DYNAMICS OF ABDUCENS NUCLEUS NEURONS IN THE AWAKE RABBIT

1. We recorded abducens neurons, identified by electrical stimulation as internuclear neurons or motoneurons, in awake rabbits. The relation ship of firing rate to eye movement was determined from responses duri ng stable fixations, sinusoidal rotation in the light (0.05-0.8 Hz), a nd triangular optokinetic stimulation at 0.1 Hz. 2. All abducens neuro ns were excited during temporal movement of the ipsilateral eye. Tempo ral and nasal saccades were associated with bursts or pauses, respecti vely, in the firing rate. 3. Motoneurons and internuclear neurons are qualitatively indistinguishable. There was no significant quantitative difference between the phase and sensitivity of the two groups for 0. 2-Hz sinusoidal rotation in the light. 4. On the basis of the response to stable eye positions, we determined static eye position sensitivit y of the abducens neuron pool to be 8.2 +/- 2.5 (SD) spikes s(-1)/degr ees, with a static hysteresis of 8.9 spikes/s (1.14 +/- 0.37 degrees). 5. We determined apparent eye position sensitivity (k) and apparent e ye velocity sensitivity (r) from the responses to sinusoidal rotation in the light. k increases and r decreases with stimulus frequency, whi ch indicates that the simplest transfer function mediating conversion of abducens nucleus (VI) firing rate to eye position (E) has two poles and one zero. 6. The VI --> E relationship has an ''amplitude nonline arity,'' manifest as a tendency for k, r, and firing rate phase lead t o decrease as eye movement amplitude increases at a fixed frequency. O n a percentage basis, phase is less affected than are the sensitivitie s. The nonlinearity becomes less pronounced for stimulus amplitudes >2 .5 degrees, and consequently a linear model of the VI --> E transforma tion remains useful, provided that consideration is restricted to the appropriate range of stimulus/response amplitudes. 7. We determined ti me constants of the linear two-pole, one-zero transfer function from t he variation of r/k versus stimulus frequency. The pole time constants were T-1 = 3.4 s and T-2 = 0.28 s, and the zero time constant (T-z) = 1.6 s. The magnitude of T-z was corroborated by measuring the time co nstant of the exponential decay in firing rate after step changes in e ye position. This transient method yielded a T-z of 1.1 s. 8. The time constants of the VI --> E transfer function are roughly 10 times larg er than those reported for the rhesus macaque. The difference is attri butable to the reported 10-fold lower stiffness of the rabbit oculomot or plant, which may in turn relate to rabbit's postulated lower degree of activation of extraocular muscles at any given position. The subst antial differences in time constants have important implications for c omparing signaling properties of ocular motoneurons and premotor neuro ns in different species.