Target dependence of motor and sensory neurons

The retrograde response to axotomy of neurons from different systems varies; possibly because neurons exhibit varying degrees of dependence on the targets they innervate. Here we have compared the response of sensory and motor neurons to reversible or permanent isolation from their peripheral targets.
Intercostal nerves T7-T10 of anaesthetised adult cats (n=2-3/group) were either crushed (reversible axotomy) or transected, ligated proximally and 3-5mm of the distal stump removed (permanent axotomy). 1d prior to perfusion-fixation, the proximal nerve stumps were newly lesioned under anaesthesia and HRP applied to label the neuronal cell bodies.
2-4d after both types of nerve injury, the orderly arrangement of the RER of Nissl bodies of α and γ motoneurons was lost and there was partial detachment of synaptic terminals. By 8d, synaptic frequency and cover had fallen to 78% and 49% of normal for α motoneurons, and to 38% and 26% for γ motoneurons. Changes in Nissl bodies and synapses peaked between 16-33d. Most α and γ motoneurons showed restoration of ordered lamellae of RER 64d after reversible axotomy, but not after permanent axotomy. Synaptic frequency and cover of both αand γ motoneurons returned to near normal values 64d after reversible axotomy, and also after permanent axotomy of γ motoneurons. No synaptic recovery was found 64d after permanent axotomy of α motoneurons. In contrast to motoneurons, disruption of RER of dorsal root ganglion neurons occurred earlier (by 1d), the retrograde response was more marked, peaked earlier (8-16d), was associated with occasional neuronal degeneration and the recovery of RER orderliness by 64d after reversible axotomy was only partial.
These results show differences in the magnitude, time course and reversibility of the retrograde response of sensory and motor neurons whose axons travel in the same spinal nerve. This may have implications for any functional recovery expected after axotomy and for the timing of therapeutic interventions.