The basal ganglia are a group of subcortical nuclei involved in a variety of processes including motor, cognitive and mnemonic functions. One of their major roles is to integrate sensorimotor, associative and limbic information in the production of context-dependent behaviours. These roles are exemplified by the clinical manifestations of neurological disorders of the basal ganglia. Recent advances in many fields, including pharmacology, anatomy, physiology and pathophysiology have provided converging data that have led to unifying hypotheses concerning the functional organisation of the basal ganglia in health and disease. The major input to the basal ganglia is derived from the cerebral cortex. Virtually the whole of the cortical mantle projects in a topographic manner onto the striatum, this cortical information is 'processed' within the striatum and passed via the so-called direct and indirect pathways to the output nuclei of the basal ganglia, the internal segment of the globus pallidus and the substantia nigra pars reticulata. The basal ganglia influence behaviour by the projections of these output nuclei to the thalamus and thence back to the cortex, or to subcortical 'premotor' regions. Recent studies have demonstrated that the organisation of these pathways is more complex than previously suggested. Thus the cortical input to the basal ganglia, in addition to innervating the spiny projection neurons, also innervates GABA interneurons, which in turn provide a feed-forward inhibition of the spiny output neurons. Individual neurons of the globus pallidus innervate basal ganglia output nuclei as well as the subthalamic nucleus and substantia nigra pars compacta. About one quarter of them also innervate the striatum and are in a position to control the output of the striatum powerfully as they preferentially contact GABA interneurons. Neurons of the pallidal complex also provide an anatomical substrate, within the basal ganglia, for the synaptic integration of functionally diverse information derived from the cortex. It is concluded that the essential concept of the direct and indirect pathways of information flow through the basal ganglia remains intact but that the role of the indirect pathway is more complex than previously suggested and that neurons of the globus pallidus are in a position to control the activity of virtually the whole of the basal ganglia.Key words : Striatum ; globus pallidus ; corticostriatal ; pallidostriatal ; GABA interneurons ; substantia nigra ; synaptic convergence. The basal ganglia are a group of subcortical nuclei involved in a variety of processes including motor, associative, cognitive and mnemonic functions. The dorsal division of the basal ganglia consists of the striatum (or caudate-putamen), the globus pallidus (GP, external segment of the globus pallidus in primates), entopeduncular nucleus (EP, internal segment of globus pallidus in primates, GPi), the subthalamic nucleus (STN) and the substantia nigra Correspondence to Dr J. P. Bolam,

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Subthalamic neurons drive basal ganglia output neurons in resting animals and relay cortical and thalamic activity to the same output neurons during movement. The first objective of this study was to determine the mechanisms underlying the spontaneous activity of subthalamic neurons in vitro and to gain insight into their resting discharge in vivo. The second objective was to determine the response of subthalamic neurons to depolarizing current injection and how intrinsic properties may shape their response to cortical and thalamic inputs during movement. Cell-attached and whole-cell recordings were made from subthalamic neurons in brain slices prepared from 3- to 4-week-old rats. The slow, rhythmic discharge of subthalamic neurons was resistant to blockade of excitatory synaptic transmission indicating that intrinsic currents underlie their spontaneous discharge. A persistent sodium current was the source of current during the depolarizing phase of the oscillation. A powerful afterhyperpolarization following each action potential was sufficient to terminate the depolarization. A long duration component of the spike afterhyperpolarization determined the period of the oscillation and was generated by an apamin-sensitive calcium-activated potassium current. Calcium entry responsible for that current was associated with action potentials. Subthalamic neurons exhibited a sigmoidal frequency-current relationship with the steeper portion starting at approximately 30-40 Hz. This property makes subthalamic neurons more sensitive to input at high firing rates associated with movement than at low rates associated with rest. We propose that the subthreshold persistent sodium current overcomes calcium activated potassium current which accumulates during high frequency firing and underlies the enhanced sensitivity to current >30 Hz.