  Refractoriness is a fundamental property of excitable elements, such as
neurons, indicating the probability for re-excitation in a given time-lag, and
is typically linked to the neuronal hyperpolarization following an evoked
spike. Here we measured the refractory periods (RPs) in neuronal cultures and
observed that an average anisotropic absolute RP could exceed 10 milliseconds
and its tail 20 milliseconds, independent of a large stimulation frequency
range. It is an order of magnitude longer than anticipated and comparable with
the decaying membrane potential timescale. It is followed by a sharp rise-time
(relative RP) of merely ~1 millisecond to complete responsiveness.
Extracellular stimulations result in longer absolute RPs than solely
intracellular ones, and a pair of extracellular stimulations from two different
routes exhibits distinct absolute RPs, depending on their order. Our results
indicate that a neuron is an accurate excitable element, where the diverse RPs
cannot be attributed solely to the soma and imply fast mutual interactions
between different stimulation routes and dendrites. Further elucidation of
neuronal computational capabilities and their interplay with adaptation
mechanisms is warranted.
