The anatomical and functional organization of sensorimotor cortex and thalamus in the Belanger’s tree shrew

Abstract

This dissertation explores the anatomical and functional organization of the sensorimotor system of Bellanger’s tree shrews. A special emphasis is placed on establishing the organization of the motor cortex and thalamus in tree shrews using multiple criteria for comparison with primates and their more distant relatives. The results of microstimulation, architectonic, and corticospinal tracing studies are presented first, which provide evidence for a primary and secondary motor cortical area (M1 and M2 respectively), and a number of cortical areas in anterior parietal cortex. The results of tracer injections into these areas are presented in the subsequent two chapters. The corticocortical connections of motor cortex provide evidence for an additional medial motor area (MMA) in cortex, as well as a number of sensorimotor areas in posterior parietal and temporal cortex. The thalamocortical tracing results are combined with detailed histological analysis of thalamus, and provide evidence for a redefinition of a number of motor and somatosensory related thalamic nuclei. A comparison of the of the functional and anatomical organization of the motor cortex and thalamus in tree shrews reveals more shared features in common with primates, especially prosimians, than more distant relatives such as rats. The findings of this dissertation indicate that 1) previous assertions that the motor system of tree shrews contains only primitive features are incorrect, 2) the secondary motor area of tree shrew motor cortex (M2) may represent a transitional form of the premotor cortex of primates, and 3) comparative studies of the motor systems of both primates and other mammals will lead to a better reconstruction of the evolution of the motor system in primates.