The role of the basal ganglia in voluntary movements is to initiate muscle activity and generate a smooth sequence of muscular actions that achieves the intended goal.
The basal ganglia are at the centre of a series of neural circuits through which movements can be initiated and sequenced appropriate to events in our environment, memory, emotions or thought processes. The association cortex concerned with each of those functions is connected to a specific area of the basal ganglia, which in turn relays infomation back to the same area of the cortex via the thalamus.
The premotor and supplementary motor cortices (Brodmann's area 6) and the bi-directional connections with the basal ganglia are essential for the planning, initiation, programming and sequencing of movements. The smooth sequencing of movements appears to depend on a balance between the activities of two neural pathways --direct and indirect - and the ability of two areas, the substantia nigra, and the subthalamic nucleus, to modulate this balance.
Each area of association cortex has connections with a specific area of the basal ganglia, which in turn relays infomation back to the same area of the cortex via the thalamus. These networks appear to perform similar tasks irrespective of the function of the cortical area under consideration, such as visual/tactile/auditory maps of 3-dimensional space, attention, emotion, memory or voluntary movements. So subregions of the basal ganglia appear to be concerned with the functions of individual areas of association cortex. All of these areas cortical areas are also linked independently by subcortical association fibres.
Afferent inputs from almost all areas of the cortex, appear to be processed within the basal ganglia using similar pathways consisting of (a) medium spiny neurones in the corpus striatum (b) neurones in the globus pallidus and (c) a relay station in the ventral thalamus, which (d) projects back to the area of cortex from which the afferents originated. A further neural looped circuit involving the substantia nigra and the modulatory effects of the subthalamic nucleus are linked into this network. The caudate nucleus, while anatomically distinct, behaves functionally as part of the striatum.
One of the important connections of the basal ganglia is with Brodmann's area 6 which is involved in planning, organising and initiating voluntary movements, and includes the supplementary motor cortex (MII) and the pre-motor cortex. These areas project to the primary motor cortex (MI Brodmann's area 4) and become active before MI during voluntary movements. Together they are involved in organising sequences of movements into meaningful, goal-directed activity.
Disorders such as Parkinson's Disease and Huntington's Chorea, in which tremors or uncontrollable involuntary movements are evident, are associated with degenerative changes within the basal ganglia.
Below is a diagram of one of the parallel circuits linking an area of cortex with the basal ganglia and returning to the same area of cortex.
GP= Globus Pallidus; Lateral GP= Globus Pallidus Externa; medial GP= Glodus Pallidus Interna
Most cells in the corpus striatum are inhibitory neurones that release GABA at their nerve endings. They have complex dendritic fields densely covered with dendritic spines, hence the name "medium spiny neurons". Most of the synapses on these dendrites are excitatory, using glutamate as a neurotransmitter, and originate in the cortex.
At rest, spiny neurons are hyperpolarised because they have a high potassium conductance, and consequently strong simultaneous excitatory inputs are necessary to fire them.
Spiny neurones project to the globus pallidus, the medial part of the basal ganglia adjacent to the internal capsule, through which it connects with the thalamus. Neurones in the substantia nigra are also inhibited by the GABAergic spiny neurones, but both can become more active by the removal of the GABAergic influence; this removal of inhibition is known as disinhibition, and is the main mechanism by which the pallidal neurones become active.
Within the sub-region of the basal ganglia associated with the premotor-cortex, these changes in the activity lead to movements, as a result of disinhibition within the striatum.
There are also a number of other types of neurones in the striatum, by they account for only a few percent of the total.