Cerebral cortex

Cortex cerebri

Also known as: grey matter of the cerebrum, neocortex, pallium

The 2–4 mm-thick folded sheet of grey matter covering the cerebral hemispheres; mostly six-layered neocortex holding ≈16 billion neurons, organised into primary, unimodal and multimodal association areas.

Overview

Thickness≈2–4 mm (thickest in motor cortex)

Surface area≈0.2 m² (≈2/3 hidden in sulci)

Neurons≈16 billion

Brodmann areas52

The cortex has a surface area of ≈0.2 m² (≈2,000–2,500 cm²), about two-thirds of which is buried in the sulci. About 90% is neocortex, with six layers. The phylogenetically older allocortex has three layers and comprises the archicortex (hippocampal formation) and the paleocortex (olfactory/piriform cortex).

Neurons are of two main types:
• pyramidal cells: glutamatergic projection neurons whose apical dendrites rise towards the surface and whose axons leave the cortex;
• non-pyramidal cells: mostly GABAergic interneurons (basket, chandelier, Martinotti) and spiny stellate cells, which are excitatory granule cells of layer IV.
Information is processed in vertical columns that cross all six layers.

Cytoarchitecture varies between regions. Brodmann distinguished 52 areas, and many coincide with functional areas, e.g. area 4 (primary motor), 3-1-2 (primary somatosensory), 17 (primary visual), 41/42 (primary auditory), 44/45 (Broca) and 22 (the Wernicke region). Agranular motor cortex has a thick layer V with giant Betz cells. Granular sensory cortex has a thick layer IV; in V1 this layer contains the visible white stria of Gennari, hence "striate cortex".

Functionally, primary areas receive or send the most direct sensory or motor signals and are mapped topographically (somatotopic, retinotopic, tonotopic). Unimodal association areas elaborate one modality, and multimodal (heteromodal) association areas in the prefrontal, posterior parietal and temporal cortex integrate several to support language, attention, planning and memory.

Location

Surface of both cerebral hemispheres, including the depths of all sulci.

Parts & subdivisions

Layer I — molecular

Few cells; apical dendritic tufts and horizontal axons.

Layer II — external granular

Small pyramidal and granule cells; corticocortical connections.

Layer III — external pyramidal

Medium pyramidal cells; main source of association and commissural (callosal) fibres.

Layer IV — internal granular

Spiny stellate cells receiving thalamic input; thickest in primary sensory cortex (stria of Gennari in V1).

Layer V — internal pyramidal

Large pyramidal cells (Betz cells in area 4) projecting to the brainstem, spinal cord and striatum (corticospinal, corticobulbar, corticostriate).

Layer VI — multiform

Polymorphic cells projecting back to the thalamus (corticothalamic feedback).

Primary motor cortex (area 4)

Precentral gyrus; motor homunculus.

Primary somatosensory cortex (areas 3, 1, 2)

Postcentral gyrus; sensory homunculus.

Primary visual cortex (area 17)

Banks of the calcarine sulcus.

Primary auditory cortex (areas 41, 42)

Transverse temporal (Heschl) gyri.

Anatomy

Histology

Six-layered neocortex (isocortex) with columns ≈0.3–0.5 mm wide; allocortex (3 layers) in the hippocampus and olfactory cortex. Pyramidal (≈70–80%, excitatory, glutamate) and non-pyramidal (≈20–30%, mostly inhibitory GABA interneurons) neurons.

Development

Neurons generated in the ventricular zone migrate along radial glia in an inside-out sequence: deep layers form first and later-born neurons migrate past them to the superficial layers. GABAergic interneurons migrate tangentially from the ganglionic eminences.

Function

Conscious sensory perception (primary sensory areas)

Initiation of voluntary movement (motor areas)

Integration of information across modalities (association areas)

Language, attention, planning, decision-making and memory storage

Generates the EEG rhythms used clinically

Physiology

The EEG records summed postsynaptic potentials of pyramidal cells:
• alpha (8–13 Hz): relaxed wakefulness with the eyes closed, over the occipital region;
• beta (>13 Hz): alert activity, and benzodiazepines;
• theta (4–7 Hz): drowsiness, and in children;
• delta (<4 Hz): deep (N3) sleep, or focal lesions in an awake adult.
Rhythms are paced by thalamocortical circuits.

Clinical significance

Epilepsy

An epileptic seizure is abnormal, excessive, synchronous cortical neuronal activity. Focal seizures arise in one network and produce symptoms of that area. In a Jacksonian march, clonic jerking spreads along the motor homunculus from hand to face to arm, and may be followed by transient post-ictal weakness (Todd paresis). Generalised seizures engage both hemispheres from the outset (tonic–clonic, absence with 3-Hz spike-and-wave). Epilepsy is a tendency to recurrent unprovoked seizures.

Neuronal migration disorders

Defects of radial migration cause lissencephaly (smooth brain, LIS1/DCX mutations), band and periventricular heterotopia, and polymicrogyria. They typically present with drug-resistant epilepsy and developmental delay.