An Anatomic Characterization of the Midbrain Near Response Neurons in the Macaque Monkey

These experiments were designed to reveal the location of the premotor neurons that have previously been designated physiologically as the midbrain near response cells controlling vergence, lens accommodation, and pupillary constriction in response to target distance.

To identify this population, the fixed N2c strain of rabies virus was injected into the ciliary body of seven Macaca fascicularis monkeys. The virus was trans-synaptically transported to the brain. Following a 58- to 76-hour survival, animals were perfused with formalin fixative. After frozen sectioning, tissue was reacted to reveal the location of the infected populations by use of a monoclonal anti-rabies antibody. Another series of sections was processed to determine which of the rabies-positive cells were cholinergic motoneurons by use of an antibody to choline acetyl transferase.

At earlier time points, only cholinergic cells in the preganglionic Edinger-Westphal nucleus ipsilateral to the injection were labeled. At later time points, an additional population of noncholinergic, premotor cells was present. These were most numerous at the caudal end of the supraoculomotor area, where they formed a bilateral band, oriented mediolaterally immediately above the oculomotor nucleus. Rostral to this, a smaller bilateral population was located near the midline within the supraoculomotor area.

Most lens preganglionic motoneurons are multipolar cells making up a continuous column within the Edinger-Westphal nucleus. A population of premotor cells that likely represents the midbrain near response cells is located in the supraoculomotor area. These cells are bilaterally distributed relative to the eye they control, and are most numerous caudally.