Morphological and physiological characteristics of inhibitory burst neurons controlling horizontal rapid eye movements in the alert cat.

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Abstract
The morphological and physiological properties of a group of inhibitory neurons, located in the brain stem medullary reticular formation, which project monosynaptically to abducens motoneurons and burst during ipsilateral horizontal rapid eye movements are described in the alert cat. Intracellular (intra-axonal) and extracellular records were obtained from inhibitory burst neurons (IBN) together with eye movement signals recorded by the search-coil technique. The morphology of some IBN was studied by injecting horseradish peroxidase (HRP) into their axons after recording their activity in the alert animals. Their location, somadendritic morphology, axonal trajectory and termination sites were determined. The quantitative relationships between the burst parameters of IBN and the eye movement parameters during spontaneous saccade and optokinetic (OKN) and vestibular nystagmus (VN) were calculated by digital computation. The somata and dendrites of IBN are located in a restricted area of the medullary reticular formation. IBN axons cross the midline and arborize exclusively on the contralateral side of the brain. Collaterals of these axons arborize and terminate extensively in the contralateral abducens nucleus. Collaterals terminate in the medial vestibular nucleus, prepositus nucleus and medullary reticular formation. IBN discharge with high-frequency bursts during all types of horizontal REM (saccades and quick phases of vestibular and optokinetic nystagmus) toward the ipsilateral side. They never discharge during pursuit or slow phases of nystagmus. For ipsilateral REM the mean lead time between burst onset and horizontal component of eye movement is 7.1 ms. This is compatible with monosynaptic inhibition of contralateral abducens motoneurons. Burst duration is proportional to the duration of the horizontal component of the eye movement. Total number of spikes in the burst is proportional to the amplitude of the horizontal eye movement component. Instantaneous firing rate shows a more complex relationship with instantaneous eye velocity. The relationship is nonlinear from eye movement onset to peak velocity and is linear from peak velocity to the end of the eye movement. Some IBN discharge during contralateral REM. Lead time is generally negative (the burst begins aftger the eye movement onset). Burst duration and eye movement duration are weakly correlated, but no correlation was found with the other eye movement parameters. IBN are the immediate premotor inhibitory neurons that provide a monosynaptic inhibition to contralateral abducens motoneurons during REM. The fact that the firing characteristics of these neurons so precisely represent eye movement variables suggests that they receive an excitatory input from the burst neurons, which drive the motoneurons during all types of REM. The morphological results indicate that the velocity and amplitude signals carried by IBN may also be important for the operation of other nuclei in the brain stem involved in eye movement generation and control.