Olfactory Cortex

Original Editor - Lucinda Hampton

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Introduction[edit | edit source]

Olfactory Cortex.png

The Olfactory Cortex is the portion of the cerebral cortex concerned with the sense of smell. It is part of the Cerebrum. It is a structurally distinct cortical region on the ventral surface of the forebrain, composed of several areas. It includes the piriform lobe and the hippocampal formation.[1]

Image: Red Olfactory Cortex

Olfactory Cortex:

  • Vital for the processing and perception of odor.
  • Is a component of the limbic system. This system is involved in the processing of our emotions, survival instincts, and memory formation and connects senses, such as odors, to our memories and emotions.
  • Has connections with limbic system structures eg The amygdala (involved in forming emotional responses, particularly fear responses, and memories); the hippocampus (indexes and stores memories); the hypothalamus (regulates emotional responses).[2]

Image 2:  Human skull. The top part of the skull. is removed. Cribriform plate shown in green and Olfactory nerve shown in yellow.

Cribriform plate and Olfactory nerve - animation.gif

Sense of Smell[edit | edit source]

Olfactory Nerve Labeled.png

Our sense of smell is a complex process that depends on sensory organs, nerves, and the brain[2]. We have about 4 million smell cells in our noses, divided into about 400 different types. Each smell cell carries just one type of receptor or 'lock' on it, the smell floats through the air, fits into the 'lock' and then activates the cell[3]

The olfactory system is often described as the most "primitive" sensory system because of its early phylogenetic development and its connections to older, subconscious portions of the brain (olfactory cortex and its' connections to the limbic system, which is important in emotional states and in memory formation. Thus, a smell frequently activates intense feelings and memories before a person even identifies the odor.[4]

  • Messages also go to conscious cortical areas. After a relay in the olfactory cortex , signals enter the thalamus, and then travel on to the frontal cortex, where identification and other related thought processes take place.
  • Thus, odor messages go to primitive brain areas where they influence emotions and memories first, and then to "higher" areas[2]

Genetic Influence[edit | edit source]

There is tremendous genetic variability within and between populations for our ability to detect odours. Studies have shown that people who are unable to smell one or one class of odors frequently have small genetic differences from the general population.

  • The inability to smell is called "anosmia," and it may be general, or specific for one odor.  About 5% of the population is anosmic .
  • "Hyperosmia," a heightened sense of smell, can be a genetic trait[4].
  • Some people are born without an olfactory bulb, the organ that was previously believed to be essential for the perception of smell. While carrying out brain imaging, a group of researchers realised that one of their normal control subjects had no apparent olfactory bulb, yet they obtained normal scores for standardised smell tests. They discovered that 0.6% of all women can smell perfectly well without an olfactory bulb. This rises to 4.3% in left-handed women. But if you are a man without an olfactory bulb, the evidence so far suggests that you are destined to a lifetime of tasteless food.
  • An exercise that helps anosmics to regain their sense of smell is “smell training”. Researchers believe that systematically exercising the olfactory neurons stimulates growth and repair, much in the same way that physiotherapy promotes injury healing. The technique was pioneered in Germany and involves actively sniffing (and concentrating) on different smells at least twice a day for several months. In a recent study of older people, smell training was shown not just to improve their olfactory function but also their verbal function and overall wellbeing, demonstrating that smell training is a good way to improve the quality of life in older people[5]

Neurodegenerative Disease[edit | edit source]

The sense of smell is today one of the focuses of interest in aging and neurodegenerative disease research. In several neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease, the olfactory dysfunction is one of the initial symptoms appearing years before motor symptoms and cognitive decline.

  • It is now being considered a clinical marker of these diseases' early stages and a marker of disease progression and cognitive decline.
  • Attention to olfactory function may help to improve chances of success for neuroprotective and disease-modifying therapeutic strategies.[6]

Neurogenesis[edit | edit source]

Smell.jpg

Neurogenesis is the process by which new neurons are formed in the brain. Neurogenesis is crucial when an embryo is developing, but also continues in certain brain regions after birth and throughout our lifespan.   

  • Scientists in New Zealand and Sweden have identified the path by which new neurones travel from a region of the olfactory cortex (subventricular zone) of the human brain, where they are born, to the olfactory bulb, demonstrating that this type of neurogenesis, so vigorous in rodents, occurs in humans, too[7].

References[edit | edit source]

  1. olfactory cortex. (n.d.) Farlex Partner Medical Dictionary. (2012). Retrieved December 27 2020 from https://medical-dictionary.thefreedictionary.com/olfactory+cortex (accessed 27.12.2020)
  2. 2.0 2.1 2.2 Thought co. Sense of smell Available from: https://www.thoughtco.com/olfactory-system-4066176 (accessed 27.12.20200
  3. Science daily How our sense of smell evolved, including in early humans Available from:https://www.sciencedaily.com/releases/2015/07/150702112110.htm (accessed 27.12.2020)
  4. 4.0 4.1 Faculty Washinton Ed Olfacation Available from: https://faculty.washington.edu/chudler/chems.html(accessed 27.12.2020)
  5. The Conversation 6 curious facts about smell Available from:https://theconversation.com/six-curious-facts-about-smell-128533 (accessed28.12.2020)
  6. Marin C, Vilas D, Langdon C, Alobid I, López-Chacón M, Haehner A, Hummel T, Mullol J. Olfactory dysfunction in neurodegenerative diseases. Current allergy and asthma reports. 2018 Aug 1;18(8):42.Available from:https://pubmed.ncbi.nlm.nih.gov/29904888/ (accessed 27.12.2020)
  7. Valeo T. Neurogenesis demonstrated in human olfactory bulb. Neurology Today. 2007 Mar 20;7(6):34-5.Available from: https://journals.lww.com/neurotodayonline/Fulltext/2007/03200/Neurogenesis_Demonstrated_in_Human_Olfactory_Bulb.15.aspx (accessed 27.12.2020)