Synaptic inputs of feline rubrospinal neurons from the parietal association cortex, pretectum and medial lemniscus, and their lesion-induced sprouting.

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Abstract
Synaptic inputs of rubrospinal (RN) neurons from the cerebral cortex, pretectal area (PRT) and medial lemniscus (ML) were investigated electrophysiologically in the cat. Stimulation of the ipsilateral parietal association cortex (PASC) and secondary sensory area (SII) produced slow-rising (about 3 ms rise time) monosynaptic EPSP [excitatory postsynaptic potential] which were, in some cases, followed by hyperpolarizations, similar to the sensorimotor cortex(SM)-induced PSP [postsynaptic potential] previously observed. Stimulation of the contralateral cerebral cortex never produced detectable PSP. Topographical arrangement of PASC-rubral projection was found. Stimulation of the lateral part of PASC induced EPSP predominantly in RN cells innervating the cervicothoracic spinal segments, while stimulation of the medial part of PASC produced EPSP predominantly in RN cells innervating the lumbosacral cord. PASC-induced EPSP were more frequently recorded at the rostral half of RN than at the caudal half. Monosynaptic EPSP and multisynaptic IPSP [inhibitory postsynaptic potential] were induced by stimulation of the ipsilateral PRT and ML. PRT- and ML-induced EPSP had times-to-peak of 1.0 .+-. 0.4 ms (mean .+-. SD) and 1.6 .+-. 0.5 ms, respectively, which were intermediate to those of the cerebral peduncle(P)- and nucleus interpositus of the cerebellum(IP)-induced EPSP. Sensitivity of amplitudes of PRT-induced EPSP to membrane hyperpolarization was intermediate to those of CP- and IP-EPSP, and that of ML-induced EPSP was lower than that of IP-EPSP. It is likely that synapses of PRT and ML fibers are formed between the distal dendrites where CP-rubral synapses terminate and soma where IP-rubral synapses terminate. PASC-induced EPSP after chronic IP and SM lesions had a new fast-rising component and the effectiveness of ML stimulation to induce the unit spike of RN cells was clearly increased in IP and SM lesioned cats. PASC-rubral fibers probably sprouted and formed new synapses at the proximal portions of soma-dendritic membranes of RN cells after IP and SM destructions. Collateral fibers to RN cells of the pyramidal tract also sprouted new synapses following IP and ML lesions.