Electron Microscopic Investigation of Monoaminergic Terminals to α-Motoneurons in the Anterior Horn of the Cat Spinal Cord

In the anterior horn of the cat thoracic cord, networks of the monoaminergic fibers surrounding the alpha-motoneurons were investigated by fluorescent microscopy and submicroscopically. Monoaminergic terminals were recognized by the administration of 5-OHDA electron microscopically. These terminals could be classified morphologically into three types. The physiological significance of monoaminergic control of alpha-motoneurons was discussed. Type I of the labeled terminals did not show any typical synaptic specialization, such as aggregation of synaptic vesicles or thickening of the pre- and postsynaptic membranes. This type did not have synaptic contact with the alpha-motoneurons. Type II showed typical synaptic contact and asymmetrical synaptic type membranous thickening. A large number of small dense-cored vesicles were accumulated in the vicinity of the presynaptic membranes. Type III contained a large number of small and large dense-cored vesicles and a few flattened small vesicles. This type had synaptic contact with the presynaptic nerve ending in which a large number of agranular vesicles were contained. This study demonstrated that alpha-motoneurons in the anterior horn receive supraspinal monoaminergic control in three ways: modulator control through Type I, monosynaptic direct control through Type II, and inhibitory control through Type III.