How do we sense different smells? The answer relates to a complex series of neural circuitry within the brain, and this allows us to interpret one odour from another. To provide an insight into how this mechanism works, a research team have been developing a ‘neural cartography of smell’.
Researchers from Columbia University have determined that the experience of every odour derives from precise brain circuitry. The research team are now revealing these inner workings of our olfactory system unprecedented detail.
The olfactory system has the ability to make sense of thousands of otherwise invisible odours. This is the product of a series of cellular and molecular steps that combine to communicate to the human brain.
The brain contains an olfactory bulb, and this is organized into thousands of segregated clusters called glomeruli. Each glomerulus responds in a specific way to the thousands of odour chemicals floating in the air.
Each glomerulus receives signals from its own subset of olfactory neurons, which are randomly distributed in the nose. For some time scientists have known that each of these subsets of olfactory neurons contains a uniquely shaped receptor protein that latches specifically onto a different odour molecule.
This led to a problem that required resolving. This is: How can each randomly located, odour-detecting cell in the nose manage to send signals to just one specific glomerulus within the olfactory bulb?
The new research has explored how the olfactory system achieves its wiring precision. This came down to identifying the central organizing process between the nose’s sensory cells and their glomeruli targets in the brain’s olfactory bulb. This research was undertaken using mice, although the same pathways occur within people.
The key to the discovery resides in the shape of each receptor protein as it assumes its unique three-dimensional form within a tubular component of the cell called the endoplasmic reticulum.
Each protein’s shape is determined by the unique sequence of its amino acid components. Furthermore, each of these amino acid sequences imposes a degree of stress on the endoplasmic reticulum. The different degrees of endoplasmic reticulum stress are considered to act as something analogous to a dial setting.
Each setting triggers a gene-directed process. Here, the sensory cells direct their axons to their target glomeruli within the olfactory bulb. Each subset of sensory cells with the same-shaped receptor protein projects its axons to the very same glomerulus.
The researchers state that without a receptor-glomerulus mapping of this kind, a rose could end up smelling like a rotten egg and vice versa.
The research appears in the journal Cell, titled “ER stress transforms random olfactory receptor choice into axon targeting precision”.
