The dynamic sensitivity of pyramidal tract neurones is influenced by previous activity

The dynamic sensitivity of Pyramidal Tract (PT) neurones has been examined in silent cells and after pre-activation with an action potential evoked at different intervals before the discharge onset. In the resting conditions the dynamic sensitivity increases after the first interspike interval of the repetitive discharge. When a conditioning spike precedes the discharge onset by 50 ms (i.e. of a time sufficient for its afterhyperpolarization, AHP, to fade out) the dynamic sensitivity becomes uniform throughout the ramp. This linearizing effect progressively declines when the conditioning interval is increased to 100 and 150 ms and is attributed to the "depression" induced by the conditioning spike on the AHP of the first spike of the ramp discharge. In natural conditions, the effect would be entirely developed when the neurones fire at their minimal discharge rate (1/AHP duration).