Neuroanatomy: Horizontal Saccades: Circuitry

Notes

The superior colliculus is commonly divided into a dorsal, visuosensory division and a ventral, motor division.
Dorsal division receives an organized retinotopic map of the contralateral visual hemifield from retinal ganglion cells and also receives afferent input from visual cortical regions, as well, including the striate, extrastriate, and frontal cortices.
Ventral division projects to the contralateral abducens nucleus.

Lie in the paramedian pontine reticular formation in the mid-pons, anterior to the superior aspect of the abducens nucleus.

Lie in the medullary reticular formation, within the rostral medulla, anterior to the plane of the abducens nucleus.

Lies along the dorsal tegmentum of the upper medulla, just anterior to the fourth ventricle.

Lie in midline in between the rootlets of the abducens nerves in the pontine tegmentum.

The frontal eye fields (think: post. middle frontal gyrus) directly excite contralateral excitatory burst neurons AND also send an indirect excitatory pathway that synapses within the superior colliculus, which, in turn, excites the contralateral excitatory burst neurons.
The excitatory burst neurons excite the ipsilateral abducens nucleus, which stimulates the final common pathway for horizontal saccades.
In addition to exciting the abducens nucleus, the excitatory burst neurons also excite the ipsilateral inhibitory burst neurons, which inhibit the contralateral abducens nucleus.
- For example, when the left abducens nucleus is stimulated, the right abducens nucleus is inactivated, which prevents both nuclei from firing at the same time.
The omnipause cells tonically inhibit the excitatory burst neurons and also the inhibitory burst neurons.
The excitatory burst neurons inhibit the omnipause neurons and inactivate their tonic suppression of the burst neurons.
The neural integrator receives fibers from and projects fibers to a wide array of nuclei, including the excitatory and inhibitory burst neurons, to sustain gaze.