Editing Brain (section)

== Physiology ==
The functions of the brain depend on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells. The [[membrane potential|electrical properties]] of neurons are controlled by a wide variety of biochemical and metabolic processes, most notably the interactions between neurotransmitters and receptors that take place at synapses.<ref name="Kandel 2000"/><!--Ch. 10-->

=== Neurotransmitters and receptors ===
[[Neurotransmitter]]s are chemicals that are released at synapses when the local membrane is depolarised and [[Calcium|Ca<sup>2+</sup>]] enters into the cell, typically when an action potential arrives at the synapse – neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell (or cells), and thereby alter the electrical or chemical properties of the receptor molecules. With few exceptions, each neuron in the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as [[Dale's principle]].<ref name="Kandel 2000"/><!--Ch 15--> Thus, a neuron can be characterized by the neurotransmitters that it releases. The great majority of [[psychoactive drug]]s exert their effects by altering specific neurotransmitter systems. This applies to drugs such as [[cannabinoids]], [[nicotine]], [[heroin]], [[cocaine]], [[Alcohol (drug)|alcohol]], [[fluoxetine]], [[chlorpromazine]], and many others.<ref>{{cite book |last1=Cooper |first1=JR |last2=Bloom |first2=FE |last3=Roth |first3=RH |title=The Biochemical Basis of Neuropharmacology |publisher=Oxford University Press US |year=2003 |isbn=978-0-19-514008-8 |url=https://books.google.com/books?id=e5I5gOwxVMkC}}</ref>

The two neurotransmitters that are most widely found in the vertebrate brain are [[glutamic acid|glutamate]], which almost always exerts excitatory effects on target neurons, and [[gamma-aminobutyric acid]] (GABA), which is almost always inhibitory. Neurons using these transmitters can be found in nearly every part of the brain.<ref>{{cite book|last1=McGeer|first1=PL|title=Basic Neurochemistry|last2=McGeer|first2=EG|publisher=Raven Press|year=1989|isbn=978-0-88167-343-2|editor=G. Siegel|location=University of Michigan|pages=311–332|chapter=Chapter 15, ''Amino acid neurotransmitters''|display-editors=etal}}</ref> Because of their ubiquity, drugs that act on glutamate or GABA tend to have broad and powerful effects. Some [[General anaesthetic|general anesthetics]] act by reducing the effects of glutamate; most tranquilizers exert their sedative effects by enhancing the effects of GABA.<ref>{{cite journal |title=Glutamate- and GABA-based CNS therapeutics |journal=Current Opinion in Pharmacology |volume=6 |pages=7–17 |year=2006 |pmid=16377242 |doi=10.1016/j.coph.2005.11.005 |last1=Foster |first1=AC |last2=Kemp |first2=JA |issue=1}}</ref>

There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. [[Serotonin]], for example—the primary target of many [[Antidepressant|antidepressant drugs]] and many dietary aids—comes exclusively from a small brainstem area called the [[raphe nuclei]].<ref>{{cite book |last1=Frazer |first1=A |last2=Hensler |first2=JG |editor=Siegel, GJ |title=Basic Neurochemistry |edition=Sixth |year=1999 |publisher=Lippincott Williams & Wilkins |isbn=978-0-397-51820-3 |chapter=Understanding the neuroanatomical organization of serotonergic cells in the brain provides insight into the functions of this neurotransmitter}}</ref> [[Norepinephrine]], which is involved in arousal, comes exclusively from a nearby small area called the [[locus coeruleus]].<ref name=Mehler>{{cite journal |year=2009 |title=Autism, fever, epigenetics and the locus coeruleus |journal=[[Brain Research Reviews]] |volume=59 |pages=388–392 |pmc=2668953 |doi=10.1016/j.brainresrev.2008.11.001 |last1=Mehler |first1=MF |last2=Purpura |first2=DP |issue=2 |pmid=19059284}}</ref> Other neurotransmitters such as [[acetylcholine]] and [[dopamine]] have multiple sources in the brain but are not as ubiquitously distributed as glutamate and GABA.<ref>{{cite book |last=Rang |first=HP |title=Pharmacology |publisher=Churchill Livingstone |year=2003 |pages=476–483 |isbn=978-0-443-07145-4}}</ref>

=== Electrical activity ===
[[File:Spike-waves.png|thumb|right|alt=Graph showing 16 voltage traces going across the page from left to right, each showing a different signal. At the middle of the page all of the traces abruptly begin to show sharp jerky spikes, which continue to the end of the plot.|Brain electrical activity recorded from a human patient during an [[Epilepsy|epileptic seizure]]]]

As a side effect of the electrochemical processes used by neurons for signaling, brain tissue generates electric fields when it is active. When large numbers of neurons show synchronized activity, the electric fields that they generate can be large enough to detect outside the skull, using [[electroencephalography]] (EEG)<ref name = "Niedermeyer">{{cite book |last1=Speckmann |first1=E-J |last2=Elger |first2=CE |chapter=Introduction to the neurophysiological basis of the EEG and DC potentials |veditors=Niedermeyer E, Lopes da Silva FH |title=Electroencephalography: Basic Principles, Clinical Applications, and Related Fields |publisher=Lippincott Williams & Wilkins |year=2004 |isbn=978-0-7817-5126-1 |pages=17–31}}</ref> or [[magnetoencephalography]] (MEG). EEG recordings, along with recordings made from electrodes implanted inside the brains of animals such as rats, show that the brain of a living animal is constantly active, even during sleep.<ref name="Buzsaki">{{cite book|last=Buzsáki|first=Gyorgy |title=Rhythms of the Brain|publisher=Oxford University Press|year=2006|isbn=9780199828234}}</ref> Each part of the brain shows a mixture of rhythmic and nonrhythmic activity, which may vary according to behavioral state. In mammals, the cerebral cortex tends to show large slow [[delta wave]]s during sleep, faster [[alpha wave]]s when the animal is awake but inattentive, and chaotic-looking irregular activity when the animal is actively engaged in a task, called [[Beta wave|beta]] and [[gamma wave]]s. During an [[epilepsy|epileptic seizure]], the brain's inhibitory control mechanisms fail to function and electrical activity rises to pathological levels, producing EEG traces that show large wave and spike patterns not seen in a healthy brain. Relating these population-level patterns to the computational functions of individual neurons is a major focus of current research in [[neurophysiology]].<ref name=Buzsaki/>

=== Metabolism ===
All vertebrates have a [[blood–brain barrier]] that allows metabolism inside the brain to operate differently from metabolism in other parts of the body. The [[neurovascular unit]] regulates cerebral blood flow so that activated neurons can be supplied with energy. [[Neuroglia|Glial cells]] play a major role in brain metabolism by controlling the chemical composition of the fluid that surrounds neurons, including levels of ions and nutrients.<ref name=Nieuwenhuys/>

Brain tissue consumes a large amount of energy in proportion to its volume, so large brains place severe metabolic demands on animals. The need to limit body weight in order, for example, to fly, has apparently led to selection for a reduction of brain size in some species, such as [[bats]].<ref>{{cite journal |last1=Safi |first1=K |year=2005 |title=Bigger is not always better: when brains get smaller |journal=Biology Letters |volume=1 |pages=283–286 |pmid=17148188 |doi=10.1098/rsbl.2005.0333 |last2=Seid |first2=MA |last3=Dechmann |first3=DK |pmc=1617168 |issue=3}}</ref> Most of the brain's energy consumption goes into sustaining the electric charge ([[membrane potential]]) of neurons.<ref name=Nieuwenhuys>{{cite book |title=The Central Nervous System of Vertebrates, Volume 1 |last1=Nieuwenhuys |first1=R |last2=Donkelaar |first2=HJ |last3=Nicholson |first3=C |publisher=Springer |year=1998 |isbn=978-3-540-56013-5 |pages=11–14}}</ref> Most vertebrate species devote between 2% and 8% of basal metabolism to the brain. In primates, however, the percentage is much higher—in humans it rises to 20–25%.<ref>{{cite journal|last1=Mink|first1=JW|last2=Blumenschine|first2=RJ|last3=Adams|first3=DB|year=1981|title=Ratio of central nervous system to body metabolism in vertebrates: its constancy and functional basis|url=https://works.bepress.com/david-adams/3|journal=American Journal of Physiology|type=Submitted manuscript|volume=241|issue=3|pages=R203–212|doi=10.1152/ajpregu.1981.241.3.R203|pmid=7282965|access-date=2021-02-10|archive-date=2020-08-17|archive-url=https://web.archive.org/web/20200817190752/https://works.bepress.com/david-adams/3/|url-status=dead}}</ref> The energy consumption of the brain does not vary greatly over time, but active regions of the cerebral cortex consume somewhat more energy than inactive regions; this forms the basis for the functional brain imaging methods of [[Brain positron emission tomography|PET]], [[fMRI]],<ref>{{cite journal |last1=Raichle |first1=M |year=2002 |title=Appraising the brain's energy budget |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=99 |pages=10237–10239 |doi=10.1073/pnas.172399499 |pmid=12149485 |last2=Gusnard |first2=DA |pmc=124895 |issue=16 |bibcode=2002PNAS...9910237R|doi-access=free }}</ref> and [[Near infrared spectroscopy|NIRS]].<ref>{{cite journal|last1=Mehagnoul-Schipper|first1=DJ|last2=Van Der Kallen|first2=BF|last3=Colier|first3=WNJM|last4=Van Der Sluijs|first4=MC|last5=Van Erning|first5=LJ|last6=Thijssen|first6=HO|last7=Oeseburg|first7=B|last8=Hoefnagels|first8=WH|last9=Jansen|first9=RW|year=2002|title=Simultaneous measurements of cerebral oxygenation changes during brain activation by near-infrared spectroscopy and functional magnetic resonance imaging in healthy young and elderly subjects |journal=Hum Brain Mapp|volume=16|issue=1|pages=14–23|doi=10.1002/hbm.10026|pmc=6871837|pmid=11870923}}</ref> The brain typically gets most of its energy from oxygen-dependent metabolism of [[glucose]] (i.e., blood sugar),<ref name=Nieuwenhuys/> but [[ketone bodies|ketones]] provide a major alternative source, together with contributions from medium chain [[fatty acid]]s ([[Caprylic acid|caprylic]] and [[Heptanoic acid|heptanoic]] acids),<ref name="Ebert 2003">{{Cite journal |last1=Ebert |first1=D. |last2=Haller |first2=RG. |last3=Walton |first3=ME. |title=Energy contribution of octanoate to intact rat brain metabolism measured by 13C nuclear magnetic resonance spectroscopy |journal=J Neurosci |volume=23 |issue=13 |pages=5928–5935 |date=Jul 2003 |pmid=12843297 |pmc=6741266 |doi=10.1523/JNEUROSCI.23-13-05928.2003 }}</ref><ref name="Marin Valencia 2013">{{Cite journal |last1=Marin-Valencia |first1=I. |last2=Good |first2=LB. |last3=Ma |first3=Q. |last4=Malloy |first4=CR. |last5=Pascual |first5=JM. |title=Heptanoate as a neural fuel: energetic and neurotransmitter precursors in normal and glucose transporter I-deficient (G1D) brain |journal=J Cereb Blood Flow Metab |volume=33 |issue=2 |pages=175–182 |date=Feb 2013 |doi=10.1038/jcbfm.2012.151 |pmid=23072752 |pmc=3564188}}</ref> [[lactic acid|lactate]],<ref name="Boumezbeur 2010">{{Cite journal |last1=Boumezbeur |first1=F. |last2=Petersen |first2=KF. |last3=Cline |first3=GW. |last4=Mason |first4=GF. |last5=Behar |first5=KL. |last6=Shulman |first6=GI. |last7=Rothman |first7=DL. |title=The contribution of blood lactate to brain energy metabolism in humans measured by dynamic 13C nuclear magnetic resonance spectroscopy |journal=J Neurosci |volume=30 |issue=42 |pages=13983–13991 |date=Oct 2010 |doi=10.1523/JNEUROSCI.2040-10.2010 |pmid=20962220 |pmc=2996729}}</ref> [[acetate]],<ref name="Deelchand 2009">{{Cite journal |last1=Deelchand |first1=DK. |last2=Shestov |first2=AA. |last3=Koski |first3=DM. |last4=Uğurbil |first4=K. |last5=Henry |first5=PG. |title=Acetate transport and utilization in the rat brain |journal=J Neurochem |volume=109 |pages=46–54 |date=May 2009 |doi=10.1111/j.1471-4159.2009.05895.x |pmid=19393008 |pmc=2722917 |issue=Suppl 1}}</ref> and possibly [[amino acid]]s.<ref>{{cite journal |last1=Soengas |first1=JL |last2=Aldegunde |first2=M |title=Energy metabolism of fish brain |journal=Comparative Biochemistry and Physiology B |year=2002 |volume=131 |pages=271–296 |pmid=11959012 |doi=10.1016/S1096-4959(02)00022-2 |issue=3}}</ref>