In the rat CST, approximately twenty-five thousand axons descend in the dorsal funiculus in search of target cells.
Early studies have shown that more axons leave the dorsal funiculus in the upper cervical region of the spinal cord and that they innervate both sides of the grey matter.
It may be acting to (1) constrain the axons in the dorsal funiculus during decent, (2) regulate branching out of the dorsal funiculus, (3) restrict the axons to one side of the grey matter once they have entered the grey matter, (4) or be involved in fine-tuning the guidance as the axons find the interneurons or (5) involved in the topographic patterning of the CST--meaning that the axons behave differently depending on the relative concentration of EphA4 and ephrin ligand at different levels of the spinal cord.
Keeping in mind that EphA4 must be membrane bound in order to signal, we need to determine exactly where it is expressed in the spinal cord, what it is interacting with in order to initiate signalling and how this relates to the timing of exit of the axons from the dorsal funiculus.
At these times the axons should have commenced branching out of the dorsal funiculus into the grey matter and completed the search for their target cells.