[Distribution pattern of lumbar epaxial, especially M. multifidus motoneurons in the spinal cord of the cat: a study by the retrograde horseradish peroxidase method].

Distribution patterns of motoneurons supplying the lumbar epaxial muscles in the spinal ventral horn have been investigated by the retrograde horseradish peroxidase (HRP) method in 6 cats. HRP (30-40%, 3.0-5.7 mg) injections into multifidus muscle in 5 cases, and for comparison, longissimus lumborum muscle in one case were made at vertebral levels from L5 to L7 at multiple sites. After two days, the animals were sacrificed and frozen sections (60 microns thick) in the frontal plane of the spinal cord were made from the L1 to L7 segments and each section reacted histochemically with the tetramethylbenzidine (TMB) method. The results are summarized as follows. Following HRP injections into the multifidus muscle, labeled cells were found in the ventral horn rostrocaudally from the L2 to L6 segments. Total number of labeled cells ranged from 176 to 436 on the injected side. Labeled cells were most numerous in the L4 segment (range: 98-244). Labeled cells were found on the medial side of the ventral horn, especially in the ventromedial part (VM), predominantly ipsilateral to the injected muscle. Following HRP injection into the longissimus lumborum muscle in one case, labeled cells were also found on the medial side, but were more restricted in the centromedial part (CM) of the ventral horn. In all cases with HRP injection into the multifidus muscle, labeled cells were occasionally observed in VM and/or the medial ventral column (VC) contralateral to the injected muscle in L3 and/or L4 segment. The total number of these cells ranged from 2 to 66. These contralaterally labeled cells were presumed to be due to the peripheral HRP spread. Labeled cells observed in VM and CM were fusiform and round in shape, respectively. These cells had mean average soma diameters (ASD) from 32 to 40 microns and were presumed to be alpha motoneurons. A few labeled cells observed in VC were fusiform in shape and were significantly smaller (mean ASD: 23-28 microns) than the cells observed in other areas (VM, CM). In some cases, the dendrites of labeled cells in VM and VC extended dorsomedially in the VC across the midline in the anterior white commissure.