Donald F. Buxton

Origins and collateralization of corticospinal, corticopontine, corticorubral and corticostriatal tracts: a multiple retrograde fluorescent tracing study

Cerebral cells of origin for the corticospinal (CST), corticopontine (CP), corticorubral (CR) and corticostriatal (CS) fibers in the rat were identified following the simultaneous retrograde transport of propidium iodide (PI), fast blue (FB), fluorogold (FG) and diamidino yellow (DY). PI was injected into the contralateral C4 spinal cord segment while FB, FG and DY were injected into the ipsilateral medial pontine nuclei, red nucleus and striatum, respectively. Labeled pyramidal neurons projecting corticospinal axons were contralateral to injection in lamina V and ranged in size from small to large. These CST neurons occupied two distinct cortical areas. The cortical neurons of origin for the corticopontine, corticorubral and corticostriatal fibers were ipsilateral to injections. Labeled neurons were localized in cortical lamina V for the corticopontine and corticorubral fibers while corticostriate neurons were located in laminae III, V and VI. The CP, CR and CS labeled cells occupied one large cortical area which topographically included parts of the medial (AGm) and lateral (AGl) agranular cortices and the primary (SI) somatosensory cortex. Considerable overlapping of the cortical neurons of origin for the four motor fiber systems was apparent. More than 98% of the labeled cells were single labeled while less than 2% were double labeled. No triple or quadruple labeled neurons were observed. Hence, morphological evidence is presented that cortical motor neurons project mainly individual, rather than collateral, axons to each of the four motor associated nuclei investigated in this study. However, only a few cortical neurons projected axons simultaneously to a maximum of two nuclei involved in the motor pathways.

Differential sites of origin and collateralization of corticospinal neurons in the rat: a multiple fluorescent retrograde tracer study

Cells of origin for corticospinal fibers in the rat were identified following retrograde transport of Fluoro-Gold (FG), Propidium iodide (PI), Fast blue (FB), and Diamidino yellow (DY) injected unilaterally into lumbar (FG), mid-thoracic (PI), cervical enlargement (FB), and cranial cervical (DY) spinal gray matter. Most labeled neurons were contralateral to injection in lamina V and ranged from small to very large. These cells occupied two distinct cortical regions: one rostral and the other larger and more caudal. Neurons of the rostral region projected axons solely to cervical spinal segments whereas neurons of the caudal region projected fibers to all spinal segments. Somatotopically, most neurons projecting to lumbar segments were most medial. More than 98% of all labeled cortical neurons contained only a single fluorescent tracer; however, within a single tissue section each of the 4 tracers could be found in these single labeled neurons. The few double labeled neurons contained only cervical (DY + FB) or thoracolumbar (PI + FG) tracers. No triple or quadruple labeled cells were seen. Hence morphological evidence is presented that corticospinal axons branch to terminate in more than one spinal region, but these collateral terminations are restricted to only a few adjacent spinal segments.

Quadruple labeling of brain-stem neurons: a multiple retrograde fluorescent tracer study of axonal collateralization

Four different fluorochromes were injected into adjacent cervical spinal cord segments, 1 unique tracer per segment. Each tracer, Fluoro-Gold, Fast Blue, Diamidino Yellow dihydrochloride and Propidium Iodide, was taken up by axonal terminals and transported intra-axonally in a retrograde direction to the cell bodies. Some, though by no means all, of these axons were stem axons with terminals in 2, 3 or 4 of the injected spinal segments. Hence as many as 4 different fluorescent tracers could be discerned simultaneously within individual neuronal somata of origin using fluorescent microscopy. These results extend the possibilities for multiple interconnection determinations within the central nervous system. Specifically, the potential for individual neurons of a nucleus to project collateral branches of a stem axon to as many as 4 different central nervous system nuclei now can be studied simultaneously using these 4 fluorescent tracers.

The lateral and medial compartments of the olfactory tubercle and their relation to olfactory-related input as determined by artificial neural network analysis

The current literature indicates that olfactory bulbar input projects throughout layer IA of the entire olfactory tubercle, with apparently more fibres in the lateral part than in the medial part of the tubercle. In addition, olfactory cortical association fibers project to layers IB, II, and III in all regions of the tubercle. This study exploited the phenomenon of transsynaptic transfer of WGA-HRP after injection into the olfactory bulb or rats to explore the degree of olfactory-related input to the tubercle. A computerized image analysis system was employed to quantify the amount of tracer transferred to layer II neurons of the tubercle. Qualitative analysis of the data indicates that the lateral tubercle consists of areas that receive little olfactory-related input. Nonparametric statistical tests and a novel application of artificial neural networks indicate regionally heterogeneous labeling across the tubercle and broad connections between homologous regions of the bulb and tubercle. These results have implications for understanding how olfactory sensory information is integrated into limbic-motor circuits by the olfactory tubercle.

A Proposed Mechanoreceptor Role for the Small Redundant Muscles which Act in Parallel with Large Prime-Movers

Small, apparently redundant, muscles frequently act in parallel with vastly larger prime-movers and we call this morphological entity the “parallel muscle combination” (PMC). The small muscles of PMCs have much higher muscle spindle densities than their large counterparts, and we call the ratio of mean spindle density of the small muscle/s to that of the large muscle/s in a PMC, the PMC SPINDLE RATIO (PMC-SR). In view of this characteristic of PMCs we proposed that the small muscles thereof play a primarily proprioceptive role (Peck et al., 1984).

The inverted multilayer perceptron as a consistency measuring device in neuroanatomical image analysis

This protocol describes the use of optical imaging and artificial neural networks to analyze the complex patterns of transsynaptic transfer of WGA-HRP from olfactory bulbar axon terminals to olfactory tubercle projection neurons. This constitutes a unique application of neural networks to pattern recognition in a neuroanatomical tract-tracing study. Furthermore, the type of neural network used in this study, i.e., the inverted multilayer perceptron, possesses a novel construction that suits it for measuring consistency in data clusters. In the experiment described, the perceptron functioned as a consistency measuring device that revealed nonlinear relationships between the site of bulbar injection and the degree of regional labeling in the olfactory tubercle. Nonparametric statistical tests were used concurrently to validate the results of this previously untested form of the multilayer perceptron.