Everything about The Neocortex totally explained
The
neocortex (
Latin for "new
bark" or "new
rind") is a part of the
brain of
mammals. It is the outer layer of the
cerebral hemispheres, and made up of six layers, labelled I to VI (with VI being the innermost and I being the outermost). The neocortex is part of the
cerebral cortex (along with the
archicortex and
paleocortex, which are cortical parts of the
limbic system). It is involved in higher functions such as
sensory perception, generation of
motor commands, spatial reasoning,
conscious thought and, in humans,
language. Other names for the neocortex include
neopallium ("new
mantel") and
isocortex ("equal rind").
Anatomy
The neocortex consists of the
grey matter, or neuronal cell bodies and
unmyelinated fibers, surrounding the deeper
white matter (
myelinated axons) in the
cerebrum. Whereas the neocortex is smooth in
rodents and other small mammals, it has deep grooves (
sulci) and wrinkles (
gyri) in
primates and other larger mammals. These folds increase the surface area of the neocortex considerably without taking up too much more volume. This has allowed primates and especially humans to evolve new functional areas of neocortex that are responsible for enhanced cognitive skills such as
working memory, speech, and language.
The neocortex contains two primary types of neurons, excitatory
pyramidal neurons (~80% of neocortical neurons) and inhibitory
interneurons (~20%). The structure of the neocortex is relatively uniform (hence the names "iso-" and "homotypic" cortex): it consists of six horizontal layers segregated principally by
cell type and
neuronal connections. However, there are many exceptions to this uniformity; for example, the
motor cortex lacks layer IV. There is some canonical circuitry within cortex; for example, pyramidal neurons in the upper layers II and III project their
axons to other areas of neocortex, while those in the deeper layers V and VI project primarily out of the cortex, for example to the
thalamus,
brainstem, and
spinal cord. Neurons in layer IV receive all of the
synaptic connections from outside the cortex (mostly from thalamus), and themselves make short-range, local connections to other cortical layers. Thus, layer IV receives all incoming sensory information and distributes it to the other layers for further processing.
The neurons of the neocortex are also arranged in vertical structures called
neocortical columns. These are patches of the neocortex with a diameter of about 0.5 mm (and a depth of 2 mm). Each column typically responds to a sensory stimulus representing a certain body part or region of
sound or
vision. These columns are similar, and can be thought of as the basic repeating functional units of the neocortex. In humans, the neocortex consists of about a half-million of these columns, each of which contains approximately 60,000 neurons.
The neocortex is divided into
frontal,
parietal,
temporal, and
occipital lobes, which perform different functions. For example, 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 enhanced in or 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. (
See Cerebral cortex and Cerebrum.)
The female human neocortex contains approximately 19 billion
neurons while the male human neocortex has 23 billion. Additionally, the female neocortex has more white matter, while the male neocortex contains more grey matter. The implications of such differences are not fully known.
Evolution
With respect to
evolution, the neocortex is the newest part of the
cerebral cortex (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.
The six-layer cortex appears to be a distinguishing feature of mammals; it has been found in the brains of all mammals, but not in any other animals. There is some debate, however, as to the cross-
species nomenclature for
neocortex. In
avians, for instance, there are clear examples of cognitive processes that are thought to be neocortical in nature, despite the lack of the distinctive six-layer neocortical structure. In a similar manner,
reptiles, such as
turtles, have primary sensory cortices. A consistent, alternative name has yet to be agreed upon.
Further Information
Get more info on 'Neocortex'.
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