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).
