The sense of smell, also known as olfaction, is a complex and vital sensory function that enables humans to detect and interpret a wide range of odorant molecules in the environment.

This subtle and powerful sense influences taste, memory, emotion, and even behavior.

Anatomy of the Olfactory System

The olfactory system begins in the nasal cavity with a specialized tissue called the olfactory epithelium, situated in the upper part of the nose near the cribriform plate of the ethmoid. The epithelium contains olfactory receptor neurons (ORNs), which are bipolar sensory cells with an apical dendrite that extends a covered in multiple cilia. These cilia are coated with mucus in which odorant molecules dissolve.

Each olfactory receptor neuron expresses a single type of olfactory receptor protein, allowing it to respond to specific odorant molecules. The axons of these neurons bundle together to form the olfactory nerve (cranial nerve I), which passes through small perforations in the cribriform plate to synapse within the olfactory bulb.

Signal Transduction and Neural Processing

When odorant molecules interact with receptors on the cilia, a signaling cascade occurs that ultimately generates an electrical signal or action potential within the olfactory neuron. These signals are relayed to the olfactory bulb, where neurons known as mitral and tufted cells form synapses in structures called glomeruli. Each glomerulus receives input from olfactory neurons that detect the same odorant, facilitating odor discrimination.

The mitral and tufted cells then transmit signals along the olfactory tract directly to the primary olfactory cortex, located in areas such as the piriform cortex, amygdala, and entorhinal cortex. Notably, unlike other sensory modalities, olfactory signals bypass the thalamus in their first processing step, allowing for rapid and direct integration with brain regions involved in emotion and memory. This neural architecture explains why certain smells evoke immediate emotional responses or vivid recollections.

Regeneration and Maintenance of Smelling Ability

A unique feature of the olfactory system is the continual regeneration of olfactory receptor neurons, which have a limited lifespan. This regenerative ability is maintained by basal stem cells within the olfactory epithelium that differentiate into new neurons, replacing those that are damaged or lost. However, factors such as aging, environmental toxins, infections, or trauma can impair this regenerative process, leading to partial or complete loss of smell, a condition known as anosmia.

The rate of olfactory neuron regeneration declines with age, contributing to reduced smelling ability in older. Certain genetic disorders involving ciliary dysfunction, such as Bardet-Biedl syndrome, also manifest with olfactory deficits due to impaired receptor function.

Physiological and Psychological Importance

Olfaction plays a critical role beyond mere scent detection. It is integral to flavor perception, significantly influencing taste sensation by combining with gustatory inputs in the orbitofrontal cortex. The sense of smell also serves as an early warning system for environmental hazards such as spoiled food or gas leaks.

Moreover, olfactory signals heavily influence emotional states and social behaviors. The close anatomical relationship between the olfactory system and the limbic system—responsible for emotions means odors can trigger mood changes, stress responses, or attraction. This linkage explains phenomena like pheromone-driven interpersonal communication in animals and the evocative power of familiar smells in humans.

Dr. Rachel Herz is an Adjunct Assistant Professor of Psychiatry and Human Behavior at Brown University. She is a leading expert on the psychological science of smell and has authored several books on the subject: "Everyone, in fact, has a unique nose. The specific receptors that are expressed in every individual for being able to detect smells is ever so slightly different because of the genetics behind it for every single person, except potentially if you have an identical twin."

The sense of smell depends on a specialized olfactory system that detects odorant molecules through receptor neurons in the nasal cavity. These neurons convert chemical stimuli into electrical signals transmitted to brain regions involved in emotional processing and memory. Smell is indispensable for flavor perception, environmental awareness, and emotional experience. Advances in olfactory science continue to enhance understanding of this intricate sensory function and its impact on human life.