Distribution patterns of dendrites in motor neuron pools of lumbosacral spinal cord of the chicken

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Abstract
The morphology of dendritic trees (dendroarchitecture) of motor neurons innervating specific hindlimb muscles (motoneuron pools, MNP) was studied in the chick spinal cord. Motoneurons were labelled by intramuscular injections of horseradish peroxidase conjugated with cholera toxin subunit B. MNPs of posterior iliotibial and femorotibial muscles were located at the dorsolateral part of lateral motor column of lumbosacral segments (LS) 1–4 and 1–3, respectively. Although the dendritic profiles of femorotibialis motoneurons were fewer than those of posterior iliotibialis, these two MNPs had a similar distribution pattern of dendrites. Dendritic profiles were about equally distributed in the gray and white matter. Dendrites from the MNP of posterior iliotibialis radiated in all directions. A large number of dendrites penetrated into the white matter, and some even reached to the subpial regions of the lateral funiculus. One array of dendrites that projected dorsomedialwards extended to the base of the posterior horn. MNPs of both the iliofibularis (LS 4–7) and caudilioflexorius (LS 6–8) had dendritic trees with similar distribution patterns. There were two main arrays of dendritic extensions; one along the dorsal, and another along the ventral border of the lateral motor column. Dendrites from the iliofibularis and caudilioflexorius motoneurons were located more frequently in the white matter than in the gray matter. A large number of dendrites extended in all directions from the MNP of the adductor muscle, which was located in the medial region of lateral motor column of LS 1–2. The distribution of dendrites from a few other MNPs was also examined. From these observations, we conclude that there are major differences in the distribution of dendrites of MNPs innervating different chick hind limb muscles. We discuss the possibility that these differences may be associated with differences in the quantity or quality of afferent inputs received by motoneurons in the various MNPs.

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