Segmental distribution of the motoneurons innervating trunk muscles in the spinal cord of the cat and rat

Abstract

The current progress in developmental biology suggests a genetically stable peripheral pathway formation. However, this may be incompatible with the variations or anomalies observed in the segmental origins of motor nerves in the mammals including the human. For the consideration of the causes raising this inconsistency, we examined the distribution of motoneurons for the serratus dorsalis cranialis muscle of the cat using a retrograde labeling method because this muscle consists of segmentally-arranged parts which receive segmental dual innervation. Consequently, the distribution of the labeled motoneurons for one part spread throughout the full extent of two spinal cord segments, while the distributions for the intercostal muscles in the cat and rat were segmental and in accordance with each spinal cord segment. This may indicate the more precise correspondence between the spinal nerve segments and the distribution of motoneurons projecting axons through them. We think, therefore, that segments of the spinal nerves supplying a given target exactly indicate the segmental levels of supplying motoneurons and suggest the segments of somites from which primordial cells of the target migrate.