Dendrite distribution of identified motoneurons in the lumbar spinal cord of the turtle Pseudemys scripta elegans

Motoneurons in the turtle lumbar spinal cord were injected with HRP by electrophoresis after being electrophysiologically identified as innervating a muscle belonging to a functional group. The distribution of dendrites was studied in transverse reconstructions of 45 motoneurons, including 11 motoneurons identified as innervating knee extensor muscles, eight motoneurons innervating hip retractor and knee flexor muscles, 14 motoneurons innervating ankle and/or toe extensors and 12 motoneurons innervating ankle and/or toe flexor muscles. The dorsal dendritic tree of motoneurons innervating distally positioned musculature (ankle and/or toe extensors and flexors) was observed to contain significantly less terminal dendritic branches compared to the dorsal dendritic trees of motoneurons innervating proximally situated (hip and knee) muscles. The distribution of dendrites within the white matter was studied by measuring the total projected length of the dendritic branches within empirically defined sectors in the transverse plane. This kind of analysis also revealed differences between the dorsal dendrites of motoneurons innervating distally and proximally positioned muscles conforming to the counts of terminal dendritic branches. It is suggested that these apparent differences in the size of the dorsal dendrite may be related to the number of synapses made by primary afferents. In the white matter, the highest dendritic density for all four groups of mononeurons was found within the central part of the lateral funiculus. However, only in the ventral funiculus could slight indications be found that the dendritic density of functionally different motoneuron groups may bear some relation to the locations of the terminations of the descending pathways known to establish monosynaptic contacts with lumbar mononeurons.