Key Points

Coordination of cortical motor commands is achieved by motor loop pathways that pass information from the cortex through the basal ganglia and thalamic nuclei and back to the cortex.

The direct (excitatory) and indirect (inhibitory) pathways of the basal ganglia loop provide opposite, counterbalanced influences on activity of the motor cortex.

The dopaminergic nigrostriatal pathway modulates activity in the direct and indirect pathways, maintaining a critical level of excitatory drive for formulation of coordinated motor commands. Specific motor disorders are caused by deficits in the dopaminergic nigrostriatal pathway (Parkinson's disease), loss of GABAergic neurons in the corpus striatum (Huntington's chorea), or damage to the subthalamus (hemiballismus).

The basal ganglia are a collection of five pairs of nuclei located in the diencephalon, deep to cortical structures (Fig. 1). One of the nuclei, the caudate, has a highly irregular shape. It is long, thin, tapered, and curved into the shape of a ram's horn. It follows the line of the lateral ventricles, with its thin tail adjacent to the temporal horn of the ventricle and its rounded head region adjacent to the lateral aspect of the anterior or frontal horn. The putamen and globus pallidus collectively are referred to as the lentiform or lenticular nucleus because of their combined lenslike shape. These two nuclei, along with the head region of the caudate, are interposed between the cortex and its pre- and postsynaptic targets in the brain stem and spinal cord. The bundles of connecting fibers passing to (spinocortical and bulbar cortical) and from (corticofugal) the cortex are merged in this region, forming a broad band of myelinated processes called the internal capsule, which penetrates the group of three basal ganglia, partitioning the caudate from the lentiform (putamen and globus pallidus) nucleus. The contrast between the gray matter of the nuclei and the white myelinated fibers of the internal capsule gives this region of brain a striped appearance. For this reason, the caudate, globus pallidus, and putamen, along with the penetrating fibers of the internal capsule, collectively are referred to as the corpus striatum. Because of functional similarities, the caudate and putamen are often considered as a single unit, referred to as the neostriatum.

A. Lateral view sensory and motor cortex internal capsule thalamus

B. Coronal section cerebral cortex

A. Lateral view

basal ganglia: caudate putamen globus pallidus

(hidden) sub thalamus substantia nigra: pars compacta pars reticulata basal ganglia caudate putamen cerebral cortex basal ganglia caudate putamen

Substantia Nigra Pars Reticulata Mouse

globus pallidus external internal sub thalamus substantia nigra: pars compacta pars reticulata

FIGURE 1 Anatomy of basal ganglia and thalamic structures. (A) Lateral view of the brain showing the basal ganglia and thalamus lying under the cerebral cortex. (B) Coronal section taken along the line indicated in (A). The basal ganglia are shown in blue; the cortex and thalamus, in gray. Combined nuclear groups of the basal ganglia form a large, knoblike structure beneath the projecting fibers (internal capsule) from the superficial cortical layers. Divisions of the basal ganglia include the long, C-shaped caudate encircling the flattened putamen. Together, these constitute the input nuclei called the neostriatum. The globus pallidus (with internal and external segments) plus the putamen form a lens-shaped mass termed the lenticular nucleus. Below these nuclei lie the subthalamus and the substantia nigra (with the subdivisions pars compacta and pars reticulata). The major targets of the basal ganglia are the ventral anterior, ventral lateral, and centromedian nuclei of the thalamus.

Two additional nuclei of the basal ganglia group, the subthalamic nucleus and substantia nigra, are located inferior and medial to the corpus striatum, just beneath the thalamus. The basal ganglia represent an important component of the motor system. Previously considered as the extra-pyramidal system, the basal ganglia system is now understood to be an integral part of the corticospinal or pyramidal system rather than a standalone circuit. Current theory holds that basal ganglia participate in a side loop pathway that fine-tunes motor instructions as they are being programmed in motor area II (Mil) (Fig. 2). This motor loop is responsible for two specific modulatory functions: (1) scaling motor patterns in the context of the task requirements, and (2) controlling the assembly of overall motor plans. For example, motor instructions for writing your name are assembled in cortical brain regions, but the basal ganglia add a scaling factor for the size of the letters so they are appropriate for large writing on a blackboard or small writing in a checkbook. The overall function of the basal ganglia is to enable automatic performance of practiced motor acts. They do not initiate movement; rather, they adjust and update motor commands in preparation for the next movement in the sequence.

THE MOTOR LOOP: DIRECT PATHWAY

The loop connection between the cortex and the basal ganglia has two branches, a direct pathway and an indirect pathway (Fig. 3). Although both interconnect the cortex, basal ganglia, and thalamus, the overall effects of these two pathways are opposite and tend to counterbalance each other. Fibers in the direct pathway originate from the entire cerebral cortex and project through the corticostriate pathway to terminate in the neostriatum (caudate and putamen). Information is then sent to the internal portion of the globus pallidus and to the substantia nigra. Fibers exit the basal ganglia and innervate the ventral anterior (VA) and centromedian (CM) nuclei of the thalamus. The loop is completed by thalamocortical fibers that project back to the supplementary motor area (SMA) region of the motor cortex. Of the four synaptic relays in this circuit, the first (cortex to basal ganglia) and last (thalamus to cortex) are

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