Dendritic distributions of motor neurons innervating fast and slow muscles of the hind limb of rats.

The purpose of the study is to determine the difference, if any, between the dendritic distributions (field) of slow and fast muscle motoneurons. A cholera toxin subunit conjugated horseradish peroxidase (CTHRPs can label retrogradely the dendrites of motoneurons and was injected into a slow muscle (soleus, SOL) and a fast muscle (extensor digitorum longus, EDL) of the hind limbs of rats. The results show that the dendrites of EDL motoneurons project into Rexed lamina VI, VII, and IX while that of SOL into V, VI, VII, VIII, and IX of the gray matter of lumbosacral spinal cord. SOL motoneurons also have many long dendrites extending into ventrolateral and ventral white matter while such extensions from EDL motoneurons are short and scanty. Thus, the dendritic field of SOL motoneurons is wider than that of EDL motoneurons. It is suggested that this difference may be related to the fact that SOL is a slow, fatigue resistant muscle and is more often used than EDL which is fast, fatigable muscle and less often used in routine muscle activity.