Editing Cerebellum (section)

==Comparative anatomy and evolution==
[[File:Porbeagle shark brain.png|thumb|right|upright=1.15|Cross-section of the brain of a [[porbeagle shark]], with the cerebellum highlighted in blue]]
The circuits in the cerebellum are similar across all [[class (biology)|class]]es of [[vertebrate]]s, including fish, reptiles, birds, and mammals.<ref name=Bell/> There is also an analogous brain structure in [[cephalopod]]s with well-developed brains, such as [[octopus]]es.<ref>{{cite journal | vauthors = Woodhams PL | title = The ultrastructure of a cerebellar analogue in octopus | journal = Journal of Comparative Neurology | volume = 174 | issue = 2 | pages = 329–45 | date = July 1977 | pmid = 864041 | doi = 10.1002/cne.901740209 | s2cid = 43112389 }}</ref> This has been taken as evidence that the cerebellum performs functions important to all animal [[species]] with a brain.

There is considerable variation in the size and shape of the cerebellum in different vertebrate species. In [[amphibian]]s, it is little developed, and in [[lamprey]]s, and [[hagfish]], the cerebellum is barely distinguishable from the brain-stem. Although the spinocerebellum is present in these groups, the primary structures are small, paired-nuclei corresponding to the vestibulocerebellum.<ref name=VB/> The cerebellum is a bit larger in reptiles, considerably larger in birds, and larger still in mammals. The large paired and convoluted lobes found in humans are typical of mammals, but the cerebellum is, in general, a single median lobe in other groups, and is either smooth or only slightly grooved. In mammals, the neocerebellum is the major part of the cerebellum by mass, but, in other vertebrates, it is typically the spinocerebellum.<ref name=VB>{{cite book |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia|page= 531|isbn=978-0-03-910284-5 |vauthors=Romer AS, Parsons TS }}</ref>

The cerebellum of [[cartilaginous fish|cartilaginous]] and [[bony fish]]es is extraordinarily large and complex. In at least one important respect, it differs in internal structure from the mammalian cerebellum: The fish cerebellum does not contain discrete [[deep cerebellar nuclei]]. Instead, the primary targets of Purkinje cells are a distinct type of cell distributed across the cerebellar cortex, a type not seen in mammals. In [[mormyridae|mormyrid fish]] (a family of weakly electrosensitive freshwater fish), the cerebellum is considerably larger than the rest of the brain. The largest part of it is a special structure called the ''valvula'', which has an unusually regular architecture and receives much of its input from the electrosensory system.<ref>{{cite journal | vauthors = Shi Z, Zhang Y, Meek J, Qiao J, Han VZ | title = The neuronal organization of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish | journal = Journal of Comparative Neurology | volume = 509 | issue = 5 | pages = 449–73 | date = August 2008 | pmid = 18537139 | pmc = 5884697 | doi = 10.1002/cne.21735 }}</ref>

The hallmark of the mammalian cerebellum is an expansion of the lateral lobes, whose main interactions are with the neocortex. As monkeys evolved into great apes, the expansion of the lateral lobes continued, in tandem with the expansion of the frontal lobes of the neocortex. In ancestral hominids, and in ''[[Homo sapiens]]'' until the middle [[Pleistocene]] period, the cerebellum continued to expand, but the frontal lobes expanded more rapidly. The most recent period of human evolution, however, may actually have been associated with an increase in the relative size of the cerebellum, as the neocortex reduced its size somewhat while the cerebellum expanded.<ref>{{cite journal | vauthors = Weaver AH | title = Reciprocal evolution of the cerebellum and neocortex in fossil humans | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 10 | pages = 3576–80 | date = March 2005 | pmid = 15731345 | pmc = 553338 | doi = 10.1073/pnas.0500692102 | bibcode = 2005PNAS..102.3576W | doi-access = free }}</ref> The size of the human cerebellum, compared to the rest of the brain, has been increasing in size while the cerebrum decreased in size.<ref name="Schoenemann, 2009">{{cite journal | vauthors = Schoenemann PT |date=December 1, 2009|title=Evolution of Brain and Language|journal=Language Learning|volume=59|pages=162–186|doi=10.1111/j.1467-9922.2009.00539.x|pmid=22230641}}</ref> With both the development and implementation of motor tasks, visual-spatial skills and learning taking place in the cerebellum, the growth of the cerebellum is thought to have some form of correlation to greater human cognitive abilities.<ref>{{cite journal | vauthors = MacLeod CE, Zilles K, Schleicher A, Rilling JK, Gibson KR | title = Expansion of the neocerebellum in Hominoidea | journal = Journal of Human Evolution | volume = 44 | issue = 4 | pages = 401–29 | date = April 2003 | pmid = 12727461 | doi = 10.1016/S0047-2484(03)00028-9 | bibcode = 2003JHumE..44..401M }}</ref> The lateral hemispheres of the cerebellum are now 2.7 times greater in both humans and apes than they are in monkeys.<ref name="Schoenemann, 2009"/> These changes in the cerebellum size cannot be explained by greater muscle mass. They show that either the development of the cerebellum is tightly linked to that of the rest of the brain or that neural activities taking place in the cerebellum were important during [[Hominidae]] evolution. Due to the cerebellum's role in cognitive functions, the increase in its size may have played a role in cognitive expansion.<ref name="Schoenemann, 2009"/>

===Cerebellum-like structures===
Most vertebrate species have a cerebellum and one or more cerebellum-like structures, brain areas that resemble the cerebellum in terms of [[cytoarchitecture]] and [[neurochemistry]].<ref name=Bell>{{cite journal | vauthors = Bell CC, Han V, Sawtell NB | s2cid = 14536411 | title = Cerebellum-like structures and their implications for cerebellar function | journal = Annual Review of Neuroscience | volume = 31 | pages = 1–24 | year = 2008 | pmid = 18275284 | doi = 10.1146/annurev.neuro.30.051606.094225 }}</ref> The only cerebellum-like structure found in mammals is the [[dorsal cochlear nucleus]] (DCN), one of the two primary sensory nuclei that receive input directly from the [[auditory nerve]]. The DCN is a layered structure, with the bottom layer containing granule cells similar to those of the cerebellum, giving rise to [[parallel fiber]]s that rise to the superficial layer and travel across it horizontally. The superficial layer contains a set of GABAergic neurons called [[cartwheel cell]]s that resemble Purkinje cells anatomically and chemically—they receive parallel fiber input, but do not have any inputs that resemble [[climbing fiber]]s. The output neurons of the DCN are [[pyramidal cell]]s. They are glutamatergic, but also resemble Purkinje cells in some respects—they have spiny, flattened superficial dendritic trees that receive parallel fiber input, but they also have basal dendrites that receive input from auditory nerve fibers, which travel across the DCN in a direction at right angles to the parallel fibers. The DCN is most highly developed in rodents and other small animals, and is considerably reduced in primates. Its function is not well understood; the most popular speculations relate it to spatial hearing in one way or another.<ref>{{cite journal | vauthors = Roberts PD, Portfors CV | title = Design principles of sensory processing in cerebellum-like structures. Early stage processing of electrosensory and auditory objects | journal = Biological Cybernetics | volume = 98 | issue = 6 | pages = 491–507 | date = June 2008 | pmid = 18491162 | doi = 10.1007/s00422-008-0217-1 | s2cid = 14393814 }}</ref>

Most species of fish and amphibians possess a [[lateral line]] system that senses pressure waves in water. One of the brain areas that receives primary input from the lateral line organ, the medial octavolateral nucleus, has a cerebellum-like structure, with granule cells and parallel fibers. In electrosensitive fish, the input from the electrosensory system goes to the dorsal octavolateral nucleus, which also has a cerebellum-like structure. In [[actinopterygii|ray-finned fishes]] (by far the largest group), the [[superior colliculus|optic tectum]] has a layer—the marginal layer—that is cerebellum-like.<ref name=Bell/>

All of these cerebellum-like structures appear to be primarily sensory-related rather than motor-related. All of them have granule cells that give rise to parallel fibers that connect to Purkinje-like neurons with [[synaptic plasticity|modifiable synapse]]s, but none have climbing fibers comparable to those of the cerebellum—instead they receive direct input from peripheral sensory organs. None has a demonstrated function, but the most influential speculation is that they serve to transform sensory inputs in some sophisticated way, perhaps to compensate for changes in body posture.<ref name=Bell/> In fact, [[James M. Bower]] and others have argued, partly on the basis of these structures and partly on the basis of cerebellar studies, that the cerebellum itself is fundamentally a sensory structure, and that it contributes to motor control by moving the body in a way that controls the resulting sensory signals.<ref>{{Cite book | vauthors = Bower JM | title = The Cerebellum: From Structure to Control | chapter = Chapter 27 is the cerebellum sensory for motor's sake, or motor for sensory's sake: The view from the whiskers of a rat? | series = Progress in Brain Research | volume = 114 | pages = 463–96 | year = 1997 | pmid = 9193161 | doi = 10.1016/S0079-6123(08)63381-6 | isbn = 978-0-444-82313-7 }}</ref> Despite Bower's viewpoint, there is also strong evidence that the cerebellum directly influences motor output in mammals.<ref>{{cite journal | vauthors = Heiney SA, Kim J, Augustine GJ, Medina JF | title = Precise control of movement kinematics by optogenetic inhibition of Purkinje cell activity | journal = Journal of Neuroscience | volume = 34 | issue = 6 | pages = 2321–30 | date = February 2014 | pmid = 24501371 | pmc = 3913874 | doi = 10.1523/JNEUROSCI.4547-13.2014 }}</ref><ref>{{cite journal | vauthors = Witter L, Canto CB, Hoogland TM, de Gruijl JR, De Zeeuw CI | title = Strength and timing of motor responses mediated by rebound firing in the cerebellar nuclei after Purkinje cell activation | journal = Frontiers in Neural Circuits | volume = 7 | pages = 133 | year = 2013 | pmid = 23970855 | pmc = 3748751 | doi = 10.3389/fncir.2013.00133 | doi-access = free }}</ref>