Editing Brain (section)

=== Cellular structure ===
[[File:Chemical synapse schema cropped.jpg|thumb|326x326px|alt=drawing showing a neuron with a fiber emanating from it labeled "axon" and making contact with another cell. An inset shows an enlargement of the contact zone.|[[Neuron]]s generate [[Action potential|electrical signals]] that travel along their [[axon]]s. When an electrical impulse reaches a junction called a [[synapse]], it causes a [[neurotransmitter]] to be released, which binds to [[Receptor (biochemistry)|receptors]] on other cells and thereby alters their electrical activity.]]

The brains of all species are composed primarily of two broad classes of [[brain cell]]s: [[neuron]]s and [[neuroglia|glial cells]]. Glial cells (also known as ''glia'' or ''neuroglia'') come in several types, and perform a number of critical functions, including structural support, metabolic support, insulation, and guidance of development. Neurons, however, are usually considered the most important cells in the brain.<ref name="Kandel 2000">{{Cite book|last1=Kandel|first1=Eric R.|url=https://archive.org/details/isbn_9780838577011|title=Principles of neural science|last2=Schwartz|first2=James Harris|last3=Jessell|first3=Thomas M.|date=2000|publisher=McGraw-Hill|isbn=978-0-8385-7701-1|edition=4th|location=New York |oclc=42073108}}</ref><!--p. 20--> In humans, the [[cerebral cortex]] contains approximately 14–16 billion neurons,<ref name="Saladin11"/> and the estimated number of neurons in the [[cerebellum]] is 55–70 billion.<ref name="JCN"/> Each neuron is connected by [[synapse]]s to several thousand other neurons. The property that makes neurons unique is their ability to send signals to specific target cells, sometimes over long distances.<ref name="Kandel 2000"/><!--p. 21--> They send these signals by means of an [[axon]], which is a thin protoplasmic fiber that extends from the cell body and projects, usually with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body. The length of an axon can be extraordinary: for example, if a [[pyramidal cell]] (an excitatory neuron) of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon, equally magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.<ref>{{cite journal |title=Neuronal circuits of the neocortex |year=2004 |volume=27 |pages=419–451 |pmid=15217339 |last1=Douglas |first1=RJ |last2=Martin |first2=KA |doi=10.1146/annurev.neuro.27.070203.144152 |journal=Annual Review of Neuroscience}}</ref> These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials constantly, at rates of 10–100 per second, usually in irregular patterns; other neurons are quiet most of the time, but occasionally emit a burst of action potentials.<ref>{{cite journal |title=The action potential |journal=Practical Neurology |volume=7 |pages=192–197 |year=2007 |pmid=17515599 |last1=Barnett |first1=MW |last2=Larkman |first2=PM |issue=3}}</ref>

Axons transmit signals to other neurons by means of specialized junctions called [[synapse]]s. A single axon may make as many as several thousand synaptic connections with other cells.<ref name="Kandel 2000"/><!--ch.10 p. 175--> When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a [[neurotransmitter]] to be released. The neurotransmitter binds to [[receptor (biochemistry)|receptor]] molecules in the membrane of the target cell.<ref name="Kandel 2000"/><!--Ch.10 -->

Synapses are the key functional elements of the brain.<ref name="ShepherdSOB">{{cite book|last=Shepherd|first=Gordon M.|url=https://books.google.com/books?id=rfcRDAAAQBAJ|title=The Synaptic Organization of the Brain|publisher=Oxford University Press US|year=2004|isbn=978-0-19-515956-1|edition=5th|location=New York, New York |chapter=1. Introduction to synaptic circuits}}</ref> The essential function of the brain is [[cell–cell interaction|cell-to-cell communication]], and synapses are the points at which communication occurs. The human brain has been estimated to contain approximately 100 trillion synapses;<ref>{{cite journal |last1=Williams |first1=RW |last2=Herrup |first2=K |title=The control of neuron number |journal=Annual Review of Neuroscience |volume=11 |pages=423–453 |year=1988 |pmid=3284447 |doi=10.1146/annurev.ne.11.030188.002231}}</ref> even the brain of a fruit fly contains several million.<ref>{{cite journal |last=Heisenberg |first=M |title=Mushroom body memoir: from maps to models |journal=Nature Reviews Neuroscience |volume=4 |pages=266–275 |year=2003 |pmid=12671643 |doi=10.1038/nrn1074 |issue=4|s2cid=5038386 }}</ref> The functions of these synapses are very diverse: some are excitatory (exciting the target cell); others are inhibitory; others work by activating [[second messenger system]]s that change the internal [[chemistry]] of their target cells in complex ways.<ref name=ShepherdSOB/> A large number of synapses are dynamically modifiable; that is, they are capable of changing strength in a way that is controlled by the patterns of signals that pass through them. It is widely believed that [[synaptic plasticity|activity-dependent modification of synapses]] is the brain's primary mechanism for learning and memory.<ref name=ShepherdSOB/>

Most of the space in the brain is taken up by axons, which are often bundled together in what are called ''nerve fiber tracts''. A myelinated axon is wrapped in a fatty insulating sheath of [[myelin]], which serves to greatly increase the speed of signal propagation. (There are also unmyelinated axons). Myelin is white, making parts of the brain filled exclusively with nerve fibers appear as light-colored [[white matter]], in contrast to the darker-colored [[grey matter]] that marks areas with high densities of neuron cell bodies.<ref name="Kandel 2000"/><!--Ch.2-->