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Internal Connectivity of the GlobusPallidus and the Arbitration System
KTH, School of Computer Science and Communication (CSC), Computational Biology, CB.
KTH, School of Computer Science and Communication (CSC), Computational Biology, CB.ORCID iD: 0000-0002-2792-1622
2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The rodent globus pallidus (homologue of primate external globus pallidus) has been shown to be composed of two types neuronal groups based on their location and local axon collaterals. The rostral outer layer near the striatopallidal border (GPr) has shorter but more dense local axon collaterals while the caudal inner layer (GPc) has wider and less dense axon collaterals. Moreover, the connection between the two segments is unidirectional with outer layer neurons sending inhibitory projections to the inner layer. Both segments inhibit the substantia nigra and the entopeduncular nucleus (homologue of primate internal globus pallidus). We have created a model of the basal ganglia arbitration subsystem composed of the subthalamic nucleus, the two segments of the pallidus as well as the entopeduncular nucleus and the substantia nigra in order to assess functional roles of the two pallidal segments. The simulations reveal that both segments of the pallidum are involved in winner-take-all structure of the arbitration system but the type of information competing is different in the two subsystems. In the STN-GPr network, strong lateral inhibition between pallidal neurons representing muscles leads to selection of a muscle which has been (due to noise or other reasons) randomly overactivated. In contrast, in STN-GPc network actions (each utilizing many muscles) compete. Our simulations suggest that both networks are active during selection and execution of movements. If overactivation of a muscle is accompanied with dopamine flow, the GPr-GPc connection together with local axonal network of GPc suppress other muscles and reinforce the muscle whose overactivity has caused the dopaminergic flow. Simulated lesions of these neuronal groups also show different results. Lesioning GPr results in synchronous activity in GPc and SNr but the mean firing rate of these nuclei remains untouched. Lesioning GPc on the other hand lifts the activity in the SNr drastically but does not create synchrony in any of the nuclei. The results suggest that STN-GPc and STN-GPr can be considered as two different subsystems working both in synergy and in competition.

Place, publisher, year, edition, pages
2013.
National Category
Neurosciences
Research subject
The KTH Railway Group - Tribology
Identifiers
URN: urn:nbn:se:kth:diva-136743OAI: oai:DiVA.org:kth-136743DiVA: diva2:677015
Note

QS 2013

Available from: 2013-12-09 Created: 2013-12-09 Last updated: 2016-02-02Bibliographically approved
In thesis
1. Subsystems of the basal ganglia and motor infrastructure
Open this publication in new window or tab >>Subsystems of the basal ganglia and motor infrastructure
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The motor nervous system is one of the main systems of the body and is our principle means ofbehavior. Some of the most debilitating and wide spread disorders are motor systempathologies. In particular the basal ganglia are complex networks of the brain that control someaspects of movement in all vertebrates. Although these networks have been extensively studied,lack of proper methods to study them on a system level has hindered the process ofunderstanding what they do and how they do it. In order to facilitate this process I have usedcomputational models as an approach that can faithfully take into account many aspects of ahigh dimensional multi faceted system.In order to minimize the complexity of the system, I first took agnathan fish and amphibians asmodeling animals. These animals have rather simple neuronal networks and have been wellstudied so that developing their biologically plausible models is more feasible. I developedmodels of sensory motor transformation centers that are capable of generating basic behaviorsof approach, avoidance and escape. The networks in these models used a similar layeredstructure having a sensory map in one layer and a motor map on other layers. The visualinformation was received as place coded information, but was converted into population codedand ultimately into rate coded signals usable for muscle contractions.In parallel to developing models of visuomotor centers, I developed a novel model of the basalganglia. The model suggests that a subsystem of the basal ganglia is in charge of resolvingconflicts between motor programs suggested by different motor centers in the nervous system.This subsystem that is composed of the subthalamic nucleus and pallidum is called thearbitration system. Another subsystem of the basal ganglia called the extension system which iscomposed of the striatum and pallidum can bias decisions made by an animal towards theactions leading to lower cost and higher outcome by learning to associate proper actions todifferent states. Such states are generally complex states and the novel hypothesis I developedsuggests that the extension system is capable of learning such complex states and linking themto appropriate actions. In this framework, striatal neurons play the role of conjunction (BooleanAND) neurons while pallidal neurons can be envisioned as disjunction (Boolean OR) neurons.In the next set of experiments I tried to take the idea of basal ganglia subsystems to a new levelby dividing the rodent arbitration system into two functional subunits. A rostral group of ratpallidal neurons form dense local inhibition among themselves and even send inhibitoryprojections to the caudal segment. The caudal segment does not project back to its rostralcounterpart, but both segments send inhibitory projections to the output nuclei of the rat basalganglia i.e. the entopeduncular nucleus and substantia nigra. The rostral subsystems is capableof precisely detecting one (or several) components of a rudimentary action and suppress othercomponents. The components that are reinforced are those which lead to rewarding stateswhereas those that are suppressed are those which do not. The hypothesis explains neuronalmechanisms involved in this process and suggests that this subsystem is a means of generatingsimple but precise movements (such as using a single digit) from innate crude actions that theanimal can perform even at birth (such as general movement of the whole limb). In this way, therostral subsystem may play important role in exploration based learning.In an attempt to more precisely describe the relation between the arbitration and extensionsystems, we investigated the effect of dynamic synapses between subthalamic, pallidal andstriatal neurons and output neurons of the basal ganglia. The results imply that output neuronsare sensitive to striatal bursts and pallidal irregular firing. They also suggest that few striatalneurons are enough to fully suppress output neurons. Finally the results show that the globuspallidus exerts its effect on output neurons by direct inhibition rather than indirect influence viathe subthalamic nucleus.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vii, 76 p.
Series
TRITA-CSC-A, ISSN 1653-5723 ; 2913:14
Keyword
Basal Ganglia, Action Selection, Motor Learning, Tectum, Superior Colliculus, Mesencephalic Locomotor Region, Reticulospinal Neurons, Computational Models
National Category
Neurosciences
Identifiers
urn:nbn:se:kth:diva-136745 (URN)978-91-7501-968-0 (ISBN)
Public defence
2013-12-19, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131209

Available from: 2013-12-09 Created: 2013-12-09 Last updated: 2014-02-11Bibliographically approved

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Ekeberg, Örjan

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