ANATOMY AND PHYSIOLOGY OF SACCADIC LONG-LEAD BURST NEURONS RECORDED IN THE ALERT SQUIRREL-MONKEY .2. PONTINE NEURONS

1. The discharge patterns and axonal projections of saccadic long-lead burst neurons (LLBNs) with somata in the pontine reticular formation were studied in alert squirrel monkeys with the use of the method of i ntraaxonal recording and horseradish peroxidase injection. 2. The larg est population of stained neurons were afferents to the cerebellum. Th ey originated in the dorsomedial nucleus reticularis tegmenti pontis ( NRTP) including its dorsal cell group (N = 5), the preabducens intrafa scicular nucleus (N = 5), and the raphe pontis (N = 1). Axons of all n eurons coursed under NRTP and entered brachium pontis without having s ynapsed in the brain stem. Three axons sent collaterals to the floccul ar lobe, but other more distant targets of these and the other cerebel lar afferents could not be determined. Movement fields of these neuron s were intermediate between vectorial and directional types. 3. Four n eurons had their somata in nucleus reticularis pontis oralis and termi nations in the brain stem reticular formation. Each neuron was differe nt, but all terminated in the region containing excitatory burst neuro ns, and most terminated in the region containing inhibitory burst neur ons. Other targets include nucleus reticularis pontis oralis and cauda lis, NRTP, raphe interpositus, and the spinal cord. Discharge patterns included both vectorial and directional types. 4. Two reticulospinal neurons had large multipolar somata either just rostral or ventral to the abducens nucleus. These neurons also projected to the medullary re ticular formation, caudal nucleus prepositus hypoglossi, and dorsal an d ventral paramedian reticular nucleus. 5. The functional implications of the connections of these LLBNs and those reported in the companion paper are extensively discussed. The fact that the efferents of the s uperior colliculus target the regions containing medium-lead saccadic burst neurons confirms the role of the colliculus in saccade generatio n. However, the finding that many other neurons project to these regio ns and the finding that superior colliculus efferents project more hea vily to areas containing reticulospinal neurons argue for a diminished role of the superior colliculus in saccade generation but an augmente d role in head movement control.