Changing synaptic connections an cell bodies of growing identified spinal motoneurons of the eel, Anguilla

As the target musculature they innervate grows throughout life, certain seg mental motoneurons from the spinal cord of Anguilla, readily identified on the basis of their form and position, also increase in size. In doing so, t hey present a steadily increasing target to the spinal and supraspinal neur ons that innervate them. How the afferent neurons respond was assessed by m easuring features of their synaptic boutons contacting the motoneuronal per ikarya, as seen with electron microscopy. About 60% of the perimeter of the perikaryal profile of each motoneuron was found to be covered with synapti c bouton profiles, a value that is independent of the size of the motoneuro n. Furthermore, the distances between synaptic profiles, their contact size s (measured as apposition length) and the number and size of the vesicles e ach profile contains were all found to be relatively constant and also inde pendent of motoneuronal size. In contrast, the number of synaptic profiles contacting a motoneuron correlated well with its perikaryal size. Our findi ngs indicate that the challenge of a growing neuronal target is met by a st eady increase in the number of contacting boutons, the form and spacing of which are held relatively constant; this strategy will require continual sy naptic realignment at the target.