Morphology of the electromotor system in the spinal cord of the electric eel, Electrophorus electricus

The spinal electromotor nucleus was studied by electron microscopy in normal eels and in eels after high spinal section. Electromotor neurons are surrounded by small myelinated fibres that degenerate after spinal section. These fibres form electrotonic synapses (characterized by gap junctions) on somata and dendrites of the electromotor neurons. Some terminals synapse with two cell processes and form a pathway for electrotonic coupling of the two postsynaptic elements. Direct dendrodendritic electrotonic synapses also occur. Within the cord electromotor axons have short internodes (ĨOμm). In rostral regions electromotor axons are smaller in diameter, have shorter internodes and are more thinly myelinated than in caudal regions. In rostral regions preterminal axons are smaller in diameter than in caudal regions. Axons of the bulbospinal (electromotor) tract are slightly larger in caudal regions. All these differences are in the right direction to explain the physiological observation that the synchronous neural volley leading to organ discharge is subject to a greater delay in transmission through the electromotor nucleus in rostral regions of the cord. This delay plays an essential role in synchronizing the discharge in widely separated regions of the organ. The very slow conduction in preterminal fibres that is indicated from physiological experiments could arise if these fibres were to have very short internode lengths similar to those observed in the electromotor axons.