Gross Anatomy and General Organization of the Central Nervous System

The Long Axis of the CNS Bends at the Cephalic Flexure

Before considering the parts of the brain in more detail, we will discuss some terms used for planes and directions in the nervous system. The midsagittal plane (often referred to simply as the sagittal plane ) goes through the midline and divides the brain into two symmetrical halves. Parasagittal planes are those parallel to the midsagittal plane; they divide the brain into asymmetrical right and left parts. Coronal planes (also called frontal planes ) are perpendicular to the sagittal plane; they divide the brain into asymmetrical anterior and posterior parts. Axial planes (also called transverse or horizontal planes ) are perpendicular to the long axis of the body; they divide the brain into asymmetrical superior and inferior parts. These terms are fairly straightforward and have the same meaning with respect to any part of the nervous system. However, directional terms such as anterior, dorsal, and rostral change their meanings relative to each other in different parts of the nervous system. The reason for this, as indicated in Fig. 3.1 , is that walking upright necessitates a bend of about 80 degrees in going from the long axis of the spinal cord and brainstem to the long axis of the forebrain. This bend is a consequence of the cephalic flexure, which appears early in the embryological development of the nervous system (see Fig. 2.7 ) and persists in the mature brain. Dorsal-ventral terminology ignores this bend, as though we had a linear CNS and walked around on all fours like most other vertebrates, so the directional meaning of “dorsal” changes by 80 degrees at the midbrain-diencephalon junction. The terms anterior and superior, in contrast, retain a constant meaning relative to the normal upright orientation of the body as a whole. This means, for example, that the ventral surface of the spinal cord is also its anterior surface, but the ventral surface of the forebrain is its inferior surface. Rostral-caudal terminology may cause additional confusion. Anatomically, rostral means “toward the nose.” However, it also has a functional connotation for many (implying “toward the telencephalon”), so the posterior end of the cerebral hemispheres could be considered rostral to all parts of the diencephalon. Use of anterior-posterior and superior-inferior terminology in reference to the cerebrum avoids any ambiguity.

Hemisecting a Brain Reveals Parts of the Diencephalon, Brainstem, and Ventricular System

Hemisection of the brain reveals many parts of the diencephalon, brainstem, and cerebellum, and additional features of the cerebral hemispheres ( Fig. 3.2B ). The cephalic flexure is visible at the junction between the brainstem and the diencephalon. The brainstem is subdivided into the midbrain, which is continuous with the diencephalon, the pons, and the medulla, which is continuous with the spinal cord at the foramen magnum. The two cerebral hemispheres are joined by a huge fiber bundle, the corpus callosum, which has an enlarged and rounded posterior splenium, a body, and an anterior, curved genu, which tapers into a ventrally directed rostrum that merges into the lamina terminalis, the site of closure of the rostral neuropore.

The nervous system develops embryologically from a neuroectodermal tube; the cavity of the tube persists in adults as a system of ventricles (see Fig. 5.1 ), part of which is apparent in the sagittal plane (see Fig. 3.2B ). Portions of the medial surfaces of the diencephalon form the walls of the narrow, slitlike third ventricle, which opens into the large lateral ventricle of each cerebral hemisphere through an interventricular foramen (of Monro). Posteriorly, the third ventricle is continuous with a narrow channel through the midbrain, the cerebral aqueduct (of Sylvius). The aqueduct in turn is continuous with the fourth ventricle of the pons and medulla, and the fourth ventricle is continuous with the central canal of the caudal medulla and the spinal cord.

Humans, Relative to Other Animals, Have Large Brains and Many Neurons

One impressive feature of the human brain is its size, to which distinctively human mental capacities are commonly attributed. An average human brain weighs about 400 g at birth; this weight triples during the first 3 years of life (resulting from addition of myelin and growth of neuronal processes, rather than the addition of more neurons). The rate of growth then slows, and the maximum brain weight of around 1400 g ( Table 3.1 ) is reached at about age 11 years. This weight holds steady until about the age of 50 years, when a slow decline sets in ( Fig. 3.3 ). The weight of 1400 g is only an average figure; brain weights for normal individuals range from 1100 g (or less) to around 1700 g. This large range of sizes is surprising, and its significance is not well understood. There is a robust, although modest, correlation between brain size and IQ.

Part of the large brain size is simply a reflection of body size: big animals tend to have big brains ( Fig. 3.4 ). Elephants, for example, have brains that weigh 5000 g. Similarly, the size difference between the bodies of human males and females explains, at least in part, why male brains are slightly larger than female brains (see Fig. 3.3 ). However, many animals that are larger than humans nevertheless have smaller brains ( Fig. 3.5 ). In different kinds of animals, brain size increases at different rates with increasing body size; primates have relatively large brains for their body size. ^a

a There are exceptions to the various brain-scaling rules found in various animal groups. In the case of primates, for example, great apes have smaller brains than would be expected for their body sizes, perhaps because they evolved to eat a diet inadequate to support the energy requirements of larger brains. Nevertheless, these smaller-than-expected brains have the neuronal packing densities and ratios of neurons to nonneuronal cells expected for primate brains.

A rodent with a brain the same size as a human's, for example, would be predicted to have a body dozens of times larger than a typical human. Humans have the largest brains among primates, and it is tempting to attribute human mental abilities to these relatively large brains, but this is an oversimplification. Not only is the size of the brain scaled to the size of the body in different ways in different kinds of animals but also different numbers of neurons grow in given volumes of these brains. In primates, many neurons are packed into the gray matter; the same hypothetical rodent with a human-size brain would contain fewer than 15% as many neurons. The net result is that humans have relatively large brains with perhaps more neurons than any other species, mostly located in the cerebral and cerebellar cortices ( Fig. 3.6 ).

Named Sulci and Gyri Cover the Cerebral Surface

A striking aspect of human cerebral hemispheres is the degree to which their surface is folded and convoluted. Each ridge is called a gyrus, and each groove between ridges is called a sulcus; particularly deep sulci are often called fissures. This folding into gyri and sulci is a mechanism for increasing the total cortical area (and the total number of cortical neurons). Each of us has enough cortex to command a surface area of about 2.5 ft ² (about 2300 cm ² ), two-thirds of which is hidden from view in the walls of sulci. The appearance of various gyri and sulci varies considerably from one brain to another ( Fig. 3.7 ). Major features, however, are reasonably constant.

In the discussion that follows, the principal surface features of the hemispheres are described, with some broad generalizations regarding the function of various cortical areas. These functional descriptions are highly oversimplified and are offered primarily for purposes of initial orientation. Cortical function is discussed in more detail in Chapter 22 .

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

Four prominent sulci—the central sulcus, the lateral sulcus (fissure), the parietooccipital sulcus, and the cingulate sulcus —together with the preoccipital notch and parts of a few other sulci are used to divide each cerebral hemisphere into five lobes ( Fig. 3.8 ):

• 1.

  The frontal lobe extends from the anterior tip of the brain (the frontal pole ) to the central sulcus (of Rolando). On the lateral surface of the hemisphere, the lateral sulcus (Sylvian fissure) separates it from the temporal lobe. On the medial surface of the brain, it extends to the cingulate sulcus and posteriorly to an imaginary line from the top of the central sulcus to the cingulate sulcus. Inferiorly it continues as the orbital part of the frontal lobe (named for its location above the orbit).

• 2.

  The parietal lobe extends from the central sulcus to an imaginary line connecting the top of the parietooccipital sulcus and the preoccipital notch. Inferiorly it is bounded by the lateral sulcus and the imaginary continuation of this sulcus to the posterior boundary of the parietal lobe. On the medial surface of the brain, it is bounded inferiorly by the subparietal and calcarine sulci, anteriorly by the frontal lobe, and posteriorly by the parietooccipital sulcus.

• 3.

  The temporal lobe extends superiorly to the lateral sulcus and the line forming the inferior boundary of the parietal lobe; posteriorly it extends to the line connecting the top of the parietooccipital sulcus and the preoccipital notch. On the medial surface, its posterior boundary is an imaginary line extending from the preoccipital notch toward the splenium of the corpus callosum, and part of its superior boundary is the collateral sulcus.

• 4.

  The occipital lobe is bounded anteriorly by the parietal and temporal lobes on the lateral and medial surfaces of the hemisphere.

• 5.

  The limbic lobe is a strip of cortex that appears to more or less encircle the telencephalon-diencephalon junction. It is interposed between the corpus callosum and the frontal, parietal, and occipital lobes and curves around to occupy part of the medial surface of what would otherwise be called the temporal lobe.

These separations correspond only approximately to functional subdivisions, but they do provide a meaningful basis for discussion and reference.

An additional area of cerebral cortex not usually included in any of the five lobes discussed lies buried in the depths of the lateral sulcus, concealed from view by portions of the frontal, parietal, and temporal lobes. This cortex, called the insula, overlies the site where the telencephalon and diencephalon fuse during embryological development (see Chapter 2 ). It can be revealed by prying open the lateral sulcus or by removing the overlying portions of other lobes ( Fig. 3.9 ). The portion of a given lobe overlying the insula is called an operculum (Latin for “lid”); there are frontal, parietal, and temporal opercula. The circular sulcus outlines the insula and marks its borders with the opercular areas of cortex.

The Frontal Lobe Contains Motor Areas

Four gyri make up the lateral surface of the frontal lobe ( Fig. 3.10 ). The precentral gyrus is anterior to the central sulcus and parallel to it, extending to the precentral sulcus. The superior, middle, and inferior frontal gyri are oriented parallel to one another and roughly perpendicular to the precentral gyrus. The superior frontal gyrus continues onto the medial surface of the hemisphere as far as the cingulate sulcus. The inferior frontal gyrus is visibly divided into three parts: (1) the orbital part, which is most anterior and is continuous with the inferior (orbital) surface of the frontal lobe; (2) the opercular part, which is most posterior and forms a portion of the frontal operculum; and (3) the wedge-shaped triangular part, which lies between the other two. The inferior, or orbital, surface of the frontal lobe is occupied primarily by a group of gyri of somewhat variable appearance that usually are collectively referred to simply as orbital gyri or orbitofrontal cortex. The only consistently named gyrus on this surface is gyrus rectus (Greek for “straight gyrus”), which is most medial and extends onto the medial surface of the hemisphere. Between gyrus rectus and the orbital gyri is the olfactory sulcus, which contains the olfactory bulb and tract. The medial surface of the lobe contains extensions of the superior frontal gyrus, precentral gyrus, and gyrus rectus; certain small cortical areas near the rostrum of the corpus callosum are part of the limbic lobe.

The frontal lobe contains four general functional areas:

• 1.

  Much of the precentral gyrus is the primary motor cortex, which contains many of the cells of origin of descending motor pathways and is involved in the initiation of voluntary movements.

• 2.

  The premotor and supplementary motor areas occupy the remainder of the precentral gyrus together with the posterior portions of the superior and middle frontal gyri; they are functionally related to the planning and initiation of voluntary movements.

• 3.

  Motor speech area (Broca's, expressive), the opercular and triangular parts of the inferior frontal gyrus of one hemisphere (usually the left), is important in the production (motor aspects) of written and spoken language.

• 4.

  The prefrontal cortex, a very large and somewhat confusingly named area (because it sounds like it is in front of the frontal lobe), occupies the remainder of the frontal lobe. Prefrontal cortex is involved with what are often referred to as executive functions, or, very generally, personality, insight, and foresight.

The Parietal Lobe Contains Somatosensory Areas

The lateral surface of the parietal lobe is divided into three areas: the postcentral gyrus and the superior and inferior parietal lobules ( Fig. 3.11 ). The postcentral gyrus is posterior to the central sulcus and parallel to it, extending to the postcentral sulcus. The intraparietal sulcus runs posteriorly from the postcentral sulcus toward the occipital lobe, separating the superior and inferior parietal lobules. The inferior parietal lobule in turn is composed of the supramarginal gyrus, which caps the upturned end of the lateral sulcus, and the angular gyrus, which similarly caps the superior temporal sulcus. The angular gyrus is typically broken up by small sulci and may overlap the supramarginal gyrus. The medial surface of the parietal lobe contains the medial extension of the postcentral gyrus (i.e., the posterior paracentral lobule ) and is completed by an area called the precuneus, which is bounded by the subparietal and parietooccipital sulci, the marginal branch of the cingulate sulcus, and part of the calcarine sulcus.

The parietal lobe is associated, in a very general sense, with three functions:

• 1.

  The postcentral gyrus corresponds to primary somatosensory cortex; it is concerned with the initial cortical processing of tactile and proprioceptive (sense of position) information; more specifically, it deals with sensory localization.

• 2.

  Much of the inferior parietal lobule of one hemisphere (usually the left), together with portions of the temporal lobe, is involved in the comprehension of language.

• 3.

  The remainder of the parietal cortex subserves complex aspects of spatial orientation and directing attention.

The Temporal Lobe Contains Auditory Areas

The lateral surface of the temporal lobe is composed of the superior, middle, and inferior temporal gyri ( Fig. 3.12 ). The superior surface of the temporal lobe extends into the lateral sulcus, where it continues into the temporal operculum. The inferior temporal gyrus continues onto the inferior surface of the lobe. The rest of the inferior surface is made up of the broad occipitotemporal (fusiform) gyrus, which is separated from the limbic lobe by the collateral sulcus. The occipitotemporal gyrus, as its name implies, is partly in the occipital lobe and partly in the temporal lobe.

The temporal lobe is associated in a general way with four functions:

• 1.

  Part of the superior surface of the temporal lobe, continuing as a small area of the superior temporal gyrus, is the primary auditory cortex.

• 2.

  Sensory speech area (Wernicke's, receptive), the posterior portion of the superior temporal gyrus of one hemisphere (usually the left), is important in the comprehension of language.

• 3.

  Much of the temporal lobe, particularly the inferior surface, is involved in higher order processing of visual information.

• 4.

  The most medial part of the temporal lobe ^b

  b The structures important in learning and memory (discussed in Chapter 23 ) are actually parts of the limbic lobe and underlying limbic-related structures and are not part of the temporal lobe as defined in this chapter. However, because of their gross anatomical location, these structures critical for memory are commonly referred to as medial temporal .

  is involved in complex aspects of learning and memory.

The Occipital Lobe Contains Visual Areas

The lateral surface of the occipital lobe is of variable configuration, and its gyri are usually referred to simply as lateral occipital gyri. On the medial surface, the wedge-shaped area between the parietooccipital and calcarine sulci is called the cuneus (Latin for “wedge”) ( Fig. 3.13 ). The gyrus inferior to the calcarine sulcus is the lingual gyrus. The lingual gyrus is adjacent to the posterior portion of the occipitotemporal gyrus, separated from it by the collateral sulcus, and usually continuous anteriorly with the parahippocampal gyrus. The transition from lingual to parahippocampal gyrus occurs at the isthmus of the cingulate gyrus ( Fig. 3.14 ).

The occipital lobe is more or less exclusively concerned with visual functions. Primary visual cortex is contained in the walls of the calcarine sulcus and a bit of the surrounding cortex. The remainder of the lobe is referred to as visual association cortex and is involved in higher order processing of visual information; visual association cortex extends into the temporal lobe as well, reflecting the importance of vision in primates.

The Limbic Lobe Is Interconnected With Other Limbic Structures, Some Buried in the Temporal Lobe

The limbic lobe (see Fig. 3.14 ) is composed primarily of the cingulate and parahippocampal gyri. The cingulate gyrus, immediately superior to the corpus callosum, can be followed posteriorly to the splenium of the corpus callosum, where it turns inferiorly as the narrow isthmus of the cingulate gyrus and continues as the parahippocampal gyrus. These two gyri give the appearance of encircling the diencephalon, and they, together with some small cortical areas near the lamina terminalis (paraterminal gyrus) and inferior to the genu of the corpus callosum (subcallosal area), make up the limbic lobe (from the Latin word limbus, meaning “border”). The anterior end of the parahippocampal gyrus hooks backward on itself, forming a medially directed bump called the uncus. The superior border of the parahippocampal gyrus is the hippocampal sulcus (see Fig. 3.24 ). Folded into the temporal lobe at the hippocampal sulcus is a differently structured area of cortex called the hippocampus ( Fig. 3.15 ). The limbic lobe is the cortical component of the limbic system, which is important in emotional responses, drive-related behavior, and memory.

The Diencephalon Includes the Thalamus and Hypothalamus

The diencephalon accounts for less than 2% of the weight of the brain but nevertheless is extremely important. It has four divisions: thalamus, hypothalamus, epithalamus, and subthalamus ( thalamus is Latin for “inner chamber”). Portions of three of these divisions can be seen on a hemisected brain ( Fig. 3.16 ); the subthalamus is an internal structure that can be seen only in sections through the brain. The epithalamus comprises the midline pineal gland and several small nearby neural structures visible in sections.