The lateral reticular nucleus in the cat. VI. Excitatory and inhibitory afferent paths.

The afferent paths from the spinal cord and from trigeminal afferents to the lateral reticular nucleus (LRN) were investigated by intracellular recording from 204 LRN neurones in preparations with a spinal cord lesion at C3 that spared only the ipsilateral ventral quadrant. Stimulation of nerves in the limbs evoked EPSPs and JPSPs in 201 of 204 tested LRN neurones. The strongest input was from the ipsilateral forelimb (iF) which evoked EPSPs in 49% and IPSPs in 73% of the LRN neurones. Each of the other limbs evoked EPSPs in approximately 20% and IPSPs in approximately 25% of the neurones. Stimulation of the ipsilateral trigeminal nerve (iTrig) evoked EPSPs in 32% and IPSPs in 46% of the neurones. The shortest latencies of the EPSPs and IPSPs indicated a disynaptic connection between primary afferents in the iF and iTrig and the LRN. The most direct pathways for excitatory and inhibitory responses from the other limbs were trisynaptic. Stimulation of the ventral part of the ipsilateral funiculus (iVLF) at C3 (C3iVLF) evoked monosynaptic responses in 189 of 201 tested LRN neurones. Monosynaptic EPSPs were recorded in 104 neurones and monosynaptic IPSPs in 126 neurones. Monosynaptic EPSPs and IPSPs were encountered in all parts of the LRN. Stimulation of the iVLF at L1 (L1iVLF) evoked monosynaptic EPSPs and IPSPs in the ventrolateral part of the LRN. The termination areas of excitatory and inhibitory fibres appeared to be the same. LRN neurones without monosynaptic EPSPs or IPSPs from the L1iVLF were located mainly in the dorsal part of the magnocellular division. Stimulation of the dorsal funiculi (DF) at C2 and the ipsilateral trigeminal nerve (iTrig) evoked excitatory and inhibitory responses in the LRN. The shortest latencies of EPSPs and IPSPs indicated disynaptic connections.