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 degrees 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 central nervous system (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 ).

Directions in the CNS.

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 of 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.

Major CNS subdivisions. Arrow, Primary fissure of the cerebellum; *, anterior commissure; B, G, S, body, genu, and splenium of the corpus callosum.

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. At the bottom of the paired septum pellucidum is a paired, long, curved bundle of axons carrying the output of the hippocampus (see Fig. 3.6 later in this chapter); this is called the fornix . Each fornix travels from the hippocampus, found in 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.

Parts of the diencephalon and nearby structures, seen in a hemisected brain. The dashed line indicates the shallow sulcus (hypothalamic sulcus) in the wall of the third ventricle that separates the thalamus and hypothalamus. 4, Fourth ventricle; A, anterior commissure; Aq, cerebral aqueduct; IA, interthalamic adhesion (a gray matter connection between the two thalami, present in most but not all brains); IF, interventricular foramen; N, nodulus (part of the cerebellar vermis); O, optic chiasm; ON, optic nerve; Pi, pineal gland.

The ventricular system continues through the midbrain as the cerebral aqueduct, then widens into the fourth ventricle of the pons and rostral medulla. The pons is characterized by a large basal portion ( basal pons ) that protrudes anteriorly.

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, a 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 lateral portion connected to the nodulus is called the flocculous as seen in Fig. 20.1 .

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. Four sulci are particularly important for defining the boundaries of cerebral lobes ( Fig. 3.4 )—the lateral sulcus (i.e., Sylvian fissure ) and central sulcus (of Rolando ) on the lateral surface of the hemisphere, and the parietooccipital and cingulate sulci on the medial surface.

Lobes of each cerebral hemisphere.

Each Cerebral Hemisphere Includes a Frontal, Parietal, Occipital, Temporal, and Limbic Lobe

Key Concepts

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  The frontal lobe contains motor areas.

  
  
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  The parietal lobe contains somatosensory areas.

  
  
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  The temporal lobe contains auditory areas.

  
  
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  The occipital lobe contains visual areas.

  
  
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  The limbic lobe is interconnected with other limbic structures buried in the temporal lobe and plays a role in emotional behavior.

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 ).

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.

Major cerebral gyri and functional areas. IPL, Inferior parietal lobule; Or, orbital gyri; OTG, occipitotemporal gyrus; PHG, parahippocampal gyrus; SPL, superior parietal lobule.

The major named gyrus of the parietal lobe is the postcentral gyrus . The postcentral gyrus corresponds to primary somatosensory cortex (i.e., ascending somatosensory pathways terminate most heavily here) and, like the precentral gyrus, extends over onto the medial surface of the parietal lobe. The rest of the lateral surface is occupied by the superior and inferior parietal lobules , separated by the deep intraparietal sulcus .

The temporal lobe is covered by four long, parallel gyri. The superior , middle , and inferior temporal gyri are exposed on the lateral surface. The inferior temporal gyrus extends around onto the inferior surface and is followed by the occipitotemporal gyrus . Most primary auditory cortex is located in transverse temporal gyri in the wall of the lateral sulcus; it extends laterally to occupy a small portion of the superior temporal gyrus.

The occipital lobe has no gyri with commonly used names. However, its medial surface is bisected by the calcarine sulcus . Primary visual cortex occupies the walls of this calcarine sulcus and extends out onto the medial surface.

The major components of the limbic lobe are the cingulate and parahippocampal gyri . The cingulate gyrus curves around adjacent to the corpus callosum, interposed between it and the frontal and parietal lobes. Near the splenium of the corpus callosum the cingulate gyrus is continuous with the parahippocampal gyrus, which proceeds parallel to the occipitotemporal gyrus. At its anterior end the parahippocampal gyrus folds back on itself to form a bump called the uncus . The parahippocampal gyrus received its name because it is continuous with a cortical region called the hippocampus , which is rolled into the interior of the hemisphere and is visible only in sections (see Fig. 3.6 ).