Topography and ultrastructure of commissural interneurons that may establish reciprocal inhibitory connections of the Mauthner axons in the spinal cord of the tench, Tinca tinca L.

This study was made to identify the inhibitory interneurons belonging to the spinal circuitry activated by the Mauthner axons in the tench (Tinca tinca L.). The histological investigations were focused on a segmental pair of commissural interneurons that were reconstructed in toto from their distinguishing topographical and ultrastructural features. These features are: (a) the adendritic soma located 100-150 microns dorsal to the central canal; (b) the first node of Ranvier which is precommissural and connected to the ipsilateral Mauthner axon via gap junction; (c) the second node of Ranvier, from which two first-order branches arise postcommissurally each supplying roughly the rostral and caudal half of the contralateral spinal cord segment; (d) their second-order branches, which arise at intervals that correspond closely to those of the Mauthner axon collaterals; (e) the postsynaptic targets of the second-order branches, which are exclusively all the motoneurons and interneurons innervated by the contralateral Mauthner axon; (f) the axon terminals of these branches, which contain F-type vesicles, form Gray type-2 synapses, and abut either on the initial segment or on the first node of Ranvier of the target neurons. Thus, it appears that this segmental interneuron has all the appropriate features that could provide the structural basis for the reciprocal fast-acting inhibitory coupling underlying the startle reflex elicited by the Mauthner neurons in response to auditory stimuli.