CORTICORETICULAR PATHWAYS IN THE CAT - I - PROJECTION PATTERNS AND COLLATERIZATION

This paper summarizes and compares the projection patterns and the rec eptive fields of cortical neurons in areas 4 and 6 that project to the pontomedullary reticular formation (PMRF). A total of 326 neurons wer e recorded in area 4 and 129 in area 6 in four awake, unrestrained cat s that were chronically implanted with arrays of electrodes in the PMR F and the pyramidal tract (PT). In area 4, 47% of the neurons projecte d to the caudal PT but not to the PMRF (PTNs); 19% were activated only from the PMRF [corticoreticular neurons (CRNs)], whereas 27% were act ivated from both the PT and the PMRF (PTN/CRNs). More PTN/CRNs conduct ed at velocities >20 m/s (82%) than did CRNs (23%). In area 6, only 19 % of the neurons were identified as PTNs, 12% were PTN/CRNs and 31% we re CRNs; a further 38% could not be activated from either structure. C ollateral branches within the PMRF conducted at maximum velocities of 20 m/s (average = 6.5 m/s). No significant differences in the conducti on velocities of the collateral branches were found either between fas t and slow PTNs or between area 4 and area 6 neurons. A large proporti on of neurons in area 4 (85/173, 49%) were activated by passive manipu lation of the more distal, contralateral forelimb, with approximately equal numbers being classed as PTNs, PTN/CRNs and CRNs. Most neurons i n area 6 for which a receptive field could be found were excited by li ghtly touching or tapping the face and neck; a receptive field could n ot be determined for 39% of the area 6 neurons compared with only 5% o f those in area 4. Finally, there was evidence that neurons in quite w idespread areas of the pericruciate cortex, including both areas 4 and 6 projected onto similar, restricted regions of the PMRF. The fact th at the cortical projection from area 4 to the PMRF includes a high per centage of fast PTNs with a receptive field on the distal forelimb is consistent with the view that this projection may serve to integrate m ovement and the dynamic postural adjustments that accompany them. The fact that the cortical projection from area 6 to the PMRF is primarily from slow PTNs with receptive fields on the face, neck and back is co nsistent with a role for this cortical area in adjusting the general p osture of the animal on which movements are superimposed.