ABSTRACT
The structures that are found within the depths of the cerebral hemispheres include the cerebral ventricles, the white matter, and the basal ganglia� The ventricles are cavities within the brain filled with cerebrospinal fluid (CSF)� The formation, circulation, and locations of the CSF are explained with Figure 7�8 in Section 3�
The ventricles of the brain are the spaces within the brain that remain from the original neural tube, the tube that was present during development� The cells of the nervous system, both neurons and glia, originated from a germinal matrix that was located adjacent to the lining of this tube� The cells multiply and migrate away from the walls of the neural tube to form the nuclei (including the basal ganglia) and cerebral cortex� As the nervous system develops, the mass of tissue grows and the size of the tube diminishes, leaving various spaces in different parts of the nervous system�
The parts of the tube that remain in the hemispheres are the cerebral ventricles, called the lateral ventricles (known as ventricles I and II, also ventricles 1 and 2)� The lateral ventricle of the hemispheres, shown here from the lateral perspective, is shaped like the letter C in reverse; it curves posteriorly and then enters the temporal lobe� Its various parts are:
• The anterior horn, which lies deep to the frontal lobes�
• The central portion or body, which lies deep to the parietal lobes�
• The atrium or trigone, where it widens and curves and then enters the temporal lobe as the inferior horn�
In addition, there may be an extension into the occipital lobes, the occipital or posterior horn, and its size varies� (The lateral ventricle has been dissected from this perspective-see Figure 9�4�)
Each lateral ventricle is connected to the midline 3rd ventricle (the 3rd ventricle is seen in Figure 1�9 and Figure 3�1) by an opening, the foramen of Monro-the interventricular foramen (see Figure 1�7)� This connection is also seen in the coronal and horizontal sections of the hemispheres (see Figure 2�9A and Figure 2�10A), and the corresponding magnetic resonance imaging scans (see Figure 2�9B and Figure 2�10B)� The ventricular system continues into the brainstem and is represented faintly (ghosted) in this illustration (and is described with the brainstem in Figure 3�1, Figure 3�2, and Figure 3�3)� The flow of CSF in the ventricular system is described in Section 3 (see Figure 7�8)�
The horizontal (axial) view was chosen to visualize the ventricles with a computed tomography (CT) scan because this is the image used most frequently in the clinical setting�
A CT image shows the skull bones (in white) and the relationship of the brain with the skull�
The outer cortical tissue is visible, with gyri and sulci, but there is not as much detail as seen with a magnetic resonance imaging (MRI) scan (shown in Figure 2�9B and Figure 2�10B)� The structures seen in the interior of the brain include the white matter, which is a “fuzzy” speckled gray; the basal ganglia and thalamus can be discerned, as well as the internal capsule�
The ventricular spaces, particularly the anterior horn of the lateral ventricles, can be easily seen�
The cerebrospinal fluid (CSF) is dark (black)� The lateral ventricles are seen at three different levels-note the different configuration depending on the level of the “cut�” The choroid plexus can be seen within the lateral ventricles (see Figure 7�8 and Figure 9�4)�
The cerebellum can be recognized, with its folia, but there is no sharp delineation between it and the hemispheres�
This is a view of the brain from an anterior (coronal) perspective, showing the location of the cerebral ventricles in each hemisphere� Note the reverse C shape of the ventricles and their position more laterally as they enter the temporal lobe�
A coronal cut of the brain through the frontal lobes would show the anterior horns of the lateral ventricles, as well as the very small space of the inferior horn of the lateral ventricle (review of Figure 2�1A may be necessary; see Figure 2�8, Figure 9�5A, and Figure 9�5B)�
Note to the Learner: The 3rd ventricle, aqueduct of the midbrain, and 4th ventricle are “faded” in this view and are shown (and discussed) in Figure 3�1�
The ventricles are again dark with these CT scans� The shape and size of the ventricle changes with the location of the horizontal “cut”—frontal horn, body, or the trigone area�
A bit of the tissue forms a small bulge that protrudes into the anterior horn, normally� This protrusion is formed by the head of the caudate nucleus (see Figure 2�5A and Figure 2�5B); this caudate bulge is also seen on a horizontal view of the brain (see Figure 2�1A and also in Figure 2�2B)�
The cut includes the temporal lobe, and the inferior (temporal) horn of the lateral ventricle is seen as a crescent-shaped slit� In this case, it is the hippocampus that protrudes into this part of the ventricle (discussed with Figure 9�4, Figure 9�5A, and Figure 9�5B)�
Note the tissue inside the ventricles on the cut through the atrium (the one on the far right)� This is some of the choroid plexus of the lateral ventricles, the site of production of the cerebrospinal fluid (see also Figure 9�4, further discussed with Figure 7�8)�
The ventricular system of the diencephalon and brainstem is shown (“ghosted”) and will be shown and
discussed with Figure 3�1, Figure 3�2, Figure 3�3, and Figure 7�8�
Note to the Learner: The same photographic image is used later to develop the understanding of the structures inside the hemispheres, namely, the basal ganglia and the thalamus (see Figure 2�8)�
A CT scan of the head is an X-ray done with computed tomography showing the brain in the interior of the skull� It can be done in seconds and this technology would now be available in most American and Canadian hospitals, even in some rural settings, but not necessarily in remote areas (such as the far north)�
CT scan is a state-of-the-art diagnostic modality which is used to investigate whether there is any “lesion” affecting the brain� For example, blood (caused by a hemorrhage) shows as a “bright” image with CT and the descriptive term for this is “hyper-density�” The loss of blood supply to a region would be seen as a “hypo-dense” region (e�g�, because of the blockage of an artery supplying a part of the brain, as discussed in Section 3)� CT scan is superior to MRI scanning when investigating lesions with high calcium content such as bony lesions or meningiomas�
CT scans can be “enhanced” using an iodinated compound injected intravenously to see whether there is a breakdown of the blood-brain barrier (for example with various tumours and other vascular lesions lesions such as aneurysms or arteriovenous malformations)�
MRI, which uses a powerful magnet and radio waves (and is not an X-ray) provides complementary information to CT scans and produces images to delineate the structures with different water and fat content inside the skull�
The computed tomography scan (particularly the image on the far right) shows an abnormal feature-cortical atrophy� There is a definite increase in the space between the brain and skull seen in these images, as well as in the sulci separating the gyri of the brain� This type of image would be seen in patients with dementia, and further studies of this patient would be warranted based on these images�
One of the other major features of the human cerebral cortex is the vast number of neurons that are devoted to communicating with other neurons of the cortex� These interneurons are essential for the processing and elaboration of information, whether generated in the external world or internally by our “thoughts�” This intercommunicating network is reflected in the enormous number of axonal connections among cortical areas and with other parts of the central nervous system�
The axons of the cortical neurons and the connections to and from these neurons are located within the depths of the hemispheres� With fixation in formalin, these myelinated axons are white, and these areas are called the white matter (see Figure 2�2B and Figure 2�3)� In the spinal cord, these are called tracts; in the hemispheres, these bundles are classified in the following way:
• Commissural bundles-connecting cortical areas across the midline�
• Association bundles-interconnecting the cortical areas on the same side (discussed with Figure 2�3)�
• Projection fibers-connecting the cerebral cortex with subcortical structures, including the basal ganglia, thalamus, brainstem, and spinal cord (see Figure 2�4); many of these
fibers are located within the internal capsule (see Figure 4�4)�
All such connections are bidirectional, including the projection fibers�
In this photograph, the brain is viewed from directly above (see Figure 1�3 and Figure 7�1), with the interhemispheric fissure opened� The dural fold between the hemispheres, the falx cerebri, has been removed from the interhemispheric fissure� This thick sheath of dura keeps the two halves of the hemispheres in place within the cranial cavity (discussed with Figure 7�4 and Figure 7�5)� A whitish structure is seen in the depths of the fissure-the corpus callosum�
The corpus callosum is the largest of the commissural bundles, as well as the latest in evolution� This is the anatomical structure required for each hemisphere to be kept informed of the activity of the other hemisphere� The axons connect to and from the lower layers of the cerebral cortex, and in most cases the connections are between homologous areas and are reciprocal� In fact, the corpus callosum was already seen previously when viewing the cortical tissue on the medial aspect of the hemispheres, as represented by the frontal, parietal, and occipital lobes (see Figure 1�7)� The corpus callosum has been divided in the process�
In this specimen, the blood vessels supplying the medial aspect of the hemispheres are present (fully described in Section 3 with Figure 8�5)� Moreover, the cerebral ventricles are located below (i�e�, inferior to) the corpus callosum (see Figure 1�7, Figure 2�9A, and Figure 2�9B)�
The clinical aspect of the corpus callosum is discussed with Figure 2�2B�
The dissection of this specimen needs some explanation� The brain is again seen from the medial view (as in Figure 1�7; its anterior aspect is on the left side of this photograph) with the corpus callosum exposed� The septum pellucidum is not present, exposing the lateral ventricle, with the head of the caudate nucleus protruding into the anterior horn (as seen previously)� Cortical tissue has been removed from this brain by using blunt dissection techniques� If this dissection is done successfully, the fibers of the corpus callosum can be followed, as well as other white matter bundles (see Figure 2�3)� These fibers intermingle with other fiber bundles that make up the mass of white matter in the depth of the hemisphere�
The corpus callosum is the massive commissure of the forebrain that connects homologous regions of the two hemispheres of the cortex across the midline� In the midline cut, the thickened anterior aspect of the corpus callosum is called the genu, and the thickened posterior portion is the splenium�
This dissection shows the white matter of the corpus callosum, followed to the cortex�
If one looks closely, looping U-shaped bundles of fibers can be seen connecting adjacent gyri; these are part of the local association fibers�
Even though the connections of the corpus callosum are well described, the function of the corpus callosum under normal conditions is difficult to discern� In rare cases, persons are born without a corpus callosum, a condition called agenesis of the corpus callosum, and these individuals as children and adults usually cannot be distinguished from anatomically normal individuals, unless specific testing is done�
The corpus callosum has been sectioned surgically in certain individuals with intractable epilepsy, which is epilepsy that has not been controllable using multiple anticonvulsant medications� The idea behind this surgical procedure is to stop the spread of the abnormal discharges from one hemisphere to the other� Generally, the surgical procedure has been helpful in well-selected cases, and there is apparently no noticeable change in the person or in his or her level of brain function�
Studies done in these individuals have helped to clarify the role of the corpus callosum in normal brain function� Under laboratory conditions, it has been possible to demonstrate in these individuals how the two hemispheres of the brain function independently, after the sectioning of the corpus callosum� These studies show how each hemisphere responds differently to various stimuli, and they also show the consequences of the failure of information to be transferred from one hemisphere to the other�
The dorsolateral aspect of the brain is viewed in this photograph (see Figure 1�3)� The lateral fissure has been opened, with the temporal lobe below; deep within the lateral fissure is the insula (as in Figure 1�4)�
Under the cerebral cortex is the white matter of the brain (see also Figure 9�4)� It is possible to dissect various fiber bundles (not easily) by using a blunt instrument (e�g�, a wooden tongue depressor)� Some of these bundles, functionally, are the association bundles, fibers that interconnect different parts of the cerebral cortex on the same side (classified with Figure 2�2A)�
This specimen has been dissected to show two of the association bundles within the hemispheres� The superior longitudinal fasciculus (fasciculus is another term for a bundle of axons) interconnects the posterior parts of the hemisphere (e�g�, the parietal lobe) with the frontal lobe� There are other association bundles present in the hemispheres that connect the various portions of the cerebral cortex� The various names of these association bundles are usually not of much importance in a general introduction to the central nervous system and are mentioned only if need be� Shorter association fibers are found between adjacent gyri (see Figure 2�2B)�
These association bundles are extremely important in informing different brain regions of on-going neuronal processing, thus allowing for integration of our activities (e�g�, sensory with motor and limbic)� One of the
major functions of these association bundles in the human brain seems to be to bring information to the frontal lobes, especially to the prefrontal cortex, which acts as the “executive director” of brain activity (see Figure 1�3 and Figure 6�13)�
One of the most important association bundles, the arcuate bundle, connects the two language areas� It connects Broca’s area anteriorly with Wernicke’s area in the superior aspect of the temporal lobe, in the dominant (left) language hemisphere (see Figure 4�5)�
Damage to the arcuate bundle from a lesion such as an infarct or tumor in that region leads to a specific disruption of language, called conduction aphasia� Aphasia is a general term for a disruption or disorder of language� In conduction aphasia, the person has normal comprehension (intact Wernicke area) and fluent speech (intact Broca area)� The only language deficit seems to be an inability to repeat what has been heard� This is usually tested by asking the patient to repeat single words or phrases whose meaning cannot be readily understood (e�g�, the phrase “no ifs, ands, or buts” or “the quick brown fox jumped over the lazy dog”)� There is some uncertainty whether this is in fact the only deficit because isolated lesions of the arcuate bundle have not yet been described�
Magnetic resonance imaging shows pathological features of the white matter-seen as hyperintense foci-in certain disease states (e�g�, multi-focal plaques such as in multiple sclerosis MS)� It is evident that disruption of communication among various functional areas disturbs the functioning brain�
This illustration was done with Diffusion Tensor Imaging (DTI, often erroneously called “tractography”), one of the latest imaging modalities but not one used clinically (at the time this text was written)� What is shown is a number of images of “fibers” within the white matter of the hemispheres� These include commissural fibers and projection fibers�
The cerebral cortex is connected to all brain structures� The fibers coming to the cortex are sensory, mainly via the thalamus (discussed with Figure 5�5) and also from
the basal ganglia (discussed with Figure 5�14)� These are sometimes called centripetal fibers� The fibers from the cortex to all “lower centers,” including motor fibers (cortico-spinal and cortico-bulbar, see Figure 5�9 and Figure 5�10), are often called centrifugal fibers� Collectively, they are classified as projection fibers�
The projection fibers course to or from the cortex (the upper figure), are found within the white matter (the corona radiata, in the middle figure), and collect in a funnel-like manner as the internal capsule (the lower figure)�
Many of the fibers to and from the cerebral cortex course through the internal capsule, which is shown in horizontal views of the brain (photograph and magnetic resonance imaging scan-see Figure 2�10A and Figure 2�10B) and is also illustrated in Figure 4�4�
The large collections of gray matter within the hemispheres, belonging to the forebrain, in addition to the white matter and the ventricles already described, are collectively called the basal ganglia� The term striatum is often used for the basal ganglia, but this term is not always used with neuroanatomical precision�
Our understanding of the functional role of the basal ganglia is derived largely from disease states affecting these neurons and their connections� In general, humans with lesions in the basal ganglia have some form of motor dysfunction, a dyskinesia (i�e�, a movement disorder)� However, as discussed in this chapter, these neurons have connections with both neocortical and limbic areas and are definitely involved in other brain functions�
The basal ganglia are described in a series of illustrations� This diagram is for orientation and terminology� Figure 2�5B contains more anatomical details and the functional aspects� The details of the connections and the circuitry involving the basal ganglia are described in Section 2 (see Figure 5�14 and Figure 5�18)�
From the strictly anatomical point of view, the basal ganglia are collections of neurons located within the hemispheres� In the upper illustration, the brain is viewed in the coronal plane partially sectioned midway between the frontal and occipital poles� The basal ganglia are seen in their anatomical location-on the proximal side and somewhat on the distal side�
The structures visualized include the caudate nucleus, the putamen, and the amygdala (in the temporal lobe; see Figure 2�9A and Figure 2�10A)� The caudate and putamen are also called the neostriatum; histologically, these are the same neurons, but in the human brain they are partially separated from each other by projection fibers located within the internal capsule (see Figure 4�4)� The development of the human brain includes the evolution of a temporal lobe, and many structures “migrate” into this lobe with the lateral ventricle� The caudate nucleus follows the curvature of the lateral ventricle into the temporal lobe (see Figure 2�5B and Figure 9�7)�
If the basal ganglia of the proximal side are removed, another structure is seen on the distal side-the globus pallidus, which is also part of the basal ganglia� As seen in Figure 2�5B and in a horizontal cut through the hemisphere (see Figure 2�10A and Figure 2�10B), the putamen and globus pallidus are anatomically grouped together in
the human brain and form a lens-like configuration, hence their collective name, the lentiform or lenticular nucleus� The name is purely descriptive (and somewhat confusing) because the two nuclei are located together in the human nervous system, yet they are functionally quite distinct and do not constitute a true nucleus�
The amygdala, also called the amygdaloid nucleus, is classically one of the basal ganglia because it is a subcortical collection of neurons (in the temporal lobe)� Most of the connections of the amygdala are with limbic structures (see Section 4), and so this nucleus is discussed in Section 4 (see Figure 9�6A and Figure 9�6B)� Another functional area of the basal ganglia has now been recognized as highly important-the nucleus accumbens� Again, this nucleus has limbic connections and is discussed in Section 4 with the limbic system� Other subcortical nuclei located in the forebrain, particularly in the basal forebrain region, have not been grouped with the basal ganglia and will be described with the limbic system (in Section 4)�
Functionally, the basal ganglia system acts as a subloop of the motor system by altering cortical activity (fully discussed in Section 2 with Figure 5�14, under the topic of Motor Modulation)� In general terms, the basal ganglia receive much of their input from the cortex, including from the motor areas and from wide areas of association cortex, as well as from other nuclei of the basal ganglia system (described later)� There are intricate connections among the various parts of the system, involving different neurotransmitters; the output is directed via the thalamus mainly to pre-motor, supplementary motor, and frontal cortical areas (see Figure 5�8 and Figure 5�18)�
The functional role of this large collection of neurons is best illustrated by clinical conditions in which this system does not function properly-Parkinson’s disease (discussed below) and Huntington’s chorea (discussed with Figure 2�9A)� These disease entities cause abnormal movements, such as chorea (jerky movements), athetosis (writhing movements), and tremors (rhythmic movements)�
The most common condition, which affects this functional system of neurons is Parkinson’s disease� The person with this disease has difficulty initiating movements, the face takes on a mask-like appearance with loss of facial expressiveness, there is muscular rigidity, a slowing of movements (bradykinesia), and a slow pill-rolling tremor of the hands at rest which goes away with purposeful movements (see also Figure 5�14)� Some individuals with Parkinson’s develop cognitive problems such as hallucinations and visuospatial problems and also emotional difficulties including anxiety and depression�
People with Parkinson’s disease also develop rigidity� In rigidity, there is an increased resistance to passive movement of the limb, which involves both the flexors and extensors, and the response is not velocity dependent�
The basal ganglia, from the point of view of functional neuroanatomy, consist of three major nuclei in each of the hemispheres (and these structures are located within the forebrain), the caudate, the putamen, and the globus pallidus-excluding the amygdala�
The caudate nucleus is described as having three portions:
• The head, located deep within the frontal lobe�
• The body, located deep in the parietal lobe�
• The tail, which goes in to the temporal lobe�
Each portion is associated anatomically with the lateral ventricle and follows its curvature (see Figure 9�7)�
The basal ganglia are shown in this series from a lateral perspective, and adjacent to each is an MRI scan done at the level indicated�
Starting with the top illustration, the various parts of the caudate nucleus are easily recognized-head, body, and tail-on the proximal side� The relationship of the caudate nucleus with the lateral ventricle can readily be described-the large head of the caudate nucleus actually intrudes into the space of the anterior horn of the lateral ventricle (see Figure 2�2B, Figure 2�9A, Figure 2�9B, Figure 2�10A, and Figure 2�10B)� The body of the caudate nucleus tapers and becomes considerably smaller, and it is found beside the body of the lateral ventricle� The tail follows the inferior horn of the lateral ventricle into the temporal lobe� This is a slender extended group of neurons, difficult to identify in sections of the temporal lobe (see Figure 9�5A)�
From this lateral perspective, it is clear that the caudate nucleus is in continuity with another large nucleus situated laterally-this is the putamen� The caudate and the putamen contain the same types of neurons and have some of the same connections; often they are collectively called the neostriatum� Strands of neuronal tissue are often seen connecting the caudate nucleus with the putamen� A very distinct and important fiber bundle, the internal capsule, separates the head of the caudate nucleus from the putamen (see Figure 4�4)� This fiber bundle fills the spaces between the cellular strands�
The adjacent axial (horizontal) T1 MRI image beside is done at the level of the foramen of Monro and is at the
same level as the horizontal cut in Figure 2�10A and the accompanying (T2) MRI image shown in Figure 2�10B�
When the putamen is removed, as in the middle illustration, another nucleus is revealed that is attached to the inner aspect of the putamen-the globus pallidus, a distinct functional part of the basal ganglia� When the putamen and globus pallidus are seen in coronal and horizontal cuts of the brain (see lowest MRI; also Figure 2�9A and Figure 2�10A), the two nuclei form a lens-shaped “nucleus”—the lentiform or lenticular nucleus� It is essential to understand that this is a descriptive term only and that there are two distinct functional parts of the basal ganglia included-the putamen (laterally) and the globus pallidus (medially)� The lentiform nucleus is situated laterally and deep in the hemispheres, within the central white matter�
The T1 MRI scan done at the level of the anterior commissure, in the horizontal (axial) plane, shows the actual connection between the caudate (head) and the putamen�
In the lower illustration, all parts of the basal ganglia of the proximal hemisphere have been removed, and one is looking at the distal side, from the medial perspective�
The lentiform nucleus is now seen to be composed of its two portions-the globus pallidus, which is medially placed, and the putamen lateral to it� In fact, the globus pallidus has two parts-an external (lateral) segment and an internal (medial) segment (see Figure 5�14 and Figure 5�18)�
The caudate nucleus (of the distal side) is now seen adjacent to the lateral ventricle (of the distal side)� (Note that the color of the caudate has changed somewhat because of the overlap of the “green” caudate with the “blue” ventricle�)
The T1 MRI scan accompanying this illustration is done in a coronal orientation also through the anterior commissure, but still shows the caudate nucleus (“bulging” slightly into the anterior horn of the lateral ventricle) and the putamen, separated by the internal capsule� The globus pallidus is difficult to distinguish radiographically because of the large numbers of fibers (white matter) within its midst�
Note to the Learner: From the functional point of view and based on the complex pattern of interconnections, two other nuclei that are not in the forebrain should be included with the description of the basal ganglia-the subthalamic nucleus (part of the diencephalon) and the substantia nigra (located in the midbrain)� The functional connections of these nuclei are discussed as part of the motor system (see Figure 5�14 and Figure 5�18)�
The diencephalon, which translates as “between brain,” is the next region of the brain to consider� The diencephalon, including both thalamus and hypothalamus and some other subparts, is situated between the brainstem and the cerebral hemispheres, deep within the brain�
As shown photographically (see Figure 1�7, Figure 1�8, and Figure 1�9) and diagrammatically (see Figure 3�1), the diencephalon sits on top of the brainstem� The enormous growth of the cerebral hemispheres in the human brain has virtually hidden or “buried” the diencephalon (somewhat like a weeping willow tree), so that it can no longer be visualized from the outside except from the inferior view (see the pituitary stalk and mammillary bodies, which are both part of the hypothalamus, in Figure 1�5 and Figure 1�6)�
In this section of the atlas, we consider the thalamus, which makes up the bulk of the diencephalon� There are two thalami, one for each hemisphere of the brain (see Figure 2�9A and Figure 2�10A), and these are often connected across the midline by nervous tissue, the interthalamic adhesion (as seen in Figure 1�7 and Figure 3�2)� As noted in Chapter 3, the 3rd ventricle is situated between the two thalami (see Figure 2�10A, Figure 2�10B, and Figure 3�1)�
The thalamus of both hemispheres is shown from the same perspective as the basal ganglia shown previously (and still shown)� The corpus callosum (cut) connecting the hemispheres is seen, below which are slit-sized (black) spaces, which are the lateral ventricles of each hemisphere� The thalamus of the proximal side is seen “behind”—that is medial to-the putamen (of the lentiform nucleus)� On the distal side, the caudate nucleus is “ghosted�” The globus pallidus on the distal side is seen, as has been explained previously�
The thalamus is usually described as the gateway to the cerebral cortex (see Figure 6�13)� This description leaves out an important principle of thalamic function,
namely, that most thalamic nuclei that project to the cerebral cortex also receive input from that area-these are called reciprocal connections� This principle does not apply, however, to all the nuclei (see later)� The various thalamic nuclei, and their functional component, are described in detail with Figure 4�3�
In this view, all structures of the proximal side have been removed, including the basal ganglia and the thalamus� The first structure seen adjacent to the midline is the thalamus (see Figure 1�7)� It is said to be the size and shape of an almond� The next structure to be seen from this perspective is the globus pallidus (refer to the lower illustration in Figure 2�5A and the lowest illustration in Figure 2�5B)� The amygdala is present in the temporal lobes, as previously shown�
Other parts of the diencephalon (not shown) include:
• The hypothalamus, one in each hemisphere (see Figure 1�7 and Figure 3�2)� It is composed of a number of nuclei that regulate homeostatic functions of the body, including water balance� It is discussed with the limbic system in Section 4�
• The pineal gland (visible in Figure 1�9) is sometimes considered a part of the diencephalon (the epithalamus)� This gland is thought to be involved with the regulation of our circadian rhythm�
The subthalamic nucleus is located below the thalamus� This nucleus is part of the circuitry associated with the basal ganglia (see Figure 5�14)�
Many people are now taking melatonin, which is produced by the pineal, to regulate their sleep cycle and to overcome jet lag when travelling eastward�
As shown in the diagram, the diencephalon is situated within the brain below the level of the body of the lateral ventricles� In fact, the thalamus forms the “floor” of this part of the ventricle (see Figure 2�9A)�
This illustration has been created from the same perspectives as Figure 2�5B-from the lateral view� This illustration in fact looks almost identical to the top illustration in Figure 2�5B, showing the basal ganglia and ventricles of the proximal hemisphere� In this illustration, a structure is seen “behind” (i�e�, on the medial aspect of the lentiform nucleus)� This is the thalamus, which is located closest to the midline�
The magnetic resonance imaging (MRI) scan (a FLAIR image mode) has been done in the horizontal (axial) plane at the level of the interthalamic adhesion (see Figure 1�7) and with the thalamus indicated (compare with the similar cut in Figure 2�5B)�
Removing the lentiform nucleus allows us to view the thalamus of the proximal hemisphere, as is shown in the middle illustration, and also a bit of the thalamus of the distal hemisphere� Looking back at Figure 2�5B, the basal ganglia nuclei are now in view-the (distal) globus pallidus and the putamen�
The (FLAIR) MRI scan beside this image that is done in the axial (horizontal) plane at the level just above the interthalamic adhesion (and includes the pineal gland) again shows the thalamus, one in each hemisphere, with the 3rd ventricle separating them�
Note to the Learner: In a horizontal section of the hemispheres, shown in the MRI scans beside the upper and middle illustrations (and in Figure 2�10B), the two thalami are located at the same level as the lentiform nucleus of the basal ganglia (see also Figure 2�10A)� This important relationship is discussed with the internal capsule (see Figure 4�4)�
The lower illustration shows the thalamus of the distal hemisphere, along with the caudate nucleus and the lateral ventricle of that side� The globus pallidus of that side is partially obscured by the thalamus� Note that the color of
the thalamus is somewhat changed because this illustration includes the midline 3rd ventricle, situated “in front” of the thalamus in this view�
The T1 MRI scan beside this illustration is again done in a coronal plane at the level of the foramen of Monro and clearly shows how the thalamus forms the “floor” of the body of the lateral ventricles�
Recreating the location of these deep structures of the hemisphere, the thalamic nuclei occupy the most medial position adjacent to the midline and the 3rd ventricle, whereas the parts of the basal ganglia-the globus pallidus and the putamen-are located more laterally� Anteriorly, the head of the caudate nucleus is found in front of the thalamus (see Figure 2�10A and Figure 2�10B)� The internal capsule separates these structures�
The major function of the thalamic nuclei is to process information before sending it on to the select area of the cerebral cortex (discussed with Figure 4�3; see also Figure 6�13)� This is particularly so for all the sensory systems, except the olfactory sense� It is possible that crude forms of sensation including pain are “appreciated” in the thalamus, but localization of the sensation to a particular spot on the skin surface requires the involvement of the cortex� Similarly, two subsystems of the motor systems, the basal ganglia and the cerebellum, relay in the thalamus before sending their information to the motor areas of the cortex (see Figure 5�18)� In addition, the limbic system has circuits that involve the thalamus (discussed in Section 4)�
Other thalamic nuclei are related to areas of the cerebral cortex that are called association areas, vast areas of the cortex that are not specifically related either to sensory or motor functions (e�g�, the dorsomedial nucleus and the prefrontal cortex, discussed with Figure 10�1B; see also Figure 6�13)� Some nuclei of the thalamus, the intralaminar and reticular nuclei (see Figure 4�3), play an important role in the maintenance and regulation of the state of consciousness, and also possibly in attention, as part of the ascending reticular activating system (ARAS; see Figure 3�6A)�
The nucleus accumbens, a functionally distinct part of the basal ganglia, is seen when looking at the medial aspect of the “distal” side, located where the head of the caudate nucleus becomes continuous with the putamen (in the middle illustration; see also Figure 2�5B)� The caudate forms part of the dorsal striatum; by definition, the nucleus accumbens would then be part of the ventral striatum� The functional aspects of the nucleus accumbens are discussed with the limbic system (in Section 4)�
The composition of the hemispheres can now be better understood, with the introduction of the various components including the ventricular system, the white matter, the basal ganglia, and the thalamus� Note that the lateral ventricle appears twice on each side on this anterior view (see Figure 2�1B), first above within the hemispheres and again within the temporal lobe�
The only additional feature, already introduced previously, is the location of the next part of the ventricular system-the third (3rd) ventricle� This slit-like ventricle is located between the thalamic nuclei in the midline and is sometimes referred to as the ventricle of the diencephalic region of the brain (see Figure 1�9 and Figure 3�1)�
The thalamus can now be visualized within the hemispheres adjacent to the midline, inferior to the lateral ventricle, and adjacent to the midline third ventricle�
Lateral to the thalamus (on both sides) is an area of white matter known as the internal capsule� Fibers from the spinal cord, the brainstem, and the thalamus to the cerebral cortex and fibers from the cerebral cortex to the
thalamus, the brainstem, and the spinal cord pass through this “funnel” (see Figure 4�4)� Lateral to the internal capsule is the lenticular (lentiform) nucleus, with the globus pallidus (medially) and the putamen (laterally)�
The ventricular system is further discussed in the following part with the brainstem (see Figure 3�1, Figure 3�2, and Figure 3�3), and the circulation of the cerebrospinal fluid is described in Section 3�
The ventricles, white matter, basal ganglia and thalamus are shown in the following illustrations, anatomically with the brain sectioned in the coronal plane (see Figure 2�9A) and in the axial (horizontal) plane (see Figure 2�10A), and also radiographically (see Figure 2�9B and Figure 2�10B)�
Note to the Learner: Many of the names of structures in the neuroanatomical literature are based upon earlier understandings of the brain, with terminology that is often descriptive and borrowed from other languages� As we learn more about the connections and functions of brain areas, this terminology often seems awkward if not obsolete, yet it persists� The term ganglia, in the strict use of the term, refers to a collection of neurons in the peripheral nervous system� Therefore, the anatomically correct name for the neurons in the forebrain should be the basal nuclei� Few texts use this term�
Most clinicians would be hard-pressed to change the name from basal ganglia to something else, so the traditional name remains�
This photographic view of the brain is sectioned in the coronal plane and shows the internal aspect of the hemispheres� On the dorsolateral view (the small figure on the upper left), the plane of section goes through both the frontal and the temporal lobes and would include the region of the basal ganglia� From the medial perspective (the small figure on the upper right), the section includes the body of the lateral ventricles with the corpus callosum above, the anterior portion of the thalamus, and the 3rd ventricle; the edge of the section also passes through the hypothalamus and the mammillary nucleus and includes the optic tracts� The section passes in front of the anterior part of the midbrain, the cerebral peduncles, and the front tip of the pons�
The cerebral cortex, the gray matter, lies on the external aspect of the hemispheres and follows its outline into the sulci in between, wherever there is a surface� The deep interhemispheric fissure is seen between the two hemispheres, above the corpus callosum (not labeled; see Figure 1�7 and Figure 2�2A)� The lateral fissure is also present, well seen on the left side of the photograph with the insula within the depths of this fissure (see Figure 1�4 and Figure 6�3)�
The white matter is seen internally; it is not possible to separate the various fiber systems of the white matter (see Figure 2�3 and Figure 2�4)� Below the corpus callosum are the two spaces, the cavities of the lateral ventricle, represented at this plane by the body of the ventricles (see Figure 2�1A and Figure 2�1B)� The small gray matter on the side of the lateral ventricle is the body of the caudate nucleus (see Figure 2�5A and Figure 2�5B)� Because the section was not cut symmetrically, the inferior horn of the lateral ventricle is found only on the right side of this photograph, in the temporal lobe�
The brain is sectioned in the coronal plane through the diencephalic region� The gray matter on either side of the third ventricle is the thalamus (see Figure 1�9 and Figure 2�8)� Lateral to this is a band of white matter, which by definition is part of the internal capsule, with the lentiform nucleus on its lateral side� The portion between the thalamus and lentiform nucleus is the posterior limb (refer to the section in the horizontal plane [Figure 2�10A, also Figure 4�4])�
The parts of the lentiform nucleus seen in this view include the putamen as well as the two portions of the globus pallidus, the external and internal segments� Because the brain has not been sectioned symmetrically, the two portions are more easily identified on the right side of the photograph�
The gray matter within the temporal lobe, best seen on the left side of the photograph, is the amygdala (see Figure 2�5A and Figure 2�6)� It is easy to understand why this nucleus is considered one of the basal ganglia, by definition� Its function, as well as that of the fornix, is explained with the limbic system (see Section 4)�
Lateral to the lentiform nucleus is another thin strip of gray matter, the claustrum (not labeled; better seen on the right side of the photograph; also seen and not labeled in Figure 2�10A, on the right side of the photograph)� The functional contribution of this small strip of tissue is not really known� Lateral to this is the cortex of the insula, inside the lateral fissure�
The other major disease that affects the basal ganglia is Huntington’s chorea, an inherited degenerative condition� This disease, which usually starts in mid-life, leads to severe motor dysfunction, as well as cognitive decline� The person whose name is most closely associated with this disease is Woody Guthrie, a legendary folk singer� There is now a genetic test for this disease that predicts whether the individual with a family history of Huntington’s chorea will develop the disease�
This is a view of the brain similar to the brain section in Figure 2�9A, in the coronal plane but slightly more anteriorly� In this T1-weighted image, the cortex is gray, the white matter is white, and the cerebrospinal fluid is dark� Note that the tables of the skull are now dark, and the bone marrow is white� The dura of the meninges can also be visualized (see Figure 7�1 and Figure 7�2)� The dural fold, the falx cerebri, is seen in the interhemispheric fissure (see Figure 7�4 and Figure 7�5), with the superior sagittal sinus seen in the midline, at the top of the falx cerebri�
The gray matter of the cerebral cortex and white matter can be easily differentiated� The corpus callosum is seen crossing the midline� The anterior horns of the lateral ventricles are seen, divided by the septum pellucidum into one for each hemisphere (see also Figure 8�6)� Again, the
plane of section has passed through the foramina of Monro, leading into the 3rd ventricle, which is situated between the thalamus on either side (see Figure 7�8)� The head of the caudate nucleus is seen, as previously (see Figure 2�2B), protruding into the space of the anterior horn of the lateral ventricle� The lentiform nucleus is still present, and the thalamus (thalami) can be seen adjacent to the 3rd ventricle�
The section has passed through the posterior limb of the internal capsule (see Figure 4�4)� Its fibers are seen as continuing to become the cerebral peduncle (see Figure 1�6, Figure 1�8, and Figure 3�1)� The plane of section includes the lateral fissure and the insula (see lower illustration in Figure 2�5B, and Figure 2�7)� The temporal lobe includes the hippocampal formation and the inferior horn of the lateral ventricle (see Figure 9�5A)�
This view also includes the brainstem-the midbrain (the cerebral peduncles), the pontine region (the ventral portion), and the medulla� The tentorium cerebelli occupies the space between the inferior aspect of the hemispheres and the cerebellum (see Figure 7�5 and Figure 7�6), with its opening or incisura at the level of the midbrain (discussed with uncal herniation; see Figure 1�6)�
In this photograph, the brain has been sectioned in the horizontal (axial) plane� From the dorsolateral view (the small figure on the upper left), the level of the section is just above the lateral fissure, and at a slight angle downward from front to back� Using the medial view of the brain (the small figure on the upper right), the plane of section goes through the anterior horn of the lateral ventricle, the thalamus, and the occipital lobe�
This section exposes the white matter of the hemispheres, the basal ganglia, and parts of the ventricular system� Understanding this particular depiction of the brain is vital to the study of the forebrain� The structures seen in this view are also of immeasurable importance clinically, and this view is most commonly used in neuroimaging studies, both computed tomography (CT, see Figure 2�1A) and magnetic resonance imaging (MRI, see Figure 2�10B)�
The basal ganglia are present when the brain is sectioned at this level (see Figure 2�5B and Figure 2�7)� The head of the caudate nucleus protrudes into the anterior horn of the lateral ventricle (seen in the CT scan, Figure 2�1B, middle)� The lentiform nucleus, shaped somewhat like a lens, is demarcated by white matter-anteromedially and posteromedially, which is the internal capsule (see Figure 4�4)�
Because the putamen and caudate neurons are identical histologically (and also developmentally), the two nuclei have the same grayish coloration� The globus pallidus is functionally different; it contains many more fibers and therefore is lighter in color� Depending on the level of the section, it is sometimes possible (in this case on both sides) to see the two subdivisions of the globus pallidus, the internal and external segments (discussed with Figure 5�14 and Figure 5�18)�
The white matter medial to the lentiform nucleus is the internal capsule (see Figure 2�9A and Figure 2�9B; also Figure 4�4)� It is divisible into an anterior limb and a posterior limb and genu� The anterior limb separates the lentiform nucleus from the head of the caudate nucleus� The
posterior limb of the internal capsule separates the lentiform nucleus from the thalamus� Some strands of gray matter located within the internal capsule represent the strands of gray matter between the caudate and the putamen (as shown in Figure 2�5B)� The base of the “V” which points medially is called the genu�
The anterior horn of the lateral ventricle is cut through its lowermost part and is seen in this photograph as a small cavity (see Figure 2�1A)� The plane of the section has passed through the connections between each of the lateral ventricles and the 3rd ventricle, the foramina of Monro (see Figure 7�8)� The section has also passed through the lateral ventricle as it curves into the temporal lobe to become the inferior horn of the lateral ventricle, the area called the atrium or trigone (better seen on the left side of this photograph)� The choroid plexus of the lateral ventricle, which follows the inner curvature of the ventricle, is present on both sides (not labeled; see Figure 2�1B)�
The 3rd ventricle is situated between the thalamus of both sides (see Figure 2�8)� The pineal gland is seen attached to the back end of the ventricle� A bit of the cerebellar vermis is visible inferior to it (see also Figure 1�9)�
The section is somewhat asymmetrical in that the occipital horn of the lateral ventricle is fully present in the occipital lobe on the left side of the photograph and not on the right side� On the right side, a group of fibers is seen streaming toward the posterior pole, and these represent the visual fibers, called the optic radiation (discussed with Figure 6�4 and Figure 6�5)� The small size of the tail of the caudate nucleus alongside the lateral ventricle can be appreciated (see Figure 2�5A, Figure 2�5B, and Figure 9�7)�
The major ascending sensory tracts from the thalamus to the cerebral cortex and the descending motor tracts from the cerebral cortex are found in the posterior limb of the internal capsule (reviewed in Section 2; see also Figure 4�4)� This is the plane of view that would be used to look for small infarcts, called lacunes, in the posterior limb of the internal capsule (discussed with Figure 8�6 in Section 3)� These infarcts are caused by occlusion of the small penetrating branches of the middle cerebral artery called the striate (lenticulostriate) branches (see Figure 8�6)�
This radiological view of the brain is not in exactly the same horizontal plane as the anatomical specimen shown in Figure 2�10A� The radiological images of the brain are often done at a slight angle to minimize the dense skull bones of the posterior cranial fossa that impair the viewing of the structures (brainstem and cerebellum) in this area� In this T2-weighted magnetic resonance image, the cerebrospinal fluid (CSF) is white, and the neuronal areas (cortex and basal ganglia) are gray�
The anterior horns of the lateral ventricle are present, and the section has passed through the foramina of Monro (see Figure 2�10A and Figure 7�8)� The lateral ventricle posteriorly is cut at the level of its widening, the atrium or
trigone, as it curves into the temporal lobe (see Figure 2�1A and Figure 2�10A)�
The structures seen in the interior of the hemispheres are the ones already identified-caudate, putamen, and internal capsule� As discussed previously, the globus pallidus is sometimes idenfiable in CT and MRI scans� The 3rd ventricle is in the midline, between the thalami� The optic radiation can be visualized on the left side of this image (see Figure 6�4 and Figure 6�5)�
A CSF cistern is seen behind the brain substance, in the midline (not labeled)� Somewhat inferior to this would be the colliculi, also known as the tectum or the quadrigeminal plate (see Figure 1�9 and Figure 3�3)� The CSF seen in this spot is found in a cistern (discussed with Figure 7�8)—the quadrigeminal cistern in the midsagittal views (see Figure 1�7, but not labeled); its “wings” are called the cisterna ambiens, a landmark for the neuroradiologist�
