The opticogravic nerve: eye-level anatomic relationships within the central nervous system.

The anatomic relationships between arterial blood supply and key structures within the central nervous system (CNS) are important in comprehending the neurophysiological effects of acceleration (+Gz) stress, including +Gz-induced loss of consciousness (G-LOC). An accurate understanding of the location of the vascular and neurologic structures at eye-level is vital, since it is possible to determine precisely when visual symptoms occur. Cerebral perfusion is supported by the pressure-equalizing effects provided by the cerebrospinal fluid. While brain and brain stem are protected by this pressure compensation, the eye is not. Decreased visual function results when retinal perfusion is compromised. G-LOC generally occurs following loss of visual function. The exact location(s) of altered perfusion within the CNS that results in G-LOC is currently unknown. To thoroughly understand G-LOC, it will be necessary to understand which CNS structures must be affected by +Gz-induced ischemia to cause G-LOC. Based on previous theoretical considerations of loss of consciousness as a protective mechanism, it is possible to consider the eye (visual system) as a dual sensor for both vision and gravitational (acceleration) stress. Due to this expanded definition of the sensory functions of the second cranial nerve, it would therefore be more appropriate to describe it as the "opticogravic" nerve. This manuscript discusses some of these considerations and the eye-level neuroanatomic relationships of vital importance for the acceleration medical subspecialist.