Macrostructure And Connections Of The Basal Ganglia

Classically, the basal ganglia consist of the globus pallidus, the putamen, and the caudate nucleus. Because of their wedge-shaped appearance in coronal (Fig. 1) and axial (Fig. 2) sections, the putamen and the globus pallidus are sometimes collectively referred to as the lentiform nucleus. The putamen is the more lateral structure of the two, and the globus pallidus is the medial, tapered end of the wedge. The globus pallidus can be divided into a lateral (or external) and a medial (or internal) segment. Despite the fact that they share the same name, the medial and lateral globus pallidus differ in terms of connectivity and function. The medial globus pallidus is in fact frequently considered a homologous structure to the substantia nigra pars reticulata. Even though the putamen and globus pallidus are adjacent and can be referred to with a collective label, they are neither structurally nor functionally homologous. The internal structures of the caudate nucleus and the putamen are quite similar. Although their connectivity is not overlapping, it can be considered to be parallel; this parallelism of connections will become evident when we discuss the connectivity of the basal ganglia. Collectively, the caudate nucleus and the putamen are referred to as the neostriatum or, often, as just the striatum.

When viewed in sagittal section (Fig. 3), the caudate nucleus forms a structure that arches over the lenti-form nucleus and the internal capsule. The head of the caudate nucleus is comparatively bulky and is embedded in the frontal horn of the lateral ventricle, lying posterolaterally to this portion of the ventricular system. The caudate nucleus follows the inner curvature of the lateral ventricle posteriorly, from the head (deep to the frontal lobe) to the body (deep to the parietal lobe) to the tail (deep within the temporal lobe), with the tail ending adjacent to the amygdala. The caudate nucleus tapers considerably as it moves posteriorly from the head through the body to the tail, until the tail becomes difficult to discern.

The internal capsule is a band of white matter that molds itself around the lentiform nucleus and the lateral portions of the caudate nucleus. The anterior limb of the internal capsule separates the caudate nucleus from the putamen and globus pallidus, although at points thin bands of gray matter traverse the internal capsule to connect the caudate nucleus and putamen. The posterior limb of the internal capsule separates the putamen and globus pallidus from the thalamus. The posterior and anterior limbs meet at the genu (or knee) of the internal capsule. The internal capsule contains fibers from the frontal cortex to the brain stem and spinal cord (e.g., corticopontine and corticospinal tracts). The internal capsule also contains reciprocal fibers between the cortex and the thalamus (i.e., corticothalamic and thalamocortical fibers). Pallidothalamic fibers compose a portion of the inferior thalamic peduncle and, therefore, traverse an inferior portion of the internal capsule.

Phylogenetically older structures bearing some similarity to the neostriatum, especially from the standpoint of connectivity, are referred to as the

Orbital Lesion Icd

Figure 1 T1-weighted coronal MRI images showing the basal ganglia. Basal ganglia structures can be traced from the more anterior sections (upper left) to the more posterior sections (lower right). The slices are 1 mm thick, with one slice every 3 mm. The frontal horn of the lateral ventricle (LVFH) is already apparent in the first slice of the top row and can be traced through consecutive slices until it becomes the body of the lateral ventricle (LVB) around the fifth slice of the second row. The head of the caudate nucleus (CNH) is embedded in the frontal horn of the lateral ventricle; it appears in the second slice of the top row and can be traced through consecutive slices until it tapers into the body of the caudate nucleus (CNB) around the fourth and fifth slices of the second row. The putamen (PUT) can be seen as early as the third slice of the first row (lateral to CNH and separated from it by the internal capsule), and remnants of it are visible in the fifth slice of the third row. The nucleus accumbens (NA) is best seen in the first slice of the second row; it occupies a position ventral to CNH and PUT and seems to join the two. The anterior limb of the internal capsule (ICAL) is the band of white matter between CNH and PUT; it is labeled in the second slice of the second row but runs from the third slice of the first row through the third slice of the second row. The globus pallidus (GP) first appears in the third slice of the second row and can be seen until the third slice of the third row. The thalamus (Thal) can first be seen clearly on the first slice of the third row. The thalamus is separated from PUT and GP by the posterior limb of the internal capsule (ICPL). The anterior commissure (AC), which connects the left and right temporal lobes, can be seen in the third slice of the second row.

Figure 1 T1-weighted coronal MRI images showing the basal ganglia. Basal ganglia structures can be traced from the more anterior sections (upper left) to the more posterior sections (lower right). The slices are 1 mm thick, with one slice every 3 mm. The frontal horn of the lateral ventricle (LVFH) is already apparent in the first slice of the top row and can be traced through consecutive slices until it becomes the body of the lateral ventricle (LVB) around the fifth slice of the second row. The head of the caudate nucleus (CNH) is embedded in the frontal horn of the lateral ventricle; it appears in the second slice of the top row and can be traced through consecutive slices until it tapers into the body of the caudate nucleus (CNB) around the fourth and fifth slices of the second row. The putamen (PUT) can be seen as early as the third slice of the first row (lateral to CNH and separated from it by the internal capsule), and remnants of it are visible in the fifth slice of the third row. The nucleus accumbens (NA) is best seen in the first slice of the second row; it occupies a position ventral to CNH and PUT and seems to join the two. The anterior limb of the internal capsule (ICAL) is the band of white matter between CNH and PUT; it is labeled in the second slice of the second row but runs from the third slice of the first row through the third slice of the second row. The globus pallidus (GP) first appears in the third slice of the second row and can be seen until the third slice of the third row. The thalamus (Thal) can first be seen clearly on the first slice of the third row. The thalamus is separated from PUT and GP by the posterior limb of the internal capsule (ICPL). The anterior commissure (AC), which connects the left and right temporal lobes, can be seen in the third slice of the second row.

archistriatum, or sometimes as the limbic striatum, because of their connection to the limbic system. The structures that comprise the archistriatum include the olfactory tubercle and the core portion of the nucleus accumbens (Fig. 1). Likewise, a portion of the basal forebrain region inferior to the anterior commissure is designated as the ventral pallidum and has connectivity similar to that of the globus pallidus except with

Olfactory Tubercle And Nucleus Accumbens

Figure 2 T1-weighted axial MRI images showing the basal ganglia. The basal ganglia can be traced from the more inferior sections (upper left) to the more superior sections (lower right). The slices are 1 mm thick, with one slice every 3 mm. The nucleus accumbens (NA) is best seen on the second slice of the first row; it occupies a position ventral to the head of the caudate nucleus (CNH) and the putamen (PUT). The PUT, CNH, frontal horn of the lateral ventricle (LVFH), and globus pallidus (GP) all can be seen beginning on the fourth slice of the first row. The thalamus (Thal) can first be seen clearly on the second slice of the second row and is visible for four or five slices. As the caudate nucleus and lateral ventricle arch over the thalamus, they become the body of the caudate nucleus (CNB) and the body of the lateral ventricle (LVB), respectively. Again, the anterior limb of the internal capsule (ICAL) separates CNH and PUT, and the posterior limb of the internal capsule separates Thal from PUT and GP.

Figure 3 T1-weighted sagittal MRI images showing the basal ganglia. The basal ganglia can be traced from the more lateral sections (upper left) to the more medial sections (lower right). The slices are 1 mm thick, with one slice every 3 mm. The putamen (PUT) is first clearly visible on the first slice of the second row. The caudate nucleus (CN) is first seen on the third slice of the second row, as it follows the lateral ventricle (LV). The globus pallidus (GP) can be seen in the third and fourth slices of the second row. The thalamus (Thal) can seen in the second slice of the second row and runs through the most medial slice of the series. Again, the anterior limb of the internal capsule (ICAL) separates CNH and PUT, and the posterior limb of the internal capsule (ICPL) separates Thal from PUT and GP.

Figure 3 T1-weighted sagittal MRI images showing the basal ganglia. The basal ganglia can be traced from the more lateral sections (upper left) to the more medial sections (lower right). The slices are 1 mm thick, with one slice every 3 mm. The putamen (PUT) is first clearly visible on the first slice of the second row. The caudate nucleus (CN) is first seen on the third slice of the second row, as it follows the lateral ventricle (LV). The globus pallidus (GP) can be seen in the third and fourth slices of the second row. The thalamus (Thal) can seen in the second slice of the second row and runs through the most medial slice of the series. Again, the anterior limb of the internal capsule (ICAL) separates CNH and PUT, and the posterior limb of the internal capsule (ICPL) separates Thal from PUT and GP.

limbic structures. Although the amygdala and the claustrum are sometimes referred to as part of the basal ganglia, we will not follow this convention here.

The organization of the basal ganglia into circuits with the cortex was described in the late 1980s by Alexander, Delong, and Strick. From the standpoint of connectivity, the basal ganglia are closely related to specific thalamic nuclei and cortical structures. As noted previously, the connectivity of the basal ganglia to these structures follows a predictable pattern that generally involves projections from the cortex to the striatum, from the striatum to the pallidum, from the pallidum to the thalamus, and from the thalamus back to the cortex (Fig. 4). Different portions of the frontal lobe project to different segments of the striatum. Grossly, anterior cingulate cortex projects to the archistriatum, premotor and motor cortex project more heavily to the putamen, and lateral prefrontal cortex projects to the caudate nucleus. The striatum projects to the lateral and medial segments of the globus pallidus. Various segments of the medial globus pallidus project to locations within the ventral lateral, ventral anterior, and dorsomedial thalamic nuclei. These loops involving projections from the cortex to the striatum, from the striatum to the globus pallidus, and from the globus pallidus to the thalamus are

Model Process Photosyntese

Figure 4 Diagram showing the relationship between components of basal ganglia loops. As conceptualized by Alexander et al., both anterior and posterior cortical areas contributed input to the loops, as shown. More recent conceptualizations of these loops (e.g., Middleton and Strick, 2000) are that they are primarily closed, i.e., they involve a single cortical input that is also the target of the loop. DM, dorsal medial nucleus of the thalamus; ILN, intralaminar nuclei; LGP, lateral globus pallidus; MGP, medial globus pallidus; SNPC, substantia nigra pars compacta; SNPR, substantia nigra pars reticulata; STN, subthalamic nucleus; Thal, thalamus; VA, ventral anterior nucleus of thalamus; VL, ventral lateral nucleus of thalamus; +glu, the excitatory neurotransmitter glutamate; —GABA, the inhibitory neurotransmitter gamma amino butyric acid, +/—DA, dopamine that can have an inhibitory or excitatory impact depending on which output neurons it affects.

Figure 4 Diagram showing the relationship between components of basal ganglia loops. As conceptualized by Alexander et al., both anterior and posterior cortical areas contributed input to the loops, as shown. More recent conceptualizations of these loops (e.g., Middleton and Strick, 2000) are that they are primarily closed, i.e., they involve a single cortical input that is also the target of the loop. DM, dorsal medial nucleus of the thalamus; ILN, intralaminar nuclei; LGP, lateral globus pallidus; MGP, medial globus pallidus; SNPC, substantia nigra pars compacta; SNPR, substantia nigra pars reticulata; STN, subthalamic nucleus; Thal, thalamus; VA, ventral anterior nucleus of thalamus; VL, ventral lateral nucleus of thalamus; +glu, the excitatory neurotransmitter glutamate; —GABA, the inhibitory neurotransmitter gamma amino butyric acid, +/—DA, dopamine that can have an inhibitory or excitatory impact depending on which output neurons it affects.

considered closed due to the thalamic projections to frontal cortex. It is worth noting that these thalamocortical projections are reciprocated by direct corticothalamic connections. These cortico-striato-pallido-thalamo-cortico loops appear to be separated from one another at all levels. Although the loops were eloquently examined by Alexander et al. it is worth noting that the influence of the motor loop on motor disorders was explored at least as far back as 1942 with Bucy's paper. The seminal work of Alexander and colleagues described five cortico-striato-pallido-thala-mo-cortical loops: a motor loop, an occulomotor loop, an anterior cingulate loop, a dorsolateral frontal loop, and an orbitofrontal loop. They also predicted that other parallel basal ganglia loops would be discovered.

Indeed, Strick and colleagues described supplementary motor area, ventral premotor, and primary motor cortex loops within the motor domain.

Several other aspects of basal ganglia connectivity should be mentioned. First are the midbrain dopaminergic projections to the striatum. Neurons in pars compacta of the substantia nigra project to the caudate nucleus and the putamen. It is the loss of the dopaminergic neurons projecting to the putamen that is responsible for the motor symptoms of Parkinson's disease. Projections from the ventral tegmental area provide dopaminergic input to the archistriatum. Second, the lateral globus pallidus projects to the subthalamic nucleus that lies below the thalamus, separated from it by an inferior portion of the internal capsule. The subthalamic nucleus, in turn, projects to the medial globus pallidus, creating a kind of subloop in the cortico-striato-pallido-thalamo-cortical loops. Third, the intralaminar nuclei of the thalamus project heavily to the striatum, constituting another source of input to this structure.

As noted previously, cortico-striato-pallido-thala-mo-cortical loops maintain separation at all levels, including the level of the thalamus. The motor loops include portions of the ventrolateral thalamus. The anterior cingulate loop passes through a portion of the dorsomedial thalamus; the oculomotor, dorsolateral prefrontal, and lateral orbitofrontal loops pass through different portions of the ventral anterior and dorsomedial thalamic nuclei.

To summarize, the basal ganglia are structures deep within the cerebral hemispheres that collectively consist of the putamen, the caudate nucleus, the archistriatum, the globus pallidus, and the ventral pallidum. Organization of the basal ganglia is dominated by multiple parallel but segregated loops that start in the cortex and project to the striatum. The striatum, in turn, projects to the medial globus pallidus and the substantia nigra pars reticulata. The latter two structures project to thalamic nuclei that project back to the cerebral cortex. Recent work suggests that the loops can best be considered as closed because the cortical target of the loops is the same as their cortical input. Multiple frontal loops exist, but temporal and parietal loops recently have been discovered. In a subloop, or indirect pathway, fibers from the lateral globus pallidus project to the subthalamic nucleus, which in turn sends fibers to the medial globus pallidus. This anatomic connectivity helps to determine the role of the basal ganglia in behavior and cognition. The nature of the neurotransmitters between these structures provides another clue to how the loops function.

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