Electrical Thresholds of Unimpaled Corticospinal Cells in the Cat

Previous measurements of cortical electrical thresholds for firing impaled pyramidal cells in the cat have now been checked on unimpaled cells, whose discharges were led from their axons in cervical spinal white matter. The thresholds are higher than those for impaled cells, but are still below the threshold for movement. Long pulses evoked repetitive discharges. Currents causing movement caused firing of cells whose lowest‐threshold foci were as far as 7 mm. distant from the lowest‐threshold motor “point”. Currents of intermediate strengths evoked synaptic potentials in spinal motoneurones. Thresholds and timing of pyramidal impulses and of synaptic potentials afford no evidence of monosynaptic connexion between pyramidal axons and motoneurones, such as occurs in primates.