The olfactory system is the sensory system used for olfaction. The accessory olfactory system senses pheromones. The olfactory system is often spoken of along with the gustatory system as the chemosensory senses because both transduce chemical signals into perception. Linda B. Buck and Richard Axel won the 2004 Nobel Prize in Physiology or Medicine for their work on the olfactory system.

Anatomy

The olfactory system is made up of a number of different areas of the brain. Here is a partial list of some of the brain structures involved:

• Outside the brain
  • Olfactory epithelium (in the nasal cavity)
  • Vomeronasal organ
  • Olfactory nerve
• In the brain
  • Olfactory bulb
  • Piriform cortex
  • Amygdala
  • Entorhinal cortex

Function

The olfactory system must accomplish several tasks:

• Create a representation of the odor
• Determine the concentration of the odor
• Distinguish a new odor from the background environmental odors
• Identify the odor across different concentrations
• Pair the odor with a memory of what the odor represents

To accomplish all of these functions, the olfactory system uses many areas of the brain. Representations of the odor may be encoded by space (a pattern of activated neurons across a given olfactory region corresponds to the odor), time (a pattern of action potentials by multiple neurons corresponds to the odor) or a combination of the two. Scientists debate whether the odor code is primarially temporal or spatial.

In mammals, odorants are inhaled through the nose where they contact the olfactory epithelium. Olfactory receptor neurons in the olfactory epithelium transduce molecular features of the odorants into electrical signals which then travel along the olfactory nerve into the olfactory bulb. Axons from the olfactory sensory neurons converge in the olfactory bulb to form tangles called glomeruli (singular glomerulus). Inside the glomulerus, the axons contact the dendrites of mitral cells and several other types of cells. Mitral cells send their axons to a number of brain areas, including the piriform cortex, the medial amygdala, and the entorhinal cortex.

The piriform cortex is probably the area most closely associated with identifying the odor. The medial amygdala is involved in social functions such as mating and the recognition of animals of the same species. The entorhinal cortex is associated with memory. The exact functions of these higher areas are a matter of scientific research and debate.

Clinical implications

Damage to the olfactory system can occur by traumatic brain injury, cancer, inhalation of toxic fumes, or neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. These conditions can cause anosmia. Doctors can detect damage to the olfactory system by presenting the patient with odors via a scratch and sniff card or by having the patient close their eyes and try to identify commonly available odors like coffee or peppermint candy.