Dendrite bundles in motoneuronal pools of trunk and extremity muscles in the rat.

The anatomical distribution and the content of dendrite bundles were studied in the spinal cord of adult rats. We injected retrogradely transported Cholera Toxin subunit B into muscles in order to label motoneurons and their dendrites. Twenty-one muscles in the trunk and in extremities were selected which are involved in a variety of motor tasks. Reconstructions were made from consecutive sections and the organization of dendrites was studied. In addition, the distributions of fiber types in the muscles were estimated on the basis of ATPase and succinic dehydrogenase-stained sections. Dendrite bundles are confined to pools of muscles which have postural tasks. Two different types can be distinguished. In the first type, longitudinal dendrite bundles extend over the length of the pool and also transversal bundles occur. Such pools innervate the axial musculature, and the soleus and flexor carpi ulnaris muscles in the extremities. These muscles contain relatively high percentages of slow twitch type I muscle fibers. In the second type, the occurrence of bundles is confined to a few zones within the pool. Such pools innervate muscles which are involved in postural tasks but in addition they also subserve other movement tasks. Distinct regions within these muscles are characterized by high percentages of type I muscle fibers.