Some aspects of the structural organization of the spinal cord of Gymnotus carapo (Teleostei, gymnotiformes) II. The motoneurons

Abstract

The use of horseradish peroxidase as neuronal marker has allowed us to distinguish in the spinal cord of the weakly electric fish Gymnotus carapo two main populations of motoneurons: the periependymal motoneurons which innervate the axial musculature and the ventral motoneurons which innervate the appendicular muscles. The ventral horns were explored by means of conventional staining procedures and divided into four dorsoventral zones (I–IV). The largest motoneurons lying in the periependymal gray have been tentatively identified as the “primary motoneurons” (PM) of this species. The PMs are located forming two columss at each side of the central canal. They are characterized by their large size (50–75 μm), the occurrence of four or five thick dendritic trunks, and the peculiar intraspinal pathways of their axons. The dendritic trunks originate exclusively from the lateral surface of each neuron and project toward the ipsilateral, most dorsal neuropiles of the ventral horns (zones I and II). The axons follow ventromedial courses, close to the ipsilateral Mauthner fiber. The ventral motoneurons innervating the muscles of the anal fin are located within zones III and IV of the anterior horns. They are arranged in groups distributed along the cord; each group consits of 15–20 small- and medium-size neurons. The muscles of the pectoral fins receive their innervation from motoneurons lying in the ventral portion of the transitional zone between the medulla and the spinal cord. Three kinds of nerve terminals have been found impinging on the motoneuron somata and dendrites: (a) terminals with the fine structural characteristics of chemical synapses, (b) terminals identified as “mixed junctions” (chemical and electrical), and (c) terminals containing flat vesicles (proposed as serving inhibitory functions). Adjacent nerve terminals show zones in which the plasma membranes lie closely apposed. At these levels there are membrane pentalaminar patches (gap junctions) similar to those found at the level of the mixed junctions. These patches may facilitate, by means of electrotonic coupling, the synchronized activity of several synaptic terminals. The functional properties of the motoneurons of Gymnotus remain unexplored. These morphological studies have been initiated to support forthcoming electrophysiological investigations.