Thick Axons Research Articles

Altered ratio between axon diameter and myelin sheath thickness in regenerated nerve fibers

The fine structure of regenerated nerve fibers, 6–24 months after a lesion, differed from that of normal nerve fibers in several respects: (1) The thickest regenerated nerve fibers were thinner than the thickest normal ones. (2) Although some axons became rather large, the myelin sheats of the thickest regenerated fibers remained relatively thin. (3) A considerable overlap of myelinated and unmyelinated fibers was found to be due to equal diameters of a proportion of aorophic, myelinated axons with extraordinarily thick sheaths, and of a number of unmyelinated, thick axons. Accordingly, the regression line for the relation between the axon diameter and the myelin sheath thickness of regenerated nerve fibers increased more gradually in slope than did that for normal nerves. These observations were made on regenerated nerve fibers both distal to nerve grafts in dogs and, under more favorable conditions for regeneration, distal to crush lesions in rats. The thickest regenerated axons, and their myelin sheaths, finally reached greater dimensions, relatively and absolutely, in rats than in dogs. It is suggested that the observed reduction of the myelin sheath thickness of the thickest regenerated nerve fibers constitutes tha main cause for the known decrease of the conduction velocity of regenerated nerve fibers.

Structural organization of the interpositus and the dentate nuclei

Summary Terminal patterns of afferent fibers to the interpositus and the dentate nuclei, and the dendritic pattern of these nuclear cells and intranuclear connections were investigated in rats and cats using the rapid Golgi and modified Golgi methods. (1) Three kinds of afferent fiber were distinguished by their fiber course, terminal pattern and fiber size: (A) thick axons terminating directly in the nuclei, which correspond to Purkinje cell axons, (B) medium-calibered, and (C) fine axons that pass through the nuclei giving off collaterals to nuclear cells. The latter two kinds of axon were identified as axons of extracerebellar origin. As in the fatigial nucleus, the interpositus and dentate nuclei are supplied by a number of collaterals derived from afferents to the cerebellar cortex and, furthermore, these afferents of extraneous origin and Purkinje cell axons converge onto nuclear cells, contributing to the formation of dense pericellular plexus. (2) Dendrites of large and medium-sized cells of the dentate nucleus exhibit a radiate type, extending in all directions for 400 μm on the average. Medium-sized cells of the anterior interpositus nucleus differ markedly from those of the posterior interpositus nucleus in their cell form and dendritic patterns. The former are provided with oval or spindle-shaped cell bodies and dendrites extending in a dorsoventral direction for 450 μm, while the latter are multipolar and extend their dendrites in all directions for 280 μm. Small cells of the interpositus and dentate nuclei are oval or spindle-shaped and extend their dendrites in all directions for 200 μm. (3) Axons of large and medium-sized cells of the nuclei contribute to the formation of the brachium conjunctivum, giving off fine and sparsely branched recurrent collaterals within the nuclei. These recurrent collaterals appear to terminate on small cells identified as short-axoned cells by their axonal patterns. This finding suggests that a certain feedback system would be established within the cerebellar nuclei between interneurons (small cells)_and projection neurons (large and medium-sized cells).

Light and electron microscopical observations of the ventral horn and ventral root in long term cultures of the spinal cord of the fetal mouse.

AbstractSegments of spinal cord from 14 to 16 day old fetal mice, grown in vitro for three weeks or longer, show some of the cytoarchitectonic subdivisions of adults cord and several characterstic fiber bundles. In the ventral horns the largest cells have thick axons which leave the explants in well defined ventral roots. These axons run a long course through the outgrowth zone and branch repeatedly there. They have no branches that ramify in the explants nor do they return to the explants. Smaller cells in the ventral horns from a distinct class of cells. Their axons ramify entirely within the explants, mainly within the homo‐ or contralateral ventral horn. Axons of dorsal horn cells also ramify within the ventral horns. Some of their branches pass into a fiber band that surrounds the explant while others enter the outgrowth zone. These last generally return to the explant and form terminal ramifications there.The development of fiber bundles and the appearance of cell groups can be followed in unstained explants, and it appears that each of the characteristic axon patterns develops in vitro.Dendrites of ventral horn cells enter the outgrowth zone and in the ventral roots they are closely related to many passing axons. At some distance from the original explant these processes from a tissue that resembles the central nervous system. New synaptic junctions are formed and the neuronal elements lie embedded amongst glial cells and their processes. The glial cells also from a more or less continuous epithelium separating the feeding solution from the neuronal elements.

Observations On the Function and the Structure of the Statocysts of Lymnaea Stagnalis (L.)

Experiments on the function of the statocyst 1. Normal, unoperated specimens of Lymnaea stagnalis show a positive geotaxis when placed upon a slope in air, and a negative geotaxis upon a slope in oxygen-poor water. The tracks deviate slightly from the perpendicular on the slope. 2. Specimens, from which both statocysts have been removed, have lost the ability of geotactic orientation upon a slope in air as well as in water. 3. Upon a slope, unilaterally statocyst-ectomized snails deviate to the intact body side during positive geotaxis, and to the operated body side when showing negative geotaxis. The structure of the statocysts Light microscopy 4. The epithelium of the statocyst is composed of two types of primary neurosensory cells (high and low sense cells) and of supporting cells. 5. The high sense cells are mainly located at the ventro-medio-caudal side and the low sense cells at the dorso-latero-rostral side of the statocyst. This morphological polarity of the statocyst is discussed with regard to the functional polarity (see 3). Electron microscopy 6. The fine structure of the three epithelial cell types is described. The differences between the high and low sense cells, which are mainly quantitative in nature, are discussed in view of the functional polarity of the statocyst. 7. The fine structure of the cilia, which are only present on the sense cells, was studied. All ciliary basal feet point away from the centre of the cell surface. 8. In the centre of the proximal part of the static nerve, which is composed of two bundles of thin and thick axons, respectively, a short, tube-like diverticulum of the lumen of the statocyst is present. 9. In normal as well as in regenerating statocysts statoliths are formed intracellularly in supporting cells and in undifferentiated cells, respectively.

Studies in neurosecretion. I. Preoptico-subcommissural neurosecretory system in the eel (Anguilla japonica).

The topography and anatomy of a neurosecretory system in the eel, Anguilla japonica, to which a name preoptico-subcommissural neurosecretory system is proposed, being composed of some of the large nerve cells of the preoptic nucleus, the so-called Reissner's fiber and the subcommissural organ, are described. It is morphologically demonstrated that the Reissner's fiber represents significant union of thick axons arising from the pars magnocellularis of the preoptic nucleus, along which neurosecretory material is traceable posteriorly and dorsally as far up as to the subcommissural organ at the base of the epiphyseal stalk. The organ, being incorporated in the neurosecretory system concerned, is deduced to be of functional significance as a storage-release center.