Cerebral cortex is the largest part of the brain that is responsible for both the regulation and control of the most complex mental processes. It comprises two parts – the neocortex and the allocortex.
What Is the Location and Function of the Allocortex?
Located deep within the medial temporal lobe, the allocortex refers to the gray matter layers and cortical structures of the brain which do not receive thalamic neuron outputs. These structures comprise the emotional network of the human brain that is involved in the processing of the sensory inputs for hearing, seeing, and somatic sensations, as well as the conscious feelings.
What Is the Structure of the Allocortex?
While the neocortex has 6 layers of neuronal bodies and comprises of the frontal, parietal, occipital, temporal, and central lobes, the allocortex is made up of 3 or 4 such layers and comprises of the following cortical structures:
- Paleocortex;
- Archicortex;
- Periallocortex.
This structure is an artificial construct located in the middle of the hemisphere. Formed from the parts of other lobes, the limbic lobe forms a ring around the corpus callosum and rostral brainstem.
Structure and Function of the Paleocortex
The paleocortex is, essentially, the olfactory cortex. It’s made up of three to five layers of cells, and the following cortical structures that are involved in translation of the sensory stimuli into smell:
- Lateral olfactory gyrus;
- Olfactory bulb;
- Piriform cortex:
- Uncus.
Lateral Olfactory Gyrus
The lateral olfactory gyrus is the main path for transmission of the sensory information between the olfactory bulb, and various cortical nuclei located in the amygdala (also labeled as caudal olfactory centers). It’s one of the two striae of the olfactory sulcus.
Olfactory Bulb
The olfactory bulb is a rounded tissue in the brain that supports the translation of the sensory information received from the nose as a smell. Olfaction is a special sensory perception of the brain that is activated when odorants bind to the olfactory receptor neurons in order to relay the sensory stimuli received through the nasal cavity.
The olfactory bulb consists of two parts: the main olfactory bulb and the accessory olfactory bulb. The nerve tissue comprising these structures is labeled as glomeruli.
The glomeruli connect the axon’s fibers of the receptor cells with the outer dendritic fibers of the interneurons. All of the glomeruli are located near the surface of the olfactory bulb.
Piriform Cortex
The piriform cortex (also labeled as the lateral olfactory gyrus) is an area located in the rhinencephalon of the brain, between the insula and the temporal lobe. It consist of the following structures:
- Cortical amygdala;
- Uncus;
- Anterior parahippocampal gyrus.
This cortical structure participates in the translation of the sensory stimuli of smell. The main neuron types of the piriform cortex that perform its functions are mostly inhibitory GABAergic cells.
Uncus
The uncus is the most anterior extremity of the middle parahippocampal gyrus. It’s a part of both the limbic system and the olfactory system.
This cortical structure is divided into:
- Anterior part – The anterior part of the uncus is most easily identified by the ambient and semilunar gyrus.
- Posterior part – The posterior part of the uncus is located in the hippocampus.
Structure and Function of the Archicortex
The archicortex is involved in the emotional expression and memory consists of only three cellular layers:
- Polymorphic,
- Pyramidal, and
- Molecular layer.
Also, we can distinguish two primary areas of the archicortex:
- The dentate gyrus
- Hippocampus
The Dentate Gyrus
The dentate gyrus is an integral part of the hippocampal formation of the brain. It’s composed mainly of granule cells that make a polymorphic cell layer. It also contains mossy fibers of the granule cell axons, that create synapses with the pyramidal cells in the C3 field of the hippocampus.
This cortical structure is comprised of distinct cell layers:
- The polymorphic layer – A layer containing interneurons (neurons that connect and enable communication between two areas of the brain).
- Stratum granulosum – A layer composed of dentate granule cells.
- Stratum moleculare, inner third – The layer where the contralateral dentate gyrus form synapses.
- Stratum moleculare, external two thirds – The deepest layer, whose cell fibers create synapses with the granule cell dendrites.
Hippocampus
Due to its location in the temporal lobe, the hippocampus plays an important role in the process of generating and storing long-term memories, as well as translating and remembering our emotions.
Hippocampus receives the neuronal connections from the primary sensory areas of the cortex, after which it translates the stimuli and relates them to similar information from past knowledge that has been previously stored in the brain. Namely, this structure of the cerebral cortex is responsible for the transformation of recently memorized experiences into long-term memories by strengthening the synapses between the neuron cells.
Structure and Function of the Periallocortex
The periallocortex refers to the area between the allocortex and the neocortex. It contains three to six cellular layers, and it’s is subdivided into:
- Cingulate gyrus;
- Insula;
- Isthmus.
Cingulate Gyrus
The cingulate cortex consists of the cingulate gyrus, which sits just above the corpus callosum, as well as the adjacent cingulate sulcus. This is the gyrus located in the middle of each of the hemispheres of the brain.
As an important part of the limbic system, the cingulate gyrus supports the regulation of both pain and emotions. In addition, the cingulate gyrus is involved in the regulation of voluntary motor movement and behavior.
Insula
The insular cortex supports our ability to analyze our emotions and the behaviors resulting from them, which adds a philosophical touch to its function. This structure of the cerebral cortex processes some of the most vital cognitive functions and social emotions in the human brain, such as disgust, pride, humiliation, guilt, atonement. By this, it predicts the internal condition of the most vital systems of the body.
Probably the most notable function of the insula is the assignment of emotional meaning to the physiological sensations and events. For example, if we feel any kind of physical pain, the insula would recognize it as a negative feeling. This makes the insula of great importance for psychopathology.
Isthmus
Located between the periinsular sulcus and the lateral ventricle, the isthmus is a white matter area that anatomically and functionally links the gray matter of the insula and central core to the remaining part of the cerebral cortex hemisphere. At the same time, it separates the insular cistern from the ventricular system of the brain.
Fun Facts
Did you know?
- Allocortex is also known as heterogenetic cortex and is the oldest cortical structure of the brain.
- EEG, PET, MRI, and fMRI, are the most common types of brain scan techniques used by researchers in order to study the brain and its functionality.
- The allocortex takes up much smaller area in the brain than the neocortex.
- The insular cortex is involved in a broad range of human behaviors, such as cravings, insights, and maternal love, for example, all based on the process of interoception.
- It’s the insula that makes us reach for one more piece of cake when we’re already full or one more cigarette.
- Granular cells are located in various regions of the brain, including the cerebellum, cerebral cortex, hippocampus, and the olfactory bulb. They produce structural proteins (keratins, loricrin, profilaggrin, and others) and lipids (ceramides, free fatty acids, and others).
- The piriform cortical regions are also labeled as: olfactory cortex, olfactory lobe or paleopallium.
- The term ‘piriform / pyriform’ has a meaning of “pear-shaped”.
- A GABAergic drug is any chemical that modifies the effects of the GABA neurotransmitter in either the body or the brain.
- The olfactory system is the only sensory system that directly makes neuronal connections to the structures of the cerebral cortex, without first creating synapses in the thalamus.