Excitatory pathways from Forel's field H to head elevator motoneurones in the cat.

Projections of neurones in Forel's field H (FFH) to the upper cervical cord and to the lower brainstem were demonstrated by retrograde labelling of the neurones with horseradish peroxidase (HRP). Systematic threshold mapping for evoking antidromic spikes of FFH neurones revealed that they projected to the neck motor nuclei and to pontomedullary reticular formation (PMRF). Stimulation of FFH evoked large monosynaptic excitatory postsynaptic potentials (EPSPs) in reticulospinal neurones (RSNs) of the PMRF, and mono- and disynaptic EPSPs in the dorsal neck motoneurones. Above EPSPs were evoked from areas confined to FFH, thus indicating that they were elicited by stimulation of FFH neurones. Monosynaptic EPSPs in motoneurones were small but disynaptic EPSPs were markedly facilitated following stimulation with train pulses, becoming several times larger than the monosynaptic EPSPs. Disynaptic EPSPs were supposed to be relayed by RSNs in the PMRF which are known to project to dorsal neck motoneurones. The mono- and disynaptic EPSPs were induced chiefly in motoneurones of the head elevator (m. biventer cervicis and complexus) and rarely of the neck lateral flexor (m. splenius). It was suggested that FFH neurones are involved in the control of vertical head movements.