Allocortex

The cerebral cortex is one of the primary components of the brain. It is the outer covering of the gray matter that is around 2 to 3 mm thick. The cerebral cortex covers the gyri and sulci. Some parts of the cortex have simple functions and they are referred to as the primary cortices.

Some of its vital functions are receiving sensory inputs such as vision, hearing, and somatic sensation. It is also involved in the production of eye movements or limb movements. Cortices associations lead to a more complex function such as language, memory, abstraction, judgment, creativity, attention, and emotion.

The cerebral cortex has two primary parts – neocortex and allocortex.  The allocortex is also known as the heterogenetic cortex.  It is called the heterogenetic cortex because its development has never lead to the six-layered architecture of the homogenetic neocortex.

It is one of the two types of the cerebral cortex. Its other type, the neocortex has six layers whereas allocortex only has three or four cellular layers. Allocortex takes up only a small area of the brain. (1)

Subtypes
of allocortex

Paleocortex

It is a thin, primitive cortical tissue consists of three layers of neuronal cellular bodies. It has three primary areas namely the olfactory tubercle, olfactory bulb, and preform cortex.

It does not develop through the cortical plate. The Paleocortex constitutes only less than 1% of the cerebral cortex’s surface area.

• Olfactory tubercle – it is a multi-sensory processing center found inside the olfactory cortex and ventral striatum. Its primary function is in reward cognition. It also plays a vital role in locomotion and attention behavior, specifically in sensory and social responsiveness. It is interconnected with brain regions such as the sensory, arousal, and reward centers. The olfactory tubercle is essential in processing sensory information and behavioral responses.
• Olfactory bulb – It is strategically located in the forebrain of the vertebrates. It receives neural input about the smell detected by the cells via the nasal cavity. The axons of the smell receptors extend into the olfactory bulb where the information about the smell is being processed.
• Piriform cortex – It is also involved in the sense of smell. (2)
• Anterior olfactory nucleus-It’s a forebrain part of the human body. It is found above the Olfactory tubercle, Piriform cortex and behind of Olfactory bulb. The main function of the Anterior olfactory nucleus is the olfaction means helping the role to process the odor information. 
• Anterior Commissure or precommissure- It’s one kind of white matter tract (an Axon bundle), performs in cerebral hemispheres and connects the 2 temporal lobes. The main function of Anterior Commissure or precommissure is help to process the pain sensation, especially the acute and sharp pain. Additionally, it consists of the decussating fibers which are very important for chemoreception and smell processing. Still, researches are going on to completely understand the full functionality of the Anterior commissure.
• Uncus- It’s one kind of parahippocampal gyrus anterior extremity. A small fissure named incisura temporalis separates it from the temporal lobe apex. It morphologically builds one of the parts of Rhinencephalon.  It has important functions in seizures due to disagreeable odor hallucinations. In conditions like edema, hemorrhage, and tumor it increases the cranial cavity pressure and ultimately causes brain heriniation.

Archicortex

Just like paleocortex, archicortex is a type of cortical tissue with four layers of neuronal cell bodies (laminae). It has two primary areas – the dentate gyrus and hippocampus. It is most prevalent in the olfactory cortex and hippocampus.

Both are responsible for the sensation and processing of smell and forming of memories. Archicortex is believed as one of the first types of tissue to develop in the primitive nervous system. (3)

Periallocortex

It is
the transitional zone between the allocortex and the neocortex. It is
subdivided into two types:

• Peripaleocortex – It is formed at the borders just in between the neocortex and the Paleocortex.
• Periarchicortex – It is at the borders just between the neocortex and archicortex.

The regions of the brain that has allocortex are the olfactory system and the hippocampus.

• Olfactory system – It is the one responsible for the sense of smell. It is one of the primary senses in the body. It is actively involved in detecting and identifying molecules in the air. Once the nose detects something, it will send signals to the brain via the nerve signals where the signals are processed. There is a direct link between the sense of taste and smell making the sense of smell one of the powerful senses. the parts of the brain like the allocortex are the ones responsible why memories, mood, and behavior can ignite once you smell something.
• Hippocampus –It is the structure of the brain embedded deep in the temporal lobe of the cerebral cortex. It is a vital part of the limbic system; a region in the brain responsible for regulating emotions, motivation, memory, and learning. It has a distinct S-shaped structure inside the medical aspect of the temporal lobe. (4)

Quick
facts about allocortex

• Allocortex handles olfaction and
  the memory of smells.
• The other term for allocortex
  is the heterogenetic cortex.

Difference between Allocortex and Neocortex

• The neocortex has a total of
  six horizontal layers of neuronal structure. On the other hand, allocortex only
  has a total of three layers.
• The neocortex has a single
  sheet of gray matter and covered the majority of the cerebral cortex.
  Allocortex only covered the boundaries of the cerebral cortex.
• The neocortex is evolutionary
  whereas the allocortex is the beginning of neocortex.
• The conscious thoughts and
  actions are governed by the neocortex while the motivations, affective
  behavior, and emotions are associated with allocortex. (5)

Conclusion

Allocortex is tagged as the oldest yet the most integral part of the cerebral cortex. It protects the gray matter in the brain. It is involved in some of the body's primary functions like receiving sensory inputs for hearing, seeing, and somatic sensation.

Eye movements and limb movements are also linked with the function of allocortex. As simple as it seems, allocortex does perform complex functions too. It includes memory retention, language, creativity, emotion, attention, and judgment.

References

1. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0058596
2. https://www.ncbi.nlm.nih.gov/pubmed/23536801
3. https://www.semanticscholar.org/topic/Allocortex/695305
4. https://www.frontiersin.org/articles/10.3389/fnins.2015.00162/full
5. https://www.jneurosci.org/content/28/14/3586