29 Cerebral cortex
Structure
Laminar organization
Cellular laminae of the neocortex (Figure 29.1)
Cell types
The three principal morphological cell types are pyramidal cells, spiny stellate cells, and smooth stellate cells (Figure 29.2).
Afferents
Afferents to a given region of the cortex are derived from five sources:
•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
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).
Investigating functional anatomy
Positron emission tomography
Image subtraction and image averaging are required for meaningful interpretation of PET studies, as explained in the caption to Figure 29.5.
Figure 29.5 Image subtraction and image averaging in PET scans.
(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.