Primate Basal Ganglia System - Pallido-nigral Set and Pacemaker - Constitution - Lateral Pallidum

Lateral Pallidum

The lateral pallidum is the lateral nucleus of the pallidum, the external "segment" of the globus pallidus (GPe, ). It is flat, curved and very extended (parasagittally and dorsoventrally). The three-dimensional shape of arborisations is discoid and flat. The arborisations are parallel to one another and to the lateral border of the pallidum. They are perpendicular to the striatal afferences. In addition to the striato-pallidal afference, the lateral pallidum receives a major connection from the subthalamic nucleus (see below). It also receives dopaminegic afferences from the nigra compacta. Contrary to two other elements of the basal ganglia core, the lateral pallidum is not a source of output to the thalamus as it sends its axons essentially to other basal ganglia elements (intrasystemic connections). To some extent, it may be seen as an inner basal ganglia regulator. Its mediator is GABA. The very fast spontaneous activity (contrary to that of medial pallidal neurons) is discontinuous with long intervals of silence "lasting up to several seconds or more". Some have low-frequency discharge. The responses to upstream stimulation of striatal neurons on pallidal (2 nuclei) in waking monkeys "consist of an initial inhibition at a mean latency of 14ms, followed by excitation, at a mean latency of 35ms". The excitation was essentially located close to the stimulation electrode and curtailed by excitation. "This arrangement suggests that excitation is used temporarily, to control the magnitude of the central striato-pallidal inhibitory signal and, spatially to focus and contrast it into a restricted number of pallidal neurons". This should be compared to morphological data. Lateral pallidal neurons are often multitargets and may correspond to several hodotypes (neuronal varieties according to the topology of their ways to their targets). From in macaques, the lateral pallidal neurons sends axons in the direction of the striatum only in 15.8%. The other lateral pallidal neurons (84,2%) project to three consecutive targets (medial pallidum, nigra reticulata and subthalamic nucleus) in 13,2% of the cases. The neurons projecting to the medial pallidum and subthalamic targets are 18,4%. Those projecting to the subthalamic nucleus and nigra reticulata 52,6%. The subthalamic nucleus is thus, in 84,2% of the cases, the target of lateral pallidal neurons. In return, the subthalamic nucleus, the privileged target of the lateral pallidum sends the majority of its axons to it (see below).