MODULE 1

THE CEREBRAL HEMISPHERE

SURFACE ANATOMY

THE LATERAL SURFACE

- - We begin with Figure 1-24. Clearly, there has to be a way to refer to different parts of this very complex organ. The next figures describe it.

• Figure 1-25. The starting point is to divide the hemisphere into four major lobes, which correspond roughly in name and position to the bones of the overlying skull.
• Figure 1-26. This view shows the general position of the four lobes present on the lateral surface of the hemisphere. Blumenfeld describes them in some detail, starting on pg. 24.
• Figure 1-27. The landmarks used to establish the borders of each lobe are shown here. The central sulcus lies about halfway between frontal and occipital poles and is significantly deeper than adjacent sulci. The lateral fissure differs from a sulcus in that it was created as tissue migrated in an arc-like fashion to form the temporal lobe. The pre occipital notch is a rather predictable indentation in the surface of the brain, and the parieto-occipital sulcus is a very constant and deep sulcus, present on the medial surface of the hemisphere. The anterior border of the occipital lobe is a line drawn between the pre occipital notch and the upper end of the parieto-occipital sulcus.
• Figure 1-28. Anteriorly, the lateral fissure separates the temporal lobe from the frontal and parietal lobes; posteriorly, the line separating the temporal and parietal lobes is a somewhat arbitrary extension of the lateral fissure.
• Figure 1-29. Here are some of these landmarks on a real brain.
• DiganatA_16A4. Here is a rotating view of the lobes.
• Figure 1-30. Before describing how each lobe is subdivided into gyri, we need to be clear on the way directions are specified when dealing with the surface of the hemisphere. The most commonly used terms are shown in this view.
• Figure 1-31. Shows the named sulci.
• Figure 1-32. Shows the gyri

- - Most textbook figures imply that the pattern of gyri and sulci on the surface of the hemisphere is quite similar from one brain to the next. In reality, there is a great deal of variability between specimens. In the brain we looked at originally, for example, the central, post central and superior temporal sulci are rather clear, with the result that the post central gyrus and the superior temporal gyrus are easily identified (Figure 1-33). The subdivisions of the inferior frontal gyrus are also pretty clear. In other brains, even the central sulcus can be difficult to identify.

- - Migration of the frontal, parietal and temporal lobes has tended to bury an "island" of cortex within the depths of the lateral fissure (Figure 1-44, DiganatS_1.4, Figure 1-45). If parts of the frontal (red) and parietal (yellow) lobes are cut away (and you can forgive the change in color code at this point) this cortical area - the insula (violet) - is revealed (Figure 1-34). Blumenfeld shows it in Figure 2.24. The insula is not truly an island, however, and its relationship to the lobes of the hemisphere is best revealed by making a cut through the brain in the plane indicated by the line and looking at the surface of the sectioned tissue (Figure 1-35).

- - For some nice views of the lateral surface of a real brain see the following Digital Anatomist views. Since this is one of the first times in which we have called up these views, we should explain to you how to use them. The Diganat slides are superb views of real brains, but they can be a little overwhelming because the authors have labeled things in great detail. Whenever possible, we want you to call each of them up and then place one of our views along side of it. Compare the two, seeing how many of the structures labeled in our idealized view can be found on the real brain. It's sort of a reality check, and you have to realize that the match won't be perfect. The bottom line is that our views define the level of detail you should learn. Treat it all as a game!

• DiganatA_1.2 - Left hemisphere, lateral view; compare with Figure 1-31 and Figure 1-32.
• DiganatA_1.3 - Left Insula; compare with Figure 1-34

THE BASAL SURFACE of the BRAIN

- - The anatomy here becomes a little more complex, because we see not only the basal surface of the hemisphere, but also the surface of the brainstem - the complex region lying between the hemisphere and the spinal cord, and the subject of Module 2. We will focus on the hemisphere, although the brainstem is also shown in these two views.

• Figure 1-36 . Identifies the lobes of the hemisphere.
• Figure 1-37 . Labels the sulci and gyri. The uncus is an important landmark on the medial edge of the temporal lobes. Compare with DiganatA_1.6 and DiganatA_1.8 (here, the brainstem has been revoved to reveal the ventral surface of the hemisphere.

THE MEDIAL SURFACE of the HEMISECTED BRAIN

- - The medial surface of the hemisected brain is the most interesting of all because it reveals not only another aspect of the hemisphere but also the mid-sagittal surfaces of the diencephalon and brainstem. We will leave the brainstem structures for Module 2 and mainly look at the hemisphere.

• Figure 1-38. An unlabeled view.
• Figure 1-39 . The gyri seen on the medial surface of the hemisphere. Compare with DiganatA_1.4
• Figure 1-40 . More anatomy, including some structures we cover in Module 2. Again, compare with DiganatA_1.4

LOCALIZATION of FUNCTION at the CORTICAL LEVEL

- - At this point, you are asking "Is that it? Do I have all the names I need to describe the surface of the cerebral cortex? If so, now tell me what each of these gyri actually does." Well, the gyri are a necessary start, but its not that simple (it never is). The gyri are useful landmarks for the neurosurgeon, the neuroradiologist and the neuropathologist and they are used in the reports that these specialists write - operative notes, and the like. But it turns out that the cellular region lying just deep to the surface of the hemisphere - the light area in Figure 1-35 - is organized into 6 layers and the thickness and cellular structure of these layers differs from one region to another. Blumenfeld describes this on Pages 29-30 and in Figure 2.14. More than 100 years ago, a worker by the name of Brodmann parceled the surface of the cortex out into more than 50 areas, based on the distinctive cellular appearance of each, and his map (Figure 1-41) is used to this day in describing the functional organization of this region.

- - We can now make a few very brief statements about localization of function at the cortical level; flip back and forth between Figure 1-41 and Figure 1-42 as you read.

- - - - - 1) The precentral gyrus is the primary motor cortex. Neurons here send descending axons to the spinal cord and brainstem; activity of these neurons results in movement. This area coincides with Brodmann's area 4.

- - - - - 2) The postcentral gyrus is the primary sensory cortex. Neurons here receive a direct input from ascending somatic sensory pathways. This are roughly coincides with Brodmann's areas 3, 1 and 2.

- - - - - 3) The cuneus (actually a gyrus) and the lingual gyrus are the primary visual cortex. Together they receive an input from the retina. This roughly coincide with area 17.

- - - - - 4) The transverse temporal gyri (part of the temporal lobe, but buried in the lateral fissure and hidden from view) are the primary auditory cortex. They receive an input from the ear and coincide with area 41

- - Many more subtle functions also are carried out in restricted cortical areas. One fascinating example concerns two regions of the brain - Broca's area and Wernicke's area (Figure 1-43). Damage of either area in the left hemisphere causes a marked impairment of speech, termed aphasia. Wernicke's aphasia is primarily a difficulty in understanding language. In contrast, subjects with Broca's aphasia understand what they have heard but have difficulty organizing a spoken response. They know what they want to say but are not able to put the words together in a fluent manner. To work properly, Broca's area requires an input from Wernicke's area and the pathway involved - the arcuate fasciculus - is well defined. You will be getting a lecture on aphasias in a few weeks and the topic is beyond the scope of this module. But, if you are interested, try to read Pages 9 - 14 in Kandel's 4th Edition.

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