The biological systems we associate with the sense of smell perform tasks unrelated to detecting odors, involving processes not connected to the conscious perception of smell. Structures and proteins thought to be exclusively for olfaction have been repurposed for entirely different physiological roles. This functional diversity demonstrates how evolution can adapt existing biological tools for novel purposes throughout the body.
Olfactory Receptors in Unexpected Places
Olfactory receptors are the specialized proteins that bind with odor molecules in the nose, initiating the sense of smell. However, these receptors are also found in tissues throughout the human body, where they perform sensing jobs that have nothing to do with smell. In these non-nasal tissues, they respond to chemical cues to regulate localized, specific biological processes.
One of the most studied examples is in the skin, where olfactory receptors contribute to wound healing. The receptor OR2AT4, when activated by a synthetic sandalwood odorant called Sandalore, has been shown to increase the proliferation and migration of skin cells known as keratinocytes. This process accelerates the re-epithelialization of wounds. This mechanism is a potential target for new therapeutic approaches in dermatology.
In the digestive system, olfactory receptors help regulate nutrient detection and hormone release. Enteroendocrine cells along the gastrointestinal tract express receptors like OR51E1, which can detect metabolites produced by gut bacteria. When these receptors are activated, they can trigger the release of hormones such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). These hormones play a part in signaling satiety and managing gut motility.
Olfactory receptors are present on the surface of sperm and appear to be involved in chemical navigation. The human olfactory receptor hOR17-4 (also known as OR1D2) is located on sperm and is activated by a compound called bourgeonal. Activation of this receptor is thought to help guide the sperm toward the egg by initiating a chemotactic response, a directed movement toward a chemical signal. This suggests a form of chemical sensing is used in fertilization, guided by the same class of receptors used for smelling.
The Olfactory Bulb’s Other Jobs
The olfactory bulb is the brain’s first processing station for smell, but its role extends beyond relaying odor information. It has direct anatomical connections to the amygdala and hippocampus, key areas of the limbic system involved in emotion and memory. This direct pathway is unique among the sensory systems and allows the olfactory system to influence mood and memory formation independently of conscious smell perception.
The influence of the olfactory bulb on the amygdala can affect emotional states and social behaviors by layering emotional context onto sensory experiences. Similarly, its connection to the hippocampus allows olfactory information to become embedded in episodic memories, which are recollections of specific events. This integration explains how certain scents can act as triggers for recalling past experiences.
The Vomeronasal System
Many animals possess a separate chemical-sensing system known as the vomeronasal organ (VNO), or Jacobson’s organ. This organ is specialized to detect non-volatile chemical cues like pheromones, which are molecules used for communication between members of the same species. The VNO is typically located in the soft tissue of the nasal septum and detects chemicals that trigger specific social or reproductive behaviors.
In animals such as snakes, lizards, cats, and dogs, the VNO is fully functional. It helps them track prey, identify predators, and assess the reproductive status of potential mates. Some mammals exhibit a characteristic behavior called the flehmen response, where they curl back their upper lip to help draw chemical cues into the VNO. The information is then sent to a distinct brain region called the accessory olfactory bulb, which processes these specific social signals.
In humans, the functionality of the VNO is debated, but the consensus is that it is a non-functional, vestigial organ. While a structure resembling a VNO is found in the human embryo, it appears to regress and lose its sensory neurons and neural connections during development. Adult humans also lack an accessory olfactory bulb, and the genes that code for VNO receptors are non-functional pseudogenes. Therefore, humans do not use a functional vomeronasal system to respond to chemical signals.