Two types of motoneurons supplying dorsal fin muscles in lamprey and their activity during fictive locomotion

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Abstract
The location and dendritic morphology of motoneurons supplying the dorsal fin muscles were studied in the lamprey spinal cord (Ichtkyomyzon unicuspis). Motoneurons were retrogradely labelled after injection of HRP into the fin muscles or after its application on the cut ends of the ventral roots. HRP‐labelled cells were subsequently reconstructed, in the horizontal and/or transverse planes. Fin motoneurons were also injected intracellularly with Lucifer Yellow and their detailed three‐dimensional structure was analysed by confocal laser‐scanning microscopy. Unlike myotomal motoneurons, which are closely spaced in the lateral cell column, fin motoneurons were distributed along the spinal cord separately or in pairs. They could be distinguished from motoneurons supplying trunk muscles by having a limited number of dendrites in the lateral part of the spinal cord. In addition, some fin motoneurons extend their dendrites into the dorsal column. The motor cells innervating fin muscles were divided into two types based on their dendritic morphology. Type I have a widespread dendritic tree in the rostrocaudal direction and, with few exceptions, completely restricted to the ipsilateral side. A proportion (25%) of these cells have dendrites extending into the dorsal column. Type II fin motoneurons extend their dendrites both ipsi‐ and contra‐laterally. The contra‐lateral dendrites pass below and above the central canal. The dendrites send off branches into the dorsal columns on both the ipsi‐ and the contra‐lateral sides. Electron microscopic analysis showed that both type I and type II fin motoneurons receive numerous synaptic contacts from dorsal column axons. During fictive locomotion both types of motoneurons are active in antiphase in relation to myotomal motoneurons and to the main locomotor burst.