Why do patients lose their sense of smell with neurodegenerative diseases? The smell nerves (olfactory nerves and tract) connect directly to the brain — mainly to the parts of the brain concerned with memory and learning (hippocampus and entorhinal cortex). Those nerves also connect to the prefrontal and parietal lobes of the brain, which are the areas most affected in Alzheimer’s disease. Interestingly, animal studies have shown that stimulating the olfactory nerves with powerful immune stimulants can produce Parkinson’s disease.

The microglia in the olfactory tracts are activated by the immune stimulant, after which these microglia then migrate and activate other microglia in the areas of the brain characteristically affected by neurodegenerative diseases.

We are exposed to a number of viruses, bacteria, and toxic chemicals through our noses, and the blood-brain barrier is incomplete in this area. Studies have shown that many substances can travel along the olfactory nerves and tracts directly into the brain. Spraying certain viruses and bacteria into the nose of animals can produce both Alzheimer’s disease and Parkinson’s disease pathology.

Studies have also shown that Alzheimer’s patients have high levels of aluminum in these nerves.

These findings explain why the earliest pathological change in cases of both Alzheimer’s and Parkinson’s disease occurs in the olfactory tracts and the gut, both of which are pathways to brain microglial activation.