Editing Brain (section)

=== 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>