In humans the cortex is 2-4 mm thick and has six layers, numbered I to VI, starting from the pial surface.

1. Layer I, the molecular layer, contains many horizontally oriented axons and a few scattered neurons and consists mainly of apical dendrites of pyramidal neurons. Layer I receives substantial inputs from the thalamus.
2. Layer II, the external granular layer contains densely packed stellate (star-shaped) cells and small pyramidal neurones.
3. Layer III, the external pyramidal layer, contains predominantly small and medium-sized pyramidal neurones. They recieve inputs form other areas of the cortex, including the contra-lateral hemisphere.
4. Layer IV, the internal granular layer, contains different types of stellate neurones and is the main target of thalamocortical pathways as well as fibres crossing over in the corpus callosum.
5. Layer V, the internal pyramidal layer, contains large pyramidal neurones whose axons project through the internal capsule to subcortical structures including the striatum, brainstem and spinal cord. The largest of these are known as Betz cells.
6. Layer VI, the polymorphic or multiform layer has precise reciprocal connections with the thalamus.

In the visual cortex (calacarine sulcus, Brodmann's area 17) there is a white band of axons visible to the naked eye and known as the Stria of Gennari. These axons carry visual information from the lateral geniculate nucleus of the thalamus to layer IV, and form a dividing line between the outer and inner layers of the primary visual cortex.

The thickness of the cortex can be measured in living humans using MRI, and there is evidence that it gets thinner in some diseases of the CNS, such as Alzheimer's disease.

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Three drawings of cortical lamination by the famous Spanish anatomist, Santiago Ramon y Cajal, each showing a vertical cross-section of the cerebral cortex, with the surface of the cortex at the top. Left: Nissl-stained visual cortex of a human adult. Middle: Nissl-stained motor cortex of a human adult. Right: Golgi-stained cortex of a 1½ month old infant. The Nissl stain shows the cell bodies of neurons; the Golgi stain shows the dendrites and axons of a random subset of neurons. (from Wikipedia)

Pyramidal neurones in the cerebral cortex are large cells with a triangular shaped cell body, rather like a pyramid. In addition to their axon these cells have complex dendrites divided into two main groups: apical and basal.

A second feature of the dentrites is the presence of dendritic spines which are the site of complex synaptic inputs.

Pyramidal neurons are found throughout the cerebral cortex, and in areas with specialised functions such as the hippocampus and amygdala.

Brodmann was an anatomist who studied the cellular composition (cytoarchitectonics) of the cerebral cortex and noticed that there were regional differences in its structure. He divided the cerebral cortex into 52 areas based on these observations.

Physiological studies indicated that there are functional differences in the cortex that correlate with the cellular structure. Thus the somatosensory cortex (areas 1,2,3) and the motor cortex (area 4) have characteristic cellular composition with differences in the numbers and distribution of neurones of different sizes.

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Dendrites

The pyramidal cells of the cerebral cortex have complex ramificacions of dendrites in different parts of the neurone.

Apical dendrites.

The apical dendrites arise from the apex of the pyramidal cell's soma. The apical dendrite is a single long thick dendrite that branches several times as distance from the soma increases.

Basal dendrites.

The basal dendrites arise from the base of the pyramidal cell's soma. The basal dendritic tree consists of three to five primary dendrites. As the distance from the soma increases, there is profuse branching of the basal dendrites.

Pyramidal cells are among the largest neurons in the brain, having cell bodies averaging around ~ 20µm. Their dendrites may be several hundred µm long and their diameter may be as large as a few µm.

The largest pyramidal neurones are known as Betz cells, and are found in an extensive area in fornt and behind the central sulcus.

Dendritic spines

Dendritic spines increase the area of receptive post-synaptic surface on pyramidal neurones, thus increasing the neuron's ability to process and integrate large amounts of information.

Dendritic spines are absent on the soma, but a typical apical dendrite has several thousand spines.

Dendritic spines receive most of the excitatory impulses that enter a pyramidal cell, and glutamate is an important excitatory neurotransmitter..

There are characteristic connections between the all cortical layers and different neuronal types, forming circuits that are arranged in vertical columns.

A cortical column is a group of neurons in the cerebral cortex with nearly identical physiological properties. In the visual cortex all cells in the column have receptive fields whose orientations are similar. In the smatosensory cortex, each column receives afferent input from a single type of sensory receptor, in skin, muscle or joints. Adjacent columns have different receptive fields, and in the illustration, they are from the same area of the visual field of the two eyes.

In the motor cortex, the cells within the column are all concerned with the direction of voluntary movement.