Somatosensory cortical input to the basal ganglia

Abstract

The mystacial vibrissae in rodents provide the major peripheral input to primary somatosensory cortex via a topographically arranged ascending system to cortex. The cortex in turn integrates this information and provides the major input to the basal ganglia and thalamus. Whilst the ascending system and the cortical integration have been studied extensively the efferent projections from cortex have not. The aim of this study was to investigate the corticostriatal pathways in a highly organised and topographically arranged cortical area using an intracellular in vivo method for the recording, filling and tracing of single corticostriatal neurons.In order to do so a successfull intracellular recording protocol had to be developed (from an existing extracellular method) to ensure maximum neuronal filling. This process was an ongoing and the various stages of development are highlighted. Alongside this a suitable biocytin method had to be developed for the visualisation of terminal fields arising from single neuronal fills. The various stages in the development of this method are also described.This study has resulted in recordings from all four types of layer V pyramidal projection neurons described in vitro . Whilst the morphology was identical to that observed in vitro for the IB and RS neurons the physiological responses to whisker deflection were more complex, especially in IB neuronal firing patterns. In general there was a lot more background synaptic activity observed in all the cellular records. The IB neurons tended to show more complex firing patterns, firing either as mainly single action potentials with occassional burst or predominantly bursts with occassional action potentials. Only rarely did they fire the typical burst firing observed in vitro.. It was apparent from this study that the RS2 and IB 1 neurons were very similar in both their physiology and morphology and were more difficult to classify.The study revealed two different classes of ipsilateral corticostriatal neurons. One class is similar to the innervation pattern seen in motor cortex. This innervation arose from a lateral branch of the axons of pyramidal tract neurons as they run through the fibre bundles within the striatum. In this case the neuron has the physiology and the morphology of an RS2 neuron. The second class of corticostriatal innervation arose from a typical IB 1 neuron that innervated the striatum in a topographic manner similar to that described in anatomical studies of rat primary somatosensory cortex. The topography appears to be arranged within rows. In the ascending vibrissal system, and within cortex, there appears to be an in -row preference generated in cortex by intracortical connections. The IB input may also be arranged in rows as it may obey the general rules observed in monkey corticostriatal connections where more heavily interconnected cortical regions are more likely to overlap and innervate the same regions of the striatum whilst still retaining a topographically arranged innervation pattern. The conclusions drawn are that the RS2 innervation may result in the initial short latency activation of the striatum whereas the topographic IB innervation may provide a longer latency and more detailed input to the striatum