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| As we navigate through our surroundings, a continuous stream
of light images impinges on our eyes. In the back of each eye is a
light-sensitive tissue, the Retina, it converts patterns of light
energy into electrical discharges known as action potentials. These
signals are conveyed along the axons of retinal ganglion cells to
connect for 20% to the SC (Superior Colliculus) and for 80% to the LGN. The Superior (SC) and
Inferior (IC) Colliculi are 2 pairs, known collectively as the Corpora
Quadrigemina (CQ / Latin, quadruplet bodies). The Superior Colliculus
(Latin: higher hill) is a part of the Tectal Area (Latin: roof) of the
dorsal part of the Midbrain, a junction of the Gray and White Matter.
The CQ receives visual, as well as auditory, inputs in its
layers and they are connected to many
sensorimotor areas of the brain. The CQ as a whole is there
to help orient the head and eyes toward something seen and heard. - The Superior Colliculus (SC): Visual processing, and control of eye movements.
- The Inferior Colliculus (IC): Auditory processing.
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SC is involved in the generation of Saccadic eye movements and eye-head coordination. - Input from: Cerebral Cortex, Inferior Colliculus, Retina, Basal Ganglia, and Spinal Cord.
- Projections to: The Paramedian Pontine Reticular formation (PPRF), Spinal Cord, and elsewhere. Signals from the Midbrain go via the Thalamus to the Cerebral Cortex for interpretation. Descending projections to the PPRF and Spinal Cord can be involved in responses to
stimuli faster than Cortical processing would allow, it can also mediate Oculomotor Movements without Cortical involvement.
SC is organized into 7 alternating
Fibrous and Cellular layers, further categorized into
Superficial and Deep: A. Superficial Layers: - Stratum Zonale (SZ)
- Stratum Griseum Superficiale (SGS)
- Stratum Optican (SO)
B. Deep Layers: - Stratum Griseum Intermedium (SGI)
- Stratum Album Intermedium (SAI)
- Stratum Griseum Profundum (SGP)
- Stratum Album Profundum (SAP)
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 |  | SC transforms signals from several sensory systems
into motor commands for guiding orienting responses of eyes and head. | 
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A. SUPERFICIAL LAYERS CONNECTIONS |
Inputs: From almost exclusively Vision areas. Retinal
- Contralateral
- Non color opponent Retinal ganglion cells
Cortical: V1
- Layers V and VI
- 6 visual cortical regions (areas 17, 18, middle
temporal, dorsomedial and medial areas and the posterior parietal region).
Cortical: Extra-striate Cortex
- Layer V of areas 18, 19, FEF, and pre-motor cortex.
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| Projections: To areas involved in Vision. - Parabigeminal Nucleus (ipselateral)
- Pretectum
- Inferior Pulvinar Nucleus
- Magnocellular dorsomedial nucleus of Thalamus
- LGN (Dorsal and Ventral)
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B. DEEP LAYERS CONNECTIONS |
Inputs: - From sensory modalities: Visual, Auditory
and Somatosensory. - From motor areas and projections from areas
neither purely motor nor sensory.
Cortical
- Layer V of FEF
- Prefrontal association cortex
- Inferior bank of intraparietal sulcus
- Layer V of temporal and occipital lobe
Sub-cortical
- Substancia Nigra (Basal Ganglia)
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Ascending projections: To areas for Saccadic eye movements.
- Dorsal Thalamus -> FEF and inferior parietal lobule.
- Rostral Interstitial Nucleus of the medial longitudinal fasciculus.
- Oculomotor complex
- Magnocellular division of the ventral anterior nucleus
- Paralamellar region of the mediodorsal nucleus
- Parafascicular nucleus
Descending projections: To Brainstem areas that have
direct connections to motor neurons in neck. Projections
thought to be involved in controlling eye movements
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Nucleus reticularis tegmenti-pontis
- Nucleus reticularis pontis oralis
- Cell groups central and medial to the abducens nucleus
- Medial accessory nucleus of the inferior olivary complex
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|  | Reseaches found that:
- MT and V3 are disynaptically connected to the SC, but V2 is not.
- The majority of labeled cells are in Layer 2b of SC, a zone that
projects heavily to the inferior Pulvinar. Few labeled cells are found
in Layer 2a, the zone that projects to LGN.
- The main route of SC disynaptic projections is likely to go
through PIL and Plm, subdivisions that may both project to MT and V3,
but not V2
This disynaptic colliculo-cortical pathway may be a unique feature of "dorsal stream" visual areas.
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