Cerebral cortex: Encyclopedia II - Cerebral cortex - Laminar pattern

Cerebral cortex - Laminar pattern

The standard areas of cortex (isocortex) is characterized as having six distinct layers. From outside inward:

1. Molecular layer
2. External granular layer
3. External pyramidal layer
4. Internal granular layer
5. Internal pyramidal layer
6. Multiform layer

After migration (interestingly, the inner layers are formed first during development), neurons form efferents and receive afferent connections characteristic of its layer.

• The molecular layer I contains few scattered neurons and consists mainly of extensions of apical dendrites and horizontally oriented axons, and some Cajal-Retzius and spiny stellate neurons can be found.
• The external granular layer II contains small pyramidal neurons and numerous stellate neurons.
• The external pyramidal layer III contains predominantly small and medium sized pyramidal neurons, as well as non-pyramidal neurons with vertically oriented intracortical axons. Layers I--III are the main target of interhemispheric corticocortical afferents, and layer III is the principal source of corticocortical efferents.
• The internal granular layer IV contains different types of stellate and pyramidal neurons, and is the main target of thalamocortical afferents as well as intra-hemispheric corticocortical afferents.
• The internal pyramidal layer V contains large pyramidal neurons (as the Betz cells in the primary motor cortex) as well as interneurons, and it is the principal source of efferent for all the motor-related subcortical structures.
• The multiform layer VI contains few large pyramidal and many small spindle-like pyramidal and multiform neurons. The layer VI sends efferent fibres to the thalamus establishing a very precise reciprocal interconnection between the cortex and the thalamus (Creutzfeldt, 1995).

The cortical layers are not simply stacked one over the other, they develop characteristic connections between different layers, which define the basic structure of the cortical columns in the mature cortex (Mountcastle, 1997).

There are no actual borders between the layers, and neurons cross layer boundaries with their dendrites and axons trees all over. The pyramidal cells (the majority of the neurons) span at least three layers, and in many cases all the layers. Thus it is not obvious that the layers have any functional significance.

Theorists such as Jeff Hawkins have posited that these layers, particularly in the neocortex, form part of a laminar memory system of classification and lateral association which underpins human cognitive function. Although new, it brings an intriguing perspective on the unusual structural consistency of the most physically large cortex of the brain.

Adapted from the Wikipedia article "Laminar pattern", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki