Organization of spinal motoneuron dendrites in bundles

EXPERIMEKTAL

28, 106-112 (1970)

NEUROLOGY

Organization

of

Spinal

Motoneuron

Dendrites

in Bundles MADGE

E.

SCHEIBEL

AND

ARNOLD

B.

SCHEIBEL’

Departments of Anatomy and Psychiatry, atzd Brairt Research Imtifzrfe, University of CaliforrliaCenter for the Health Sciences, Los Angeles, California 90024 Received

March

lo,1970

In the spinalventral horn of the mature cat and monkey, most motoneuron dendritesshowhigh degreesof organizationalong the rostrocaudalaxis. Horizontal sagittal sections of Golgi-impregnated cord indicate that the majority of thesedendritesare organizedin denselypackedbundles.Each bundleconsistsof dendrite shaftsfrom neuronsof different motor cell columnsrepresentingdifferent functional groups. It is suggested that these dendrite bundles, utilizing both synapticand extrasynapticmechanisms, constitutesubcenters for the integrationof motor output. Introduction

Most histological studies of the spinal cord, including the classic investigations of Ram6n y Cajal (14)) are based on transverse sections which emphasize the radiative, apparently unpatterned aspects of dendrite organization in the ventral horn (17, 23). Several recent studies of sagittally cut spinal cord have revealed a longitudinal orientation of many of the motoneuron dendrites (2, 7, 18, 24) in sharp contrast to the rostrocaudally compressedchiplike dendritic domains of most spinal proprioneurons (18, 20, 22). We now call attention to another aspect of motoneuron dendrite organization apparently seen most clearly in horizontal sagittal sections and, conceivably, of considerable functional interest. Methods The study is based on examination of approximately 500 Golgi-stained of lumbosacral spinal cord cut in the horizontal plane. Material was derived from one adult monkey (formalin fixed) two adult cats (one formalin perfused and one immersion fixed in formalin), three half-grown cats approximately 5 months of age and two of 4 months. Several variasections

1 This study was supported by U. S. Public Health Grant NB-01063. 106

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DEKDRITES

107

tions of the rapid Golgi method (lS, 19) were used in impregnating this material. The resulting specimens were compared to and interpreted in the light of a group of approximately 25,000 Golgi-stained sections from kitten spinal cord (l-90 daysj cut in transverse and sagittal planes and used in previous studies ( 1620. 22). Sissl- and reduced silver-stained preparations were used as controls. Relevant sections were photographed and drawn directly from the microscope. Results

Horizontal sections through the lumbosacral portion of the ventral horn of spinal cord in mature cats and monkeys show that the great majority of motoneuron dendrites stream in the rostrocaudal direction. A very large number (though apparently not all) of these dendrites tend to cluster in groups, running together to form a number of closely packed bundles. Each aggregate is usually made up of a number of dendrites from different motor cells in varying positions and frequently in different motor pools. Each bundle may maintain its individual identity over distances of several millimeters. Since the average motoneuron dendrite is seldom longer than SOO-1200 p ( 1, 1s j, there is a tendency for the constitution of each bundle to change as individual shafts are added or subtracted. Dendrite branches from propriospinal neurons may also be included for variable distances in the bundle although the details of this relationship are not yet clear. Figure 1 shows that cells in different positions in the ventral horn may contribute shafts to the same bundle. On the right side of this figure, the interrelations of three such neurons are examined. Cells A and B are motoneurons of different cell columns while cell C apparently represents a large propriospinal neuron of lamina S. The centrally placed dendrite bundle, a, contains closely packed dendritic components from all three cells as does the laterally lying bundle b. The medial-lying bundle, c, shares dendritic elements of cells B and C alone. Such Golgi dissections depict only a small number of the elements involved (< 10% ‘j, so it is likely that the bundles often include appreciably larger numbers of shafts. The schematic diagram (Fig. 2) shows in greatly simplified form the general organizational plan of the lumbosacral ventral horn based on analyses of horizontal sections. Two important facts are emphasized here. The first is that dendrites from cells of different motor cell columns, and therefore presumably of different functional groups, share the same bundle. The second is that individual dendrites of a single cell may enter different bundles. thereby operating with functionally different dendrite groupings. As examples of the former, bundle s contains dendritic elements from cell 2 of column -1, cells 1 and 5 from column C, and cell 6 from column D.

108

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AND

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600 u

1. Drawing of a horizontal region showing arrangement

FIG.

S-l

sagittal section of 5-month cat spinal of dendrites. Left side: a number of

cord at L-7, motoneurons

and propriospinal neuronIs and their sagittally oriented dendrite systems. Several bundle systems are apparent. Right side: a more lightly stained area showing three neurons and the relations of their dendrite systems. Cells A and B appear to be motoneurons while C is a large medially placed propriospinal neuron. Well formed dendrite bundles are seen at a, b, and c, while a number of transversely oriented dendrites, d, cross the midline white matter as a dendritic commissure. Arrows point to sites where a dendrite divides, sending branches to different bundles. Other abbreviations include vm, ventromedial white matter; VI. ventrolateral white matter. Inset diagram shows plane of section. X 200.

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109

FIG. 2. Horizontal section through lumbosacral cord in diagrammatic form to show relations of cells and dendrites to each other, Motoneuron pools A, B, C, and D, contain motoneurons 1 through 6 while cell 7 is a propriospinal neuron. Various combinations of these dendrites form dendritic bundles r, s, t, and u. All dendrite domains are simplified and individual shafts are shortened to simplify the diagram. See text for details.

Bundle t, on the other hand, contains shafts from cell 1 of column A, cell 3 of column B, and cell 4 of column C. Short dendritic lengths of the propriospinal interneuron, 7, are included in bundles r and u. The second significant point can be illustrated by almost any of the neurons pictured. For instance, cell 1 sends a major dendrite into bundle r and another into bundle t. while cell 3 sends shafts into bundles t and u, respectively. Dendrites estending in the other direction and indicated here only by short stubs can be expected to show similar diversity. Quantitative evaluation of these bundles is not complete, but order-ofmagnitude values can be proposed at this time. Each bundle may contain as few as three or four dendrites or as many as 15 to 25. The diameter of each bundle is highly variable (lo-70 PL) as is its length. We have traced bundles running long the lateral border of the anterior horn for 2500 p or more. The average length of a single clendritic component while running in a bundle is likely to average 400-600 p. Nre conclude that the dendrite population of each bundle changes appreciably during its trajectory through gain and loss of dendrite shafts. We also noted that a certain proportion of

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dendrites (+ 20%) seen in horizontal sections appear not to enter dendritic bundles. Within the individual bundle, dendrite packing is frequently very tight. In many that we have traced, individual shafts remain within 2 p or less of each other, continuously, for several hundred micra. In some cases, the extraneuronal space between two or more dendrite shafts has not been resolvable with high-dry or oil-immersion objective leading us to suspect intradendritic distances of several tenths of a micra at the maximum. The majority of these dendrites are spine-bearing, but the distribution is not nearly as regular as in cortical systems (21) and definitive statements cannot yet be made about their role in possible interactions among elements of each dendritic bundle. Electron microscopic evaluation of these structures is clearly indicated. Discussion

We are familiar with only two references to complex relationships of this sort among sagittally organized dendrites of spinal motoneurons. The first occurs in a comment by Barron in the Ciba Foundation Symposium on spinal cord (3) in which he refers to some rabbit spinal cord preparations of Laruelle where the dendrites were “absolutely interwoven.” The second appears in a recent brief article by Marsh (8) who bases his work on reconstructions from series of transverse Nissl sections showing groups of irregular profiles which he interprets as dendrite bundles. In this study, the bundle formations could be traced for 40-50 p at the most. In addition, since the worker depended on the presence of chromophilic substances for identification of dendrite profiles, he was limited to the proximal portions of dendrite shafts. The significance of this dendrite arrangement remains to be determined, but several possibilities can be suggested. When Renshaw ( 15) first demonstrated that antidromic activation of certain groups of motoneurons condition reflex excitability in other motoneurons, in addition to the well known recurrent collateral hypothesis, he suggested the possibility of perineuronal current flow as a source of the effect. More recently, Nelson and Frank (10) reported field potentials about an active motoneuron for distances of at least 500 p mediolaterally and TOO p dorsoventrally. Some of the measurable extracellular voltage gradients approached 10 mv/mm. Nelson (9) later noted short latency facilitation representing as much as a 307% reduction in firing threshold of adjacent neurons. Rall and his associates (13) have offered evidence for direct dendrodendritic synaptic interactions in the olfactory bulb. They suggest that inhibition is mediated by nonpropagated depolarization of the dendritic tree and present electron microscopic data indicating that both mitral and granule cell dendrites have presynaptic

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and postsynaptic relationships with each other. Somewhat similar mechanisms have been proposed as operating in other parts of the nervous system (4-6) where lateral inhibition appears to contribute processes of interface enhancement or noise suppression (or both). While it may be premature to suggest similar models for the ventral horn of the spinal cord, the antidromic inhibition of Renshaw (15. 16) is generally conceded to represent. or at least to mimic, a mechanism for enhancement of output precision based on lateral or surround inhibition. Dendrites represent SO-90% of the avai!able synaptic surface of motoneurons ( 1 ), and it now appears that much of this membrane area is characteristically packed into bundles. Furthermore, these dendrite bundles are of heterogenous composition and clearly transcend the columnar grouping of cell bodies which also characterizes the ventral horn. Theoretical and empirical studies of Rall and his associates (11. 12) emphasize the capacity of motoneuron dendrites to modulate firing behavior of the neurons and relate the nature and effectiveness of this control to the site of development of the postsynaptic potential along the dendrite. We have previously pointed out that because of the sequential distribution of motoneurons and the consequent partial overlapping of their dendrites aligned in the sagittal plane, each member of a group of such dendrites receiving terminal synaptic clusters from individual primary afferent collaterals could theoretically compute a different function of the same input signal (22). We now further suggest that each dendritic bundle may represent an integrative subcenter for spinal effector activity where, through both synaptic and extrasynaptic mechanisms, the complex processes of motor control are shaped to the momentary needs of the organism. References 1. AITICEK, the 2.

J., and

J. BRIDGER.

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: Topographie,

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neurone und Zwischenneurone in den Segmenten

Arch.

der MotoPsychiat.

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