Axonal trajectories of single Forel's field H neurones in the mesencephalon, pons and medulla oblongata in the cat.

We studied axonal trajectories of single Forel's field H (FFH) neurones (n = 19) in the mesencephalon, pons and medulla by systematic antidromic threshold mapping in cats and differentiated them into two major types. Type I neurones were characterized by projections to the oculomotor nucleus (IIIn) and type II neurones by lack of projections to the IIIn. 2. Type I neurones (11/19) were further classified into three subtypes by the lowest level of projections; type Ic (n = 3) which projected to the cervical cord and type Ib (n = 7) which terminated at the ponto-medullary level and type Ia (n = 1) at more rostral level. In the mesencephalon, stem axons passed just lateral to the IIIn and projected collaterals to the IIIn and the ventral part of the periaqueductal gray matter. In the lower brain stem, stem axons of type Ib and Ic neurones passed in the dorsal part of the reticular formation or in the medial longitudinal fasciculus and projected collaterals to the dorsal part of the nucleus reticularis pontis caudalis (NRPC) and the nucleus reticularis gigantocellularis (NRG) and the reticular formation underlying the nucleus prepositus hypoglossi (PH) and the raphe region. Projections to the superior colliculus were observed in two cases. 3. Type II neurones (8/19) were classified into 2 type IIb projecting to the ponto-medullary reticular formation and 6 type IIc projecting to the cervical spinal cord. In the mesencephalon, stem axons passed through a more lateral region than those of type I and projected collaterals to the mesencephalic reticular formation and the red nucleus. In the lower brain stem, the stem axons passed in the ventral part of the reticular formation corresponding to the central tegmental tract and projected collaterals to the ventral part of the NRPC and NRG. Projections to the interstitial nucleus of Cajal, the inferior olive and the reticular formation underlying the PH were also observed. 4. The dorsal and ventral location of, respectively, stem axons of type I and type II neurones in the lower brain stem was confirmed in a larger number of neurones in experiments with restricted mapping. 5. There was not much difference in location of cell bodies of type I (totally n = 50) and type II (n = 46) neurones. The proportion of spinal-projecting neurones were larger in type II (21/46, 46%) than in type I (7/50, 14%) neurones.