Ramanathan, Sankari

2018-05-22T12:47:19Z

2018-05-22T12:47:19Z

2003

The information from whiskers is processed in layer IV of the cortex by groups of neurones arranged in discrete functional units known as barrels. Each barrel processes input obtained from a single whisker. The barrel cortex can be differentiated into the cytochrome rich barrel centres and the septal cells surrounding the barrels. Previous work in the laboratory had established two cortical inputs to striatum from the barrel cortex. One of these arises from septal cells and is bilateral and composed of thin calibre fibres. The other route involves the barrel centres, is only unilaterally represented and is composed of topographically arranged, thick fibres. Based on these morphological differences, the postsynaptic targets of the two pathways with reference to the two output pathways of the striatum were examined. A method was also developed to examine the physiological consequences of stimulation of the two pathways upon the striatal output cells of the anaesthetised rat in both normal and dopamine-depleted animals. An anatomical study of the cortical input to the GABAergic intemeurones was also undertaken as these cells strongly modulate the output of striatal neurones.

The pathways differ in their connectivity, with the bilateral pathway contacting the neurones of the striatopallidal pathway more often than the fibres of the topographic system. The stimulation of the two pathways can depolarise striatal cells and give rise to EPSPs, which can be differentiated based on their rise times. EPSPs in response to whisker pad stimulation have a rapid rise time, while the contralateral cortically derived EPSPs are slower to rise and the spike initiation latency more variable. Both pathways interacted at the level of a single striatal cell and gave rise to a summation of EPSPs at a time interval of 10ms, followed by a period of inhibition, the extent of which was dependent on the order and source of the stimuli. This pattern ofinteraction was not seen in cortical neurones. In dopamine depleted animals both stimuli were also able to depolarise the spiny neurones to their firing threshold. However the EPSPs to whisker pad stimulation were significantly slower to rise compared to control animals and were similar to the rise times of EPSPs in response to contralateral cortical stimulation. The interaction of the two pathways was also affected by the loss of dopamine and the summation of EPSP amplitude observed when stimuli were delivered 1 Oms apart in control animals was no longer present. The anatomical study revealed that GABAergic intemeurones receive convergent cortical input from both motor and sensory cortices and that their pattern of innervation is different from the cortical innervation of striatal output neurones

The results of this thesis suggest that the two inputs from the barrel cortex differ in their physiological influence on striatal neurones, and that they might convey different aspects of somatosensory information to the striatum. The changes observed in dopamine-lesioned animals indicate that the topographic, ipsilateral pathway is selectively affected by the loss of dopamine suggesting that dopaminedepletion does not have a generalized action that is independent of presynaptic or postsynaptic origins. Rather its effects are specific to the neuronal subtype affected as well to the origin of the synapses. The complex pattern of innervation of striatal intemeurones suggests that these cells play a very important role in striatal physiology and that their modulation by dopamine may serve as a possible explanation for the effects seen after lesion in this study

http://hdl.handle.net/1842/30669

The University of Edinburgh

Annexe Thesis Digitisation Project 2018 Block 19

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Somatosensory cortical input to the striatum

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PhD Doctor of Philosophy