basal ganglia

basal ganglia In classical neuroanatomy this terms refers to the masses of grey matter lying deeply within each cerebral hemisphere, separated from the outer shell of cerebral cortex by a wide band of white matter. Together these large aggregations of nerve cells or ‘nuclei’ are described as the ‘striate body’ (corpus striatum — striped because it is partly split by bands of white matter). However as a result of much clinical and animal-based basic research, other structures lower in the brain are now included with the basal ganglia on a functional basis, notably the subthalamic nuclei and the substantia nigra.

A rich variety of chemical transmitters was identified within the basal ganglia, and sophisticated anatomical tracing techniques employing radioactive or fluorescent ‘tracers’ to mark out nerve pathways, soon disclosed that they receive information from throughout the frontal lobe cerebral cortex in addition to the motor cortex and frontal eye fields, and also from the substantia nigra in the uppermost part of the brainstem. Moreover, it was shown that the darkly staining neuronal cell bodies of the substantia nigra, responsible for its name, contained the neurotransmitter dopamine, and that the number of these neurons was severely reduced in the brains of patients who were suffering from Parkinsonism at the time of death. Furthermore, dopamine, and other transmitters such as noradrenaline and serotonin, were found to be depleted also in the basal ganglia of these same patients. These observations and the clues they provided to the functional links between these structures led to the remarkable twentieth-century discovery that the substance L-DOPA, the metabolic precursor of dopamine, when given orally in adequate quantities, was very effective in diminishing or abolishing the disabling tremor of what in earlier times was called ‘the shaking palsy’. This localization of the site of the problem promoted more research based on stereotactic surgery (three-dimensional positioning of micro-surgical instruments) which, when combined with electrophysiological and imaging procedures, has greatly benefitted patients so severely disabled by tremor that the surgical relief of symptoms has been necessary. Another feature of Parkinson's disease is ‘akinesis’ — paucity of movement and slowness in starting or finishing movements. Although initiated by an act of will, most movements are carried out automatically; they are implemented through motor programmes refined by practice throughout life. This is the domain that the basal ganglia appear to be involved in. Crucial to this is the fact that the output from the basal ganglia is not only passed to brain stem centres and relayed on to the spinal cord; it also reaches the areas of the thalamus that transmit information back to the cerebral cortex, as well as mediating the control of automated movement by the cerebellum. Still more recent research indicates that this system does not simply function by processing the signal flow in a serial mechanism (as suggested by the classical anatomical studies of connectivity between the cortex, basal ganglia, thalamus, and back to motor cortex). Instead, the system consists of multiple segregated pathways, involving the entire frontal cortex, drawing on parallel processing to permit the planning, execution, and co-ordination of eye and limb movements and, by inference, other frontal lobe processes including those of the ‘limbic system’.

Tom Sears

See also brain; dopamine; grey matter; limbic system; movement, control of.