CHAPTER 12. THE CEREBRAL CORTEX

OBJECTIVES:
  1. BE ABLE TO IDENTIFY THE CEREBRAL CORTEX AND ITS PYRAMIDAL CELLS IN HISTOLOGICAL PREPARATIONS.
  2. BE ABLE TO LOCATE THE CORTICAL GYRI AND FUNCTIONAL AREAS IN BRAIN SPECIMENS.
  3. BE ABLE TO IDENTIFY THE MAJOR ASSOCIATION BUNDLES AND COMMISSURES IN DISSECTED BRAIN SPECIMENS AND SLICES.

Microscopic Features

The chief cytological characteristics of the six layers of the neocortex are as follows:

I molecular (plexiform) layer - most superficial, containing few cells;

II external granular layer - densely packed granule cells;

III  external pyramidal layer - numerous small to medium size pyramidal cells;

IV internal granular layer - densely packed granule cells;

V  internal pyramidal layer - largest pyramidal cells;

VI multiform layer - numerous neurons of different shapes and sizes.

The general characteristics of the cerebral cortex can be observed in an hematoxylin and eosin preparation (Sl. 70).

This is the postcentral gyrus which contains abundant granule cells, especially in layers II and IV. Using these as guidelines, the boundaries of the other layers can be approximated, although with difficulty! More striking than the horizontal lamination in this preparation, is the alignment of neurons into vertical columns that form the functional units of the neocortex.

The lamination in the cortex can be better observed in a silver preparation of the precentral gyrus (Sl. 71).

Layer I is at the surface and relatively devoid of neurons. Layer II is approximately the same thickness as layer I, but contains numerous small neurons, mainly of the granule type. Layer III is almost twice as thick as layers I and II combined, and is relatively devoid of cells but does contain numerous pyramidal neurons that increase in size from external to internal. Layer IV is thin and contains clumps of densely- packed granule neurons. Layer V is comprised of many large pyramidal cells. Layer VI has fewer cells and these are arranged in a more random fashion.

Additional histological features can be observed in Slide 72, a higher magnification of laminae III, IV, and V of Slide 71.

The abundance of pyramidal neurons in laminae III and V is obvious, with those in V being largest. The small clumps of granule cells occurring between these layers form layer IV.

The vertical orientation of the cortex is chiefly due to the shape and arrangement of the pyramidal cells. The morphological features of a group of pyramidal cells can be observed in Slide 73.

The cell bodies are triangular and give off apical and basal dendrites. The apical dendrite proceeds vertically toward the surface of the cortex. The basal dendrites emerge from the sides of the cell body and pass horizontally. Note the spines on many of the branches of the apical and basal dendrites. The axon arises from the base of the pyramidal cell and proceeds vertically through the deeper layers to the subcortical white matter. The morphological features of the cell bodies, apical and basal dendrites, and the axons can also be observed in Slide 74, a higher magnification of two pyramidal cells. The pyramidal cells are the efferent cortical neurons and their axons become the projection, commissural, or association fibers of the hemispheric white matter.

Gyri and Areas

Frontal Cortex

Using brain specimens, identify in the precentral gyrus and the adjacent parts of the paracentral lobule the approximate locations of areas 4 and 6 (Pls. 1, 4, 5). The primary motor area is found especially in that part of the precentral gyrus forming the anterior wall of the central sulcus, as well as in its continuation into the anterior part of the paracentral lobule.

Movements in various parts of the body are localized in specific parts of the primary motor area. Thus, jaw, tongue, and face movements are represented in the ventral part (the part nearer the lateral fissure), upper limb movements more dorsally, and lower limb movements at the superior margin and in the paracentral lobule.

The premotor area is found in the precentral gyrus anterior to the primary motor area. Dorsally it extends slightly into the posterior part of the superior frontal gyrus. The supplementary motor area is in the superior frontal gyrus laterally and medially. The frontal eye field is chiefly in the posterior part of the middle frontal gyrus. The triangular and opercular parts of the inferior frontal gyrus form (Pl. 4).the Broca speech center, which is in the hemisphere dominant for language, usually the left.

The prefrontal cortex is in the superior, middle, and inferior frontal gyri anterior to the motor areas, frontal eye field, and the Broca area, as well as on the medial and inferior surface of the frontal lobe. This massive prefrontal association area forms almost a fourth of the entire neocortex and, based chiefly on its connections, is subdivided into dorsolateral and orbitofrontal parts. The dorsolateral part consists of the gyri on the convexity of the hemisphere. It is closely associated with higher mental functions and is sometimes referred to as the executive cortex. The orbitofrontal part comprises the inferior and medial surfaces of the frontal lobe and is associated with emotions and behavior.

Parietal Cortex

Identify the postcentral gyrus (Pls. 1, 4) and its continuation onto the medial aspect of the hemisphere, in the posterior part of the paracentral lobule (Pl. 5). This is the primary somatosensory area where the body parts are represented in an upside down fashion similar to the primary motor cortex.

Locate the more posterior parts of the parietal lobe which are associational and include the superior and inferior parietal lobules that are more or less demarcated from each other by the intraparietal sulcus (Pl.4). The superior parietal lobule expands onto the medial surface and well into the precuneus (Pl.5). The inferior parietal lobule is divided into the supramarginal and angular gyri. The supramarginal gyrus is more anterior. It arches over the ascending ramus of the lateral sulcus and becomes continuous with the postcentral gyrus anteriorly and the superior temporal gyrus posteriorly and inferiorly. The angular gyrus arches over the ascending part of the superior temporal sulcus and becomes continuous posteriorly and inferiorly with the middle temporal gyrus

The superior parietal lobule is associated with stereognosis and also has connections with cortical gaze centers. The supramarginal gyrus is continuous with the superior temporal gyrus which is associated with speech. The angular gyrus has strong connections with somatosensory, auditory, and visual areas and makes us aware of our body and its surroundings.

Occipital Cortex

Identify on the medial surface of the occipital lobe the very deep calcarine fissure which separates the cuneus from the lingual gyrus (Pl. 5). The walls of this sulcus form the primary visual area. It is also called the striate area because of the very conspicuous strip or band of fibers within its layer IV which is called the line of Gennari and can be observed with the naked eye in unstained slices. It is especially prominent in myelin stained sections (Sl. 97). The visual association areas, the parastriate and the peristriate comprise the rest of the occipital lobe on its medial and lateral surfaces.

Retinotopic representation exists in the primary visual cortex with macular vision being in the posterior half, paramacular anterior to it, and peripheral most anteriorly. In addition, the right and left halves of the visual field are represented contralaterally and upside down, e.g. the right upper quadrant is represented in the left lingual gyrus and the right lower quadrant is represented in the left cuneus.

Temporal Cortex

The anterior part of the temporal lobe is ventral to the lateral fissure. Its posterior part is continuous with the parietal and occipital lobes on the lateral surface of the hemisphere and with the limbic and occipital lobes on the inferomedial surface of the hemisphere. The superior temporal gyrus borders the lower wall of the lateral fissure and deep within the floor of the fissure are the transverse temporal gyri (Pl. 14) of Heschl, the primary auditory area.

The posterior part of the superior temporal gyrus (Pl. 4) forms Wernicke's speech area in the side dominant for language. Also frequently included in this area are the posterior part of the middle temporal gyrus and the supramarginal gyrus.

Examine the right and left hemispheres and compare their lateral fissures. It has been reported that in most brains the right lateral fissure extends posteriorly about 2.5 cm beyond the central sulcus and then arches upward into the inferior parietal area. In contrast, the left lateral fissure continues almost 4 cm beyond the central sulcus before either arching upward or ending in a short bifurcation. In addition to a longer lateral fissure, the left side of most brains also contains a larger planum temporale, which is the part of the superior surface of the superior temporal gyrus behind the primary auditory area. The planum temporale is a part of Wernicke's speech area and, hence, these asymmetries are thought to be related to the language function of the left hemisphere.

The massive posterior parts of the middle and inferior temporal gyri and the occipitotemporal gyri on the inferior surface are intimately associated with memory storage and retrieval.

The most anterior part of the temporal lobe, the temporal pole, has strong connections with the limbic system; in fact, the most inferomedial part of the temporal lobe, the parahippocampal gyrus, is a part of the limbic lobe.

Association and Commissural Fibers

The locations of the major association, commissural, and projection bundles as seen in the frontal plane can be observed in Plates 22, 23, 24, 25, 26, 27, 28, 29, 30. and in MRIS 1, 2, 3, 4, 5, 6,and 7.

The corpus callosum and anterior commissure can be observed in the bisected brain specimen. Note the four parts of the corpus callosum, which are from anterior to posterior: rostrum, genu, trunk or body, and splenium (Pls. 5, 12; MRI 15). The rostrum and genu interconnect the most anterior parts of the hemispheres via bundles of fibers that arch into the prefrontal areas as the forceps minor (Pl. 31). The occipital lobes are interconnected by the splenium and its posterior extensions, the forceps major (Pl. 31). The trunk chiefly interconnects the rest of the cortex with the exception of the inferior temporal gyri. The latter are connected by the anterior commissure which can be observed as a small round bundle crossing the midline just behind the point at which the rostrum of the corpus callosum and the lamina terminalis meet. It can also be observed at the midline in Slides 45 and 48 and Plate 33 and more laterally in the hemisphere in Slides 43 and 49 and Plates 24 and 25.