The Nervous System - Advanced Version / The Brain

submitted by Dr. Gary Farr Last Updated June, 24, 2002

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Beneath the cerebral cortex is a mass of white matter, which is composed of nerve fibers projecting to and from the cerebral cortex, commissural systems connecting the two hemispheres via the corpus callosum, and association fibers connecting different regions of a single hemisphere. Myelinated fibers projecting to and from the cerebral cortex form a concentrated fan-shaped band, known as the internal capsule. In horizontal sections of the brain, the internal capsule can be seen to consist of two parts: (1) an anterior limb, between the caudate nucleus and the putamen; and (2) a larger posterior limb, running between the thalamus and the globus pallidus and putamen. These two limbs form an obtuse angle with the apex directed toward the centre of the brain; the junction is called the genu.

Deep within the white matter are fluid-filled cavities that form the ventricular system. These cavities include a pair of C-shaped lateral ventricles with anterior, inferior, and posterior “horns” protruding into the frontal, temporal, and occipital lobes, respectively. Most of the cerebrospinal fluid is produced in the ventricles. About 70 percent of the fluid produced by the central nervous system is secreted by the choroid plexus, a collection of blood vessels in the walls of the lateral ventricles. The fluid drains via interventricular foramina, or openings, into a slitlike third ventricle, which, situated along the midline of the brain, separates the symmetrical halves of the thalamus and hypothalamus. From there it passes through the cerebral aqueduct in the midbrain and into the fourth ventricle in the hindbrain. Openings in the fourth ventricle permit cerebrospinal fluid to enter so-called subarachnoid spaces surrounding both brain and spinal cord.
 

Deep within the cerebral hemispheres, large gray masses or nerve cells, called nuclei, form components of the basal ganglia. Four nuclei can be distinguished:

(1) the caudate nucleus,
(2) the putamen,
(3) the globus pallidus, and
(4) the amygdala.

The amygdala is the oldest of the basal ganglia and is therefore often referred to as the archistriatum; the globus pallidus is known as the paleostriatum, and the caudate nucleus and putamen are together known as the neostriatum, or simply striatum. The putamen and the adjacent globus pallidus are referred to descriptively as the lentiform nucleus, while the caudate nucleus, putamen, and globus pallidus form the corpus striatum.

The putamen lies deep within the cortex of the insular lobe, while the caudate nucleus has a C-shaped configuration that parallels the lateral ventricle. The head of the caudate nucleus protrudes into the anterior horn of the lateral ventricle, the body lies above and lateral to the thalamus, and the tail is in the roof of the inferior horn of the lateral ventricle. The tail of the caudate nucleus ends in relationship to the amygdaloid nuclear complex, which lies in the temporal lobe beneath the cortex of the uncus.

There is an enormous number of neurons within the caudate nucleus and putamen; these are of two basic types, spiny and aspiny. Spiny striatal neurons are medium-size cells with radiating dendrites that are studded with spines. Axons of these cells project beyond the boundaries of the neostriatum. All afferent systems entering the neostriatum terminate upon the dendritic spines of spiny striatal neurons, and all output is via axons of the same neurons. Chemically, spiny striatal neurons are heterogeneous—that is, most contain more than one neurotransmitter. Neurotransmitters identified in spiny striatal neurons are gamma-aminobutyric acid (GABA), substance P, and enkephalin, with overwhelming dominance by GABA.

Aspiny striatal neurons have smooth dendrites and short axons confined to the caudate nucleus or putamen. Small aspiny striatal neurons secrete GABA, neuropeptide Y, somatostatin, or some combination of these. The largest aspiny neurons are evenly distributed cholinergic neurons that play a crucial role in maintaining the balance of dopamine and GABA.

Because the caudate nucleus and putamen receive varied and diverse inputs from multiple sources that utilize different neurotransmitters, they are regarded as the receptive component of the corpus striatum. The most massive input originates from virtually all regions of the cerebral cortex, with the connecting corticostriate fibers containing the excitatory neurotransmitter glutamate. In addition, afferent fibers originating from the substantia nigra in the midbrain or from intralaminar thalamic nuclei in the diencephalon project to the caudate nucleus or the putamen. The neurotransmitter secreted by thalamostriate neurons has not been identified, while neurons from the substantia nigra synthesize dopamine. All striatal afferent systems terminate in patchy arrays referred to as strisomes; areas not receiving terminals are called the matrix. Striatal efferent systems—that is, spiny neurons containing GABA, substance P, and enkephalin—project in a specific pattern onto the globus pallidus and the substantia nigra; GABA, an inhibitory neurotransmitter, is dominant in all neostriatal projections.

The globus pallidus, consisting of two cytologically similar wedge-shaped segments, the lateral and the medial, lies between the putamen and the internal capsule. Fibers terminating on the pallidum arise mostly from the caudate nucleus and putamen; these so-called striatopallidal fibers converge on the globus pallidus like spokes of a wheel. Both segments of the pallidum receive GABAergic terminals, but in addition the medial segment receives substance P fibers, and the lateral segment receives enkephalinergic projections. The output of the entire corpus striatum (i.e., the caudate nucleus, putamen, and globus pallidus together) arises from GABAergic cells in the medial pallidal segment and in the substantia nigra, both of which receive fibers from the striatum. GABAergic cells in the medial pallidal segment and the substantia nigra project to different nuclei in the thalamus; these in turn influence distinct regions of the cortex concerned with motor function. The lateral segment of the globus pallidus, on the other hand, projects almost exclusively to the subthalamic nucleus (in the ventral thalamus), from which it receives a reciprocal input. No part of the corpus striatum projects fibers to spinal levels.

Pathological processes involving the corpus striatum and related nuclei are associated with a variety of specific syndromes characterized by abnormal involuntary movements (collectively referred to as dyskinesia) and significant alterations of muscle tone. Parkinson's disease and Huntington's disease are among the more prevalent syndromes; each appears related to deficiencies in the synthesis of particular neurotransmitters.

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