1 Dynamics of Abducens Nucleus Neurons in the Awake Mouse 2 ( Revised

25 26 The mechanics of the eyeball and orbital tissues (the “ocular motor plant”) are a 27 fundamental determinant of ocular motor signal processing. The mouse is used 28 increasingly in ocular motor physiology, but little is known about its plant mechanics. 29 One way to characterize the mechanics is to determine relationships between extraocular 30 motoneuron firing and eye movement. We recorded abducens nucleus neurons in mice 31 executing compensatory eye movements during 0.1-1.6 Hz oscillation in the light. We 32 analyzed firing rates to extract eye position and eye velocity sensitivities, from which we 33 determined time constants of a viscoelastic model of the plant. The majority of abducens 34 neurons were already active with the eye in its central rest position, with only 6% 35 recruited at more abducted positions. Firing rates exhibited largely linear relationships to 36 eye movement, although there was a nonlinearity consisting of increasing modulation in 37 proportion to eye movement as eye amplitudes became small (due to reduced stimulus 38 amplitude or reduced alertness). Eye position and velocity sensitivities changed with 39 stimulus frequency as expected for an ocular motor plant dominated by cascaded 40 viscoelasticities. Transfer function poles lay at approximately 0.1 and 0.9s. In 41 comparison to previously studied animal species, the mouse plant is stiffer than the 42 rabbit, but laxer than cat and rhesus. Differences between mouse and rabbit can be 43 explained by scaling for eye size (allometry). Differences between the mouse and cat or 44 rhesus can be explained by differing ocular motor repertoires of animals with and without 45 a fovea or area centralis. 46 47