DENDRITE BUNDLES IN MOTONEURONAL POOLS OF TRUNK AND EXTREMITY MUSCLESIN THE RAT

The anatomical distribution and the content of dendrite bundles were s tudied in the spinal cord of adult rats. We injected retrogradely tran sported Cholera Toxin subunit B into muscles in order to label motoneu rons and their dendrites. Twenty-one muscles in the trunk and in extre mities were selected which are involved in a variety of motor tasks. R econstructions were made from consecutive sections and the organizatio n of dendrites was studied. In addition, the distributions of fiber ty pes 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 dist inguished. In the first type, longitudinal dendrite bundles extend ove r the length of the pool and also transversal bundles occur. Such pool s innervate the axial musculature, and the soleus and flexor carpi uln aris 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 m uscle fibers. (C) 1996 Academic Press, Inc.