The ultrastructure of the lateral spinal nucleus

Réthelyi, Miklós

ABSTRACT

The lateral spinal nucleus (LSN) is located inthe dorsal region of the lateral funiculus in therodent spinal cord. Four to six medium-sizedneurons were found in the cross section of theLSN. The neurons were surrounded by thefibers of the lateral funiculus. Small-diametermyelinated fibers and numerous unmyelinatedfibers were seen. The unmyelinated fiberswere 0.15 to 0.20 µm in diameter; their diametersdid not change, and they did not havesynapsing en passant widening. Axon terminalsin the dorsolateral funiculus (DLF) withvarious types of synaptic vesicles gatheredaround the neuronal perikarya as well as followedthe straight dendrites and establishedaxosomatic and axodendritic synapses. Themost common type of axonal varicosities wasthe small-sized boutons 1.0 to 2.0 µm indiameter loosely filled with round, regularsynaptic vesicles (Type I boutons). Dense corevesicles were regularly present in the varicosities.Type I boutons were found alone, butmore often in groups. A characteristic synapticarrangement was seen, in which Type Ivaricosities completely surrounded a thinnerdendrite (rosette-shaped synaptic complexes).Type I boutons made both symmetrical andasymmetrical synaptic contacts. In the rosetteshapedsynaptic complexes, the synapses wereinvariably of the asymmetrical type.Axon varicosities densely filled with ovoidsynaptic vesicles (Type II boutons) were rarelyseen. Axon varicosities of different diameterscontaining neurosecretory vesicles 130-140nm in diameter were also found (Type III boutons).No synaptic specializations were foundin connections with Type III varicosities. Substance-P labeled axon terminals outlined thedendrites in longitudinally cut sections. Atultrastructural level the SP positive boutonscould be classified as Type I varicosities. Thedendrites of the LSN neurons frequentlycoursed towards the pial surface of the spinalcord. Dendrites were found directly below thebasal lamina or within the range of the mostsuperficial 1 µm-thick layer of the DLF.Superficially located dendrites often receivedsynapsing axon terminals. Numerous LSNneurons could be labeled with antiserumdetecting Type 1a metabotrop glutamatereceptors. The labeled dendrites followed astraight course, approached the pial surface,and received numerous synapsing axon terminals.Some of the neurons could be labeledwith NK-1 receptor antiserum (AU)