The SC is divided into 2 parts: (1) superficial (dorsal) and (2) deep (ventral). The sensory signal (input from the visual cortex and retina) is processed mainly by the superficial SC. The motor signal originates within the deeper layers (the stratum griseum profundum and stratum album profundum) that receive position information from the more superficial layers. The SC projects contralaterally to multiple locations throughout the brainstem, including the DLPN.
Both the parietal and frontal lobe supranuclear pathways travel mainly to the SC; few fibers connect directly to brainstem premotor neurons. The supranuclear pathways also go through the BG (caudate nucleus, putamen nucleus, and substantia nigra pars reticulate). The BG appears to have several roles in the saccadic system, including inhibiting unnecessary reflexive saccades during fixation and helping control voluntary saccades.
Figure 1-24 Pursuit system. A moving object creates signals in the striate cortex (V1), and from there to the extrastriate cortex (V2, V3). These signals are subsequently relayed to the middle temporal (MT) area and medial superior temporal (MST) areas, the posterior parietal cortex, and the frontal eye field (FEF). The descending cortical pathways of the temporal, parietal, and frontal lobes innervate the ipsilateral dorsal lateral pontine nucleus (DLPN). The neurons in the DLPN decussate and project to the vermis and flocculus in the contralateral cerebellar lobe, which innervates the medial vestibular nucleus (VN). VN neurons then decussate and project to the contralateral CN VI nucleus. The nucleus of CN VI initiates conjugate horizontal eye movements by innervating the ipsilateral lateral rectus muscle and, via internuclear neurons that travel in the medial longitudinal fasciculus, the contralateral medial rectus muscle.
(Illustration by Rob Flewell, CMI.)
The thalamus (internal medullary lamina and pulvinar) is involved in programming saccades. It receives information from the cortex and brainstem and projects only to the cortex and BG. Therefore, the thalamus appears to relay messages from the brainstem to the cortical eye fields.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.