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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