The central lobe comprises the [[precentral gyrus]] and the [[postcentral gyrus]] and is included since it forms a distinct functional role.<ref name="Ribas"/><ref name="Frigeri">{{cite journal |pmid=25555079|year=2015|last1=Frigeri|first1=T.|title=Microsurgical anatomy of the central lobe|journal=Journal of Neurosurgery|volume=122|issue=3|pages=483–98|last2=Paglioli|first2=E.|last3=De Oliveira|first3=E.|last4=Rhoton Jr|first4=A. L.|doi=10.3171/2014.11.JNS14315 }}</ref>

The [[brainstem]], resembling a stalk, attaches to and leaves the cerebrum at the start of the [[midbrain]] area. The brainstem includes the midbrain, the [[pons]], and the [[medulla oblongata]]. Behind the brainstem is the [[cerebellum]] ({{langx |la|little brain}}).{{sfn|Gray's Anatomy|2008|pp=227-9}}

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 more delicate inner [[pia mater]]. Between the arachnoid mater and the pia mater is the [[Meninges#Subarachnoid space|subarachnoid space]] and [[subarachnoid cisterns]], which contain the [[cerebrospinal fluid]].{{sfn|Purves|2012|p=724}} The outermost membrane of the cerebral cortex is the basement membrane of the pia mater called the [[glia limitans]] and is an important part of the [[blood–brain barrier]].<ref name="Anatomy and Ultrastructure">{{Cite book |last1=Cipolla |first1=M.J. |chapter=Anatomy and Ultrastructure |title=The Cerebral Circulation |via=NCBI Bookshelf |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK53086/#s2.2 |publisher=Morgan & Claypool Life Sciences |date=January 1, 2009 |url-status=live |archive-url=https://web.archive.org/web/20171001170945/https://www.ncbi.nlm.nih.gov/books/NBK53086/#s2.2 |archive-date=October 1, 2017  }}</ref> In 2023 a fourth meningeal membrane has been proposed known as the [[subarachnoid lymphatic-like membrane]].<ref name="Fluids barriers">{{cite journal |vauthors=Plá V, Bitsika S, Giannetto MJ, Ladron-de-Guevara A, Gahn-Martinez D, Mori Y, Nedergaard M, Møllgård K |title=Structural characterization of SLYM-a 4th meningeal membrane |journal=Fluids Barriers CNS |volume=20 |issue=1 |pages=93 |date=December 2023 |pmid=38098084 |pmc=10722698 |doi=10.1186/s12987-023-00500-w |doi-access=free |url=}}</ref><ref name="Neuhuber">{{cite journal |vauthors=Neuhuber W |title=An "outer subarachnoid space": fact or artifact? A commentary on "Structural characterization of SLYM- a 4th meningeal membrane" fluids and barriers of the CNS (2023) 20:93 by V. Plá et al |journal=Fluids Barriers CNS |volume=21 |issue=1 |pages=48 |date=June 2024 |pmid=38831302 |pmc=11149175 |doi=10.1186/s12987-024-00539-3 |doi-access=free |url=}}</ref> The living brain is very soft, having a gel-like consistency similar to soft tofu.<ref name="NPR">{{cite news |title=A Surgeon's-Eye View of the Brain |url=https://www.npr.org/templates/story/story.php?storyId=5396115 |website=NPR |series= Fresh Air |date=May 10, 2006 |url-status=live |archive-url=https://web.archive.org/web/20171107023155/http://www.npr.org/templates/story/story.php?storyId=5396115 |archive-date=November 7, 2017  }}</ref> The cortical layers of neurons constitute much of the cerebral [[grey matter]], while the deeper subcortical regions of [[myelin]]ated [[axon]]s, make up the [[white matter]].{{sfn|Gray's Anatomy|2008|pp=227-9}} The white matter of the brain makes up about half of the total brain volume.<ref name="Neuron">{{cite journal |last1=Sampaio-Baptista |first1=C |last2=Johansen-Berg |first2=H |title=White Matter Plasticity in the Adult Brain |doi-access=free |journal=Neuron |date=December 20, 2017 |volume=96 |issue=6 |pages=1239–1251 |doi=10.1016/j.neuron.2017.11.026 |pmid=29268094|pmc=5766826 }}</ref>
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| header            = Structural and functional areas of the human brain

<!--image 1-->| image1            = Sobo 1909 624.png
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| alt1              = A diagram showing various structures within the human brain
| caption1          = Human brain bisected in the [[sagittal plane]], showing the white matter of the corpus callosum

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| alt2              = A diagram of the functional areas of the human brain
| caption2          = Functional areas of the human brain. Dashed areas shown are commonly left hemisphere dominant.
}}

====Cerebrum====
{{Main|Cerebrum|Cerebral cortex}}
[[File:Gray726.png|thumb|Major gyri and sulci on the lateral surface of the cortex]]
[[File:Gehirn, medial - Lobi en.svg|thumb|Lobes of the brain]]

The cerebrum is the largest part of the brain and is divided into nearly [[Symmetry in biology#Bilateral symmetry|symmetrical]] left and right [[cerebral hemisphere|hemisphere]]s by a deep groove, the [[longitudinal fissure]].<ref name="Davey">{{cite book |author=Davey, G. |title=Applied Psychology |isbn=978-1-4443-3121-9 |publisher=[[John Wiley & Sons]] |year=2011 |page=153 |url=https://books.google.com/books?id=K1qq1SsgoxUC&pg=PA153}}</ref> Asymmetry between the lobes is noted as a [[Paleoneurobiology#Asymmetry|petalia]].<ref>{{cite journal|pmid=30601110|year=2019|last1=Arsava|first1=E. Y.|title=Occipital petalia as a predictive imaging sign for transverse sinus dominance|journal=Neurological Research|volume=41|issue=4|pages=306–311|last2=Arsava|first2=E. M.|last3=Oguz|first3=K. K.|last4=Topcuoglu|first4=M. A.|doi=10.1080/01616412.2018.1560643|s2cid=58546404}}</ref> The hemispheres are connected by five [[Commissural fiber#Structure|commissures]] that span the longitudinal fissure, the largest of these is the [[corpus callosum]].{{sfn|Gray's Anatomy|2008|pp=227-9}}
Each hemisphere is conventionally divided into four main [[lobes of the brain|lobes]]; the [[frontal lobe]], [[parietal lobe]], [[temporal lobe]], and [[occipital lobe]], named according to the [[skull |skull bones]] that overlie them.{{sfn|Gray's Anatomy|2008|pp=335-7}} Each lobe is associated with one or two specialised functions though there is some functional overlap between them.<ref name=Ackerman/> The surface of the brain is [[gyrification|folded]] into ridges ([[gyrus|gyri]]) and grooves ([[sulcus (neuroanatomy)|sulci]]), many of which are named, usually according to their position, such as the [[frontal gyrus]] of the frontal lobe or the [[central sulcus]] separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds.{{sfn|Larsen|2001|pp=455–456}}

The outer part of the cerebrum is the [[cerebral cortex]], made up of [[grey matter]] arranged in layers. It is {{convert|2 |to |4 |mm}} thick, and deeply folded to give a convoluted appearance.<ref>{{cite book |last=Kandel |first=E.R. |author2=Schwartz, J.H. |author3=Jessel T.M. |title=Principles of Neural Science |year=2000 |publisher=McGraw-Hill Professional |isbn=978-0-8385-7701-1 |page=[https://archive.org/details/isbn_9780838577011/page/324 324] |url=https://archive.org/details/isbn_9780838577011/page/324 }}</ref> Beneath the cortex is the cerebral [[white matter]]. The largest part of the cerebral cortex is the [[neocortex]], which has six neuronal layers. The rest of the cortex is of [[allocortex]], which has three or four layers.{{sfn|Gray's Anatomy|2008|pp=227–9}}

The cortex is [[brain mapping|mapped]] by divisions into about fifty different functional areas known as [[Brodmann's areas]]. These areas are distinctly different when [[Histology|seen under a microscope]].{{sfn|Guyton & Hall|2011|p=574}} The cortex is divided into two main functional areas – a [[motor cortex]] and a [[sensory cortex]].{{sfn|Guyton & Hall|2011|p=667}} The [[primary motor cortex]], which sends axons down to [[motor neuron]]s in the brainstem and spinal cord, occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The [[primary sensory areas]] receive signals from the [[sensory nerve]]s and [[nerve tract|tracts]] by way of [[Thalamus#Thalamic nuclei|relay nuclei]] in the [[thalamus]]. Primary sensory areas include the [[visual cortex]] of the [[occipital lobe]], the [[auditory cortex]] in parts of the [[temporal lobe]] and [[insular cortex]], and the [[somatosensory cortex]] in the [[parietal lobe]]. The remaining parts of the cortex 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 [[cognition|cognitive processes]] of [[perception]], [[thought]], and [[decision-making]].<ref>Principles of Anatomy and Physiology 12th Edition – Tortora, p. 519.</ref> The main functions of the frontal lobe are to [[Attentional control|control attention]], abstract thinking, behaviour, problem-solving tasks, and physical reactions and personality.<ref name="Freberg">{{cite book |author=Freberg, L. |title=Discovering Biological Psychology |publisher=[[Cengage Learning]] |year=2009 |pages=44–46 |isbn=978-0-547-17779-3 |url=https://books.google.com/books?id=-zyTMXAjzQsC&pg=PA44}}</ref><ref name="Kolb">{{cite book |last1=Kolb |first1=B. |last2=Whishaw |first2=I. |title=Fundamentals of Human Neuropsychology |publisher=[[Macmillan Publishers|Macmillan]] |year=2009 |pages=73–75 |isbn=978-0-7167-9586-5 |url=https://books.google.com/books?id=z0DThNQqdL4C&pg=PA73}}</ref> The occipital lobe is the smallest lobe; its main functions are visual reception, visual-spatial processing, movement, and [[Color vision#Color in the human brain|colour recognition]].<ref name="Freberg"/><ref name="Kolb"/> There is a smaller occipital lobule in the lobe known as the [[cuneus]]. The temporal lobe controls [[Echoic memory|auditory]] and [[visual memory|visual memories]], [[Language processing in the brain|language]], and some hearing and speech.<ref name="Freberg"/>

[[File:Visible Human head slice.jpg|thumb|upright|Cortical folds and white matter in horizontal bisection of head]]

<!--Ventricles and subcortical detail is here.-->The cerebrum contains the [[ventricular system|ventricles]] where the cerebrospinal fluid is produced and circulated. Below the corpus callosum is the [[septum pellucidum]], a membrane that separates the [[lateral ventricles]]. Beneath the lateral ventricles is the [[thalamus]] and to the front and below is the [[hypothalamus]]. The hypothalamus leads on to the [[pituitary gland]]. At the back of the thalamus is the brainstem.{{sfn|Pocock|2006|p=64}}

The [[basal ganglia]], also called basal nuclei, are a set of structures deep within the hemispheres involved in behaviour and movement regulation.{{sfn|Purves|2012|p=399}} The largest component is the [[striatum]], others are the [[globus pallidus]], the [[substantia nigra]] and the [[subthalamic nucleus]].{{sfn|Purves|2012|p=399}} The striatum is divided into a ventral striatum, and dorsal striatum, subdivisions that are based upon function and connections. The ventral striatum consists of the [[nucleus accumbens]] and the [[olfactory tubercle]] whereas the dorsal striatum consists of the [[caudate nucleus]] and the [[putamen]]. The putamen and the globus pallidus lie separated from the lateral ventricles and thalamus by the [[internal capsule]], whereas the caudate nucleus stretches around and abuts the lateral ventricles on their outer sides.{{sfn|Gray's Anatomy|2008|pp=325-6}} At the deepest part of the [[lateral sulcus]] between the [[insular cortex]] and the striatum is a thin neuronal sheet called the [[claustrum]].<ref name="Goll">{{cite journal |last1=Goll |first1=Y. |last2=Atlan |first2=G. |last3=Citri |first3=A. |title=Attention: the claustrum |journal=Trends in Neurosciences |date=August 2015 |volume=38 |issue=8 |pages=486–95 |doi=10.1016/j.tins.2015.05.006 |pmid=26116988|s2cid=38353825 }}</ref>

Below and in front of the striatum are a number of [[basal forebrain]] structures. These include the [[nucleus basalis]], [[diagonal band of Broca]], [[substantia innominata]], and the [[medial septal nucleus]]. These structures are important in producing the [[neurotransmitter]], [[acetylcholine]], which is then distributed widely throughout the brain. The basal forebrain, in particular the nucleus basalis, is considered to be the major [[cholinergic]] output of the central nervous system to the striatum and neocortex.<ref name="Goard">{{cite journal |last1=Goard |first1=M. |last2=Dan |first2=Y. |title=Basal forebrain activation enhances cortical coding of natural scenes |journal=Nature Neuroscience |date=October 4, 2009 |volume=12 |issue=11 |pages=1444–1449 |doi=10.1038/nn.2402|pmid=19801988 |pmc=3576925 }}</ref>

====Cerebellum====
[[File:Sobo 1909 623.png|thumb|upright=1.25|Human brain viewed from below, showing cerebellum and brainstem]]
{{Main|Cerebellum}}

The cerebellum is divided into an [[anterior lobe of cerebellum|anterior lobe]], a [[posterior lobe of cerebellum|posterior lobe]], and the [[flocculonodular lobe]].{{sfn|Guyton & Hall|2011|p=699}} The anterior and posterior lobes are connected in the middle by the [[cerebellar vermis|vermis]].{{sfn|Gray's Anatomy|2008|p=298}} Compared to the cerebral cortex, the cerebellum has a much thinner outer cortex that is narrowly furrowed into numerous curved transverse fissures.{{sfn|Gray's Anatomy|2008|p=298}}
Viewed from underneath between the two lobes is the third lobe the flocculonodular lobe.<ref>{{cite book |last1=Netter |first1=F. |title=Atlas of Human Anatomy Including Student Consult Interactive Ancillaries and Guides. |date=2014 |publisher=W B Saunders Co |location=Philadelphia, Penn. |isbn=978-1-4557-0418-7 |page=114 |edition=6th}}</ref> The cerebellum rests at the back of the [[posterior cranial fossa|cranial cavity]], lying beneath the occipital lobes, and is separated from these by the [[cerebellar tentorium]], a sheet of fibre.{{sfn|Gray's Anatomy|2008|p=297}}

It is connected to the brainstem by three pairs of [[nerve tract]]s called [[cerebellar peduncle]]s. The [[superior cerebellar peduncle|superior pair]] connects to the midbrain; the [[middle cerebellar peduncle|middle pair]] connects to the medulla, and the [[inferior cerebellar peduncle|inferior pair]] connects to the pons.{{sfn|Gray's Anatomy|2008|p=298}} The cerebellum consists of an inner medulla of white matter and an outer cortex of richly folded grey matter.{{sfn|Gray's Anatomy|2008|p=297}} 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 of [[Equilibrioception|balance]]{{sfn|Guyton & Hall|2011|pp=698–9}} although debate exists as to its cognitive, behavioural and motor functions.{{sfn|Squire|2013|pp=761–763}}

====Brainstem====
{{Main|Brainstem}}
The brainstem lies beneath the cerebrum and consists of the [[midbrain]], [[pons]] and [[medulla oblongata|medulla]]. It lies in the [[posterior cranial fossa|back part of the skull]], resting on the part of the [[base of the skull|base]] known as the [[clivus (anatomy)|clivus]], and ends at the [[foramen magnum]], a large [[:wikt:foramen|opening]] in the [[occipital bone]]. The brainstem continues below this as the [[spinal cord]],{{sfn|Gray's Anatomy|2008|p=275}} protected by the [[vertebral column]].

Ten of the twelve pairs of [[cranial nerve]]s{{efn|Specifically the [[oculomotor]], [[trochlear nerve]], [[trigeminal nerve]], [[abducens nerve]], [[facial nerve]], [[vestibulocochlear nerve]], [[glossopharyngeal nerve]], [[vagus nerve]], [[accessory nerve]] and [[hypoglossal nerve]]s.{{sfn|Gray's Anatomy|2008|p=275}}}} emerge directly from the brainstem.{{sfn|Gray's Anatomy|2008|p=275}} The brainstem also contains many [[cranial nerve nucleus|cranial nerve nuclei]] and [[nucleus (neuroanatomy)|nuclei]] of [[nerve|peripheral nerves]], as well as nuclei involved in the regulation of many essential processes including [[breathing]], control of eye movements and balance.{{sfn|Guyton & Hall|2011|p=691}}{{sfn|Gray's Anatomy|2008|p=275}} The [[reticular formation]], a network of nuclei of ill-defined formation, is present within and along the length of the brainstem.{{sfn|Gray's Anatomy|2008|p=275}} Many [[nerve tract]]s, which transmit information to and from the cerebral cortex to the rest of the body, pass through the brainstem.{{sfn|Gray's Anatomy|2008|p=275}}

===Microanatomy===
The human brain is primarily composed of [[neuron]]s, [[glial cell]]s, [[neural stem cell]]s, and [[blood vessel]]s. Types of neuron include [[interneuron]]s, [[pyramidal cell]]s including [[Betz cell]]s, [[motor neuron]]s ([[upper motor neuron|upper]] and [[lower motor neuron]]s), and cerebellar [[Purkinje cell]]s. Betz cells are the largest cells (by size of cell body) in the nervous system.{{sfn|Purves|2012|p=377}} The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells.<ref name=":1" /> Out of these neurons, 16 billion (19%) are located in the cerebral cortex, and 69 billion (80%) are in the cerebellum.<ref name="Bigos"/><ref name=":1">{{cite journal |last1=Azevedo |first1=F. |display-authors=etal |title=Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain |journal=The Journal of Comparative Neurology |date=April 10, 2009 |volume=513 |issue=5 |pages=532–541 |doi=10.1002/cne.21974 |quote=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. |pmid=19226510|s2cid=5200449 }}</ref>

Types of glial cell are [[astrocyte]]s (including [[Bergmann glia]]), [[oligodendrocyte]]s, [[ependymal cell]]s (including [[tanycyte]]s), [[radial glial cell]]s, [[microglia]], and a subtype of [[oligodendrocyte progenitor cell]]s. Astrocytes are the largest of the glial cells. They are [[stellate cell]]s with many processes radiating from their [[soma (biology)|cell bodies]]. Some of these processes end as perivascular [[endfeet]] on [[capillary]] walls.<ref>{{Cite book |last1=Pavel |first1=Fiala |last2=Jiří |first2=Valenta |title=Central Nervous System |url=https://books.google.com/books?id=LPlSBAAAQBAJ&pg=PA79 |publisher=Karolinum Press |page=79 |date=January 1, 2013|isbn=978-80-246-2067-1 }}</ref> The [[glia limitans]] of the cortex is made up of [[Astrocyte endfeet|astrocyte endfeet processes]] that serve in part to contain the cells of the brain.<ref name="Anatomy and Ultrastructure"/>

[[Mast cell]]s are [[white blood cell]]s that interact in the [[neuroimmune system]] in the brain.<ref name="Mast cell neuroimmmune system">{{cite journal | last1=Polyzoidis |first1=S. |last2=Koletsa |first2=T. |last3=Panagiotidou |first3=S. |last4=Ashkan |first4=K. |last5=Theoharides |first5=T.C. | title=Mast cells in meningiomas and brain inflammation | journal=Journal of Neuroinflammation | volume=12 | issue=1 | page=170 | year=2015 | pmid=26377554 | pmc=4573939 | doi=10.1186/s12974-015-0388-3 |doi-access=free }}</ref> Mast cells in the central nervous system are present in [[Mast cell#In the nervous system|a number of structures]] including the meninges;<ref name="Mast cell neuroimmmune system" /> they mediate neuroimmune responses in inflammatory conditions and help to maintain the blood–brain barrier, particularly in brain regions where the barrier is absent.<ref name="Mast cell neuroimmmune system" />{{sfn|Guyton & Hall|2011|pp=748–749}} Mast cells serve the same general functions in the body and central nervous system, such as effecting or regulating allergic responses, [[Innate immune system|innate]] and [[Adaptive immune system|adaptive immunity]], [[autoimmunity]], and [[inflammation]].<ref name="Mast cell neuroimmmune system" /> Mast cells serve as the main [[effector cell]] through which pathogens can affect the [[gut-brain axis|biochemical signaling that takes place between the gastrointestinal tract and the central nervous system]].<ref name="pmid24833851">{{cite journal | last1=Budzyński |first1=J |last2=Kłopocka |first2=M. | title=Brain-gut axis in the pathogenesis of Helicobacter pylori infection | journal=World J. Gastroenterol. | volume=20 | issue=18 | pages=5212–25 | year=2014 | pmid=24833851 | pmc=4017036 | doi=10.3748/wjg.v20.i18.5212 |doi-access=free }}</ref><ref name="Microbiome-CNS-ENS">{{cite journal | last1=Carabotti |first1=M. |last2=Scirocco |first2=A. |last3=Maselli |first3=M.A. |last4=Severi |first4=C. | title=The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems | journal=Ann Gastroenterol | volume=28 | issue=2 | pages=203–209 | year=2015 | pmid=25830558 | pmc=4367209}}</ref>

Some 400 [[gene]]s are shown to be brain-specific. In all neurons, [[ELAVL3]] is expressed, and in pyramidal cells, [[NRGN]] and [[REEP2]] are also expressed. [[GAD1]] – essential for the biosynthesis of the neurotransmitter [[GABA]] – is expressed in interneurons. Proteins expressed in glial cells include astrocyte markers [[Glial fibrillary acidic protein|GFAP]] and [[S100B]] whereas [[myelin basic protein]] and the transcription factor [[OLIG2]] are expressed in oligodendrocytes.<ref>{{Cite journal|last1=Sjöstedt|first1=Evelina|last2=Fagerberg|first2=Linn|last3=Hallström|first3=Björn M.|last4=Häggmark|first4=Anna|last5=Mitsios|first5=Nicholas|last6=Nilsson|first6=Peter|last7=Pontén|first7=Fredrik|last8=Hökfelt|first8=Tomas|last9=Uhlén|first9=Mathias|date=June 15, 2015|title=Defining the human brain proteome using transcriptomics and antibody-based profiling with a focus on the cerebral cortex|journal=PLOS ONE |volume=10|issue=6|page=e0130028 |doi=10.1371/journal.pone.0130028|pmid=26076492 |pmc=4468152|issn=1932-6203|bibcode=2015PLoSO..1030028S|doi-access=free}}</ref>

===Cerebrospinal fluid===
[[Image:Blausen 0216 CerebrospinalSystem.png|thumb|[[Cerebrospinal fluid]] circulates in spaces around and within the brain]]
{{Main|Cerebrospinal fluid}}
Cerebrospinal fluid is a clear, colourless [[transcellular fluid]] that circulates around the brain in the [[subarachnoid space]], in the [[ventricular system]], and in the [[central canal]] of the spinal cord. It also fills some gaps in the subarachnoid space, known as [[subarachnoid cisterns]].{{sfn|Gray's Anatomy|2008|pp=242–244}} The four ventricles, two [[lateral ventricle|lateral]], a [[third ventricle|third]], and a [[fourth ventricle]], all contain a [[choroid plexus]] that produces cerebrospinal fluid.{{sfn|Purves|2012|p=742}} The third ventricle lies in the midline and [[Interventricular foramina (neuroanatomy)|is connected]] to the lateral ventricles.{{sfn|Gray's Anatomy|2008|pp=242–244}} A single [[duct (anatomy)|duct]], the [[cerebral aqueduct]] between the pons and the cerebellum, connects the third ventricle to the fourth ventricle.{{sfn|Gray's Anatomy|2008|p=243}} Three separate openings, the [[Medial aperture|middle]] and two [[lateral aperture]]s, drain the cerebrospinal fluid from the fourth ventricle to the [[cisterna 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.{{sfn|Gray's Anatomy|2008|pp=242–244}}
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.{{sfn|Gray's Anatomy|2008|pp=242–244}}

A [[glymphatic system]] has been described as the lymphatic drainage system of the brain.<ref name="Yankova2021">{{cite journal |last1=Yankova |first1=Galina |last2=Bogomyakova |first2=Olga |last3=Tulupov |first3=Andrey |title=The glymphatic system and meningeal lymphatics of the brain: new understanding of brain clearance |journal=Reviews in the Neurosciences |date=1 November 2021 |volume=32 |issue=7 |pages=693–705 |doi=10.1515/revneuro-2020-0106|pmid=33618444 }}</ref><ref name="Glymphatic system and brain waste clearance 2017 review" /> The brain-wide glymphatic pathway includes drainage routes from the cerebrospinal fluid, and from the [[meningeal lymphatic vessels]] that are associated with the [[dural sinuses]], and run alongside the cerebral blood vessels.<ref name="D-O">{{cite journal|last1=Dissing-Olesen|first1=L.|last2=Hong|first2=S. |last3=Stevens|first3=B. |title=New brain lymphatic vessels drain old concepts |journal=eBioMedicine |date=August 2015|volume=2|issue=8|pages=776–7|doi=10.1016/j.ebiom.2015.08.019|pmid=26425672|pmc=4563157}}</ref><ref name="Sun">{{cite journal |last1=Sun |first1=BL |last2=Wang |first2=LH |last3=Yang |first3=T |last4=Sun |first4=JY |last5=Mao |first5=LL |last6=Yang |first6=MF |last7=Yuan |first7=H |last8=Colvin |first8=RA |last9=Yang |first9=XY |title=Lymphatic drainage system of the brain: A novel target for intervention of neurological diseases. |journal=Progress in Neurobiology |date=April 2018 |volume=163–164 |pages=118–143 |doi=10.1016/j.pneurobio.2017.08.007 |pmid=28903061|s2cid=6290040 }}</ref> The pathway drains [[interstitial fluid]] from the tissue of the brain.<ref name="Sun"/>

===Blood supply===
{{Main|Cerebral circulation}}
[[File:Circle of Willis en.svg|thumb|upright|Two circulations joining at the circle of Willis (inferior view)]]
[[File:Gray769-en.svg|thumb|Diagram showing features of cerebral [[meninges|outer membranes]] and supply of blood vessels]]
<!-- Arteries -->The [[internal carotid arteries]] supply [[Blood#Oxygen transport|oxygenated blood]] to the front of the brain and the [[vertebral arteries]] supply blood to the back of the brain.{{sfn|Gray's Anatomy|2008|p=247}} These two circulations [[anastomosis|join]] in the [[circle of Willis]], a ring of connected arteries that lies in the [[interpeduncular cistern]] between the midbrain and pons.{{sfn|Gray's Anatomy|2008|pp=251-2}}

The internal carotid arteries are branches of the [[common carotid arteries]]. They enter the [[cranium]] through the [[carotid canal]], travel through the [[cavernous sinus]] and enter the [[subarachnoid space]].{{sfn|Gray's Anatomy|2008|p=250}} They then enter the [[circle of Willis]], with two branches, the [[anterior cerebral arteries]] emerging. These branches travel forward and then upward along the [[longitudinal fissure]], and supply the front and midline parts of the brain.{{sfn|Gray's Anatomy|2008|p=248}} One or more small [[anterior communicating artery|anterior communicating arteries]] join the two anterior cerebral arteries shortly after they emerge as branches.{{sfn|Gray's Anatomy|2008|p=248}} The internal carotid arteries continue forward as the [[middle cerebral arteries]]. They travel sideways along the [[sphenoid bone]] of the [[orbit (anatomy)|eye socket]], then upwards through the [[insula cortex]], where final branches arise. The middle cerebral arteries send branches along their length.{{sfn|Gray's Anatomy|2008|p=250}}

The vertebral arteries emerge as branches of the left and right [[subclavian arteries]]. They travel upward through [[Vertebra#Cervical vertebrae|transverse foramina]] which are spaces in the [[cervical vertebrae]]. Each side enters the cranial cavity through the foramen magnum along the corresponding side of the medulla.{{sfn|Gray's Anatomy|2008|p=250}} They give off [[Posterior inferior cerebellar artery|one of the three cerebellar branches]]. The vertebral arteries join in front of the middle part of the medulla to form the larger [[basilar artery]], which sends multiple branches to supply the medulla and pons, and the two other [[Anterior inferior cerebellar artery|anterior]] and [[Superior cerebellar artery|superior cerebellar branches]].{{sfn|Gray's Anatomy|2008|p=251}} Finally, the basilar artery divides into two [[posterior 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.{{sfn|Gray's Anatomy|2008|p=251}} Each posterior cerebral artery sends a small [[posterior communicating artery]] to join with the internal carotid arteries.

====Blood drainage====
<!--Veins-->
[[Cerebral veins]] drain [[Blood#Oxygen transport|deoxygenated blood]] from the brain. The brain has two main networks of [[vein]]s: an exterior or [[Superior cerebral veins|superficial network]], on the surface of the cerebrum that has three branches, and an [[Internal cerebral veins|interior network]]. These two networks communicate via [[anastomosis|anastomosing]] (joining) veins.{{sfn|Gray's Anatomy|2008|pp=254-6}} The veins of the brain drain into larger cavities of the [[dural venous sinuses]] usually situated between the dura mater and the covering of the skull.{{sfn|Elsevier's|2007|pp=311–4}} Blood from the cerebellum and midbrain drains into the [[great cerebral vein]]. Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into the [[spinal veins]] or into adjacent cerebral veins.{{sfn|Gray's Anatomy|2008|pp=254-6}}

The blood in the [[Anatomical terms of location#deep|deep]] part of the brain drains, through a [[venous plexus]] into the [[cavernous sinus]] at the front, and the [[superior petrosal sinus|superior]] and [[inferior petrosal sinus]]es at the sides, and the [[inferior sagittal sinus]] at the back.{{sfn|Elsevier's|2007|pp=311–4}} Blood drains from the outer brain into the large [[superior sagittal sinus]], which rests in the midline on top of the brain. Blood from here joins with blood from the [[straight sinus]] at the [[confluence of sinuses]].{{sfn|Elsevier's|2007|pp=311–4}}

Blood from here drains into the left and right [[transverse sinus]]es.{{sfn|Elsevier's|2007|pp=311–4}} These then drain into the [[sigmoid sinus]]es, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large [[internal jugular vein]]s.{{sfn|Elsevier's|2007|pp=311–4}}{{sfn|Gray's Anatomy|2008|pp=254-6}}

====The blood–brain barrier====
The larger arteries throughout the brain supply blood to smaller [[capillaries]]. These smallest of [[blood vessel]]s in the brain, are lined with cells joined by [[tight junction]]s and so fluids do not seep in or leak out to the same degree as they do in other capillaries; this creates the [[blood–brain barrier]].{{sfn|Guyton & Hall|2011|pp=748–749}} [[Pericyte]]s play a major role in the formation of the tight junctions.<ref name="Daneman">{{cite journal |last1=Daneman |first1=R. |last2=Zhou |first2=L. |last3=Kebede |first3=A.A. |last4=Barres |first4=B.A. |title=Pericytes are required for blood-brain barrier integrity during embryogenesis |journal=Nature |date=November 25, 2010 |volume=468 |issue=7323 |pages=562–6 |pmid=20944625 |doi=10.1038/nature09513 |pmc=3241506|bibcode=2010Natur.468..562D }}</ref> The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including [[anaesthetic]]s and alcohol).{{sfn|Guyton & Hall|2011|pp=748–749}} The blood-brain barrier is not present in the [[circumventricular organs]]{{Em dash}}which are structures in the brain that may need to respond to changes in body fluids{{Em dash}}such as the [[pineal gland]], [[area postrema]], and some areas of the [[hypothalamus]].{{sfn|Guyton & Hall|2011|pp=748–749}} There is a similar [[Choroid plexus#Function|blood–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.{{sfn|Guyton & Hall|2011|pp=748–749}}<ref name="BCSF">{{cite book |last1=Laterra |first1=J. |last2=Keep |first2=R. |last3=Betz |first3=L.A. |title=Basic neurochemistry: molecular, cellular and medical aspects |date=1999 |publisher=Lippincott-Raven |location=Philadelphia |edition=6th |section-url=https://www.ncbi.nlm.nih.gov/books/NBK27998/ |section=Blood–cerebrospinal fluid barrier |display-authors=etal}}</ref>