THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES Physiology of the Basal Ganglia: An Overview

The major anatomical connections of the basal ganglia are reviewed, emphasizing the inputs to the striatum and efferent projections from the major output nuclei, the internal segment of globus pallidus and the pars reticulata of substantia nigra. The results from lesioning experiments, electrical stimulation, and chronic recording of single neuron activity have provided a wealth of data concerning the physiology of the basal ganglia. Although the deficits resulting from disease of the basal ganglia are well recognized, the specific role which these structures play in the control of normal movements remains speculative. RESUME: Nous revisons les principales connections anatomiques des noyaux gris centraux, insistant particulierement sur les entries au striatum et les projections efferentes des principaux noyaux de sortie: le segment interne du globus pallidus et la pars reticulata de la substance noire. Les resultats d'experiences de lesions, de stimulations et d'enregistrements chroniques monocellulaires, produisent une plethore de donnees sur la physiologie de ces noyaux. Malgre qu'on connaisse bien les deficits secondaires aux maladies de ces noyaux, on ne connait pas encore bien le role specifique de ces structures dans le controle des mouvements normaux. Can. J. Neurol. Sci. 1984; 11:124-128 The basal ganglia have attracted the interest of some of the best known figures in clinical neurology such as Kinnier Wilson, Purdon Martin and Denny-Brown. As a result of careful clinical observations by these individuals and many others, we have some appreciation of the wide variety of neurological deficits which result from disease processes affecting the basal ganglia. Disorders of the basal ganglia are characteristically associated with disturbances of posture and movement, and it has seemed a logical step to assume that these structures play an important role in the control of normal motor activity. However, despite the fact that the basal ganglia have been the focus of intensive anatomical and physiological investigation for the past two decades, the exact nature of this role remains unclear. No attempt will be made to touch on all aspects of basal ganglia physiology in this brief survey. The topic has been covered in depth in several recent reviews (Delong and Georgopoulos, 1981; Kitai, 1981; Marsden, 1982; Penney and Young, 1983). The aim here will be to provide an overview of the normal function of the basal ganglia to serve as a basis for consideration of the pathophysiology of Parkinson's disease (see papers by Marsden and Tatton in in this volume). Anatomical Considerations It is impossible to discuss function without first considering structure and we shall begin by reviewing some of the functionally important anatomical connections in the basal ganglia. The anatomy of the basal ganglia is complex and most of the individual nuclei are interconnected to some degree with all the other nuclei in the basal ganglia. However, there are several general principles of anatomical organization which may provide clues concerning function. First, the majority of the inputs to the basal ganglia originate from the cerebral cortex. In contrast to the cerebellum, the basal ganglia receive no direct sensory input from the periphery. Secondly, a well developed somatotopic organization is maintained at all levels as information passes through the basal ganglia. Thirdly, most of the output from the basal ganglia is directed via the thalamus back to the cerebral cortex. There are no known output projections extending below the level of the mesencephalon. Therefore, the basal ganglia have no direct influence on the activity of spinal or brainstem motoneurons. Their main action is through "intermediaries", particularly various areas of the cerebral cortex. A simplified schematic wiring diagram is presented in Figure 1. Anatomical purists may be disturbed by the fact that this illustration only remotely resembles the normal appearance of the basal ganglia. To emphasize functional connections and the normal flow of information through the basal ganglia, considerable liberty has been taken in the placement of various structures and a number of known anatomical connections have been omitted. The illustration is arranged so that the major inputs to the basal ganglia appear on the left and output connections appear on the right. As a result, components of some structures such as the substantia nigra and globus pallidus have become widely separated in the illustration even though they are normally in close anatomical proximity. The striatum is the major receiving area for afferent input to the basal ganglia. The majority of the inputs of the striatum originate from various regions of the cerebral cortex. These projections have an exitatory effect on striatal neurons and utilize glutamate as a neurotransmitter. Another important input is the well known nigrostriatal dopaminergic system which originates from the pars compacta of the substantia nigra (SNpc). There has been a long debate as to whether dopaminergic inputs exert an excitatory or inhibitory effect on target neurons in the striatum, but there is now general consensus that the main effect is inhibitory. The striatum receives additional inputs from a number of other structures including the intralaminar nuclei and centrum medianum of the thalamus and the dorsal raphe nuclei of the midbrain, but there is only limited information concerning the function of these pathways. From the Department of Clinical Neurosciences, Faculty of Medicine, University of Calgary Reprint requests to: Dr. Robert G. Lee, Department of Clinical Neurosciences, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada T2N 4N1 124 Physiology of the Basal Ganglia — Lee https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0317167100046278 Downloaded from https://www.cambridge.org/core. IP address: 54.191.40.80, on 14 Jul 2017 at 04:41:13, subject to the Cambridge Core terms of use, available at LE JOURNAL CANADIEN DES SCIENCES NEUROLOGIQUES