The olfactory cortex, or olfactory brain, is the three-layered part of the cerebrum located above the eye sockets that is responsible for olfactory perception and processing. Although it has little more cortical capacity in humans, it allows discrimination of up to a trillion different odors and projects olfactory perceptions directly to brain areas of memory and emotional processing. In the context of various diseases, so especially as a result of degenerative diseases, this area of the brain can take cellular damage, which distorts the perception of smell or even makes it impossible.
What is the olfactory cortex?
The olfactory cortex is also known as the olfactory brain and corresponds to the part of the cerebrum that enables the processing and perception of odors. Thus, the system corresponds to the part of the olfactory pathway located in the cerebrum and is also known as the primary olfactory cortex. In the context of the brain’s layered structure, the olfactory cortex belongs to the allocortex, or cerebral cortex, which itself is composed of three to five layers. The olfactory brain differs in its structure from all other brain areas related to receptive tasks. While the olfactory cortex in humans hardly lives up to its name in the true sense, it is much more pronounced in primates. Especially the olfactory stalk and the equilateral olfactory bulb of humans show only a low cell number and have thus almost completely lost their cortical properties. Because of this low expression, the olfactory cortex was interpreted as a nerve in prehistoric times. To this day, the structures of the olfactory brain are actually incorrectly known as the first cranial nerve, the so-called olfactory nerve.
Anatomy and structure
The olfactory system is located above the orbits, has three layers, and is closely associated with the hippocampus. It can be differentiated into the primary and secondary olfactory centers. In humans, this system is extremely small in area, as they have little olfactory capacity. The olfactory brain is composed of fibers that project to specific brain areas. This projection mostly targets the piriform cortex, the amygdala, and the entorhinal cortex, the latter of which is associated with the processing of emotions and memory, which is responsible for the emotional loading of olfactory perceptions. In the olfactory cortex, the cerebral cortex runs out anteriorly in one strand to form the olfactory bulb and stalk, or pedunculus olfactorius. The centripetal pathways of the olfactory brain are formed by the tractus olfactorii lateralis et medialis and the trigonum olfactorium. The secondary, olfactory cortex areas, which are primarily responsible for odor identification, overlap with the secondary areas of taste at the orbital prefrontal cortex.
Function and tasks
The task of the olfactory brain, in its broadest sense, is the perception and processing of odors. This processing includes, among other things, the ability to remember specific odor perceptions. Olfactory perceptions are the only perceptions that reach the thalamus without intermediate circuits and enter the cortex by a direct route. Together with the nasal-trigeminal system for tactile and chemical stimuli and the gustatory system for taste stimuli, the olfactory cortex is responsible for all olfactory perceptions. An odor is picked up via the sensory cells of the olfactory mucosa by its molecules docking with the receptor molecules of the mucosa. Via holes in the sieve plate of the ethmoid bone, the action potentials of the odor reach the interior of the brain, where in the olfactory cortex sometimes more than 1,000 axons converge simultaneously on a downstream neuron, allowing the signals to converge via scattered sensory cells. Besides the connection of the two olfactory bulb sides, there is a connection from here to memory storage, odor identification and the area of emotions and motivations. Even humans are thus said to be able to distinguish about a trillion different olfactory mixtures via the olfactory brain.
Diseases
In particular, when the cells or fibers of the olfactory cortex are destroyed, confusion or even the demise of the ability to smell occurs.Such destruction of fibers and cells may be due to inflammatory diseases or strokes, for example. Tumors in this area of the brain can also alter the sense of smell or cause it to be suspended if their growth alters the structures of the olfactory cortex. However, degenerative diseases such as Alzheimer’s or Parkinson’s disease, in which parts of the brain perish, are among the most common causes of complaints related to olfactory perception. The olfactory ability is tested by the doctor during olfactometry. As a rule, this test is the responsibility of the ear, nose and throat specialist. This olfactory procedure offers the possibility of early diagnosis of Parkinson’s and Alzheimer’s diseases, because even in the early stages of both diseases, the sense of smell changes in about 80 percent of those affected due to irreversible damage to the olfactory system. Even if the olfactory cortex is damaged, however, this does not usually mean that olfactory perception can no longer take place at all. As explained above, the gustatory system also plays a role in the perception of odors. Therefore, many affected individuals can still smell even after the demise of parts of the olfactory cortex, but depending on the location of the damage, they can no longer classify odors, for example.
