CHAPTER 6. THE BASAL GANGLIA

OBJECTIVE:
    BE ABLE TO IDENTIFY IN GROSS SPECIMENS, BRAINSTEM SECTIONS, AND WHEN POSSIBLE IN MRIs, THE BASAL GANGLIA AND THE TRACTS, NUCLEI, AND CEREBRAL CORTICAL AREAS ASSOCIATED WITH THEIR INPUT AND OUTPUT.
Corpus Striatum

Identify the corpus striatum, comprised of the caudate and lentiform nuclei which are continuous with each other at the level of the anterior commissure (Pl. 33; Sls. 48, 49). They are also connected by streaks of gray matter passing through the anterior limb of the internal capsule (Pls. 22, 23). These streaks provide the striated appearance after which this nuclear mass was originally named.

The caudate nucleus is a comma-shaped structure located in the wall of the lateral ventricle. It is divided into three parts: head, body, and tail. The head is larger and can be seen protruding into the lateral wall of the anterior horn of the lateral ventricle in brain specimens, MRIs, and slides (Pls. 22, 23, 24, 31, 32; MRI 1, 2, 3, 8, 9, 10; Sl. 45). It is located anteromedial to the anterior limb of the internal capsule (Pls. 31, 32, 33; Sl 47). Posteriorly, the head tapers and, at about the level of the interventricular foramen, it becomes the body which is located dorsolateral to the thalamus (Pls. 25, 26, 27, 28, 29; MRIs 4, 5, 6; Sls. 37, 39, 43). The tail of the caudate nucleus is continuous with the body and arches downward and forward into the temporal lobe. It lies in the roof of the inferior horn (Pls. 27, 28, 29, 30, 31, 32, 33; MRIs 8, 9, 10, 11; Sl. 50) and is continuous with the amygdaloid nucleus in the anterior part of the temporal lobe.

The lentiform nucleus is wedge-shaped and consists of a lateral part, the putamen, and a medial part, the globus pallidus. The putamen is larger and lies medial to the external capsule (Sls. 47, 48, Pls. 22, 23, 24, 25, 26, 27, 31, 32, 33). The globus pallidus is located between the putamen and the posterior limb of the internal capsule (Sls. 37, 38, 39, 40, 41, 43, 44; MRIs 3, 4, 10, 11; Pl. 25, 26). It consists of lateral and medial segments. Ventrally, the medial segment is further divided into inner and outer parts (Sl. 40).

Note in slide 38, the close anatomical relationships of the medial segment of the pallidum to the posterior limb of the internal capsule medially and the optic tract ventrally. It is at this site that pallidotomy was performed for the partial relief of Parkinsonism.

Subthalamic Nucleus

The subthalamic nucleus is located in the subthalamus, the ventral part of the diencephalon that is bordered dorsally by the thalamus, medially by the hypothalamus, and ventrally and laterally by the posterior limb of the internal capsule and the cerebral crus (Pl. 26; Sls. 37, 38). The entire subthalamus contains three nuclei: the zona incerta dorsolaterally, the prerubral field dorsomedially, and the subthalamic nucleus ventrally. The subthalamic nucleus is the largest and appears as a prominent biconvex structure nestled in the arm of the most rostral part of the cerebral crus (Sls. 37, 38). Currently the subthalamic nucleus is the target of choice for the deep brain stimulation that relieves most Parkinson abnormalities.

Substantia Nigra

The substantia nigra is the largest nuclear mass of the midbrain extending throughout its length and even overlapping with the subthalamus rostrally (Sls. 31, 32, 33, 35, 36, 37, 38). It consists of two parts: compact and reticular. The compact part is more dorsal and its cells contain the melanin that gives this nucleus the black color after which it was named (Pls. 16, 27, 28). The cells of the compact part are rich in dopamine and their degeneration results in Parkinson disease. The reticular part is more ventral and intermingles with the fiber bundles of the cerebral crus. It extends more rostrally than the compact and becomes fused with the medial segment of the globus pallidus (Sl. 40).

Afferent Connections

The most prominent input comprises corticostriate projections from the cerebral cortex. Most of these reach the caudate nucleus and putamen directly from the surrounding white matter.

Interconnections

The main internuclear connections of the basal ganglia include reciprocal striatonigral and pallidosubthalamic loops, and a massive striatopallidal projection. The latter can be observed as bundles of myelinated fibers between the putamen and posterior parts of the globus pallidus (Sls. 47, 48, 49) and between the head of the caudate nucleus and anterior parts of the globus pallidus directly (Sls. 48, 49), and via the anterior limb of the internal capsule (Sl. 47).

Efferent Connections

The globus pallidus is the chief efferent nucleus of the basal ganglia and most of its projections terminate in the thalamus. These pallidothalamic fibers arise chiefly from the medial segment and are gathered in two bundles: the lenticular fasciculus and the ansa lenticularis. The lenticular fasciculus (Sls. 37, 38, 39, 40) is more posterior and, after its fibers gather at the dorsal aspect of the medial pallidal segment, they reach the subthalamus by passing medially through the internal capsule. In the subthalamus the lenticular fasciculus is ventral to the zona incerta until it reaches the prerubral field where its fibers turn dorsally to enter the thalamic fasciculus (Sls. 37, 38). The ansa lenticularis is more anterior (Sl. 49) and its fibers gather along the ventral border of the globus pallidus (Sls. 39, 40). They then pass ventral to the internal capsule and loop dorsally around its medial edge (Sls. 40, 41, 42) to enter the prerubral field of the subthalamus. Here they intermingle with the fibers of the lenticular fasciculus (and with fibers from the cerebellum).

Fibers from the pallidum reach the thalamus in the thalamic fasciculus (Sls. 37, 38, 39, 40, 41, 42) which courses anterolaterally just above the zona incerta. The anatomical relationships of the pallidothalamic fibers in the ansa lenticularis, lenticular fasciculus, and thalamic fasciculus can be observed in a horizontal section (Sl. 49).

The pallidothalamic fibers terminate chiefly in the ventral anterior nucleus (VA), which in our slides is located at the levels of the optic chiasm (Sls. 43, 44) and the anterior commissure (Sls. 45, 46).

In addition to these well-known pallidothalamic connections, the reticular part of the substantia nigra also projects directly to the thalamus. These nigrothalamic fibers, chiefly concerned with head and eye movements, terminate in the part of the ventral anterior nucleus near the mamillothalamic tract (Sl. 43, 44).

The ventral anterior nucleus projects to the premotor area and adjacent parts of the frontal lobe chiefly through the anterior limb of the internal capsule.