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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.IN previous work on the responses of cortical pyramidal neurones to electrical stimulation of the pia-covered cortex of cats, intracellular recording was used [Phillips, 1956 b]. The intracellular method has unique value in giving accurate information of the quantity, timing and direction (excitatory or inhibitory) of subliminal responses of the neuronal membrane. It may, however, be misleading if one wishes to measure the threshold current needed to cause pyramidal discharge. The membrane potential of cells penetrated by micro-electrodes may be abnormally low, through leakage at the site of puncture, a tear elsewhere in the membrane, or distortion of the cell by pressure and traction. A background of pathological depolarization would then keep the membrane potential nearer than normal to the firing level. Transient additional depolarizations, whether due to direct electrical stimulation or to synaptic stimulation, might then cause a discharge of impulses which normally they would not do, because their amplitude would not be adequate, without the background depolarization, to bring the membrane potential to the firing level. Thus, previous measurements of cortical electrical thresholds for minimal pyramidal discharge might have been spuriously low, and the observed discrepancy between cellular and motor thresholds might have been spuriously large. The repetitive firing of pyramidal cells in response to 10 msec. cortical shocks might also have been an artefact of impalement. We have, therefore, repeated the experiments with measured cortical currents, but have recorded the discharge of impulses from single corticospinal axons in the cervical spinal white matter. We confirm that the thresholds for repetitive pyramidal discharge are below the threshold for movement. A long pulse which just causes a flick movement of the forelimb causes firing of cells whose lowest-threshold foci may be as far as 7 mm. distant from the lowest-threshold motor "point". We have * M.R.C. Scholar.

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