Chapter 1, pp. 1-21
Appendix (Survey of Human Neuroanatomy), pp. 717-744
Atlas plates 1-6, pp. 745-759
Lab 5 Protocol
Examine slabs through the human forebrain
Learning objective: to recognize the principal features of the forebrain (and brainstem) that are visible
with the unaided eye, including major gray matter and white matter structures in the cerebral
hemispheres.
Specimens: whole brain slabs cut in the coronal, axial/horizontal or parasagittal plane; Sylvius4 Online
Activities:
Beginning with a set of forebrain slabs cut in the coronal plane, identify in the tissue all gray and
white matter structures that are identified in Figures 5.8,5.9, 5.10, 5.11, and 5.12.
Pay particular attention to Challenge 5.1 with a goal of identifying the internal capsule and the
deep gray matter structures that surround it.
Work through Challenge 5.2 by focusing on the medial temporal lobe. Consider the relative location
of the amygdala and the hippocampus in relation to the temporal horn of the lateral ventricle
Overview
Now that you have a acquired a framework for understanding the regional anatomy of the human brain, as viewed from the surface, and some understanding of the blood supply to both superficial and deep brain structures, you are ready to explore the internal organization of the brain. This chapter will focus on the sectional anatomy of the forebrain (recall that the forebrain includes the derivatives of the embryonic prosencepahlon). Given the complexity of the brainstem and its importance for diagnosis and clinical practice, that portion of the brain will be addressed in a separate chapter. But before beginning to study the internal anatomy of the brain, it will be helpful to familiarize yourself with some common conventions that are used to describe the deep structures of the central nervous system.
For the rest of this chapter, we will discuss the appearance of sections through the forebrain, so that you can learn to identify the structures that are not visible on a surface view. The anatomy of the forebrain as seen in sections is relatively simple; however the geometry of some of these deep structures can be a challenge to appreciate. For example, simply note the appearance of the hippocampal formation and the lateral ventricle in the view of partially dissected brain in Figure 5.1. You will soon learn why these structures appear where they do.
As you work through the remainder of this chapter, be sure to recognize and locate the following structures on sections cut in any of the three standard planes:
Let’s begin our study of internal forebrain anatomy with the cerebral cortex. The cerebral cortex is a thin layer of gray matter that covers the entire surface of the hemispheres. Most of the cortex that is visible from the surface in humans is known as neocortex, cortex which is made up of six layers of neurons (see Appendix 5 regarding the Histology of the Nervous System). Phylogenetically older cortex, which has fewer cell layers (paleocortex and archicortex), is found on the inferior surface of the temporal lobe, separated from neocortex by the rhinal fissure (Figure 5.2). The cortex with the fewest layers (three) is known as the hippocampus (archicortex of the parahippocampal
gyrus). The hippocampus is the medial edge of temporal cortex that becomes double-folded into the medial aspect of
the temporal lobe; it is visible only in dissected brains (see Figure 5.1) or in sections. It is worth remembering that the
entire cerebral cortex is derived from the walls of the largest and most anterior swelling of the embryonic brain, the
prosencephalon. Thus, despite its deep sulci and fissures and phylogenetic divisions, the entire cerebral cortex in one hemisphere is a continuous sheet of neural tissue.
The cortex is made up of neuronal cell bodies, their dendrites, and the terminal arborizations of axons coming from the thalamus and other sources, mainly from other neurons in the cerebral cortex. Indeed, many neurons in the cortex send axons that travel some considerable distance in the central nervous system to make synaptic connections with other neurons. Axons that enter and leave the cortex form the white matter that makes up a large part of the hemispheres. We often speak of axons as though they were moving, using words such as ‘entering,’ ‘leaving,’ ‘descending,’ ‘traveling,’ ‘projecting,’ etc. Of course, their place is fixed in the adult, and what we are actually referring to are the directions in which action potentials normally propagate along the axons.
Buried deep within the hemispheres are the basal ganglia (Figure 5.3), which are large gray matter structures concerned with modulating thalamic interactions with the frontal lobe. (The term ‘ganglion’ is not usually used for clusters of neurons inside the central nervous system; this is an exception.) The basal ganglia lie partly rostral and partly lateral to the diencephalon (refer to the the chart from Figure A24 (Neuroscience, 5th ed.) for their embryonic relations). They can be divided into four main structures: the caudate, the putamen, the nucleus accumbens, and the globus pallidus.
Structurally and functionally, the caudate, putamen and nucleus accumbens are similar, and they are often referred to collectively as the striatum, because of the stripes or “striations” of gray matter that run through a prominent bundle of white matter (the internal capsule) that otherwise separates the caudate from the putamen. (The caudate and putamen are also called the “neostriatum” to emphasize their evolutionary and functional relation to neural circuits in the neocortex.) Ventral to the caudate and putamen are additional divisions of the striatum, which are important for understanding motivated behavior and addiction. The most prominent of these structures in this so-called ventral striatum is the nucleus accumbens.
These three divisions of the striatum receive inputs from different portions of the telencephalon that define the
functional roles of each striatal division. In general terms, the striatum (and the circuits through the basal ganglia that
begin here) regulates movement, with the three divisions of the striatum governing different domains of movement.
Thus, it should be instructive to remember that:
the putamen is concerned with the regulation of bodily movement;
the caudatenucleus (especially its large anterior ‘head’) regulates movement of the mind and eyes (which often
indicate what we are thinking about); and
the nucleus accumbens is concerned with movement of emotion or motivated behavior.
Obviously, we are speaking of the concept of movement in loose terms. Nevertheless, it is important to recognize that
each striatal division (and the distinct circuits through the basal ganglia that derive from each) share common structural
and functional motifs that help explain their contribution to the modulation of behavior. Each circuit is involved in the initiation or suppression of some program for behavior. To accomplish these functions, each division of the striatum
projects to some division of the pallidum; the globus pallidus is the largest division of the pallidum and it receives input
mainly from the putamen. The pallidum in turn regulates thalamo-cortical interactions. A full consideration of basal
ganglia circuitry is beyond the scope of this survey; but these important circuits will be considered elsewhere in the
course and in Appendix 3.
Now let’s turn our attention from gray matter to white matter.
There are three bundles of axons in the hemisphere that have already been identified on sagittal views: the corpus callosum, anterior commissure and fornix (see Lab 2). One additional system of axonal fibers should now be appreciated. Many of the axons entering or leaving the cortex do not assemble into compact bundles, except in the vicinity of the thalamus and the basal ganglia, where they form a structure known as the internal capsule.
The internal capsule lies just lateral to the diencephalon, and as mentioned briefly above, a portion of it separates the caudate from the putamen. Many of the axons in the internal capsule terminate or arise in the thalamus. Other systems of axons descending from the cortex, course through the internal capsule, and continue past the diencephalon to enter the cerebral peduncles of the midbrain. Between the cortex and the internal capsule, the axons of the white matter are not so tightly packed. Here they are sometimes called the ‘corona radiata’, a reference to the way they appear to radiate out from the compact internal capsule to reach multiple areas of cortex. (Individual groups of axons may also be indicated in this way. For example, you will hear reference to the visual radiations or the auditory radiations, axons that travel from the thalamus to the visual and auditory cortices, respectively.) We will return to consider the internal capsule in relation to the important deep gray matter structures after reviewing cross-sectional views through the forebrain.
There are several other bundles of axons that run through the white matter of the forebrain longitudinally in each cerebral hemisphere, connecting different cortical areas (associational white matter); but you need not be concerned with identifying them now.
That’s almost it for structures in the hemispheres! But there are two other gray matter structures you should know.
One of the additional gray matter structures to know is a group of complex nuclei, known as the basal forebrain nuclei, which have become associated with the signs and symptoms of diseases such as Alzheimer’s disease (see Figure 5.9). Like the basal ganglia, the basal forebrain nuclei are made up of clusters of cells (rather than layers); but unlike the basal ganglia, these clusters are much smaller and typically much less compact. They are located ventral to the anterior commissure and below the basal ganglia, between the ventral striatum and the hypothalamus.
The other additional gray matter structure is the amygdala, which is a large mass of gray matter buried in the anterior-medial part of the temporal lobe, anterior to the lateral ventricle and the hippocampus (see Figure 5.10). The amygdala is an important component of ventral-medial forebrain circuitry and it is involved in the experience and expression of emotion. It was once classified as part of the basal ganglia; however, it is structurally and functionally heterogeneous, with systems of neurons and intrinsic connections that are comparable to those in striatum and the cerebral cortex.
There is one slight complication that you will encounter as you begin to identify the structures of the forebrain in sections. That is, sometimes you see the same structures twice in the same section in the same hemisphere. To understand why this is so, refer to Figures 5.3&5.4.
The diencephalon comes to lie medial to the hemispheres. The thalamus is the largest subdivision of the diencephalon (see Figure 5.5). It is egg-shaped and is made up of many subdivisions (e.g. the VPL and VPM nuclei associated with somatic sensation from the limbs and face, respectively) . The hypothalamus lies ventral to the thalamus. Anterior to the hypothalamus is the optic chiasm. (Clinically, the close physical proximity of the chiasm to the pituitary gland is very important, since a combination of visual and endocrine problems is a strong indication of a pituitary tumor.) The mammillary bodies are a part of the hypothalamus lying in its caudal part just at its junction with the midbrain.
Finally, before taking on the challenge of viewing the forebrain in cross-sections, study the photograph of a brain with the cerebellum cut away so that you see the dorsal surface of the brainstem and the posterior surface of the dorsal thalamus (Figure 5.6). Likewise, consider the three-dimensional configuration of the ventricular system within the forebrain and brainstem (Figure 5.7 and the Sylvius Self-Study Exercise below). It is important to appreciate the relationships among these structures so that you will understand why cross-sections through the brain in various planes appear as they do (as you do so, you may want to again refer to the chart from Figure A24 (Neuroscience, 5th ed.).
With some study of the chart from Figure A24 (Neuroscience, 5th ed.) and Figure 1.1, you can became quite conversant with the ventricular compartments of the human brain, their relation to surrounding subdivisions of the adult brain, and their embryological precursors from which these complex shapes arose.
As a point of emphasis for this self-study, remember that the ventricles are the product of the morphogenic events that bent, pinched and expanded the lumen of the embryological neural tube and greatly increased the thickness and complexity of its walls (now that’s an understatement!). The objective of this exercise is to master the visual recognition of the various compartments that constitute the ventricular system of the adult brain. This will entail recognizing four principal ventricles, the paired lateral ventricles, the third ventricle, and the fourth ventricle, as well as one narrow channel, the cerebral aqueduct. Along the way, you will also become introduced to standard views of the forebrain and brainstem in cross-section.
To begin, be sure you are familiar with the illustrations in Figure 5.7; they provide the foundation for your exploration of the ventricles in sectional views of the brain. Next, open Sylvius4 and go to Sectional Anatomy, Photographic Atlas, and then click on Ventricles. Now, view the most rostral coronal section (“Coronal 1” should appear if your mouse lingers over the correct thumbnail image in the navigation window). Begin sectioning this brain from rostral to caudal (click on the rightward arrowhead in the navigation window) and note the appearance of the frontal horn of the lateral ventricle in coronal section 3. With your attention on the lateral ventricle, continue sectioning and note the appearance of the temporal horn of the lateral ventricle in the medial temporal lobe (coronal sections 5 & 6). Finally, note the caudal extension of the lateral ventricle as it penetrates the occipital lobe as the occipital horn of the lateral ventricle (coronal sections 7 & 8).
Now, re-slice the forebrain in the axial (horizontal) plane from dorsal to ventral (click on an axial thumbnail image or grab the dorsal-ventral slider in the navigation box to select an axial image). Look for these same compartments within the lateral ventricle. Do you notice how the lateral ventricle opens widely in its central part or body (horizontal section 2), then appears more posteriorly in a region called the atrium (horizontal section 3) before appearing more anteriorly in the temporal lobe (horizontal section 4)? Refer back to the illustrations in Figure 5.7 as you section in the axial plane.
To appreciate the third ventricle, compare horizontal section 3 with coronal sections 4 & 5. Do you see the narrow slit-like space defined by the third ventricle at the medial base of the diencephalon? By what means does cerebrospinal fluid flow from the lateral ventricle, where it is synthesized by choroid plexus, into the third ventricle? (Refer to Figure A22 on page 743 of Neuroscience, 5th Ed. or Netter 103 and Netter 96A for the answer)
The third ventricle communicates with the fourth ventricle by means of a narrow channel through the dorsal midbrain (mesencephalon) called the cerebral aqueduct. This channel is barely visible in (coronal section 6 because of its very small diameter). To better view the cerebral aqueduct, enter the Brainstem Cross Sectional Atlas and select All Structures; then view the section labeled “2 – Midbrain” (second section from the top of thumbnail list). See the small space along the dorsal midline of the section? That’s the cerebral aqueduct and it is a principal landmark that will always help you identify transverse sections through the midbrain, as inferred from the chart from Figure A24 (Neuroscience, 5th ed.). From here, continue sectioning through the brainstem in the caudal direction and note the gradual expansion of the cerebral aqueduct as you enter the pons.
By section “6 – Pons”, the cerebral aqueduct has fully opened up into the fourth ventricle (click on the fourth ventricle to highlight this space). This most caudal ventricle in the adult brain lies between the dorsal surface of the pons and the large stalks of white matter (the cerebellar peduncles; “peduncle” means stalk) that connect the cerebellum to the brainstem.
Now return to the forebrain and view the midsagittal plane (go to Sectional Anatomy > Photographic Atlas > Ventricles). As you view the four major components of the ventricular system again, appreciate their structural continuity, their physical relation to surrounding brain regions and consider again their embryological origins.
As one final challenge—section through the brain in the MR Atlas (go to Sectional Anatomy > MR Atlas > All Structures) and relate what you see in serial sections to the colorized views of the ventricles in the Photographic Atlas and to Figure 5.7.
Lab 5 Protocol
Examine slabs through the human forebrain
Learning objective: to recognize the principal features of the forebrain (and brainstem) that are visible
with the unaided eye, including major gray matter and white matter structures in the cerebral
hemispheres.
Specimens: whole brain slabs cut in the coronal, axial/horizontal or parasagittal plane; Sylvius4 Online
Activities:
Beginning with a set of forebrain slabs cut in the coronal plane, identify in the tissue all gray and
white matter structures that are identified in Figures 5.8,5.9, 5.10, 5.11, and 5.12.
Pay particular attention to Challenge 5.1 with a goal of identifying the internal capsule and the
deep gray matter structures that surround it.
The five coronal sections through the brain shown in (Figures 5.8,5.9, 5.10, 5.11, and 5.12), were taken from Sylvius4 and should resemble the brains that are available for examination in the laboratory. Remember, areas with little or no myelin appear dark and are considered gray matter, and areas containing myelinated axons appear light and are called white matter.
The section in Figure 5.13 is cut lateral to the midline in the location indicated on the insets of coronal and horizontal sections. After spending some time studying sectional views of the brain in isolation, it is worth recalling how the brain and spinal cord are related to the surrounding structures of the cranium and vertebral column, as is shown in this section through a cadaveric specimen.
Non-invasive imaging views of the human central nervous system
Figures 5.14, 5.15, and 5.16 contain T1-weighted, magnetic resonance imaging (MRI) views of the head from the MR Atlas in Sylvius4. Sets of scans in the coronal, sagittal, and horizontal planes are shown, with insets to remind you of their orientation. Use these images along with others in the MR Atlas in Sylvius4 to work through the Sylvius Self-Study Exercise below:
Challenge 5.1—internal capsule and deep gray matter
One of the most difficult challenges in human brain anatomy is gaining an appreciation for the 3D arrangement of deep gray and white matter within the forebrain. But be encouraged! There is a principled means of simplifying this challenge. You must first understand the positional relations among the major components of the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus), thalamus, and the internal capsule. Then, you should recognize how the lateral ventricle fits in. Once you do so, you can interpret any section through the forebrain in any plane of section, be it a standard anatomical plane or an oblique plane.
Here’s the key to framing your 3D understanding: the deep gray matter structures identified above are always found on one side of the internal capsule or the other. Specifically...
the caudate nucleus and the thalamus are medial to the internal capsule;
the putamen and globus pallidus are lateral to the internal capsule.
These relations reflect the course of the outgrowing axons that formed the internal capsule in fetal development as they navigated through the anlage of deep gray matter in the embryonic brain. As you carefully inspect sections through the forebrain (in the next few pages and in lab), note the appearance of the internal capsule and the deep gray matter. There are several additional details to observe and remember:
the caudate and putamen become continuous around the rostral margin of the internal capsule and just inferior to the anterior limb of the internal capsule, these deep gray matter structures fuse to form a ventral, anterior component called the nucleus accumbens;
the globus pallidus is a relatively small structure located near the middle of the basal ganglia;
the globus pallidus is located between the internal capsule and the putamen;
the thalamus occupies a more posterior volume of brain-space than the bulk of the basal ganglia;
the caudate nucleus has a long “tail” that follows the course of the lateral ventricle into the temporal lobe (see again Figure 5.3 and 5.4).
the anterior limb of internal capsule separates the head of the caudate from putamen and globus pallidus, while the posterior limb of internal capsule mainly separates thalamus from globus pallidus
After gaining an understanding of these points in the set of coronal sections in Figures 5.8,5.9, 5.10, 5.11, and 5.12, open Sylvius4 Online and enter the Basal Ganglia image set of the Photographic Atlas in the Sectional Anatomy group; then view the most rostral coronal section and review these same sections (and a few not reproduced here). Next, re-slice this digital brain in the axial plane and then in the parasagittal plane. The internal capsule may be more difficult to appreciate in these other planes of section; but here’s a tip: look-up the internal capsule in the Visual Glossary and you will see it labeled in the axial plane. In this plane, you will readily appreciate its anterior and posterior limbs. The anterior limb of internal capsule mainly separates the anterior caudate from putamen and globus pallidus, and the posterior limb of internal capsule mainly separates thalamus from globus pallidus.
So now that you are primed to interpret the internal anatomy of the forebrain, lay out a set of coronal or axial sections on a tray and identify each of the structures and relations numbered (1) to (6) above.
Now that you have worked through views of the internal anatomy of the forebrain (and parts of the brainstem) in coronal sections, try identifying all of the same structures that are labeled in Figures 5.8,5.9, 5.10, 5.11, and 5.12 and in the other planes of section that are available in Sylvius4, namely the horizontal (axial) and parasagittal planes.
In principle, this is precisely the same type of self-study exercise that you may have worked through while studying the ventricles; only now, the challenge is to key in on a variety of structures that may not be quite so obvious as you pass through the forebrain. So open Sylvius4 and go to Sectional Anatomy, Photographic Atlas, and then click on one of the filter sets, depending on which structures you wish to have highlighted, such as Ventricles, Limbic System, or Basal Ganglia, or perhaps you simply wish to view the specimen Unlabeled. Now, view the most dorsal horizontal (axial) section (“Horizontal 1” should appear if your mouse lingers over the correct thumbnail image in the navigation window). Begin sectioning this brain from dorsal to ventral (click on the rightward arrowhead in the navigation window) and note the appearance of colorized structures of interest (depending, again, on which filter set you selected). After passing up and down through the specimen a few times, re-slice the forebrain in the parasagittal plane (click on a parasagittal thumbnail image or grab the medial-lateral slider in the navigational box to select a parasagittal image). Look for these same structures as you pass through the brain from medial to lateral and back again.
Try repeating the same process while passing through the T1-weighted image set in the MR Atlas (go to Sectional Anatomy > MR Atlas > All Structures) and find each of the numbered structures from Figures 5.8,5.9, 5.10, 5.11, and 5.12. An additional resource is the MRI atlas at headneckbrainspine.com.
Tip—In the MR Atlas, you can open multiple images windows if you click and hold on the thumbnail until a small textbox appears that says “Open image in new window”; while still holding down the mouse button, move the mouse over this text to select this option and a new image window will open when you release the mouse button.
Tip—Also in the MR Atlas, you can search for a structure in the Structure window to the right; just scroll through the list, select the structure of interest, and that structure will be highlighted in every thumbnail image that contains that structure (look for the tiny yellow blocks that label the selected structure).
Refer to the unlabeled, printed versions of the images from the Sylvius4MR Atlas above to reinforce your recognition of several of the major structures that define the internal organization of the forebrain.
Challenge 5.2—amygdala & hippocampus
Lay out in front of you on a tray a set of coronal or axial sections through a cerebral hemisphere. Now that you are oriented to the basal ganglia, thalamus and internal capsule, you are ready for a simpler challenge. Turn your attention to the medial portions of the temporal lobe and identify the amygdala and the hippocampus. As you sort out where these structures are and what they look like, address the following set of questions:
Which is more anterior, the amygdala or the hippocampus?
Do the amygdala or the hippocampus overlap?
Is the amygdala cortex, basal ganglia, or something different?
Why is the amygdala called the amygdala? (Latin for “almond”)
What is the relationship between the hippocampus and the lateral ventricle?
Why is the hippocampus called the hippocampus? (Latin—from the Greek—for “sea horse” or “sea monster”)
How do the amygdala and the hippocampus communicate with structures in the hypothalamus and nearby ventral-medial forebrain?
Challenge 5.3—modeling deep gray matter
Now that you have some experience with the sectional anatomy of the forebrain and a growing appreciation for the relations of deep gray matter structures in brain-space, you are ready to get your hands dirty. No sequence of neuroanatomical study is complete until learners are challenged to build or model their own brain. Obtain a set of colored Play-Doh™; five colors would be best, with one being white. Your goal will be to construct a simple, but accurate model of the spatial relations in the brain that you discovered in Challenge 5.1. In particular, construct a clay (dough) model of the major components of the basal ganglia, thalamus and internal capsule, as described in Challenge 5.1, with special attention to the five numbered points of detail.
How to start? Begin by constructing the internal capsule: flatten out a white (for white matter, of course) lump of clay into an elongated fan shape. In the brain, the wide end of the fan (called the corona radiata) penetrates into the subcortical white matter and the narrow end penetrates the diencephalon and brainstem, where it forms the cerebral peduncle and, eventually, the medullary pyramid; the basal ganglia and thalamus reside near the middle of the fan. Next, add a colored lump of clay for the globus pallidus (but on what side of the internal capsule, lateral or medial?). Then, encompass your ‘faux’ globus pallidus with the putamen and fashion at least the rostral and dorsal portions of the caudate nucleus. When ready to be more ambitious, try creating a more complete caudate nucleus that includes its temporal tail. Finally, add an egg-shaped lump for the thalamus (remember its position relative to the internal capsule?). How does it look … anything like Figure 5.3? Don’t worry if your first attempt(s) are less than edifying. What is most important about this exercise is the visualization of spatial relations that comes from wrestling with both substance (modeling clay) and abstraction (imagined brain-space).
One additional tip for this modeling exercise: Sylvius4 contains illustrations and an interactive virtual model of a standard brainstem model that is often used in neuroanatomical laboratories (including ours). This model includes the diencephalon, basal ganglion and internal capsule; refer to this model and interact with the “Atlas extras” feature (available via the folder in the navigation window in the upper left) for additional views of the relation among these structures.
Now that you have in front of you a clay model of the deep gray matter of the human brain, try actually sectioning your model in one of the three standard neuroanatomical planes (the coronal plane is a good starting plane for deconstruction). This should be easily done with a standard dissection knife or a thin wire. Assuming the clay (dough) is of the proper consistency and has survived sectioning, do you recognize the spatial relations among your modeled gray matter structures that you discovered in the human brain? Try comparing different planes of section through your model with sectional views of the digital brain in Sylvius4. You might even try re-attaching your sections with a little gentle kneading and then re-sectioning in an orthogonal plane (try axial next). With some persistence and patience, working through this exercise will foster a more cogent understanding of 3D relations within the deepest substratum of the human forebrain.
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