Accommodative behavior of cat pyramidal tract cells investigated with intracellular injection of currents

Summary1. Single glass microelectrodes were inserted into pyramidal tract (PT) cells of cat's cerebral cortex. Accommodative properties of their membranes were investigated by intracellular injection of depolarizing currents. 2. The threshold-latency curve was derived by applying linearly rising currents. When the rising slope of the current was decreased gradually, the threshold intensity first decreased, and then later increased, reaching a certain constant value in the manner of a ceiling. 3. Factors determining this characteristic threshold-latency curve were analyzed by using current steps. A special form of the membrane impedance was thus revealed; the time course of the potential changes induced by current steps can be approximated by the sum of three exponential curves, just as in motoneurones. Threshold-latency curves calculated for a triple exponential model of the membrane fit closely to the actual ones. It implies that the PT cells have no genuine accommodation under normal conditions. 4. Local responses and alterations in the critical depolarization were considered to contribute only minutely to the threshold-latency curve of the PT cell membrane.