THE INTRINSIC ANATOMICAL ORGANIZATION OF THE CUNEO-GRACILE NUCLEI COMPLEX OF THE RAT

Abstract

The studies making up the thesis were aimed towards elucidating the intrinsic structural organization of the cuneo-gracile nuclei complex. The rat was the experimental animal of choice but limited data were collected from cats and monkeys. The cytoarchitectonic organization of the rat cuneo-gracile complex was studied to provide the background to other studies. Five neuronal types, each with its differential density distribution were identified within the cuneo-gracile nuclei. These neurons were classified as (a) small round (b) medium round to oval, (c) large round to globular (d) fusiform and (e) polygonal according to their soma shape. The distribution characteristics of these neurons were such as to permit a regionalisation of the rat cuneo-gracile complex into rostral, middle and caudal portions along the rostro-caudal axis. The reference point was the obex and the rostral region was 1.3-1.5 mm in front of the obex. The middle region was 1.3 mm behind the obex. The caudal region was the supervening 1.0 mm caudal of the middle region. of all the three regions, the middle one had the largest cell density distribution. With Golgi techniques it was possible to further define the morphological properties of these five basic neurons. Furthermore, the medium round to oval and the large round to globular neurons were identified to project axons into the medial lemniscus. The small round, fusiform and polygonal neurons were identified as interneurons. One of these, the polygonal neuron, appear to be a special type of interneuron since it not only had local axonal projection but also one which was capable of integrating activities between the different regions of the complex and the adjoining reticular formation; a function in keeping with that of a proprioneuron. On the basis of neuronal dispositions in relation to the afferent domain, sub-species of projecting and non-projecting cells were shown to exist. One species, dorsally placed were predominantly under dorsal column afferent influence. Another species ventrally placed received convergent inputs from the dorsal column, pyramidal tract and reticular formation. A member of the species with convergent input projected its axon dorsally thereby integrating ventral and dorsal on-going activities. Certain complex synaptic networks identified in Golgi studies were confirmed by electron-microscopic studies and in fact the possible substrate for sensory integration and modulation was deemed to reside in these complex synaptic networks. When all of the data obtained by the different techniques are brought together, it is possible to formulate local circuit pathways conceivably operative in the rat cuneo-gracile nuclei complex. It is also possible to utilise the structural data to propose substrates for certain functional mechanisms of somato-sensory processing which have been observed at this level. The results obtained were also further discussed in the light of existing knowledge and possibilities for future research were highlighted especially from the view point of technique.