Cerebral cortex

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29 Cerebral cortex

Chapter Summary

Structure

Laminar organization
Columnar organization
Cell types
Afferents
Efferents
Cortical areas

Investigating functional anatomy
Sensory areas

Somatic sensory cortex
Somatic sensory association area
Superior parietal lobule
Inferior parietal lobule
Intraparietal cortex
Secondary somatic sensory area
Visual cortex
Auditory cortex
Motor areas

Primary motor cortex
Premotor cortex
Supplementary motor area
Cortical eye fields
CLINICAL PANEL

Stiff person syndrome

Study Guidelines

1 The cerebral cortex is the part of the body that makes us truly human. Its structure is enormously complex, and assignment of functions to different parts is made difficult, and often unrealistic, by the multiplicity of interconnections.
2 Sensory, motor, and cognitive areas of the cortex are taken in turn.
3 Although damage often leads to permanent disability, the plasticity of the cortex is of special interest to all concerned with neurorehabilitation. Examples are taken from sensory and motor areas.

Structure

The cerebral cortex, or pallium (Gr. ‘shell’), varies in thickness from 2 to 4 mm, being thinnest in the primary sensory areas and thickest in the motor and association areas. More than half of the total cortical surface is hidden from view in the walls of the sulci. The cortex contains about 50 billion neurons; about 500 billion neuroglial cells; and a dense capillary bed.

Microscopy reveals the cortex to have both a laminar and a columnar structure. The general cytoarchitecture varies in detail from one region to another, permitting the cortex to be mapped into dozens of histologically different ‘areas’. Although considerable progress has been achieved in relating these to specific functions, the ‘areas’ are merely nodal points having widespread connections with other parts of the brain.

Laminar organization

A laminar (layered) arrangement of neurons is apparent in sections taken from any part of the cortex. Phylogenetically old elements, including the paleocortex of the uncus (concerned with olfaction), and the archicortex of the hippocampus in the medial temporal lobe (concerned with memory), are made up of three cellular laminae, whereas six laminae are seen in the neocortex (neopallium) covering the remaining 90% of the brain.

Cellular laminae of the neocortex (Figure 29.1)

I The molecular layer contains the tips of the apical dendrites of pyramidal cells (see below), and the most distal branches of axons projecting to the cortex from the intralaminar nuclei of the thalamus.
II The outer granular layer contains small pyramidal and stellate cells.
III The outer pyramidal layer contains medium-sized pyramidal cells and stellate cells.
IV The inner granular layer contains stellate cells receiving afferents from the thalamic relay nuclei. Stellate cells are especially numerous in the primary somatic sensory cortex, primary visual cortex, and primary auditory cortex. The term granular cortex is applied to these areas. In contrast, the primary motor cortex contains relatively few stellate cells in lamina IV and is called agranular cortex.
V The inner pyramidal layer contains large pyramidal cells projecting to the corpus striatum, brainstem, and spinal cord.
VI The fusiform layer contains modified pyramidal cells projecting to the thalamus.
image

Figure 29.1 Cerebral (six-layered) isocortex. (A) Somatic sensory cortex. Cortical laminas

I–VI are numbered on the left. Abbreviations, left to right: N-S, non-specific; CT, corticothalamic fiber passing from fusiform neuron to ventral posterior nucleus of thalamus; PT, pyramidal tract fiber running from large pyramidal neuron to a sensory relay nucleus; TC, thalamocortical afferent from ventral posterior nucleus of thalamus; SAF, short association fiber passing to the motor cortex. (B) Primary motor cortex. Cortical laminas are numbered on the right. Abbreviations, left to right: SAF, short association fiber collateral passing to somatic sensory cortex; PT, pyramidal tract fiber running to motor nucleus in brainstem or spinal cord; CT, corticothalamic fiber passing to ventral lateral nucleus of thalamus; TC, thalamocortical afferent from ventral lateral nucleus of thalamus; PT as above; C/AF, cholinergic or aminergic fiber.

Columnar organization (Figure 29.1)

In the somatic sensory cortex, the neurons were discovered (in monkeys) to be arranged functionally in terms of columns 50–100 µm in diameter extending radially through all laminae. Within each column, all of the cells are modality-specific. For example, a given column may respond to movement of a particular joint but not to stimulation of the overlying skin. Subsequent research has shown that cell columns comprising several hundred neurons are the functional units or modules of the cortex. Some modules are activated by specific thalamocortical inputs, others by corticocortical inputs from the same hemisphere, others again by inputs from the opposite hemisphere. Aggregates of modules create a cortical mosaic.

Cell types

The three principal morphological cell types are pyramidal cells, spiny stellate cells, and smooth stellate cells (Figure 29.2).

• Pyramidal cells have cell bodies ranging in height from 20 to 30 µm in laminae II and III to more than twice that height in lamina V. Tallest of all, at 80–100 µm, are the giant cells of Betz in the motor cortex. The single apical dendrite of each pyramidal cell reaches out to lamina I. The several basal dendrite branches arising from the basal ‘corners’ of the cell extend radially within their respective laminae. The apical and basal dendrites branch freely and are studded with dendritic spines. The axons of all pyramidal cells give off recurrent branches, capable of exciting neighboring pyramidal cells, before leaving the gray matter. All pyramidal cells are excitatory, and use glutamate (or closely related aspartate) as transmitter.
• Spiny stellate cells have spiny dendrites and in general are excitatory. They receive most of the afferent input from the thalamus and from other areas of the cortex, and they form glutamatergic synapses upon pyramidal cells.
• Smooth stellate cells have non-spiny dendrites and are inhibitory. They receive recurrent collateral branches from pyramidal cells and they form GABAergic synapses upon other pyramidal cells. Inhibitory, GABA-secreting neurons make up about 25% of all neurons in the cerebral cortex. Some synapse upon the bases of dendritic spines of pyramidal cells, some synapse upon their somas, and some synapse upon their initial axonal segments (Figure 29.3). The most powerful of the three types is the chandelier cell, so named because of the candle-shaped clusters of axoaxonic boutons. As is the case in the cerebellar cortex (Ch. 25), the GABA neurons exert a focusing action in the cerebral cortex by silencing weakly active cell columns.
• Bipolar cells are found mainly in the outer laminae. Most contain one or more peptides, such as vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), or somatostatin. Peptides are also co-liberated with GABA from many smooth stellate cells.

image

Figure 29.2 Input–output connections. Arrows indicate directions of impulse traffic. +/− signs denote excitation/inhibition. TC, thalamocortical fiber. Pyramidal cell P1 is excited by the spiny stellate cell; it excites P2 within its own cell column; P3 within a neighboring column is inhibited by the smooth stellate cell.

image

Figure 29.3 Three morphological types of GABAergic inhibitory neuron. A, axodendritic cell, synapsing upon the shaft of the apical dendrite of a pyramidal cell. B, basket cell, forming axosomatic synapses on a pyramidal cell; C, chandelier cell, forming axoaxonic synapses (*) upon the initial segments of the two pyramidal cell axons shown, and upon four other initial segments not shown.

(Based on DeFelipe 1999 with permission.)

Human cortical neurons can be captured in fragments of biopsies taken for other purposes. They can be kept alive for several hours and examined for responses to transmitters and transmitter analogs. It appears that a single pyramidal cell may have as many as 10 different kinds of receptor scattered over its surface. It also appears that the response of the neuron to a particular transmitter is not completely predictable, being modified by concurrent effects of other transmitters.

Afferents

Afferents to a given region of the cortex are derived from five sources:

1 Long and short association fibers from small and medium-sized pyramidal cells occupying other parts of the ipsilateral cortex.
2 Commissural fibers from medium-sized pyramidal cells projecting through the corpus callosum from matching areas in the opposite hemisphere.
3 Thalamocortical fibers from the appropriate specific or association nucleus, e.g. from the ventral posterior thalamic nucleus to the somatic sensory cortex, from the dorsomedial thalamic nucleus to the prefrontal cortex (defined below).
4 Non-specific thalamocortical fibers from the intralaminar nuclei.
5 Cholinergic and aminergic fibers from basal forebrain, hypothalamus, and brainstem. These fibers are represented in green in Figure 29.1. The relevant nuclei of origin, and the transmitters/modulators involved, are as follows:

•basal forebrain nuclei acetylcholine
•tuberoinfundibular (hypothalamus) histamine
•tegmentum (midbrain) dopamine
•raphe nucleus (midbrain) serotonin
•cerulean nucleus (pons) norepinephrine

These five sets of neurons are of particular relevance to psychiatry and are considered in Chapter 34.

Efferents

All efferents from the cerebral cortex are axons of pyramidal cells, and all are excitatory in nature. Axons of some pyramidal cells contribute to short or long association fibers. Others form commissural or projection fibers.

• Examples of short association fiber projections are those entering the motor cortex from the sensory cortex and vice versa (Figure 29.1). Examples of long association fiber projections are the numerous backward projections from the prefrontal cortex – the cortex anterior to the motor areas (see below) to sensory association areas.
• The commissural fibers of the brain are entirely composed of pyramidal cell axons running across in the corpus callosum and anterior commissure (and in other, minor commissures) to matching areas in the opposite hemisphere.
• Projection fibers from the primary sensory and motor cortex form the largest input to the basal ganglia (Ch. 33). The thalamus receives projection fibers from all parts of the cortex. Other major projection systems are corticopontine (to the ipsilateral nuclei pontis), corticonuclear (to contralateral motor and somatic sensory cranial nerve nuclei in pons and medulla), and corticospinal (to anterior horn motor neurons).

Cortical Areas

The most widely used reference map is that of Brodmann, who divided the cortex into 47 areas on the basis of cytoarchitectural differences. Most of these areas are shown in Figure 29.4. Colored in that figure are the three principal primary sensory areas (somatic, visual, auditory) and the single primary motor area, together with the respective unimodal association areas. The rest of the neocortex comprises multimodal (polymodal) association areas receiving association fibers from more than one unimodal association area (e.g. receiving tactile and visual inputs, or visual and auditory).

image

Figure 29.4 Cytoarchitectural areas of Brodmann.

Colored areas

Motor (red)

4 primary motor cortex
6 on medial surface, supplementary motor area
6 on lateral surface, premotor cortex

Sensory (blue)

3/1/2 primary somatic sensory cortex
40 secondary somatic sensory cortex
17 primary visual cortex
18, 19 visual association cortex
41, 42 primary auditory cortex
22 auditory association cortex

(Note: The primary auditory cortex is not in fact visible from the side, being entirely on the upper surface of the superior temporal gyrus.)

Investigating functional anatomy

Two dominant methods are in use for localization of functions in the human brain. Both techniques depend upon the local increases in blood flow that meet the additional oxygen demand imposed by localized neural activity.

Positron emission tomography

Positron emission tomography (PET) measures oxygen consumption following injection of water labeled with oxygen-15 into a forearm vein. 15O is a positron-emitting isotope of oxygen; the positrons react with nearby electrons in the blood to create gamma rays which are counted by gamma-ray detectors. Alternatively, fluorine-18-labeled deoxyglucose may be used to measure glucose consumption. 18F-deoxyglucose is taken up by neurons as readily as glucose.

Image subtraction and image averaging are required for meaningful interpretation of PET studies, as explained in the caption to Figure 29.5.

image

Figure 29.5 Image subtraction and image averaging in PET scans.

Top: The middle image is from a control mode, where the subject lies at rest. Uptake of 15O is active throughout the cortex and subcortical gray matter. The left image is from the same subject staring at dots moving on a screen. The high level of background activity obscures the effect. The right image is produced by subtracting the control value to reveal the additional activity in the visual cortex produced by the staring task.

Middle: Four other subjects have performed the same task. Subtraction of background ‘noise’ reveals varying differences among the five. Because brains vary in size between individuals, activities in all five brains have been projected onto a common, ‘average’ brain (hence the identical brain profiles in this row).

Bottom: A mean value for the five brains produces a ‘mean difference image’ representative of the five as a group.

(Adapted from Posner and Raichle, Images of Mind, Sci. Amer. Library, 1994, p. 65, with permission.)

For specialized investigations, radiolabeled drugs are used to quantify receptor function, e.g. radiolabeled dopamine in the corpus striatum in relation to Parkinson’s disease (Ch. 33); radiolabeled serotonin in brainstem and cortex in relation to depression (Ch. 26), and radiolabeled acetylcholinesterase in relation to Alzheimer disease (Ch. 34).

Functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) (Figure 29.6) does not require introduction of any extraneous material. It depends upon the different magnetic susceptibility of oxygenated versus deoxygenated blood. As it happens, the local increases in blood flow are more than sufficient to meet oxygen demands, and it is the increase in the ratio of oxyhemoglobin to deoxyhemoglobin that is exploited to generate the MR signal.

image

Figure 29.6

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