Our brain is sensitive to essential oils due to the complex network of receptors and pathways involved in processing olfactory information. 

When we inhale an essential oil, its aromatic molecules enter the nose and bind to olfactory receptor cells, which are specialised cells in the nasal cavity that detect odours. These receptor cells send signals to the olfactory bulb, which is a structure located in the brain that processes olfactory information.

From there, the signals travel to other parts of the brain, including the limbic system and the cerebral cortex. The limbic system is involved in emotion, memory, and behaviour, and essential oils can influence mood and memory through this pathway. 

The cerebral cortex, on the other hand, is involved in higher-level cognitive functions such as attention, perception, and decision-making, and some essential oils have been shown to improve cognitive performance and alertness.

Essential oils can also affect the brain by modulating the activity of neurotransmitters, which are chemical messengers that transmit signals between neurons. For example, some essential oils may increase the activity of neurotransmitters such as serotonin, which is involved in regulating mood, while others may inhibit the activity of neurotransmitters such as acetylcholine, which is involved in cognitive processes.

Essential oils can affect the brain in a number of ways, including through their aroma, absorption through the skin, and ingestion. 

Aromatherapy: The scent of essential oils can have a direct effect on the brain through the olfactory system. When we inhale an essential oil, its aroma molecules travel through the nose and stimulate the olfactory nerve, which sends signals to the brain’s limbic system. The limbic system is involved in emotion, memory, and behaviour, and essential oils can influence mood, memory, and cognitive performance through this pathway.

Absorption through the skin: Some essential oils can be absorbed through the skin and enter the bloodstream, where they can circulate throughout the body and cross the blood-brain barrier. When essential oils are applied topically, they can influence the brain and nervous system through this route.

Ingestion: Some essential oils can be ingested in small amounts, either in food or as a dietary supplement. When ingested, the active compounds in essential oils can enter the bloodstream and cross the blood-brain barrier, where they can directly affect the brain and nervous system.

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The present study thus suggests that blended essential oil can significantly change brain activity and functional connections in human beings, which may improve human selective attention:

Behavioural and Neural Changes Induced by a Blended Essential Oil on Human Selective Attention: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6815549/ 

This study discusses the clinical effects of EOs on mood regulation and emotional disturbances as well as the cellular and molecular mechanisms of action:

Therapeutic Effect and Mechanisms of Essential Oils in Mood Disorders: Interaction between the Nervous and Respiratory Systems 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8125361/