Neocortex

Tuesday, May 26, 2009

The neocortex is the part of the cerebrum which forms the outer layer of the cerebral hemispheres. The neocortex consists of six layers neurons, labelled I to VI, with VI being the innermost and I being the outermost. It is the center of higher mental functions, such as sensory perception, generation of motor commands, spatial reasoning, conscious thought, and language. The neocortex contains about 100 billion cells, each with 1,000 to 10,000 synapses. "Neocortex" means in Latin "new bark."

The neocortex is composed of the grey matter, which is made up of the neuronal cell bodies and unmyelinated axons. It surrounds the deeper white matter (myelinated axons) in the cerebrum. The neocortex is smooth in rodents and other small mammals, but it has deep grooves (sulci) and wrinkles (gyri) in primates and specially human beings. These deep folds increase the surface area of the neocortex considerably without taking up too much more volume.

The neocortex is formed by two primary types of neurons, excitatory pyramidal neurons, which make up about 80% of neocortex, and inhibitory interneurons (~20%). The structure of the neocortex is relatively uniform. It consists of six horizontal layers segregated principally by cell type and neuronal connections. The neocortex is divided into frontal, parietal, temporal, and occipital lobes, which perform different functions. The occipital lobe contains the primary visual cortex, and the temporal lobe contains the primary auditory cortex. Further subdivisions or areas of neocortex are responsible for more specific cognitive processes. In humans, the frontal lobe contains areas devoted to abilities that are unique to our species, such as complex language processing localized to the ventrolateral prefrontal cortex (Broca's area) and social and emotional processing localized to the orbitofrontal cortex.

The neocortex is the newest part of the cerebral cortex to evolve (hence the name "neo"); the other parts of the cerebral cortex are the paleocortex and archicortex, collectively known as the allocortex. The cellular organization of the allocortex is different from the six-layer structure mentioned above. In humans, 90% of the cerebral cortex is neocortex.