Smell has a main airway path and a fluid-sensing sidekick
The olfactory system is the sensory gear for smell. Most mammals and reptiles run a main system for airborne chemicals plus an accessory system for fluid-phase cues. Together with taste it forms the chemosensory duo that reports what molecules are in food and air.
Smell is one of the special senses tied to a dedicated organ. Most mammals and reptiles actually run two smell systems: a main one tuned to airborne substances and an accessory one that senses chemicals dissolved in fluid. Smell and taste together are often called the chemosensory system, because both report the chemical makeup of things to the brain through transduction.
The peripheral hardware is the nostrils, the ethmoid bone, the nasal cavity and the olfactory epithelium, thin mucus-covered layers holding membranes, glands, sensory neurons and nerve fibres. Molecules arrive two ways: through the nostrils on an inhaled breath, or up from the throat when the tongue pushes air backwards during chewing and swallowing, a route called retronasal olfaction. Mucus dissolves the molecules, glands add enzymes to it, and receptors on the neurons' cilia bind them and generate an electrical response.
From there, nerve fibres pass through the cribriform plate, a perforated shelf of the ethmoid bone separating nose from brain, to reach the olfactory bulbs of the limbic system. The main bulb feeds two kinds of cell. Mitral cells fire slowly and are easily silenced by neighbours; tufted cells fire fast and resist inhibition. Both help judge concentration from when clusters of neurons fire, a timing code, and they register differences between very similar smells to aid later recognition. Researchers can partly describe this circuit with a mathematical model.
The olfactory cortex lies in the uncus and includes the piriform cortex, amygdala, olfactory tubercle and parahippocampal gyrus. The tubercle alone has 27 inputs and 20 outputs, linking to areas as varied as the thalamus, retina and auditory cortex; it helps confirm a signal came from a real odour, shapes social behaviour driven by smell and feeds reward circuits. The amygdala handles pheromone-type signals, though in humans that processing largely goes unnoticed.
Source: Olfactory system
