A surgical access system comprising a tissue dilation assembly and a tissue retraction assembly, both of which may be equipped with one or more electrodes for use in detecting the existence of (and optionally the distance and/or direction to) neural structures. A retractor assembly for creating an operative corridor to a spinal surgical target site is disclosed, comprising: a retractor body and a plurality retractor blades extending generally perpendicularly to the retractor body, the retractor body being operable to separate the at plurality of retractor blades relative to each； and a supplemental retractor blade assembly attachable to at least two of the plurality of retractor blades, the supplemental retractor blade assembly comprising an elongated supplemental retractor blade and a crossbar connector, the crossbar connector configured to be attached to said at least two retractor blades while engaging the supplemental retractor blade.
