The structure of the neocortex

The neocortical grey matter is usually described as having six layers ( Plate 1(.a.).). Wide variation in the nature of this microscopic lamination underlies the subdivision of neocortex into a multiplicity of (usually numbered) areas. At its simplest, two types of neurones make up the grey matter—pyramidal and non-pyramidal (or granular) cells. An apparent predominance of one or other type gives the extremes of granular and agranular cortex, equating with sensory areas (granular) and the motor cortex (agranular). In fact, the proportion of different cell types is constant in all areas. Indeed, with the single exception of the primary visual cortex, the numbers of neurones under a fixed surface area is also constant in all cortical areas. Variations in the size of pyramidal cells in particular lead to an apparent change in proportions. These variations probably reflect differences in the axonal volume of individual pyramidal cells, reflecting the distance and volume of projection fibres from a cortical area.

Pyramidal cells have a single main apical dendrite ascending towards the pial surface, and several horizontally spreading basal dendrites. All dendrites bear dendritic spines, which receive synapses (P]ate...1..(.b)). All pyramidal cells use excitatory amino acids as neurotransmitters and have axons which enter the subcortical white matter; hence they are all projection neurones. They constitute approximately 60 per cent of all the neurones in the cortex. A second spiny neuronal type, the spiny stellate cells, is the next most numerous. These also use an excitatory amino acid, most probably glutamate, as their neurotransmitter. Unlike pyramidal cells, however, their axons remain confined to the cortical grey matter; they are interneurones, accounting for a further 25 per cent of cortical neurones. All the other neurones are inhibitory interneurones, using g-aminobutyric acid ( GABA) as their major neurotransmitter. Many also contain one or more neuropeptides, and their content of specific calcium-binding proteins varies. They have a wide range of axonal and dendritic forms, and have been multiply classified in the past. Broadly speaking, they can be grouped into those with horizontal axonal arborizations, those whose axons ramify at right angles to the pial surface, i.e. through the depth of the cortex, and those with radial axons (Fig 7).

Fig. 7 Inhibitory (GABA-ergic) neurones of the neocortex: (a) vertical; (b) horizontal.

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