The three-dimensional organization of the spinal nerve roots at the
level of the conus medullaris has not been described previously, to our
knowledge. In this study, we used a newly developed technique of in situ
fixation and embedding to define the cross-sectional anatomy at the level
of the conus medullaris in ten fresh human cadavera. A highly organized
overlapping pattern of nerve roots was demonstrated in all specimens. The
nerve roots form a peripheral rim around the spinal cord at the levels of
the tenth and eleventh and the eleventh and twelfth thoracic intervertebral
discs. More caudally, the cord diminishes in size and the nerve roots
predominate. The most cephalad roots lie laterally, with the motor roots
ventral to their sensory counterparts. The more caudad roots overlap toward
the midline, and the motor and sensory portions of each root are separated
by spinal cord tissue. An intricate web of arachnoid membrane holds the
nerve roots in a fixed relationship to each other.