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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Projections to the Basal Ganglia

The basal ganglia are divided into several functionally distinct groups of nuclei (Figure 18.1). The first and larger of these groups is called the corpus striatum, which includes the caudate and putamen. These two subdivisions of the corpus striatum are the input zone of the basal ganglia, their neurons being the targets of most of the pathways that reach this complex from other parts of the brain (Figure 18.2). The name (which means “striped body”) is given because the axon fascicles that pass through the caudate and putamen give them a striped appearance when cut in cross section. The destination of the incoming axons from the cortex are onto the dendrites of a class of cells called medium spiny neurons in the corpus striatum (Figure 18.3). The large dendritic trees of these neurons allow them to integrate inputs from a variety of cortical, thalamic, and brainstem structures. The axons arising in turn from the medium spiny neurons converge on neurons in the globus pallidus and the substantia nigra pars reticulata. The globus pallidus and substantia nigra pars reticulata are the main sources of output from the basal ganglia complex.

Figure 18.1. Motor components of the human basal ganglia.

Figure 18.1

Motor components of the human basal ganglia. (A) Basic circuits of the basal ganglia pathway: (+) and (-) denote excitory and inhibitory connections. (B) Idealized coronal section through the brain showing anatomical locations of structures involved in (more...)

Figure 18.2. Anatomical organization of the inputs to the basal ganglia.

Figure 18.2

Anatomical organization of the inputs to the basal ganglia. An idealized coronal section through the human brain, showing the projections from the cerebral cortex and the substantia nigra pars comparta to the caudate and putamen.

Figure 18.3. Neurons and circuits of the basal ganglia.

Figure 18.3

Neurons and circuits of the basal ganglia. (A) Medium spiny neurons in the caudate and putamen. (B) Diagram showing convergent inputs onto a medium spiny neuron from cortical neurons, dopaminergic cells of the substantia nigra, and local circuit neurons. (more...)

Nearly all regions of the neocortex project directly to the corpus striatum, making the cerebral cortex the largest input to the basal ganglia by far. Indeed, the only cortical areas that do not project to the corpus striatum are the primary visual and primary auditory cortices (Figure 18.4). Of those cortical areas that do innervate the striatum, the heaviest projections are from association areas in the frontal and parietal lobes, with substantial contributions from the temporal, insular, and cingulate cortices as well. All of these projections, referred to collectively as the corticostriatal pathway, travel through the internal capsule to reach the caudate and putamen directly (see Figure 18.2).

Figure 18.4. Regions of the cerebral cortex (shown in purple) that project to the caudate, putamen, and ventral striatum (see Box C) in both lateral (A) and medial (B) views.

Figure 18.4

Regions of the cerebral cortex (shown in purple) that project to the caudate, putamen, and ventral striatum (see Box C) in both lateral (A) and medial (B) views. The caudate, putamen, and ventral striatum receive cortical projections primarily (more...)

The cortical inputs to the caudate and putamen are not equivalent, however, a fact that reflects functional differences between these two nuclei. The caudate nucleus receives cortical projections primarily from multimodal association cortices, and from motor areas in the frontal lobe that control eye movements. As the name implies, the association cortices do not process any one type of sensory information; rather, they receive inputs from a number of primary and secondary sensory cortices and associated thalamic nuclei (see Chapter 26). The putamen, on the other hand, receives input from the primary and secondary somatic sensory cortices in the parietal lobe, the secondary (extrastriate) visual cortices in the occipital and temporal lobes, the premotor and motor cortices in the frontal lobe, and the auditory association areas in the temporal lobe. The fact that different cortical areas project to different regions of the striatum implies that the corticostriatal pathway consists of multiple parallel pathways serving different functions. This interpretation is supported by the observation that the segregation is maintained in the structures that receive projections from the striatum, and in the pathways that project from the basal ganglia to other brain regions.

There are other indications that the corpus striatum is functionally subdivided according to its inputs. For example, visual and somatic sensory cortical projections are topographically mapped within different regions of the putamen. Moreover, the cortical areas that are functionally interconnected at the level of the cortex give rise to projections that overlap extensively in the striatum. Anatomical studies by Ann Graybiel and her colleagues at Massachusetts Institute of Technology have shown that cortical regions concerned with the hand (see Chapter 9) converge in specific rostrocaudal bands within the striatum; conversely, regions in the same cortical areas concerned with the leg converge in other striatal bands. These rostrocaudal bands, therefore, appear to be functional units concerned with the movement of particular body parts. Another study by the same group showed that the more extensively cortical areas are interconnected, the greater the overlap in their projections to the striatum.

A further indication of functional subdivision within the striatum is the spatial distribution of different types of medium spiny neurons. Although medium spiny neurons are distributed throughout the striatum, they occur in clusters of cells called “patches” or “striosomes” and in a surrounding “matrix” of neurochemically distinct cells. Whereas the distinction between the patches and matrix was originally based only on differences in the types of neuropeptides contained by the medium spiny cells in the two regions, the cell types are now known to differ in the sources of their inputs from the cortex and in the destinations of their projections to other parts of the basal ganglia. For example, even though most cortical areas project to medium spiny neurons in both these compartments, limbic areas of the cortex (such as the cingulate gyrus) project more heavily to the patches, whereas motor and somatic sensory areas project preferentially to the neurons in the matrix. These differences in the connectivity of medium spiny neurons in the patches and matrix further support the conclusion that functionally distinct pathways project in parallel from the cortex to the striatum.

The nature of the signals transmitted to the caudate and putamen from the cortex is not understood. It is known, however, that collateral axons of the corticocortical, corticothalamic, and corticospinal pathways all make excitatory glutamatergic synapses on the dendritic spines of medium spiny neurons (see Figure 18.3B). The arrangement of these cortical synapses is such that the number of contacts established between an individual cortical axon and a single medium spiny cell is very small, whereas the number of spiny neurons contacted by a single axon is extremely large. This divergence of cortical axon terminals allows a single medium spiny neuron to integrate the influences of thousands of cortical cells.

The medium spiny cells also receive noncortical inputs from interneurons, from the midline and intralaminar nuclei of the thalamus, and from most of the brainstem aminergic nuclei. In contrast to the cortical inputs, the motor circuit neuron and thalamic synapses are made on the dendritic shafts and close to the cell soma, where they can modulate the effectiveness of cortical synaptic activation arriving from the more distal dendrites. The aminergic ones are dopaminergic synapses from a subdivision of the substantia nigra called pars compacta because of its densely packed cells. These synapses are, in contrast, located on the base of the spines in close proximity to the cortical synapses, where they more directly modulate cortical input (see Figure 18.3B). As a result, inputs from both the cortex and the substantia nigra pars compacta are relatively far from the initial segment of the medium spiny neuron axon, where the nerve impulse is generated. Accordingly, the medium spiny neurons must simultaneously receive many excitatory inputs from cortical and nigral neurons in order to become active. The medium spiny neurons are, therefore, usually silent.

When the medium spiny neurons do become active, their activity is associated with the imminent occurrence of a movement. Extracellular recordings show that these neurons typically increase their rate of discharge just before an impending movement. Neurons in the putamen tend to discharge in anticipation of body movements, whereas caudate neurons fire prior to eye movements. These anticipatory discharges are evidently part of a movement selection process; in fact, they can precede the initiation of movement by as much as several seconds. Similar recordings have also shown that the discharges of some striatal neurons vary according to the location in space of the target of a movement, rather than with the starting position of the limb relative to the target. Thus, the activity of these cells may encode the decision to reach toward the target, rather than simply the direction and amplitude of a movement as such.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10988

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