Smell sense loss is often the first symptom of Parkinson’s disease. Ninety percent of individuals with Parkinson’s disease suffer from defects of the sense of smell long before the appearance of the motor symptoms that are characteristic of the disease. Now researchers at the Max Planck Research Unit for Neurogenetics in Frankfurt and the University of Auckland in New Zealand have conducted a study comparing the olfactory bulbs of individuals with and without Parkinson’s disease. The researchers found that in Parkinson’s cases the total volume occupied by the functional units in the olfactory bulb (the glomeruli) is only half that in normal individuals and the distribution of the glomeruli within the olfactory bulb is altered. The olfactory bulbs of normal cases had 70 percent of their glomerular component in the bottom half of the olfactory bulb, but the olfactory bulbs of Parkinson’s disease cases contained only 44 percent in the bottom half. The question remaining is which type of neurons in the olfactory bulb is affected first or foremost in Parkinson’s disease. The researchers are planning future studies that would identify the neurons in the olfactory bulb that are the most vulnerable.