Simple Science

Cutting edge science explained simply

# Biology# Neuroscience

How Our Brain Processes Smells: Recent Insights

Recent research sheds light on how the olfactory system operates within the brain.

― 6 min read


Smell Processing in theSmell Processing in theBrainsystem's role and aging effects.New insights into the olfactory
Table of Contents

The Olfactory System helps us detect and respond to smells. It plays an important role in our daily lives and influences our social behaviors. This system works by combining smells with other senses and past experiences. Humans can detect a wide variety of smells due to many different receptors that specifically react to different odor molecules. While our sense of smell may be more critical for some animals lower on the evolutionary ladder, it still matters a lot for humans and higher primates. Smells can affect how we feel and behave.

One interesting aspect of how the olfactory system is organized is that it does not involve a part of the brain called the thalamus, which is typically involved in processing information from other senses. The brain regions that deal with smells operate differently from those that handle other types of sensory information. The first step in processing smells occurs in an area called the olfactory epithelium before the information goes to the olfactory bulb. From there, it is sent to various parts of the brain involved in smell processing, which includes the anterior olfactory nucleus and the Piriform Cortex.

Research has mostly focused on examining the connections and the workings of these smell-processing areas. However, it seems that looking at just these areas might not be enough to understand how smells affect brain functions. New approaches suggest we should consider how different regions of the brain work together when processing smells. Additionally, conditions like aging, certain diseases, and even recent illnesses like COVID-19 have been linked to problems in the olfactory system, indicating a need for a comprehensive look at how the whole olfactory network functions.

Challenges in Researching the Olfactory System

To study the entire olfactory system, researchers need sophisticated imaging techniques that can measure brain activity. However, traditional methods such as fMRI can be challenging. One issue is that the brain tends to become less responsive to smells when they are repeatedly presented, which makes it hard to track brain activity effectively. Another challenge is that many different odors need to be tested, complicating the research process.

Past studies using fMRI on both animals and humans have mainly identified the main areas responsible for processing smells. However, there are still many unknowns, especially regarding the interactions between different brain regions when processing smells. Some key areas might not have been identified or fully studied. To address these gaps, researchers are developing new strategies to stimulate the olfactory-specific neurons in critical regions using advanced imaging techniques. This can help explore the connected brain areas and how they respond to smell stimulation.

Recent Research Efforts

In this research, scientists used a method called optogenetic fMRI. This technique allows them to stimulate specific groups of neurons in the olfactory bulb, anterior olfactory nucleus, and piriform cortex. By doing so, they could see how different areas of the brain activate in response to smells. The study revealed that different parts of the olfactory system recruit different brain networks.

For example, stimulating the anterior olfactory nucleus led to more activity in the hippocampus and striatum, while activating the piriform cortex engaged areas related to emotions and behavior. Interestingly, the research showed that repeated stimulation of certain areas, like the anterior olfactory nucleus, led to decreased activity in the overall network, while stimulating the piriform cortex did not have the same effect.

Using dynamic causal modeling, researchers could determine how different regions affect each other when processing smells. They discovered that there is an inhibitory effect coming from the anterior olfactory nucleus to other brain regions. In contrast, the piriform cortex showed excitatory connections, meaning it promotes neural activity in its connected areas.

The Role of Different Regions in Smell Processing

To better understand the role of specific regions, researchers looked at how activating the olfactory bulb affected various brain networks. When the researchers stimulated the olfactory bulb, they observed strong activation in various brain areas, including those associated with emotions, Memory, and bodily sensations. These findings suggest the olfactory bulb interacts with higher-order functions beyond just processing smells.

Stimulation of the anterior olfactory nucleus primarily activated networks linked to memory and reward systems. Conversely, the piriform cortex mainly engaged limbic areas, which are crucial for emotional responses, indicating that this region has its unique role in processing smells.

The research also investigated how the brain adapts to repeated stimulation. They found that activation in response to prior stimulation weakened over time, a phenomenon known as Habituation. When the anterior olfactory nucleus was stimulated repeatedly, there was a noticeable decline in the activation of downstream areas. In contrast, repeated stimulation of the piriform cortex did not show the same decline, indicating different adaptive mechanisms at play in these regions.

Investigating Aging and Its Effects on the Olfactory System

With the growing interest in the impact of aging on smell processing, researchers also examined how aging affects the olfactory system. Using a rodent model for accelerated aging, they found that while the overall pattern of brain activation remained similar to younger animals, the intensity of these activations decreased significantly. This was especially true in key regions involved in the initial processing of smells and emotional reactions.

The analysis also revealed that the connection between the anterior olfactory nucleus and the piriform cortex changed with age. In younger animals, the anterior olfactory nucleus facilitated the activity of the piriform cortex. However, in older rodents, this relationship became inhibitory. This suggests that there are significant changes in how the olfactory network operates as animals age, potentially leading to the olfactory impairments commonly seen in older adults.

Importance of the Findings

The insights gained from this research have important implications for understanding how the olfactory system works and how it is affected by age and disease. By mapping the interactions between olfactory-specific regions and their downstream targets, researchers can uncover how smells influence both emotional and cognitive functions. Understanding these networks will be crucial for developing therapeutic strategies for various neurological conditions characterized by olfactory dysfunction.

The findings also suggest that the inhibitory role of the anterior olfactory nucleus can influence the overall responses to odors, emphasizing the need to consider these interactions in olfactory research. This study enhances our knowledge of the dynamic properties of the olfactory system and how different brain regions collaborate to process smells effectively.

Conclusion

The olfactory system is a complex network that plays a vital role in detecting and responding to smells, influencing various aspects of behavior, emotion, and cognition. Recent studies using advanced techniques have begun to map out how different regions of the olfactory system work together to process smells and how these interactions can change with age or disease.

Understanding the detailed workings of this system can provide valuable insights into how our sense of smell relates to our overall brain function, and it could lead to the development of new treatments for olfactory dysfunctions associated with neurological disorders. Further research in this area will help clarify the roles of various brain regions and how they affect our ability to perceive and respond to our environment through smell.

Original Source

Title: Olfactory cortical outputs recruit and shape distinct brain-wide spatiotemporal networks

Abstract: Odor information is transmitted from the olfactory bulb to several primary olfactory cortical regions in parallel, including the anterior olfactory nucleus (AON) and piriform cortex (Pir). However, the specific roles of the olfactory bulb and cortical outputs in wider interactions with other interconnected regions throughout the brain remain unclear due to the lack of suitable in vivo techniques. Furthermore, emerging associations between olfactory-related dysfunctions and neurological disorders underscore the need for examining olfactory networks at the systems level. Using optogenetics, fMRI, and computational modeling, we interrogated the spatiotemporal properties of brain-wide neural interactions in olfactory networks. We observed distinct downstream recruitment patterns. Specifically, stimulation of excitatory projection neurons in OB predominantly activates primary olfactory network regions, while stimulation of OB afferents in AON and Pir primarily orthodromically activates hippocampal/striatal and limbic networks, respectively. Temporally, repeated OB or AON stimulation diminishes neural activity propagation brain-wide in contrast to Pir stimulation. Dynamic causal modeling analysis reveals a robust inhibitory effect of AON outputs on striatal and limbic network regions. In addition, experiments in aged rat models show decreased brain-wide activation following OB stimulation, particularly in the primary olfactory and limbic networks. Modeling analysis identifies a dysfunctional AON to Pir connection, indicating the impairment of this primary olfactory cortical circuit that disrupts the downstream long-range propagation. Our study for the first time delineates the spatiotemporal properties of olfactory neural activity propagation in brain-wide networks and uncovers the roles of primary olfactory cortical, AON and Pir, outputs in shaping neural interactions at the systems level.

Authors: Ed X. Wu, T. Ma, X. Wang, X. Lin, J. Wen, L. Xie, P.-L. Khong, P. Cao, A. T. L. Leong

Last Update: Oct 31, 2024

Language: English

Source URL: https://www.biorxiv.org/content/10.1101/2024.07.19.604242

Source PDF: https://www.biorxiv.org/content/10.1101/2024.07.19.604242.full.pdf

Licence: https://creativecommons.org/licenses/by/4.0/

Changes: This summary was created with assistance from AI and may have inaccuracies. For accurate information, please refer to the original source documents linked here.

Thank you to biorxiv for use of its open access interoperability.

More from authors

Similar Articles