Introduction to Olfaction: Physiology

Anatomical Organization Of The Olfactory System

The structures that make up vertebrate olfactory systems can be divided into peripheral and central components (Allison, 1953). In the periphery, the nasal cavities enclose an olfactory epithelium (OE) that is responsible for sampling and detecting chemical cues in the environment. This information is then relayed to central systems by the Olfactory Nerve (ON), the first of twelve cranial nerves that interface the brain with the external world. In fish, lamellar folds of the olfactory sensory epithelium occupy a capsule that communicates with the external aqueous medium through a pair of openings or nares. In air breathing vertebrates, the olfactory epithelium resides on a series of convoluted shelves of bony cartilage, the nasal turbinates, that are housed in a nasal cavity positioned between the oral cavity and the brain (Figures 1a, 2a). The olfactory epithelium is a specialized sensory area of mucus-lined nasal tissues (nasal mucosa) (Graziadei, 1971). It contains several million Olfactory Sensory Neurons (OSNs), which are specialized neurons with a bipolar morphology (Figures 2b–d, 3c). Each OSN cell body (or soma) extends a long dendrite to the epithelial surface, where an apical knob sprouts fine cilia containing the odorant detection apparatus. A thin cable or axon extends basally from each OSN cell body (Figure 2c), and axons bundle together to form the olfactory nerve which enters the skull through fine perforations of the bony cribriform plate separating the nasal cavity from the brain. The OSNs are embedded in a matrix of sustentacular cells, which are positioned between the OSN dendrites.