Editing Superior colliculus (section)

==Structure==
{{See also|Nuclei colliculi superioris}}
[[File:Gray712.png|thumb|Section of mid-brain at level of superior colliculi.]]
[[File:Gray719.png|thumb|Hind- and mid-brains; postero-lateral view. Superior colliculus labeled in blue.]]
The superior colliculus is a paired structure of the [[Dorsum (anatomy)|dorsal]] [[midbrain]] and is part of the [[midbrain tectum]]. The two superior colliculi are situated inferior/caudal to the [[pineal gland]] and the [[splenium of corpus callosum]]. They are overlapped by the [[Pulvinar nuclei|pulvinar]] of the thalamus, and a [[medial geniculate nucleus]] of the thalamus is situated lateral to either superior colliculus.'''<ref>{{Cite book |last=Sinnatamby |first=Chummy S. |title=Last's Anatomy |publisher= Elsevier Australia|year=2011 |isbn=978-0-7295-3752-0 |edition=12th |pages=476}}</ref>''' The two [[inferior colliculus|inferior colliculi]] are situated immediately inferior/caudal to the superior colliculi; the inferior and superior colliculi are known collectively as the [[corpora quadrigemina]] (Latin for ''quadruplet bodies''). The superior colliculi are larger than the inferior colliculi, though the inferior colliculi are more prominent.<ref name="Grays">{{cite web |title=IX. Neurology. 4b. The Mid-brain or Mesencephalon. Gray, Henry. 1918. Anatomy of the Human Body. |url=https://www.bartleby.com/107/188.html |website=www.bartleby.com |access-date=10 October 2019}}</ref>

The '''brachium of superior colliculus''' (or '''superior brachium''') is a branch that extends laterally from the superior colliculus, and, passing to the [[thalamus]] between the pulvinar and the medial geniculate nuclei, is partly continued into an eminence called the [[lateral geniculate nucleus]], and partly into the [[optic tract]].{{Citation needed|date=June 2023}}

The superior colliculus is associated with a nearby structure called the '''parabigeminal nucleus''', often referred to as its satellite. In the optic tectum this nearby structure is known as the [[nucleus isthmi]].<ref>{{cite journal |last1=Wang |first1=SR |date=2003 |title=The nucleus isthmi and dual modulation of the receptive field of tectal neurons in non-mammals |journal=Brain Research Reviews |volume=41 |issue=1 |pages=13–25 |doi=10.1016/s0165-0173(02)00217-5 |pmid=12505645}}</ref>

The superior colliculus is a synaptic layered structure.<ref name="Ito" /> The microstructure of the superior colliculus and of the optic tectum, varies across species. As a general rule, there is always a clear distinction between superficial layers, which receive input primarily from the visual system and show primarily visual responses, and deeper layers, which receive many types of input and project to numerous motor-related brain areas. The distinction between these two zones is so clear and consistent that some anatomists have suggested that they should be considered separate brain structures.

In mammals, seven layers are identified<ref>[[#refHuerta|Huerta & Harting, 1984]]</ref> The top three layers are called ''superficial'':
*'''Lamina I''' or '''SZ''', the ''stratum zonale'', is a thin layer consisting of small myelinated axons together with marginal and horizontal cells.
*'''Lamina II''' or '''SGS''', the ''stratum griseum superficiale'' ("superficial gray layer"), contains many neurons of various shapes and sizes.
*'''Lamina III''' or '''SO''', the ''stratum opticum'' ("optic layer"), consists mainly of axons coming from the [[optic tract]].

Next come two ''intermediate layers'':
*'''Lamina IV''' or '''SGI''', the ''stratum griseum intermedium'' ("intermediate gray layer"), is the thickest layer, and is filled with many neurons of many sizes. This layer is often as thick as all the other layers together. It is often subdivided into "upper" and "lower" parts.
*'''Lamina V''' or '''SAI''', the ''stratum album intermedium'' ("intermediate white layer"), consists mainly of fibers from various sources.

Finally come the two ''deep layers'':
*'''Lamina VI''' or '''SGP''', the ''stratum griseum profundum'' ("deep gray layer"), consists of loosely packed neurons and myelinated fibers.
*'''Lamina VII''' or '''SAP''', the ''stratum album profundum'' ("deep white layer"), lying directly above the [[periaqueductal gray]], consists entirely of fibers.

===Neural circuitry===
The superficial layers receive input mainly from the retina, vision-related areas of the cerebral cortex, and two tectal-related structures called the [[pretectum]] and ''parabigeminal nucleus''. The retinal input encompasses the entire superficial zone, and is bilateral, although the contralateral portion is more extensive. The cortical input comes most heavily from the [[primary visual cortex]] (area 17, V1), the secondary visual cortex (areas [[Brodmann area 18|18]] and [[Brodmann area 19|19]]), and the [[frontal eye fields]]. The parabigeminal nucleus plays a very important role in tectal function that is described below.

In contrast to the vision-dominated inputs to the superficial layers, the intermediate and deep layers receive inputs from a very diverse set of sensory and motor structures. Most areas of the cerebral cortex project to these layers, although the input from "association" areas tends to be heavier than the input from primary sensory or motor areas.<ref>{{cite journal |last1=Agster |first1=Kara L. |last2=Burwell |first2=Rebecca D. |title=Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat |journal=Hippocampus |date=December 2009 |volume=19 |issue=12 |pages=1159–1186 |doi=10.1002/hipo.20578 |pmid=19360714 |pmc=3066185 }}</ref> However, the cortical areas involved, and the strength of their relative projections, differ across species.<ref>{{cite journal |vauthors=Clemo HR, Stein BE |title=Topographic organization of somatosensory corticotectal influences in cat |journal=Journal of Neurophysiology |volume=51 |issue=5 |pages=843–858 |year=1984 |pmid=6726314 |doi=10.1152/jn.1984.51.5.843 }}</ref> Another important input comes from the [[substantia nigra]], [[pars reticulata]], a component of the [[basal ganglia]]. This projection uses the inhibitory neurotransmitter [[GABA]], and is thought to exert a "gating" effect on the superior colliculus. The intermediate and deep layers also receive input from the [[spinal trigeminal nucleus]], which conveys somatosensory information from the face, as well as the [[hypothalamus]], [[zona incerta]], [[thalamus]], and [[inferior colliculus]].

In addition to their distinctive inputs, the superficial and deep zones of the superior colliculus also have distinctive outputs. One of the most important outputs goes to the [[pulvinar nuclei|pulvinar]] and lateral intermediate areas of the thalamus, which in turn project to areas of the cerebral cortex that are involved in controlling eye movements. There are also projections from the superficial zone to the pretectal nuclei, [[lateral geniculate nucleus]] of the thalamus, and the parabigeminal nucleus. The projections from the deeper layers are more extensive. There are two large descending pathways, traveling to the brainstem and spinal cord, and numerous ascending projections to a variety of sensory and motor centers, including several that are involved in generating eye movements.

Both colliculi also have descending projections to the [[paramedian pontine reticular formation]] and spinal cord, and thus can be involved in responses to stimuli faster than cortical processing would allow.

==== Mosaic structure ====
On detailed examination, the collicular layers are actually not smooth sheets, but divided into a honeycomb arrangement of discrete columns.<ref name="Chevalier">[[#refChevalier|Chavalier & Mana, 2000]]</ref> The clearest indication of columnar structure comes from the cholinergic inputs arising from the parabigeminal nucleus, whose terminals form evenly spaced clusters that extend from top to bottom of the [[tectum]].<ref name="Illing">[[#refIlling|Illing, 1996]]</ref> Several other neurochemical markers including calretinin, parvalbumin, GAP-43, and NMDA receptors, and connections with numerous other brain structures in the brainstem and diencephalon, also show a corresponding inhomogeneity.<ref>[[#refMana|Mana & Chevalier, 2001]]</ref> The total number of columns has been estimated at around 100.<ref name="Chevalier" /> The functional significance of this columnar architecture is not clear, but it is interesting that recent evidence has implicated the cholinergic inputs as part of a recurrent circuit producing winner-take-all dynamics within the tectum, as described in more detail below.

All species that have been examined — including mammals and non-mammals — show compartmentalization, but there are some systematic differences in the details of the arrangement.<ref name=Illing /> In species with a streak-type retina (mainly species with laterally placed eyes, such as rabbits and deer), the compartments cover the full extent of the SC. In species with a centrally placed fovea, however, the compartmentalization breaks down in the front (rostral) part of the SC. This portion of the SC contains many "fixation" neurons that fire continually while the eyes remain fixed in a constant position.