Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

14. Telencephalon (Cerebrum)

14.1 Telencephalon, Development and External Structure

A Terms of location and direction

Midsagittal section viewed from the left side. Terms of location and direction in the telencephalon and diencephalon are based on the Forel axis (2), which runs horizontally through the forebrain. (1) Brainstem axis (Meynert axis).

В Development of the cerebral cortex

a Embryonic brain; b adult brain.

The cerebral hemispheres can be divided into phylogenetically ancient (“paleo”), old (“archi”) and new (“neo”) parts (see D). The cerebral cortex together with associated areas of underlying white matter is called the pallium, a term sometimes used interchangably with “cortex”.

C Cray and white matter in the telencephalon

a Coronal section showing the distribution of gray and white matter in the brain.

Gray matter:

• Cerebral cortex: contains most of the gray matter of the telencephalon. It is divided on histological grounds into two parts:

- Isocortex: corresponds to the neocortex (see B); largest part of the cerebral cortex, consisting of six layers (see p. 200).

- Allocortex: corresponds to the paleo- and archicortexes (see B); consists of three or four layers (see p. 204).

 Subcortical nuclei: Basal ganglia: the caudate nucleus and putamen (collectively called the corpus striatum), and the globus pallidus. Note: The basal ganglia are often called the basal nuclei. Because they are located in the CNS, however, the term “ganglia” is more appropriate than “nuclei."

• Other gray-matter nuclei that are not included among the basal ganlgia of the telencephalon:

- Amygdala: often considered a transitional form between the two types of gray matter—cortex and basal ganglia—based on its location (see p. 207)

- Claustrum

White matter: tissue below the cerebral cortex and surrounding the subcortical nuclei. Note: The white matter also contains nuclei of the diencephalon (see p.215).

b Lateral view of the left hemisphere. Part of the cerebral cortex sinks below the surface during development, forming the insula. The portions of the cerebral cortex that overlie deeper cortical areas are called opercula (“little lids”).

D Phylogenetic origins of major compoments of the telencephalon

Phylogenetic term

Structure In the embryonic brain

Structure(s) in the adult brain

Cortical structure

Paleopallium (oldest part)

Floor of the hemispheres

• Rhinencephalon (= olfactory bulb plus surrounding region)

Allocortex (see p. 204)

Archipallium (old part)

Medial portion of hemispheric wall

• Ammon’s horn (largest part, not shown here)

• Indusium griseum

• Fornix

Allocortex

Neopallium (newest part)

Most of the brain surface plus the deeper corpus striatum

• Neocortex (= cortex), largest part of the cerebral cortex

• Insula

• Corpus striatum

Isocortex (see p. 200)

E Division of the cerebral hemispheres into lobes

a Left lateral view of the left hemisphere; b Left lateral view of the right hemisphere; c Basal view with the brainstem removed, showing the cut surface of the midbrain (mesencephalon).

The two cerebral hemispheres are the externally visible part of the telencephalon. They are separated from each other by the longitudinal cerebral fissure and are each subdivided into six lobes:

 Frontal lobe

 Parietal lobe

 Temporal lobe

 Occipital lobe

 Insular lobe (insula, see Cb)

 Limbic lobe (limbus)

The surface contours of the cerebral hemispheres are highly variable between individuals. A histological subdivision into cortical areas is more meaningful in terms of functional brain organization than a macroscopic subdivision into gyri and sulci (see p. 200).

14.2 Cerebral Cortex, Histological Structure and Functional Organization

A Histological structure of the cerebral cortex

A six-layered (laminar) structure is found throughout most of the neocortex. The silver impregnation (a) or Nissl staining of the cell bodies (b) allows for histological division of the neocortex according to the dominant structure of each layer:

I Molecular layer: (outermost layer); relatively few neurons.

II External granular layer: mostly stellate and scattered small pyramidal neurons.

III External pyramidal layer: small pyramidal neurons.

IV Internal granular layer: stellate an small pyramidal neurons.

V Internal pyramidal layer: large pyramidal neurons.

VI Multiform layer: (innermost layer); neurons of varied shape and size.

Cortical areas that are concerned primarily with information processing (e.g., primary somatosensory cortex) are rich in granule cells; the granular layers of these regions (granular cortex, see Ba) are also exceptionally thick. Areas in which information is transmitted out of the cortex (e.g. the prinary motor cortex) are distinguished by prominent layers of pyramidal cells and known as the agranular cortex (see Bb). Analysis of the distribution of nerve cells in the cerebral cortex allows for identification of functionally distinct areas (cytoarchitectonies, see A, p. 203).

В Examples of granular and agranular cortex

a Granular cortex (koniocortex from the Greek konis = sand): The primary somatosensory cortex, in which the afferents from the thalamus terminate (at layer IV), is located in the postcentral gyrus. It is thinner overall than the primary somatomotor cortex (see b). A striking feature in the primary somatosensory cortex is that the external and internal granular layers (II and IV) where the large sensory tracts terminate are markedly widened. By contrast, the pyramidal cell layers (III and V) are thinned.

b Agranular cortex: The efferents fibers that project to the motor nuclei of the cranial nerves and motor columns of the spinal cord originate in the primary somatomotor cortex, located in the precentral gyrus. Its pyramidal layers (III and V) are greatly enlarged. Exceptionally large pyramidal neurons (Betz cells after the author who first described them) are found in the some areas of layer V. Their long axons extend into the sacral spinal cord (see p. 267).

C Columnar organization of the cortex (after Klinke and Silbernagl) While morphological considerations divide the cerebral cortex into horizontal layers (see A), functional considerations lead to its division into distinct units or modules (see C). Encompassing all six layers, these modules consist of vertically-arranged cortical columns of neurons that are interconnected a serve a common function, despite showing no distinct histological boundaries. In total, there are several million such modules in the cerebral cortex, with a variable width between 50 and 500 µm each. One cortical column has here been magnified to display its constituent neurons and connections in separate panels. Panels a-c show the principal types of cells participating in a cortical column: several thousand stellate neurons of various subtype and one hundred or so large and small pyramidal neurons (panel a). Panel b isolates the small pyramidal cells whose axons tend to terminate within the cortex

D Types of neuron in the cerebral cortex (simplified)

itself. In contrast, the deeper, large pyramidal neurons (panel c) have axons that generally project to subcortical structures. Large pyramidal cells are responsible for tracts of corticobulbar and corticospinal motor axons, which project to the brainstem and spinal cord, respectively. They may also send recurrent collateral fibers which end in the local cortex. Panels d-f contain axons projecting into the cerebral cortex. Panel d isolates thalamocortical projections that enter from the thalamus and synapse mostly on the stellate neurons of layer IV. Incoming association fibers of the nearby cortex and commissural fibers of the contralateral hemisphere frequently terminate on the dendrites of the small pyramidal neurons (panel e). Panel f shows the large pyramidal neurons whose apical dendrites reach from layer V to layer I. These large pyramidal neurons integrate inputs from various other local neurons and incoming fibers.

Neuron

Definition

Properties

Stellate neuron (layers II and IV)

Cell with short axon for local information processing; various types: basket, candelabra, double-bouquet cells

Inhibitory interneuron in most cortical areas; primary information-processing neuron (in layer II), especially in primary sensory areas

Small pyramidal neuron (layer III)

Cell with long axon that often ends within the cortex, either as:

• Association fiber: axon ends in same hemisphere but different cortical area, or as

• Commissural fiber: axon ends in opposite hemisphere but cortical area of similar function

Projection neuron whose axons end within the cortex

Large pyramidal neuron (layer V)

Cell with very long axon that projects outside the cortex, sometimes reaching distant structures

Excitatory projection neuron whose axons end outside the cortex

Granule cell (layers II and IV)

Generic term for small neuron, most often with stellate morphology

Depends on the cell type (see entries for stellate and small pyramidal neurons)

14.3 Neocortex, Cortical Areas

A Brodmann areas in the neocortex

a Midsagittal section of the right cerebral hemisphere, viewed from the leftside; b Lateral view of the left cerebral hemisphere.

As noted earlier, the surface of the brain consists macroscopically of lobes, gyri, and sulci. Microscopically, however, subtle differences can be found in the distribution of the cortical neurons, and some of these differences do not conform to the gross surface anatomy of the brain. Portions of the cerebral cortex that have the same basic microscopic features are called cortical areas or cortical fields. This organization into cortical areas is based on the distribution of neurons in the different layers of the cortex (cytoarchitectonies, see A, p. 200). In the brain map shown at left, these areas are indicated by different colors. Although the size of the cortical areas may vary between individuals, the brain map pictured here is still used today as a standard reference chart. It was developed in the early 20th century by the anatomist Korbinian Brodmann (18681918), who spent years painstakingly examining the cellular architecture of the cortex in a single brain. It has long been thought that the map created by Brodmann accurately reflects the functional organization of the cortex, and indeed, modern imaging techniques have shown that many of the cytologically defined areas are associated with specific functions. There is no need, of course, to memorize the location of all the cortical areas, but the following areas are of special interest:

 Areas 1,2, and 3: primary somatosensory cortex

 Area 4 primary motor cortex

 Area 17: primary visual cortex (striate area, the extent of which is best appreciated in the midsagittal section)

 Areas 41 and 42: auditory cortex

В Visual cortex (striate area)

a Right hemisphere viewed from the left side; b Coronal section (plane of section shown in a), anterior view.

The primary visual cortex (striate area, shaded yellow) is the only cortical area that can be clearly recognized by its macroscopic appearance. It extends along both sides of the calcarine sulcus at the occipital pole. In an unstained coronal section (b), the stria of German can be identified as a prominent white stripe within the gray cortical area. This stripe contains cortical association fibers that synapse with the neurons of the internal granular layer (IV, see p. 201). The pyramidal cell layers (efferent fibers) are attennated in the visual cortex, while the granular cell layers where the afferent fibers from the lateral geniculate nucleus terminate are markedly enlarged.

14.4 Allocortex, Overview

A Overview of the allocortex

View of the base of the brain (a) and the median surface of the right hemisphere (b). Structures belonging to the allocortex are indicated by colored shading (see listing of allocortical structures in D, p. 199).

The allocortex consists of the phylogenetically old part of the cerebral cortex. It is very small in relation to the cortex as a whole. Unlike the isocortex, which has a six-layered structure, the allocortex (allo = “other”) usually consists of three layers that encompass the paleo- and archicortexes. Additionally, there exist /bur- layered transitional areas between the allocortex and isocortex: the peripaleocortex (not indicated separately in the drawing) and the periarchicortex (indicated by pink shading). An important part of the allocortex is the rhinencephalon (“olfactory brain”). Olfactory impulses that are perceived by the olfactory bulb are the only sensory afferent impulses that do not reach the cerebral cortex by way of the dorsal thalamus. Another important part of the allocortex is the hippocampus and its associated nuclei (see p. 206). As in the isocortex, the gyral patterns of the allocortex do not always conform to its histological organization.

В Organization of the archipallium: deeper parts

Lateral view of the left hemisphere. The archicortex described in A is the only part of the archipallium that is located on the brain surface. The deeper parts of the archipallium, which lie within the white matter, are the hippocampus (“sea horse”), indusium griseum (“gray covering”), and fornix (“arch”). All three structures are part of the limbic system (see p. 374), and together form a border (“limbus”) around the corpus callosum as a result of their migration during development.

C Topography of the fornix, corpus callosum, and septum pellucidum (after Feneis)

Occipital view from upper left. The fornix is a tract of the archicortex that is closely apposed but functionally unrelated to the corpus callosum. The corpus callosum is the largest neocortical commissural tract between the hemispheres, serving to interconnect cortical areas of similar function in the two hemispheres (see also p. 376). The septum pellucidum is a thin plate that stretches between the corpus callosum and fornix, forming the medial boundary of the lateral ventricles. Between the two septa is a cavity of variable size, the cavum septi pellucidi. The cholinergic nuclei in the septa, which are involved in the organization of memory, are connected to the hippocampus by the fornix (see p. 206).

D Topography of the hippocampus, fornix, and corpus callosum

Viewed from the upper left and oral aspect. This drawing shows the hippocampus on the floor of the inferior horn of the lateral ventricle. The left and right crura of the fornix unite to form the commissure of the fornix (see C) and the body of the fornix, which divides anteriorly into left and right bundles, the columns of the fornix. The fornix is a white matter tract connecting the hippocampus to the mammillary bodies in the diencephalon. Contained within the fornix are hippocampal neurons whose axons project to the septum, mammillary bodies, contralateral hippocamous, and other structures. This important pathway is part of the limbic system (see p. 374).

14.5 Allocortex: Hippocampus and Amygdala

A Left hippocampal formation

Lateral view. Most of the left hemisphere has been dissected away, leaving only the corpus callosum, fornix, and hippocampus. The intact right hemisphere is visible in the background.

The hippocampal formation is an important component of the limbic system (see p.374). It consists of three parts:

 Subiculum (see Cb)

 Hippocampus proper (Ammon’s horn)

 Dentate gyrus (fascia dentata)

The fiber tract of the fornix connects the hippocampus to the mammillary body. The hippocampus integrates information from various brain areas and influences endocrime, visceral, and emotional processes via its efferent output. It is particulary associated with the establishment of short-term memory. Lesions of the hippocampus can therefore cause specific defects in memory formation.

Besides the hippocampus, which is the largest part of the archicortex, we can recognize another component of the archicortex, the indusium griseum.

В Right hippocampal formation and the caudal part of the fornix

Left medial view. Compare this medial view of the right hippocampal formation with the lateral view in A above. A useful landmark is the calcarine sulcus, which leads to the occipital pole. The cortical areas that border the hippocampus (e.g., the parahippocampal gyrus) are particulary apparent in this view.

C Left temporal lobe with the inferior horn of the lateral ventricle exposed

a Transverse section, posterior view of the hippocampus on the floor of the inferior (temporal) horn. The following structures can be identified from lateral to medial: hippocampus, fimbria, dentate gyrus, hippocampal sulcus, and parahippocampal gyrus.

b Coronal sections of the left hippocampus. The hippocampus appears here as a curled band (Ammon’s horn = the hippocampus proper), which shows considerable structural diversity in its different portions. The junction between the entorhinal cortex (entorhinal region) in the parahippocampal gyrus and Ammon’s horn is formed by a transitional area, the subiculum. The entorhinal region is the “gateway” to the hippocampus, through which the hippocampus receives most of its afferent fibers.

D Relationship of the amygdala to Internal brain structures

Lateral view of the left hemisphere. The amygdala (amygdaloid body) is located below the putamen and anterior to the tail of the caudate nucleus. The fibers of the pyramidal tract run posterior and medial to the amygdala.

E Amygdala

a Coronal section at the level of the interventricular foramen. The amygdala extends medially to the inferior surface of the cortex of the temporal lobe. For this reason, it is considered to be part of the cortex as well as a nuclear complex that has migrated into the white matter. Stimulation of the amygdala in humans leads to changes in mood, ranging from rage and fear to rest and relaxation depending on the emotional state of the patient immediately prior to stimulation. Since the amygdala functions as an “emotional amplifier,” lesions affect the patient’s evaluation of events’ emotional significance. The surrounding periamygdaline cortex and the corticomedial half of the amygdala are part of the primary

olfactory cortex. Hence these portions of the amygdala are considered part of the paleocortex, while the deeper portion is characterized as “nuclear.”

b Detail from a showing the two main groups of nuclei in the amygdala:

 Phylogenetically old corticomedial group:

- Cortical nucleus

- Central nucleus

 Phylogenetically new basolateral group:

- Basal nucleus

- Lateral nucleus

The basal nucleus can be subdivided into a parvocellular medial part and a macrocellular lateral part.

14.6 Telencephalon: White Matter and Basal Ganglia

A Teased fiber preparation of the white matter of the telencephalon Lateral view of the left hemisphere. This dissection shows the superficial layer of white matter located between the basal ganglia and the gray matter of the cerebral cortex. A special preparation technique was used to display the fiber structure of the white matter, which normally has a uniform white appearance. The fiber structure is defined by the tracts (bundles of myelinated axons) th at interconnect different areas of the gray matter. For example, we can identify the short cerebral arcuate fibers (II fibers) that run between two adjacent gyri as well as the association fibers that span multiple gyri (e.g., the superior longitudinal and frontotemporal fasciculi). When these tracts are damaged (in multiple sclerosis, for example) the communication pathways within the brain cease to function normally. This may lead to central paralysis, visual disturbances (optic nerve damage), and behavioral changes (damage to the frontal cortex).

В Projection of the basal ganglia onto the brain surface and ventricular system

a Viewfrom the upper left anterior aspect. The basal ganglia are masses of gray matter deep within the brain that contain the cell bodies of neurons. Further details on the basal ganglia are shown in C.

b Left lateral view. The caudate nucleus is closely applied to the concave lateral wall of the lateral ventricle. It is connected to the putamen by numerous streak-like bands of gray matter (corpus striatum, see C).

C Basal ganglia

a Transverse section through the cerebrum at the level of the corpus striatum (see D), viewed from above. In a strict anatomical sense, the basal ganglia consist of the caudate nucleus, putamen, and globus pallidus. Developmentally, the globus pallidus is a part of the diencephalon (see D, p.211) that has migrated into the telencephalon, but it is still counted among the basal ganglia. The basal ganglia are an essential component of the extra pyramidal motor system (its functional significance is described on p.340). The claustrum (“barrier”) is a strip of gray matter lateral to the putamen. It is not part of the basal ganglia but instead has reciprocal connections with sensory areas of the cerebral cortex.

b Coronal section through the cerebrum at the level of the olfactory tract, anterior view. This section demonstrates how the caudate nucleus and putamen are separated from each other by the fibrous white matter of the internal capsule. The caudate nucleus and putamen together constitute the corpus striatum (often shortened to “striatum”; see D). The globus pallidus is not visible because it is occipital to this plane of section.

D Relationship between the corpus striatum and lentiform nucleus The caudate nucleus and putamen together constitute the corpus striatum, while the putamen and globus pallidus make up the lentiform (“lens-shaped”) nucleus. Although the globus pallidus and the putamen are anatomically juxtaposed, the putamen is functionally associated instead with the caudate nucleus. Developmentally, the putamen is part of the telencephalon and the globus pallidus is part of the diencephalon.