Descending projections of Forel's field H neurones to the brain stem and the upper cervical spinal cord in the cat

Summary 1. Descending projections from Forel's field H (FFH) to the brain stem and upper cervical spinal cord were studied in cats. 2. Following implantation of HRP pellets into the spinal gray matter (C1-C3) or in the ponto-medullary reticular formation, the nucleus reticularis pontis caudalis (NRPC) or in the nucleus reticularis gigantocellularis (NRG), numerous neurones were retrogradely labelled in FFH on the ipsilateral side. In the former cases, the sizes of labelled neurones were medium-large (2040 m in diametre) while both small and medium-large neurones were labelled in the latter cases. 3. The lowest levels of spinal projection of single FFH neurones (n=70) were assessed by antidromic spikes elicited by stimulating electrodes placed in C1, C3 and C7. The majority (59%) projected to C1 (but not to C3), about 27% to C3 (but not to C7), and only 14% to C7. 4. Axonal trajectories of single FFH neurones in C1-C3 segments were investigated by antidromic threshold mapping methods. The stem axons of spinal-projecting FFH neurones descended in the ventral or in the ventrolateral funicli and the collaterals were projected to neck motor nuclei (lamina IX, Rexed 1954) and laminae V–VIII. The conduction velocities were estimated as 8–37 m/s from the antidromic latencies. 5. Axonal trajectories of 7 FFH neurones were investigated in the ponto-medullary reticular formation. All were antidromically activated from C1. In six neurones, the stem axons were located in the ventral part of the central tegmental tract and collaterals were projected to the NRPC and/or the NRG. Some of them projected to the inferior olive and the nucleus prepositus hypoglossi as well. The stem axon, in the remaining cell, was in the most dorso-medial part of the medial longitudinal fasciculus and collaterals were projected mainly to the dorsal part of the NRPC and the NRG, and also to the medial vestibular nucleus. 6. Anterograde transport of WGA-HRP injected into FFH revealed that in the upper cervical spinal cord, stem axons were found in the ventral funiculus and ventral part of the lateral funiculus. Collateral projections and presumed bouton-like deposits were observed in the laminae VI–IX, especially in their medial part. In the brain stem, dense bundles of the descending fibres were found in the central and the medial tegmental tracts and in the medial longitudinal fasciculus. FFH neurones projected densely to the caudal half of the NRPC and to the rostral half of the NRG. Extremely dense projections to the inferior olive were noted.Abbreviations AM anteromedian nucleus of the Edinger-Westphal - BCC m. biventer cervicis and complexus - CCN central cervical nucleus - CP cerebral peduncle - CTT central tegmental tract - DAB diaminobenzidine - DAO dorsal accessory nucleus of the inferior olive - DW nucleus Darkschewitsch - FFH Forel's field H - FMN fasciculus mammillothalamics - FR fasciculus retroflexus - G genu of the facial nerve - HB habenula - HRP horseradish peroxidase - INC interstitial nucleus of Cajal - IO inferior olive - LGN lateral geniculate nucleus - LHT lateral hypothalamic nucleus - MAO medial accessory nucleus of the inferior olive - MB mammillary body - MLF medial longitudinal fasciculus - MTT medial tegmental tract - MVN medial vestibular nucleus - NRG nucleus reticularis gigantocellularis - NRPC nucleus reticularis pontis caudalis - NRTP nucleus reticularis tegmenti pontis - OT optic tract - PAG periaqueductal gray matter - PC posterior commissure - PCN posterior commissure nucleus - PF parafascicular nucleus - PH nucleus prepositus hypoglossi - PHT posterior hypothalamic nucleus - PN pontine nucleus - PO principal nucleus of the inferior olive - Py pyramid - RN red nucleus - RSNs reticulospinal neurones - SN substantia nigra - SNr substantia nigra pars reticulata - sPF subparafascicular nucleus - STH subthalamic nucleus - TB trapezoid body - TMB tetramethylbenzidine - V3 third ventricle - ZI zona incerta - VI abducens nucleus/nerve - XII nucleus hypoglossi