Effects of trans-membrane polarization and TEA injection on monosynaptic actions from motor cortex, red nucleus and group Ia afferents on lumbar motoneurons in the monkey.

Abstract

Abstract The monosynaptic EPSPs evoked from supraspinal structures and muscle afferents in alpha-motoneurons were investigated in the rhesus monkeys with the aid of intracellular recording. There was a statistically significant difference in the time course of cortico-motoneuronal, rubro-motoneuronal and group Ia monosynaptic EPSPs but a relatively small difference between these inputs in the sensitivity to the artificial polarization. No correlation was found between the sensitivity of monosynaptic actions to injected currents and the rise time of EPSP. Tetraethylammonium (TEA) injection into the neuroplasm induced a large increase in the ability of the applied currents to affect the magnitude of the monosynaptic EPSPs, and shifted the extrapolated equilibrium potential in the negative direction. TEA accentuated the trend of cortico-motoneuronal EPSPs toward a more negative equilibrium potential as compared with the group Ia and rubro-motoneuronal EPSPs recorded in the same cell. The early phase of all monosynaptic EPSPs could not be reversed by depolarizing currents in normal and TEA-treated motoneurons and when superimposed on the plateau of depolarization of TEA-prolonged action potential. The results are discussed in relation to the ionic mechanisms of excitatory subsynaptic membrane and to the location of the synaptic sites.