Walsh & Hoyt: Phasic Velocity and Tonic Position Commands to Orbital Forces that Determine Ocular Motility

Before discussing the central organization of the different ocular motor systems, it is useful to review the nature of the mechanical properties of the orbital tissues, since passive forces must be overcome to move the eyes and, once the eyes are in position, to hold them in place. The predominant orbital forces are elasticity and viscosity. The moment of inertia of the human eye is so small relative to other tissue forces that for practical purposes it can be ignored. Viscous forces are the major hindrance to movement of the globe within the orbit. For fast eye movements, therefore, a powerful phasic contraction of the extraocular muscles is necessary to overcome viscous drag. This high-frequency phasic discharge provides a ""burst"" of neural activity that stimulates muscle contraction. The very-high-frequency burst that is required to drive the eyes at saccadic speeds is called a pulse of innervation.