Basal Ganglia: Anatomy And Physiology
The basal ganglia consist of the caudate and putamen (dorsal or neostriatum; NS), the internal and external segments of the globus pallidus (GPi and GPe, respectively), the pars reticulata and pars compacta of the substantia nigra (SNr and SNc, respectively) and the subthalamic nucleus (STN).
· The NS is the main receiving area of the basal ganglia and receives information from the whole cortex in a somatotopic fashion as well as the intralaminar nuclei of the thalamus (IL). The major outflow from the basal ganglia is via the GPi and SNr to the ventroanterior – ventrolateral nuclei of the thalamus (VA–VL) which in turn project to the premotor cortex (PMC), supplementary motor area (SMA) and prefrontal cortex. In addition, there is a projection to the brainstem, especially to the pedunculopontine nucleus (PPN), which is involved in locomotion (see Chapter 37), and to the superior colliculus, which is involved with eye movements (see Chapters 25 and 56).
· The basal ganglia also have a number of loops within them that are important. There is a striato – nigral – striatal loop with the latter projection being dopaminergic (DA) in nature. There is also a loop from the GPe to the STN which then projects back to the GPi and SNr. This pathway is excitatory in nature and is important in controlling the level of activation of the inhibitory output nuclei of the basal ganglia to the thalamus. However, although a marked degree of convergence and divergence can be seen throughout the basal ganglia, the projections do form parallel pathways, which at the most simplistic level divide into a motor pathway through the putamen and a non-motor pathway through the caudate nucleus.
· The NS consists of patches or striosomes that are deficient in the enzyme acetylcholinesterase (AChE). These are embedded in an otherwise AChE-rich striatum, which forms the large extrastriosomal matrix. In general, the striosomes are closely related to the dopaminergic nigrostriatal pathway and prefrontal cortex and amygdala, while the matrix is more involved with sensorimotor areas. However, the relationship of these two components of the neostriatum to any parallel pathways is not clear.
• This non-motor role of the basal ganglia is perhaps more clearly seen with the ventral extension of the basal ganglia which consists of the ventral striatum (nucleus accumbens), ventral pallidum and substantia innominata (not shown in the figure). It receives a dopaminergic input from the ventral tegmental area that lies adjacent to the SNc in the midbrain, and projects via the thalamus to the prefrontal cortex and frontal eye fields. These structures are intimately associated with motivation and drug addiction (see Chapter 47).
• The neurophysiology of the basal ganglia shows that many of the cells within it have complex properties that are not clearly sensory or motor in terms of their response characteristics. For example, some units in the NS respond to sensory stimuli but only when that sensory stimulus is a trigger for a movement. In contrast, many units in the pallidum respond maximally to movement about a given joint before any electromyographic (EMG) changes. Thus, from a neurophysiological point of view, the basal ganglia take highly processed sensory information and convert it into some form of motor programme. This is supported by the clinical disorders that affect the basal ganglia (see Chapters 42 and 55).