Basal Ganglia

Overview

The term basal ganglia designates the areas of the basal forebrain and midbrain known to be involved in the control of movement and motor learning ( Fig. 26.1 ). The basal ganglia comprise the following:

• The striatum (caudate nucleus, putamen, and nucleus accumbens; the nucleus accumbens and the olfactory tubercle are known as the ventral striatum )

• The globus pallidus , which is comprised of an external (lateral) segment (GPe) and an internal (medial) segment (GPi); together they are known as the dorsal pallidum . The internal segment extends into the midbrain as the pars reticulata (or reticular part) of the substantia nigra (SNpr) .

  Anteriorly and inferiorly at the level of the anterior commissure the lentiform nucleus is indented by the commissure, and that portion of the globus pallidus below the commissure is called the ventral pallidum .

• The subthalamic nucleus (STN)

• The pigmented pars compacta (or compact part) of the substantia nigra (SNc)

• The putamen and pallidum together are also called the lentiform nucleus .

Most neurons (90%) within the striatum are GABAergic inhibitory projection neurons (medium-sized spiny neurons) that form two functional subgroups expressing different receptors. One subgroup projects to the GPi and the SNpr; this constitutes what is called the direct pathway that promotes activity (‘go’). The other subgroup projects to the GPe, which then projects to the STN; this is the indirect pathway that elicits inhibition (‘no go’).

Both groups of projection neurons receive excitatory glutaminergic cortical inputs as well as modulatory dopaminergic inputs from the SNc. While different receptors exist on striatal projection neurons, dopaminergic receptors appear ‘unique’ and receive their input from nigrostriatal (SNc) afferents. Dopamine receptors on direct pathway projection neurons are referred to as D ₁ -type and neurons of the indirect pathway are called D ₂ -type ; however, dopaminergic input is excitatory for D ₁ neurons, promoting the direct pathway, and inhibitory for the D ₂ neurons of the indirect pathway.

The striatum can be subdivided into multiple topographic areas that receive their input from different cortical areas and project to different thalamic nuclei or SNc and functional areas called the matrix and striosome . The striatal neurons of the direct and indirect pathways are within the matrix and receive input from the cortex, thalamus, and portions of the SNc. Those neurons within the striosome receive their input from the limbic cortex, amygdala, and different areas of the SNc, and a mechanism by which the basal ganglia influence and are influenced by the limbic system.

The GPi and SNpr represent the output nuclei of the basal ganglia; these are tonically active inhibitory GABAergic neurons that project to the thalamus and brainstem nuclei, and whose activity is modified by the direct and indirect pathway ( Fig. 26.2 ). (There is one other projection from the cerebral cortex directly to the subthalamic nucleus, the hyperdirect pathway , which allows the cerebral cortex to ‘bypass’ the striatum and directly activate this nucleus to inhibit motor activity.) While shown as separate pathways, the direct and indirect pathways’ main role is to reinforce a chosen motor act, inhibit those that are unchosen and thereby ensure the activation of the specific motor program that is required.

The remaining striatal neurons (10%) are interneurons. Most use acetylcholine as their neurotransmitter (giant aspiny cholinergic interneurons) and the remainder are GABAergic. These interneurons have a direct modulating effect on both subgroups of striatal projection neurons through their presynaptic effects on glutamate release from corticostriatal pathways and dopamine release from nigrostriatal terminals. Some of these interneurons receive dopaminergic input from the SNc, while others modulate the activity of striatal interneurons.

Basic Circuits

It is possible to demonstrate at least four circuits that predominantly begin in the cerebral cortex, traverse the basal ganglia, and return to the cortex:

• 1.

  A motor loop , concerned with voluntary and learned movements.

• 2.

  A cognitive loop , concerned with planning and motor intentions.

• 3.

  A limbic loop , concerned with emotional aspects of movement.

• 4.

  An oculomotor loop , concerned with voluntary eye movements, saccades.

Motor Loop

The motor loop commences in the sensorimotor cortex and returns there via the striatum (predominantly the putamen), thalamus, and the supplementary motor area (SMA).

Fig. 26.2 is a schematic wiring diagram depicting the component parts of this motor loop. Two pathways are known. The direct pathway involves five consecutive sets of neurons (see Fig. 26.2A ). The indirect pathway adds the STN to the circuitry and involves seven sets of neurons (see Fig. 26.2B ). Two pathways comprise the output from the GPi ( ansa lenticularis and lenticular fasciculus ) and project to the thalamus, as shown in Fig. 26.3 .

All projections from the cerebral cortex arise from pyramidal cells and are excitatory (glutaminergic). So too is the projection from the thalamus to the SMA. Those from the striatum and from both segments of the globus pallidus (GPi, GPe, and the ventral pallidum) arise from medium-sized spiny neurons and are inhibitory. They are GABAergic and also contain neuropeptides of an uncertain role.

The nigrostrial pathway projects from the SNc to the striatum, where it forms two types of synapses upon those projection neurons (see Fig. 26.2 ): those synapsing upon direct pathway neurons are facilitatory, by way of dopaminergic type 1 (D ₁ ) receptors on their dendritic spines; and those synapsing upon indirect pathway neurons are inhibitory, by way of dopaminergic type 2 (D ₂ ) receptors. Cholinergic interneurons within the striatum are excitatory to projection neurons and are inhibited by dopamine.

A healthy substantia nigra is tonically active, favouring activity in the direct pathway. Facilitation of this pathway is necessary for the SMA to become active before and during movement. SMA activity immediately prior to movement can be detected by means of recording electrodes attached to the scalp. This activity is known as the (electrical) readiness potential , and its manner of production is described in the caption to Fig. 26.4 . Impulses pass from the SMA to the motor cortex, where a cerebello–thalamo–cortical projection selectively enhances pyramidal and corticoreticular neurons within milliseconds prior to discharge.

The putamen and globus pallidus are somatotopically organised, permitting selective facilitation of neurons relevant to (say) arm movements via the direct route, with simultaneous inhibition of unwanted (say) leg movements via the indirect route. For suppression of unwanted movements, the STN, acting upon that segment of the body map in the GPi, is especially important. Progressive failure of dopamine production by the SNpc is the precipitating cause of Parkinson disease (PD) ( Clinical Panel 26.1 ).

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