The lateral reticular nucleus in the cat. VII. Excitatory and inhibitory projection from the ipsilateral forelimb tract (iF tract).

Intracellular recording from neurones in the lateral reticular nucleus (LRN) demonstrated that, in addition to the previously identified excitatory ipsilateral forelimb tract (iF tract) (Clendenin et al. 1974c) there is an inhibitory tract mediating information from the ipsilateral forelimb to the LRN. The excitatory and inhibitory tracts were similarly organized. The tract neurones were monosynaptically activated by afferents in the ipsilateral forelimb and projected to the same area of the LRN. They will be considered as excitatory and inhibitory components of the iF tract and denoted the excitatory and inhibitory iF tract (EiF and IiF tracts). Stimulation of the descending ipsilateral dorsolateral funiculus (iDLF) in the C3 segment evoked disynaptic EPSPs and IPSPs in LRN neurones contacted by the EiF and IiF tracts. The responses in individual LRN neurones evoked from the iDLF were similar to the responses evoked from the forelimb nerves suggesting that the EiF and IiF tracts are monosynaptically activated by fibres in the iDLF. The dorsal portion of the magnocellular part of the LRN constituted the main termination area of both the EiF and IiF tracts. Neurones in this area have previously been shown to project ipsilaterally to lobule V in the pars intermedia of the cerebellar anterior lobe and to the paramedian lobule (Clendenin et al. 1974a). IPSPs evoked from the IiF tract in LRN neurones outside the main termination area had smaller amplitudes and longer latencies. This finding suggests that these responses were generated by thin axon collaterals given off from dorsally located stem axons.