The axonal microtubular density is higher than normal in fibres innervating spastic muscles.

In infants with spastic disorders and with congenital dysplasia of the hip (CDH), the microtubular content of nonmedullated and myelinated fibres of the anterior obturator nerves was studied with the electron microscope. In CDH infants, the microtubular content of nonmedullated and myelinated fibres was similar to values reported for experimental animals. In spastic patients, the microtubular content of nonmedullated fibres was similar to that found in CDH patients while the microtubular densities of 3- and 10-microns myelinated fibres were 37.5 and 16.7 microtubules/micron2, respectively, or 100% and 43% greater than corresponding values of CDH patients. We propose that the high firing rate determines the abnormally high microtubular content of myelinated axons supplying spastic muscles.