3 Gross Anatomy and General Organization of the Central Nervous System
A useful way to start studying the brain is to learn some of the vocabulary that refers to its major parts, and to understand in a vague way what they do. These major parts can then serve as reference points to build on in later chapters.
The Long Axis of the CNS Bends at the Cephalic Flexure
Most creatures move through the world with their spinal cords oriented horizontally. In humans, the cephalic flexure of the embryonic neural tube persists in the adult brain as a bend of about 80° between the midbrain and the diencephalon, allowing us to walk around upright. Terms like dorsal and ventral, however, are used as though the flexure does not exist, the CNS is still a straight tube, and we walk around on all fours. The result is that in the spinal cord and brainstem dorsal has the same meaning as posterior, whereas in the forebrain dorsal has the same meaning as superior (Fig. 3-1).
Hemisecting a Brain Reveals Parts of the Diencephalon, Brainstem, and Ventricular System
The cerebral hemispheres of humans are so big that they cover over much of the rest of the CNS. The medial surface of a hemisected brain, however, reveals all the major divisions (Fig. 3-2), still arranged in the same sequence as in the embryonic neural tube: cerebral hemisphere-diencephalon-brainstem/cerebellum-spinal cord.
Two fiber bundles interconnect the cerebral hemispheres. The corpus callosum interconnects most cortical areas, extending from an enlarged genu in the frontal lobe through a body to an enlarged splenium in the parietal lobe. The much smaller anterior commissure performs a similar function for parts of the temporal lobes. Beneath the corpus callosum in an accurately hemisected brain is a membrane called the septum pellucidum. This is a paired membrane (one per hemisphere) that separates those parts of the lateral ventricles adjacent to the midline (THB6 Figures 3-19 to 3-21, pp. 69 and 70). At the bottom of the septum pellucidum is the fornix, a long curved fiber bundle carrying the output of the hippocampus (see Fig. 3-6 later in this chapter) from the temporal lobe to structures like the hypothalamus at the base of the brain.
Hemisection passes through the middle of the third ventricle, exposing the thalamus and hypothalamus in its walls (Fig. 3-3). Each interventricular foramen connects the third ventricle to the lateral ventricle of that side. The optic chiasm, in which about half the fibers in each optic nerve cross the midline, is attached to the bottom of the hypothalamus. The pineal gland (part of the diencephalon) is attached to the roof of the third ventricle, near the diencephalon-brainstem junction.
The cerebellum is divided, in one gross anatomical sense, into a midline portion called the vermis (Latin for “worm”) and a much larger hemisphere on each side. In another gross anatomical sense, the deep primary fissure divides the bulk of the cerebellum into an anterior lobe and a substantially larger posterior lobe. Hence the anterior and posterior lobes have both vermal and hemispheral portions. Finally, there is a small flocculonodular lobe. The vermal part (the nodulus) can be seen in Fig. 3-3; the flocculus can be seen in THB6 Figures 3-16 and 3-17, pp. 65 and 66.
Named Sulci and Gyri Cover the Cerebral Surface
The surface of each cerebral hemisphere is wrinkled up into a series of gyri and sulci, constant from one brain to another in their general configuration but not in their details (THB6 Figure 3-6, p. 58). Four sulci are particularly important for defining the boundaries of cerebral lobes (Fig. 3-4)—the lateral sulcus (= Sylvian fissure) and central sulcus (of Rolando) on the lateral surface of the hemisphere, and the parietooccipital and cingulate sulci on the medial surface.
Each Cerebral Hemisphere Includes a Frontal, Parietal, Occipital, Temporal, and Limbic Lobe
The frontal lobe is above the lateral sulcus and in front of the central sulcus. The parietal lobe is right behind the frontal lobe, extending back to the occipital lobe (which is defined by landmarks more easily visible on the medial surface of the hemisphere). The temporal lobe is below the lateral sulcus. All four of these lobes continue onto the medial surface of the hemisphere, extending as far as the limbic lobe. The limbic lobe is a ring of cortex that encircles the junction between the cerebral hemisphere and the diencephalon. In addition, the insula, not part of any of the preceding lobes, is buried in the lateral sulcus, covered over by parts of the frontal, parietal, and temporal lobes (see Fig. 2-6; and see THB6 Figure 3-8, p. 60).
The lateral surface of the frontal lobe is made up of the precentral gyrus and the superior, middle, and inferior frontal gyri (Fig. 3-5). The precentral gyrus is located immediately in front of the central sulcus and most of it is primary motor cortex (i.e., much of the corticospinal tract originates here). The other three are broad, parallel gyri that extend anteriorly from the precentral gyrus. The precentral and superior frontal gyri extend over onto the medial surface of the frontal lobe, where they end at the cingulate sulcus. The inferior (or orbital) surface of the frontal lobe is made up of a series of unnamed orbital gyri together with gyrus rectus, which is located adjacent to the midline.
