The structural organization of the ventral posterolateral nucleus in the rat

Abstract

The structural plan of the ventral posterolateral nucleus (VLP) in the rat was analyzed by using a variety of techniques to study the pattern of distribution of the ascending afferent fibers and the synaptology of the neuropil within this somatosensory relay nucleus. Golgi stains, Fink-Heimer methods, HRP labeling methods, and electron microscopy were all used in the analysis. The neurons in VPL are aligned in rostrocaudal and dorsoventral rows that are roughly parallel to the curvature of the external medullary lamina (EML) and curve partially around the rostral pole of the ventral posteromedial nucleus (VPM). Golgi-impregnated sections reveal that the dendritic trees of the VPL neurons conform in general to the laminar pattern of VPL. Thick proximal dendrites extend about 25 mu m from the cell bodies. Most proximal dendrites are aligned with the laminae of VPL but the distal dendrites spread over many laminae within VPL. The inputs from the dorsal column nuclei (DCN) end only on proximal dendrites as large, round-vesicle terminals. About 20--25% of the small round-vesicle terminals originate in the cerebral cortex and synapse only on the distal dendrites. The third type of synapse contains many flattened vesicles and is of unknown origin. No serial synapses or vesicle-containing dendrites were observed. Input from the spinal cord projects to two segregated zones which are transitional between the ventral lateral nucleus (VL) and VPL rostrally and between the posterior thalamic complex (PO) and VPL caudally. Each transition zone contains neurons characteristic of both VPL and the adjacent region. Ascending afferent projections were demonstrated by the antegrade transport of HRP following injections into the ventral mesencephalon and by Fink-Heimer stains of degeneration resulting from small lesions of the DCN. Both methods indicate that large-caliber axons course parallel to each other and give off collaterals that diverge to widespread areas of the VPL. The widespread terminal fields that result do not conform to the laminar pattern of the nucleus. Small punctate lesions of DCN result in sparse degeneration that is also widespread in VPL. Structures which appear to be clusters of terminal arborizations of the ascending afferent input were also observed in VPL. These results suggest that somatotopy and modality separation in VPL may be determined either by intrinsic and/or corticothalamic morphological relationships and not by precise topographical ordering of sensory input.

Maturation and Fine Structure of Thalamic Reticular Neurons Transplanted into the Adult Rat CNS

“…M ajor resulting abnormali ties are described here, namely membrane appositions between RT and VB neuronal somata as well as GABA ergic boutons that simultaneously make synaptic contact with GABA-and non-GABA-ir dendrites. Somal mem brane appositions have been described in the RT [17] although no specific membrane differentiation has been demonstrated, but not in the VB [37,38]. It is not clear whether these appositions sustain specific cell-to-cell in teractions or represent only a cytoarchitectural organiza tion [see discussion in 17], In the transplants, they may be due either to a passive phenomenon related to the prox imity of the two populations or to a specific ability o f RT neurons to promote the formation of such appositions.…”

Target-controlled differentiation of axon terminals and synaptic organization.

“…29; D.F., unpublished data). In the rat, these axons terminate in large boutons containing round synaptic vesicles and dark mitochondria (LR-boutons) (16,17). LR-boutons in the normal hamster VB (Fig.…”

“…This can be done when a population of axons projects abnormally to a novel target. We and others (9,11,12,16,17) show that the synaptic complexes of ascending, specific sensory axons in the LGd differ from those in the primary somatosensory (ventrobasal) thalamic nucleus (VB). Therefore, in this study we induced RGC axons in newborn hamsters to form permanent, functional, abnormal retinal projections to VB (18)(19)(20).…”

Synaptic Arrangements in the Ventral Posterolateral Nucleus of the Squirrel

“…In the cat, the Ft pro file is also seen in complex arrangements with the other terminal types. In the rat VPL, terminals are only presynaptic to dendrites and somata but not to other vesi cle-containing profiles [Tripp and Wells, 1978;McAllister and Wells, 1981].…”

“…Although the murine rodents have many features of the organi zation of VPL that are similar to that of cats, there are no complex synaptic ar rangements in \PL [Tripp and Wells, 1978;McAllister and Wells, 1981], which is in sharp contrast to the complex arrange ments that are seen in the VPL of cats [Ral ston and Herman, 1969]. The rat and mouse VPL have no serial synapses, no vesicle-containing dendrites and, very likely, no intrinsic neurons , The question naturally arises whether the relative simplicity of the rat and mouse VPL is a rodent characteristic.…”