Lobes

The surfaces of the two cerebral hemispheres are furrowed by sulci, the intervening ridges being called gyri. Most of the cerebral cortex is concealed from view in the walls of the sulci. Although the patterns of the various sulci vary from brain to brain, some are sufficiently constant to serve as descriptive landmarks. Deepest sulci are the lateral sulcus (Sylvian fissure) and the central sulcus (Rolandic fissure) (Figure 2.1A). These two serve to divide the hemisphere (side view) into four lobes, with the aid of two imaginary lines, one extending back from the lateral sulcus, the other reaching from the upper end of the parietooccipital sulcus (Figure 2.1B) to a blunt preoccipital notch at the lower border of the hemisphere (the sulcus and notch are labeled in Figure 2.3). The lobes are called frontal, parietal, occipital, and temporal.

Figure 2.1 The five lobes of the brain. (A) Lateral surface of right cerebral hemisphere. (B) Medial surface of right hemisphere.

Figure 2.3 (A) Lateral and (B) medial views of the right cerebral hemisphere, depicting the main gyri and sulci.

The blunt tips of the frontal, occipital, and temporal lobes are the respective poles of the hemispheres.

The opercula (lips) of the lateral sulcus can be pulled apart to expose the insula (Figure 2.2). The insula was mentioned in Chapter 1 as being relatively quiescent during prenatal expansion of the telencephalon.

Figure 2.2 Insula, seen on retraction of the opercula.

The medial surface of the hemisphere is exposed by cutting the corpus callosum, a massive band of white matter connecting matching areas of the cortex of the two hemispheres. The corpus callosum consists of a main part or trunk, a posterior end or splenium, an anterior end or genu (‘knee’), and a narrow rostrum reaching from the genu to the anterior commissure (Figure 2.3B). The frontal lobe lies anterior to a line drawn from the upper end of the central sulcus to the trunk of the corpus callosum (Figure 2.3B). The parietal lobe lies behind this line, and it is separated from the occipital lobe by the parietooccipital sulcus. The temporal lobe lies in front of a line drawn from the preoccipital notch to the splenium.

Figures 2.3–2.6 should be consulted along with the following description of surface features of the lobes of the brain.

Figure 2.4 ’Thick slice’ surface anatomy brain MRI scan from a healthy volunteer.

(From Katada, 1990.)

Figure 2.5 Cerebrum viewed from inferior aspect, depicting the main gyri and sulci.

Figure 2.6 The diencephalon and its boundaries. CC, corpus callosum.

Diencephalon

The largest components of the diencephalon are the thalamus and the hypothalamus (Figures 2.6 and 2.7). These nuclear groups form the side walls of the third ventricle. Between them is a shallow hypothalamic sulcus, which represents the rostral limit of the embryonic sulcus limitans.

Figure 2.7 Sagittal MRI ‘slice’ of the living brain.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California, USA.)

Midline sagittal view of the brain

Figure 2.8 is taken from a midline sagittal section of the head of a cadaver, displaying the brain in relation to its surroundings.

Figure 2.8 Sagittal section of fixed cadaver brain.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Thalamus, caudate and lentiform nuclei, internal capsule

The two thalami face one another across the slot-like third ventricle. More often than not, they kiss, creating an interthalamic adhesion (Figure 2.9). In Figure 2.10, the thalamus and related structures are assembled in a mediolateral sequence. In contact with the upper surface of the thalamus are the head and body of the caudate nucleus. The tail of the caudate nucleus passes forward below the thalamus but not in contact with it.

Figure 2.9 Thalamus and corpus striatum, seen on removal of the trunk of the corpus callosum and the trunk of the fornix.

Figure 2.10 Diagrammatic reconstruction of corpus striatum and related structures. The vertical lines on the left in A and B indicate the level of the coronal sections on the right. (A) Ventricular system. (B) Thalamus and caudate nucleus in place. (C) Addition of projections to and from cerebral cortex. (D) Lentiform nucleus in place.

The thalamus is separated from the lentiform nucleus by the internal capsule, which is the most common site for a stroke resulting from local arterial embolism (blockage) or hemorrhage. The internal capsule contains fibers running from thalamus to cortex and from cortex to thalamus, brainstem, and spinal cord. In the interval between cortex and internal capsule, these ascending and descending fibers form the corona radiata. Below the internal capsule, the crus of the midbrain receives descending fibers continuing down the brainstem.

The lens-shaped lentiform nucleus is composed of two parts: putamen and globus pallidus. The putamen and caudate nucleus are of similar structure, and their anterior ends are fused. Behind this, they are linked by strands of gray matter that traverse the internal capsule, hence the term corpus striatum (or, simply, striatum) used to include the putamen and caudate nucleus. The term pallidum refers to the globus pallidus.

The caudate and lentiform nuclei belong to the basal ganglia, a term originally applied to a half-dozen masses of gray matter located near the base of the hemisphere. In current usage, the term designates four nuclei known to be involved in motor control: the caudate and lentiform nuclei, the subthalamic nucleus in the diencephalon, and the substantia nigra in the midbrain (Figure 2.11).

Figure 2.11 Nomenclature of basal ganglia.

In horizontal section, the internal capsule has a dog-leg shape (see photograph of a fixed-brain section in Figure 2.12, and the living-brain magnetic resonance image [MRI] ‘slice’ in Figure 2.13). The internal capsule has four named parts in horizontal sections:

Figure 2.12 Horizontal section of fixed cadaver brain at the level indicated at top. IC, internal capsule.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Figure 2.13 Horizontal MRI ‘slice’ in the plane of Figure 2.12. IC, internal capsule.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California, USA.)

The corticospinal tract (CST) descends in the posterior limb of the internal capsule. It is also called the pyramidal tract, a tract being a bundle of fibers serving a common function. The CST originates mainly from the cortex within the precentral gyrus. It descends through the corona radiata, internal capsule, and crus of midbrain and continues to the lower end of the brainstem before crossing to the opposite side of the spinal cord.

From a clinical standpoint, the CST is the most important pathway in the entire central nervous system (CNS), for two reasons. First, it mediates voluntary movement of all kinds, and interruption of the tract leads to motor weakness (called paresis) or motor paralysis. Second, it extends the entire vertical length of the CNS, rendering it vulnerable to disease or trauma in the cerebral hemisphere or brainstem on one side, and to spinal cord disease or trauma on the other side.

A coronal section through the anterior limb is represented in Figure 2.14; a corresponding MRI view is shown in Figure 2.15. A coronal section through the posterior limb from a fixed brain is shown in Figure 2.16; a corresponding MRI slice is shown in Figure 2.17.

Figure 2.14 Drawing of a coronal section through the anterior limb of the internal capsule.

Figure 2.15 Coronal MRI ‘slice’ at the level of Figure 2.14.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California, USA.)

Figure 2.16 Coronal section of fixed cadaver brain at the level indicated at top.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Figure 2.17 Coronal MRI ‘slice’ at the level of Figure 2.16.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California, USA).

Lateral to the lentiform nucleus are the external capsule, claustrum, and extreme capsule.

Hippocampus and fornix

During embryonic life, the hippocampus (crucial for memory formation) is first seen above the corpus callosum. The bulk of it remains in that position in lower mammals. In primates, it retreats into the temporal lobe as this lobe develops, leaving a tract of white matter, the fornix, in its wake. The mature hippocampus stretches the full length of the floor of the inferior (temporal) horn of the lateral ventricle (Figures 2.18 and 2.19). The mature fornix comprises a body beneath the trunk of the corpus callosum, a crus, which enters it from each hippocampus, and two pillars (columns), which leave it to enter the diencephalon. Intimately related to the crus and body is the choroid fissure, through which the choroid plexus is inserted into the lateral ventricle.

Figure 2.18 Tilted view of the ventricular system, showing the continuity of structures in the body and inferior horn of the lateral ventricle. Note: The amygdala, stria terminalis, and tail of caudate nucleus occupy the roof of the inferior horn; the hippocampus occupies the floor.

(The choroid plexus is ‘reduced’ in order to show related structures.)

Figure 2.19 Coronal section through the body and inferior horn of the lateral ventricle.

Association and commissural fibers

Fibers leaving the cerebral cortex fall into three groups:

Association fibers (Figure 2.20)

Short association fibers pass from one gyrus to another within a lobe.

Figure 2.20 (A) Medial view of ‘transparent’ right cerebral hemisphere. (B) Lateral view of ‘transparent’ left hemisphere. (C) Coronal section, showing position of short and long association fiber bundles.

Long association fibers link one lobe with another. Bundles of long association fibers include:

• the superior longitudinal fasciculus, linking the frontal and occipital lobes;

• the inferior longitudinal fasciculus, linking the occipital and temporal lobes;

• the arcuate fasciculus, linking the frontal lobe with the occipitotemporal cortex;

• the uncinate fasciculus, linking the frontal and anterior temporal lobes;

• the cingulum, underlying the cortex of the cingulate gyrus.

Cerebral commissures

Corpus callosum

The corpus callosum is the largest of the commissures linking matching areas of the left and right cerebral cortex (Figure 2.21). From the body, some fibers pass laterally and upward, intersecting the corona radiata. Other fibers pass laterally and then bend downward as the tapetum to reach the lower parts of the temporal and occipital lobes. Fibers traveling to the medial wall of the occipital lobe emerge from the splenium on each side and form the occipital (major) forceps. The frontal (minor) forceps emerges from each side of the genu to reach the medial wall of the frontal lobe.

Figure 2.21 Horizontal section through genu and splenium of corpus callosum. Fibers passing laterally from the trunk intersect the corona radiata.

Minor commissures

The anterior commissure interconnects the anterior parts of the temporal lobes, as well as the two olfactory tracts.

The posterior commissure and the habenular commissure lie directly in front of the pineal gland.

The commissure of the fornix contains some fibers traveling from one hippocampus to the other by way of the two crura.

Lateral and third ventricles

The lateral ventricle consists of a body within the parietal lobe, and anterior (frontal), posterior (occipital), and inferior (temporal) horns (Figure 2.22). The anterior limit of the central part is the interventricular foramen, located between the thalamus and anterior pillar of the fornix, through which it communicates with the third ventricle. The central part joins the occipital and temporal horns at the atrium (Figures 2.23 and 2.24).

Figure 2.22 Ventricular system. (A) Isolated cast. (B) Ventricular system in situ.

Figure 2.23 Coronal section of fixed cadaver brain at the level indicated at top.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Figure 2.24 Coronal MRI ‘slice’ at the level indicated at top.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California, USA.)

The relationships of the lateral ventricle are listed below.

The third ventricle is the cavity of the diencephalon. Its boundaries are shown in Figure 2.6. A choroid plexus hangs from its roof, which is formed of a double layer of pia mater called the tela choroidea. Above this are the fornix and corpus callosum. In the side walls are the thalamus and hypothalamus. The anterior wall is formed by the anterior commissure, the lamina terminalis, and the optic chiasm. In the floor are the infundibulum, the tuber cinereum, the mammillary bodies (also spelt ‘mamillary’), and the upper end of the midbrain. The pineal gland and adjacent commissures form the posterior wall. The pineal gland is often calcified, and the habenular commissure is sometimes calcified, as early as the second decade of life, thereby becoming detectable even on plain radiographs of the skull. The pineal gland is sometimes displaced to one side by a tumor, hematoma, or other mass (space-occupying lesion) within the cranial cavity.

Box 2.1 Brain planes

Figure Box 2.1.1 (A) Planes of reference for the central nervous system as a whole. In this presentation, only the brainstem (owing to its obliquity) differs from the standard for gross anatomy. However, some authors use the terms ventral and dorsal instead of anterior and posterior with respect to the spinal cord, and some use the terms rostral and caudal to signify superior and anterior with respect to spinal cord and/or brainstem. The horizontal line represents the bicommissural plane. AC, PC, anterior and posterior commissures. (B) The brain sectioned in the bicommissural plane.

(Adapted from Kretschmann and Weinrich 1998, with permission of Thieme and the authors.)

Box 2.2 Magnetic resonance imaging

Magnetic resonance imaging of the CNS is immensely useful for detection of tumors and other space-occupying lesions (masses). When properly used, it is quite safe, even for young children and pregnant women. As will be shown later on, it can be adapted to the study of normal brain physiology in healthy volunteers.

The original name for the technique is nuclear MRI, because it is based on the behavior of atomic nuclei in applied magnetic fields. The simplest atomic nucleus is that of the element hydrogen, consisting of a single proton, and this is prevalent in many substances (e.g. water) throughout the body.

Nuclei possess a property known as spin (Figure Box 2.2.1), and it may be helpful to visualize this as akin to a spinning gyroscope. Normally, the direction of the spin (the axis of the gyroscope in our analogy) for any given nucleus is random. Spin produces a magnetic moment (vector) that makes it behave like a tiny dipole (north and south) magnet. In the absence of any external magnetic field, the dipoles are randomly arranged.

Figure Box 2.2.1 Basis of nuclear magnetic resonance and its manipulation by radio waves. (A) The proton of the hydrogen nucleus is in a constant state of spin analogous to that of a gyroscope. (B) At rest, the orientation of the axes of the spinning protons is random. (C) When the external magnet is switched on, all the axes become oriented along its longitudinal, z axis. The great majority are parallel, with a small minority antiparallel, as indicated. (D) At the same time, the magnetic moments immediately precess around the axis like a wobbling gyroscope, being oriented in an intermediate state between the z axis of the magnetic field and the x–y axis at right angles to it. (E) An excitatory radio-frequency pulse at right angles to the axis of the external magnetic field tips the net magnetic moment along a ‘snail shell’ spiral into the x–y plane. (F) While the radio-frequency transceiver pulse is ‘on’, the nuclei are precessing in phase. (G) Switching off the radio frequency allows the nuclei to dephase immediately, with a brief T2 time constant. (H) Conical precession is resumed under the influence of the external magnet with a longer, T1 time constant.

(The assistance of Professor Hugh Garavan, Department of Psychology, Trinity College, Dublin, is gratefully appreciated.)

In the presence of a magnetic field, however, the dipoles will orient themselves along the direction of the magnetic field z (vertical) line.

The cylindric external magnet of an MRI machine (Figure Box 2.2.2) is immensely powerful, capable of lifting the weight of several cars at one time. When the magnet is switched on, individual nuclear magnetic moments undergo a process called – precession analogous to the wobbling of a gyroscope – whereby they adopt a cone-shaped spin around the z axis of the external magnetic field.

Figure Box 2.2.2 The MRI machine. Outermost is the magnet. Innermost is the radio-frequency transceiver. In between are the gradient coils.

Excitatory pulses are transmitted from radio-frequency coils set at right angles to the z axis of the external magnetic field. The effect is to tilt the net nuclear magnetic moment into the x–y axis, with all the nuclei precessing ‘in phase’. When the radio-frequency coils are switched off, the nuclei ‘dephase’ while still in the x–y axis, and then relax back to vertical alignment. The time constant involved is called T2. The external magnet then restores the conical precession around the z axis; the time constant here is much slower and is called T1.

Because the spinning, precessing nuclei behave like little magnets, if they are surrounded by a coil of wire, they will induce a current in that coil that can then be measured. As it happens, the radiotransmitter coil is able to receive and measure this current, hence term transceiver in the diagram.

This is the basic principle of nuclear magnetic resonance. However, to be able to construct an actual image, we require to spatially resolve the detected signal. This can be achieved by introducing gradient coils. Superimposition of a second magnetic field, set at right angles to that of the main magnet, causes the resonant frequency to be disturbed along the axis of the new field, the proton spin being highest at one end and lowest at the other end. The magnetic resonance machine in fact contains three gradient coils, one being set in each of the three planes of space. The three coils are activated sequentially, allowing three-dimensional localization of tissue signals. In this way, it is possible to ‘slice’ through the patient, detecting the signal emitted from different components in each selected plane of the patient, and building up an image piece by piece.

The varying densities within the magnetic resonance images reflect the varying rates of dephasing and of relaxation of protons in different locations. The protons of the cerebrospinal fluid, for example, are free to resonate at maximum frequency, whereas in the white matter they are largely bound to lipid molecules. The gray matter has intermediate values, some protons being protein-bound. The radio-frequency pulses can be varied to exploit these differences. Almost all the images shown in textbooks (including this one) are T1-weighted, favoring the very weak signal provided by free protons during the relaxation period. This accounts for the different densities of CSF, gray matter, and white matter, the last being strongest. The reverse is true for T2-weighted images. T2-weighted images are especially useful in detection of lesions in the white matter. For example, they can indicate an increase in free protons resulting from patchy loss of myelin sheath lipid in multiple sclerosis (see Ch. 6), or local edema of brain tissue resulting from a vascular stroke.

The standard orientation of coronal and axial slices is shown in Figure Box 2.2.3.

Figure Box 2.2.3 Standard orientation of magnetic resonance images. Coronal sections are viewed from in front. Axial sections are viewed from below.

References

Jones DK. Fundamentals of diffusion MRI imaging. In: Gillard JH, Waldman AD, Barker PB, editors. Fundamentals of neuroimaging. Cambridge: Cambridge University Press; 2005:54-85.

Mitchell DG, Cohen MS. MRI principles, ed 3. Philadelphia: Saunders; 2004.

Saper CB, Iversen S, Frackoviak R, Kandel ER, Schwarz JH, Jessell TJ, editors. Principles of neural science, ed 4. New York 2000,, McGraw-Hill, pp 370-375.

Box 2.3 Diffusion tensor imaging

Terms

• Diffusion tensor imaging is a technique developed in the mid-1990s that uses MRI to measure diffusion constants of water molecules along many (>6) directions and that characterizes diffusion anisotropy.

• An isotropic Iiquid has uniform diffusion properties on all sides, e.g. a drop of milk diffuses uniformly all around when released in water. Isotropy is uniformity in all directions and can be represented by a sphere.

• An anisotropic (Gr. ‘not isotropic’) liquid diffuses along a preferred axis and can be represented graphically as an ellipsoid.

• A tensor describes the shape of the ellipsoid. Diffusion tensors are second-order tensors (special cases of tensors are scalar [zero order or single digits] and vectors [first order or {1 × n} matrices]). A tensor can be reduced to its component axes (eigenvalues), termed lambda 1, 2 and 3, that describe the relative rate of diffusion along the length, breadth and width of the ellipsoid (eigenvectors).

• Fractional anisotropy (FA) describes the relationship between lambda 1, 2 and 3 as a fraction. The value of FA therefore ranges from 0 to 1.

Figure Box 2.3.1 Deterministic tractography across the entire brain performed using *ExploreDTI software. This image uses the common convention in diffusion tensor and tractography images where tracts in the anterior–posterior orientation are represented in green, superior–inferior in blue and right–left in red. SCP, superior cerebellar peduncle.

Figure Box 2.3.2 Fractional anisotropy is shown in three planes of space, with the same color-coding as in Figure 2.3.1. c.c., corpus callosum; MCP, middle cerebral peduncle; TFP, transverse fibers of pons.

In the nervous system, diffusion of intracellular water in white matter is restricted by the cell membranes. Extracellular water circulating in the ventricles and subarachnoid space and water in gray matter diffuses in a more isotropic manner. The interstitial fluid among myelinated fiber bundles preferentially diffuses parallel to the long axis of the fibers. The higher the fractional anisotropy, the more compact and uniform the bundles of fibers. This is particularly useful when comparing the relative integrity of matching myelinated pathways on each side of the brain or spinal white matter. One can reconstruct the three-dimensional trajectories of white matter tracts using tractography together with color encoding to denote direction. The reconstruction algorithm is based on fiber orientation information obtained from diffusion tensor imaging. A more advanced method that addresses the limitations of the tensor model (i.e. that summarizes information to one principal direction as the basis of tractography) and results in more accurate reconstruction is constrained spherical deconvolution (CSD). CSD uses information in multiple directions for each voxel and has begun to address the problems for tractography that occur in regions where fiber bundles cross.

*ExploreDTI.com provided by Dr. Alexander Leemans, Image Sciences Institute, University Medical Center, Utrecht.

(The assistance of Dr. Dara M. Cannon, Co-Director, Clinical Neuroimaging Laboratory, Department of Psychiatry, National University of Ireland, Galway is gratefully acknowledged.)

References

Hess CP, Mukherjee P. Visualizing white matter pathways in the living human brain: diffusion tensor imaging and beyond. Neuroimag Clin N Am. 2007;17:407-426.

Mori S. Introduction to diffusion tensor imaging. Amsterdam: Elsevier; 2007.