Morphology of dendrite bundles in the cervical spinal cord of the rat: a light microscopic study

Histological staining techniques and Golgi-Cox impregnation revealed three discrete dendrite bundles in the ventral horn of the rat cervical spinal cord. A midline dendrite bundle (MDB) traversed the ventromedial gray matter (C3-6), a central dendrite bundle (CDB) coursed the medial aspect of the ventral horn (C3-5), and a lateral dendrite bundle (LDB) traveled in the ventrolateral gray matter (C2-4). At the light microscopic level, the three dendrite bundles were composed of longitudinally oriented intertwined dendrites that coursed in close apposition among motoneuron perikarya, neuroglia, and capillaries. A gradient of packing density of dendrites in the bundles existed, the MDB displaying the greatest packing density and the LDB forming the most loosely interwoven dendritic plexus. Dendrites contributing to the bundles originated from several different motoneuron pools. Smaller transverse dendrite bundles radiated from the longitudinal dendrite bundles at right angles and appeared to interconnect the MDB, CDB, and LDB. Transverse dendrite bundles also exited the MDB and LDB to course into the anterior and lateral funiculi, respectively. The presence of dendrite bundles among fields of motoneurons suggests that dendrite bundles may provide an anatomical substrate for the synchronization of neuronal activity for coordination of muscle groups involved in particular movements. Dendrite bundles also would provide a means whereby functionally similar motoneurons can receive and integrate similar synaptic inputs, and thus allow these inputs to modulate and coordinate groups of neurons that act as a functional unit. The presence of transverse dendrite bundles interconnecting the longitudinal bundles may permit the fine tuning of motoneuron activity for better coordination of movements involving synergistic and antagonistic muscle groups.