The basal ganglia and movement disorders.

Brain Update provides practicing nurses with featured aspects of brain biology. PPC offers this feature in an effort to enable nurses to integrate brain biology into their practice. The purpose of this column is to focus on the biological causes of mental illness. Topics of future columns will include the limbic system, the extraphyramidal system, and the role of biology in schizophrenia. Key words: Basal ganglia, caudate nucleus, globus pallidus, lentiform nucleus, putamen, striatum The basal ganglia are a group of subcortical nuclei that lie below the outer rim of brain gray matter. The basal ganglia are composed of three major nuclei: the caudate nucleus, the putamen, and the globus pallidus (Figure 1). Other authors include the subthalamic and substantia nigra nuclei (Kandler, Schwartz, & Jessell, 1991). The caudate and putamen are collectively referred to as the striatum, while the putamen and globus pallidus are referred to as the lentiform nucleus. The basal ganglia, which are located deep within the brain, bridge the cerebral cortex, the diencephalon (various thalamic nuclei), and the midbrain. Visible only when the brain is cut, the basal ganglia are rich both with afferent and efferent projections. Most input (afferent) projections arise from the cerebral cortex, while output (efferent) projections are directed first to the thalamus and then to the prefrontal, premotor, and motor areas of the cortex (Albin, Young, & Penney, 1995; Zweig, 1995). These connections are named in such a way as to identify the beginning and ending point of a particular projection. For example, the striatopallidal pathway begins in the striatum and ends in the globus pallidus; the nigrostriatal pathway begins in the substantia nigra and projects to the striatum. These extensive connections and interconnections form a generous network of parallel loops, which underscores the interdependence of these afferent and efferent pathways. Functions of the Basal Ganglia Brooks (1995) lists several functions of the basal ganglia: 1) preparation for movement, 2) inhibition of unwanted movement, 3) adaptation to novel circumstances, 4) facilitation of reward actions, and 5) promotion of motor learning and planning. Given that basal ganglia projections reach the prefrontal, premotor, and motor cortices, the dysfunctions associated with basal ganglia pathology have clarified that these nuclei are part of the extrapyramidal system. The pyramidal system, or corticospinal tract, begins in the motor area of the cortex and is responsible for voluntary movement. The extrapyramidal system supports the pyramidal system primarily through the preparation for and execution of movement (Middleton & Strick, 1994). Movement disorders occur when extrapyramidal influences are lifted. Basal ganglia also may facilitate reward activities of the brain. Apparently, the use of cocaine affects the uptake and release of basal ganglia catecholamines, including dopamine (Graybiel, 1995). Catecholamine flooding causes intense perceptions of pleasure and is "addicting" in the broader sense of the word. The basal ganglia, with their rich connectivity to frontal areas, also play a role in cognitive function. Verbal fluency, card sorting, planning, and spatial and digit working memory are impaired in clients with known basal ganglia deficits (e.g., parkinsonism) (Brooks, 1995). Graybiel (1995) ponders whether movement disorders have analogues in thought control disorders, such as obsessive-compulsive disorder (OCD). Tics, a group of stereotyped repetitive motor disorders that have both motor and cognitive components, are sometimes associated with OCD (Albin et al., 1995). For example, uncontrolled verbal obscenities are part of the symptoms found in clients with Tourette's syndrome. Disorders of the Basal Ganglia Most knowledge of basal ganglia function stems from postmortem examination of basal ganglia nuclei in Parkinson and Huntington's disorder clients. …