Projections of individual axons in lamprey spinal cord determined by tracings through serial sections

Abstract

The brain and spinal cord of one adult sea lamprey, Petromyzon marinus, 12 cm in length, were fixed in glutaraldehyde and osmium, stained in toluidine blue, embedded in Araldite, and sectioned serially at 5 P. Over 50 large axons were traced through the spinal cord to their probable cells of origin.The axon of one giant interneuron was followed from the posterior spinal cord rostrally into the lateral medulla of the brain.A formerly unrecognized pair of Muller axons was found to originate from the reticular cells Iz.An unusual feature of some Muller axons was their migration out of the ventral columns and their subsequent branching.The central nervous system of the lamprey contains a number of large neurons which are obvious in isolated preparations and in histological sections. The midbrain and medulla contain about 50 large reticulospinal neurons, the most prominent of which are the Muller and Mauthner cells. Axons of these cells are large, unmyelinated, and usually unbranched; thus, they are well suited to the technique of tracing through serial sections. Locations of Muller and Mauthner axons in the anterior spinal cord have been determined by previous tracings (Rovainen, '67a), but they have not been followed individually to the posterior end of the cord.Giant interneurons are located in the posterior spinal cord and their axons extend towards the brain. These axons have been traced fOr a few centimeters (Rovainen, '67b), but not to their terminations. In the present study the entire brain stem and spinal cord of one animal were sectioned serially, and an attempt was made to trace axons of reticular cells and of giant interneurons over their entire lengths. 4" and was anesthetized by immersion in gm/ml Tricaine. Its brain and spinal cord were exposed by dissection under iced Ringer fluid, and then removed from the notochord and ventral skull. The central nervous system was fmed by immersion in 6% glutaraldehyde and 2% acrolein in 120 mM Tris-HC1 buffer, pH 7.2, on ice for two hours, washed in buffer, and then fixed again in buffered 1% Os04. After dehydration in ethanol it was stained with 0.5% toluidine blue in ethanol and then was embedded in Araldite. Over 20,000 serial transverse sections were cut at 5 p on a steel knife. Sections were arranged on slides in ten rows of 20 each for easy reference to their position along the animal.Large and medium-sized axons were obvious as light, oval structures in transverse sections (figs. 3-5), and they usually changed very little in position and shape in adjacent sections. Reference diagrams of labelled axons in brain and spinal cord were made at intervals of 100 p , patterns of large axons were checked at least at 50 p intervals, and smaller axons were traced through individual sections. The most difficult portion of an axon for tracing was near its cell of origin, where it narrowed to a few microns and lost contrast with the surrounding tissue. Axon

The Transformation of a Unilateral Locomotor Command into a Symmetrical Bilateral Activation in the Brainstem

“…Anatomical experiments also argue against the possibility of a bilateral coupling between RS neurons in lampreys. The dendrites (Martin, 1979) and branching axons (Rovainen et al, 1973) of large RS cells do not cross the midline. However, we cannot exclude that some of the smaller RS cells on one side may project contralaterally to interact with RS cells on the other side.…”

“…The lamprey brainstem is uniquely organized with pairs of identifiable RS cells on the basis of their position, size, and shape (Rovainen, 1967(Rovainen, , 1978Rovainen et al, 1973;Bussières, 1994). Their large size (up to 150 m) combined with their easy access for microelectrode impalement under visual guidance make the brainstem of lampreys an excellent model for examining possible bilateral symmetry pattern in the MLR command.…”

The Contribution of Synaptic Inputs to Sustained Depolarizations in Reticulospinal Neurons

“…In all experiments, the recorded RS cells were located in the middle rhombencephalic reticular nuclei (MRRN) on both sides of the brain. The recorded cells were in all cases the giant Müller cells visually identifiable and bilaterally paired (Rovainen, 1967;Rovainen et al, 1973). Single and paired (bilaterally homologous cells) intracellular recordings of RS cells were performed.…”

Recovery of Neurofilament Expression Selectively in Regenerating Reticulospinal Neurons

“…Similar heterogeneity in regenerative ability was reported previously (Swain, 1989;Davis and McClelland, 1994a,b). Regenerating neurites grow through the same scar domain, and axons of both regenerating and nonregenerating neurons project in the same spinal tracts proximal to the transection (Rovainen et al, 1973;Rovainen, 1976;Selzer, 1978;Yin and Selzer, 1983;Lurie and Selzer, 1991a,c). Furthermore, because neurons whose axons regenerated well are located adjacent to neurons whose axons regenerate poorly, the location of the perikaryon did not determine probability of neurite outgrowth.…”

“…Bifurcations of giant reticulospinal axons have been described in lamprey, but these are exceptional (Rovainen et al, 1973). Although lamprey Mauthner cells have one or more medial dendrites that cross the midline, the mutual inhibition seen between Mauthner cells of teleosts is absent in the lamprey (Rovainen, 1967).…”