Monkey flexor and abductor pollicis brevis motoneuron pools: Proximal dendritic trees and small motoneurons

Transverse sections of the monkey cervical spinal cord from a previous study (Jenny and Inukai, 1983 [1]) were reanalyzed using Neurolucida to create a three-dimensional display of flexor pollicis brevis and abductor pollicis brevis (FAbPBr) motoneurons and dendrites that had been jointly labeled with horse radish peroxidase (HRP). These data were correlated with similar data from a reanalysis of an extensor digitorum communis (EDC) motoneuron pool (Jenny, Cheney, and Jenny, 2018 [2]). The FAbPBr motoneuron columns were located in the C8 (caudal) and T1 segments of the spinal cord and within the most dorsal and medial regions of the motor column pools that innervate hand muscles. Small motoneurons (cell body areas less than 500 µm² and presumed to be gamma motoneurons) comprised about four percent of the motoneurons and were located throughout the length of the motoneuron pool. HRP labeled dendrites extended radially (360°) from the motoneuron soma but greater numbers of dendrites were directed either dorsomedial to the base of the dorsal horn or medial to the ventromedial gray matter. The longer HRP labeled dendrites and their branch dendrites usually continued in the same radial direction as when originating from the cell body or proximal dendrite. As such we considered the radial direction of the longer HRP labeled dendrites to be a reasonable estimate of the radial direction of the more distal dendritic trees [2]. Both the EDC and FAbPBr motoneuron groups had a greater number of dendrites oriented in dorsal and medial directions from the motoneuron column. Our data continue to suggest that motoneuron dendritic trees have direction-oriented dendrites that extend toward functional terminal regions.