Neuronal gap junctions and morphologically mixed synapses in the spinal cord of a teleost, Sternarchus albifrons (gymnotoidei)

Abstract

Spinal cord motoneurons in the gymnotid, Sternarchus albifrons, were studied electron microscopically with special reference to the freeze-fracture method. Two types of motoneurons were identified. Electromotor neurons are monopolar and are located in a midline column dorsal to the ventral gray. These cells have a small fraction of their surface covered by synapses from descending axons, often at nodes. The synapses have multiple gap junctions, but few presynaptic vesicles or other correlates of chemical transmission. The gap junctions have an ordinary appearance in freeze-fracture replicas and exhibit a highly ordered substructure. The not infrequent appositions between the cell bodies of electromotor neurons exhibit no junctional specializations. Ordinary motoneurons are multipolar and densely covered with axosomatic and axodendritic synapses. In thin sections these synapses can be divided into two groups according to whether the vesicles are spherical or flattened. Gap junctions occur only at the first type, thus forming ‘morphologically mixed’ synapses. In freeze-etch replicas of motoneurons, the gap junctions are often found near clusters of postsynaptic E face particles elsewhere associated with excitatory chemical transmission. In addition, vesicle attachment sites occur in the presynaptic membranes of some synapses with gap junctions. The morphological observations are consistent with dual chemical and electrical transmission at these particular synapses, i.e. electrical excitation across gap junctions and chemical excitation at active zones with spherical vesicles and post-synaptic E face particles.