Editing Hippocampus (section)

=== Hippocampal theta rhythm ===
{{Main|Theta wave}}
[[File:eeg theta.svg|thumb|Example of a one-second EEG theta wave|350px|right]]
The underlying currents producing the [[theta wave]] are generated mainly by densely packed neural layers of the entorhinal cortex, CA3, and the dendrites of pyramidal cells. The theta wave is one of the largest signals seen on EEG, and is known as the hippocampal theta rhythm.<ref name=Buzsaki2002>{{cite journal | vauthors = Buzsáki G | title = Theta oscillations in the hippocampus | journal = Neuron | volume = 33 | issue = 3 | pages = 325–340 | date = January 2002 | pmid = 11832222 | doi = 10.1016/S0896-6273(02)00586-X | ref = refBuzsaki2002 | s2cid = 15410690 | doi-access = free }}</ref> In some situations the EEG is dominated by regular waves at 3 to 10&nbsp;Hz, often continuing for many seconds. These reflect subthreshold [[membrane potential]]s and strongly modulate the spiking of hippocampal neurons and synchronize across the hippocampus in a travelling wave pattern.<ref>{{cite journal | vauthors = Lubenov EV, Siapas AG | title = Hippocampal theta oscillations are travelling waves | journal = Nature | volume = 459 | issue = 7246 | pages = 534–539 | date = May 2009 | pmid = 19489117 | doi = 10.1038/nature08010 | url = https://authors.library.caltech.edu/14755/2/Lubenov2009p4508Nature_supp.pdf | ref = refLubenov2009 | access-date = 2019-07-13 | url-status = live | s2cid = 4429491 | bibcode = 2009Natur.459..534L | archive-url = https://web.archive.org/web/20180723181835/https://authors.library.caltech.edu/14755/2/Lubenov2009p4508Nature_supp.pdf | archive-date = 2018-07-23 }}</ref> The [[trisynaptic circuit]] is a relay of [[neurotransmission]] in the hippocampus that interacts with many brain regions. From [[animal testing on rodents|rodent studies]] it has been proposed that the trisynaptic circuit generates the hippocampal theta rhythm.<ref>{{cite journal | vauthors = Komisaruk BR | title = Synchrony between limbic system theta activity and rhythmical behavior in rats | journal = Journal of Comparative and Physiological Psychology | volume = 70 | issue = 3 | pages = 482–492 | date = March 1970 | pmid = 5418472 | doi = 10.1037/h0028709 | author1-link = Barry Komisaruk }}</ref>

Theta rhythmicity previously clearly shown in rabbits and rodents has also been shown in humans.<ref>{{cite journal | vauthors = Cantero JL, Atienza M, Stickgold R, Kahana MJ, Madsen JR, Kocsis B | title = Sleep-dependent theta oscillations in the human hippocampus and neocortex | journal = The Journal of Neuroscience | volume = 23 | issue = 34 | pages = 10897–10903 | date = November 2003 | pmid = 14645485 | pmc = 6740994 | doi = 10.1523/JNEUROSCI.23-34-10897.2003 | ref = refCantero2003 }}</ref> In [[laboratory rat|rats]] (the animals that have been the most extensively studied), theta is seen mainly in two conditions: first, when an animal is walking or in some other way actively interacting with its surroundings; second, during [[Rapid eye movement sleep|REM sleep]].<ref>{{cite journal | vauthors = Vanderwolf CH | title = Hippocampal electrical activity and voluntary movement in the rat | journal = Electroencephalography and Clinical Neurophysiology | volume = 26 | issue = 4 | pages = 407–418 | date = April 1969 | pmid = 4183562 | doi = 10.1016/0013-4694(69)90092-3 }}</ref> The function of theta has not yet been convincingly explained although numerous theories have been proposed.<ref name=Buzsaki2006>{{cite book | vauthors = Buzsáki G | title = Rhythms of the Brain | publisher = Oxford University Press | year = 2006 | isbn = 978-0-19-530106-9 | ref = refBuzsaki2006 }}</ref> The most popular hypothesis has been to relate it to learning and memory. An example would be the phase with which theta rhythms, at the time of stimulation of a neuron, shape the effect of that stimulation upon its synapses. What is meant here is that theta rhythms may affect those aspects of learning and memory that are dependent upon [[synaptic plasticity]].<ref>{{cite journal | vauthors = Huerta PT, Lisman JE | title = Heightened synaptic plasticity of hippocampal CA1 neurons during a cholinergically induced rhythmic state | journal = Nature | volume = 364 | issue = 6439 | pages = 723–725 | date = August 1993 | pmid = 8355787 | doi = 10.1038/364723a0 | ref = refHuerta1993 | s2cid = 4358000 | bibcode = 1993Natur.364..723H }}</ref> It is well established that lesions of the [[medial septum]]{{snd}}the central node of the theta system{{snd}}cause severe disruptions of memory.<ref>{{cite journal | vauthors = Numan R, Feloney MP, Pham KH, Tieber LM | title = Effects of medial septal lesions on an operant go/no-go delayed response alternation task in rats | journal = Physiology & Behavior | volume = 58 | issue = 6 | pages = 1263–1271 | date = December 1995 | pmid = 8623030 | doi = 10.1016/0031-9384(95)02044-6 | url = https://www.sciencedirect.com/science/article/abs/pii/0031938495020446 | ref = refNuman1995 | access-date = 2020-03-09 | url-status = live | s2cid = 876694 | archive-url = https://web.archive.org/web/20210427200148/https://www.sciencedirect.com/science/article/abs/pii/0031938495020446 | archive-date = 2021-04-27 }}</ref> However, the medial septum is more than just the controller of theta; it is also the main source of [[cholinergic]] projections to the hippocampus.<ref name="Anderson"/> It has not been established that septal lesions exert their effects specifically by eliminating the theta rhythm.<ref>{{cite journal | vauthors = Kahana MJ, Seelig D, Madsen JR | title = Theta returns | journal = Current Opinion in Neurobiology | volume = 11 | issue = 6 | pages = 739–744 | date = December 2001 | pmid = 11741027 | doi = 10.1016/S0959-4388(01)00278-1 | ref = refKahana2001 | s2cid = 43829235 }}</ref>