Neuronal pathways from low-threshold hindlimb cutaneous afferents to motoneurons innervating trunk muscles in low spinal cats

Postsynaptic potentials (PSPs) evoked in motoneurons innervating the back a nd abdominal muscles in the lumbar part of the body by stimulating hindlimb cutaneous afferents were investigated in unanesthetized decerebate and spi nal cats. Various types of PSP: pure excitatory postsynaptic potential (EPS P), pure inhibitory postsynaptic potential (IPSP), and mixed PSP (i.e., EPS P followed by IPSP, EPSP/IPSP; and IPSP followed by EPSP, IPSP/EPSP) were o bserved. The weak stimulation at 2 times threshold (2T) produced predominan tly the EPSP, while at 5T the incidence of IPSP or EPSP followed by IPSP wa s increased. In about 20-50% of the various groups of motoneurons, PSPs evo ked by ipsi- and contralateral nerves were qualitatively and quantitatively similar. For the other motoneurons, PSPs evoked by ipsi- and contralateral nerves were markedly different with respect to magnitude and/or polarity. These findings suggest that, within each motoneuron pool, some neurons act to increase stiffness of the trunk or to move vertically in response to an increased activity of cutaneous afferents, while the other motoneurons act to produce lateral bending of the trunk.