Both afferent and efferent connections influence postnatal growth of motoneuron dendrites in the rat

Spinal motoneurons from mature rats, which had received one of 5 different surgical procedures neonatally, were retrogradely labelled with a cholera toxin-horseradish peroxidase conjugate and their dendritic morphology was analysed. The motoneurons studied were those innervating extensor digitorum longus and the procedures disrupted their motor and sensory connections to varying degrees. Disruption of motor contact with the target muscle retarded dendritic growth in the transverse plane, particularly in the dorso-medial direction. Disruption of sensory as well as motor contact resulted additionally in an increase in dendritic density in the longitudinal plane, largely along the rostral-caudal axis. The findings suggest that dendritic development of motoneurons is influenced by both afferent and efferent target contacts and that these effects can be differentiated.