  Refractory periods are an unavoidable feature of excitable elements,
resulting in necessary time-lags for re-excitation. Herein, we measure neuronal
absolute refractory periods (ARPs) in synaptic blocked neuronal cultures. In so
doing, we show that their duration can be significantly extended by dozens of
milliseconds using preceding evoked spikes generated by extracellular
stimulations. The ARP increases with the frequency of preceding stimulations,
and saturates at the intermittent phase of the neuronal response latency, where
a short relative refractory period might appear. Nevertheless, preceding
stimulations via a different extracellular route does not affect the ARP. It is
also found to be independent of preceding intracellular stimulations. All these
features strongly suggest that the anisotropic ARPs originate in neuronal
dendrites. The results demonstrate the fast and significant plasticity of the
neuronal ARP, depending on the firing activity of its connecting neurons, which
is expected to affect network dynamics.
