| Neocortex | |
|---|---|
 A representativecolumn of neocortex. Cell body layers are labeled on the left, and fiber layers are labeled on the right. | |
| Identifiers | |
| MeSH | D019579 |
| NeuroNames | 2314 |
| NeuroLex ID | birnlex_2547 |
| TA98 | A14.1.09.304 A14.1.09.307 |
| TA2 | 5532 |
| FMA | 62429 |
| Anatomical terms of neuroanatomy | |
Theneocortex, also called theneopallium,isocortex orsix-layered cortex, is a set of layers of themammaliancerebral cortex involved in higher-order brain functions such assensory perception,cognition, generation ofmotor commands,[1]spatial reasoning, andlanguage.[2] The neocortex is further subdivided into thetrue isocortex and theproisocortex.[3]
In thehuman brain, thecerebral cortex consists of the larger neocortex and the smallerallocortex, respectively taking up 90% and 10%.[4] The neocortex is made up ofsix layers, labelled from the outermost inwards, I to VI.
The term is fromcortex,Latin, "bark" or "rind", combined withneo-,Greek, "new".Neopallium is a similar hybrid, from Latinpallium, "cloak".Isocortex andallocortex are hybrids with Greekisos, "same", andallos, "other".
The neocortex is the most developed in its organisation and number of layers, of the cerebral tissues.[5] The neocortex consists of thegrey matter, or neuronal cell bodies andunmyelinated fibers, surrounding the deeperwhite matter (myelinatedaxons) in thecerebrum. This is a very thin layer though, about 2–4 mm thick.[6] There are two types of cortex in the neocortex, theproisocortex and the true isocortex. The pro-isocortex is a transitional area between the true isocortex and theperiallocortex (part of theallocortex). It is found in thecingulate cortex (part of thelimbic system), inBrodmann's areas24,25,30 and32, theinsula and theparahippocampal gyrus.
Of all the mammals studied to date (including humans), a species ofoceanic dolphin known as thelong-finned pilot whale has been found to have the most neocortical neurons.[7]
The neocortex is smooth inrodents and other small mammals, whereas inelephants,dolphins andprimates and other larger mammals it has deep grooves (sulci) and ridges (gyri). These folds allow the surface area of the neocortex to be greatly increased. All human brains have the same overall pattern of main gyri and sulci, although they differ in detail from one person to another.[8] The mechanism by which the gyri form during embryogenesis is not entirely clear, and there are several competing hypotheses that explain gyrification, such as axonal tension,[9] cortical buckling[10] or differences in cellular proliferation rates in different areas of the cortex.[11]
The neocortex contains both excitatory (~80%) and inhibitory (~20%)neurons, named for their effect on other neurons.[12] The human neocortex consists of hundreds of different types of cells.[13] The structure of the neocortex is relatively uniform (hence the alternative names "iso-" and "homotypic" cortex), consisting of six horizontal layers segregated principally bycell type andneuronal connections.[14] However, there are many exceptions to this uniformity; for example, layer IV is small or missing in theprimary motor cortex. There is some canonical circuitry within the cortex; for example,pyramidal neurons in the upper layers II and III project theiraxons to other areas of neocortex, while those in the deeper layers V and VI often project out of the cortex, e.g. to thethalamus,brainstem, andspinal cord. Neurons in layer IV receive the majority of thesynaptic connections from outside the cortex (mostly from thalamus), and themselves make short-range, local connections to other cortical layers.[12] Thus, layer IV is the main recipient of incoming sensory information and distributes it to the other layers for further processing.
The neocortex is often described as being arranged in vertical structures calledcortical columns, patches of neocortex with a diameter of roughly 0.5 mm (and a depth of 2 mm, i.e., spanning all six layers). These columns are often thought of as the basic repeating functional units of the neocortex, but their many definitions, in terms of anatomy, size, or function, are generally not consistent with each other, leading to a lack of consensus regarding their structure or function or even whether it makes sense to try to understand the neocortex in terms of columns.[15]
The neocortex is derived embryonically from the dorsaltelencephalon, which is therostral part of theforebrain. The neocortex is divided into regions demarcated by the cranial sutures in the skull above, intofrontal,parietal,occipital, andtemporal lobes, which perform different functions. For example, the occipital lobe contains theprimary visual cortex, and the temporal lobe contains theprimary auditory cortex. Further subdivisions or areas of neocortex are responsible for more specific cognitive processes. In humans, thefrontal lobe contains areas devoted to abilities that are enhanced in or unique to our species, such as complex language processing localized to theventrolateral prefrontal cortex (Broca's area).[12] In humans and other primates, social and emotional processing is localized to theorbitofrontal cortex.
The neocortex has also been shown to play an influential role in sleep, memory and learning processes.Semantic memories appear to be stored in the neocortex, specifically the anterolateraltemporal lobe of the neocortex.[16] It is also involved ininstrumental conditioning; responsible for transmitting sensory information and information about plans for movement to thebasal ganglia.[16] The firing rate of neurons in the neocortex also has an effect onslow-wave sleep. When the neurons are at rest and arehyperpolarizing, a period of inhibition occurs during a slowoscillation, called the down state. When the neurons of the neocortex are in the excitatorydepolarizing phase and are firing briefly at a high rate, a period of excitation occurs during a slow oscillation, called the up state.[16]
Lesions that develop inneurodegenerative disorders, such asAlzheimer's disease, interrupt the transfer of information from the sensory neocortex to the prefrontal neocortex. This disruption of sensory information contributes to the progressive symptoms seen in neurodegenerative disorders such as changes in personality, decline in cognitive abilities, anddementia.[17] Damage to the neocortex of the anterolateral temporal lobe results insemantic dementia, which is the loss of memory of factual information (semantic memories). These symptoms can also be replicated bytranscranial magnetic stimulation of this area. If damage is sustained to this area, patients do not developanterograde amnesia and are able to recallepisodic information.[18]
The neocortex is the newest part of thecerebral cortex to evolve (hence the prefixneo meaning new); the other part of the cerebral cortex is theallocortex. The cellular organization of the allocortex is different from the six-layered neocortex. In humans, 90% of the cerebral cortex and 76% of the entire brain is neocortex.[12]
For a species to develop a larger neocortex, the brain must evolve in size so that it is large enough to support the region. Body size, basalmetabolic rate and life history are factors affecting brain evolution and thecoevolution of neocortex size and group size.[19] The neocortex increased in size in response to pressures for greater cooperation and competition in early ancestors. With the size increase, there was greater voluntary inhibitory control of social behaviors resulting in increased social harmony.[20]
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.[2] There is some debate,[21][22] however, as to the cross-species nomenclature forneocortex. Inavians, 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.[23] Evidence suggest theavian pallium to be broadly equivalent to the mammalian neocortex.[24][25][26] In a similar manner,reptiles, such asturtles, have primary sensory cortices. A consistent, alternative name has yet to be agreed upon.
The neocortex ratio of a species is the ratio of the size of the neocortex to the rest of the brain. A high neocortex ratio is thought to correlate with a number of social variables such asgroup size and the complexity of social mating behaviors.[27] Humans have a large neocortex as a percentage of total brain matter when compared with other mammals. For example, there is only a 30:1 ratio of neocortical gray matter to the size of themedulla oblongata in the brainstem of chimpanzees, while the ratio is 60:1 in humans.[28]
The neocortex is the part of the brain responsible for execution of higher-order brain functions, including cognition, sensory perception, and sophisticated motor control.
For example, in the human brain, the auditory cortex presents an expansion of cortical surface, with additional gyri and with a much larger inter-individual variability...
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