The sense of smell begins with an odorant binding to olfactory sensory neurons that reside in the olfactory mucosa of the nasal cavity. These olfactory sensory neurons face the external environment, which is unique to just the sense of smell. Because of the exposure to the environment, olfactory sensory neurons are much more vulnerable to infections and changes in the airway surface liquid or mucus. Thus, the olfactory mucosa is designed to protect these sensory neurons and undergo repair if necessary.
The olfactory system is also unusual compared to other sensory systems, as its sensory receptors are true neurons that project directly to the central nervous system. This is a potential route to transport bacteria or other infectious agents from the external environment to the central nervous system. This is why olfactory sensory neurons can be damaged easily. Recent studies have shown, however, that the olfactory system can repair itself particularly better than other sensory systems. This ability allows the olfactory system to maintain its sensory function.
Anatomy and Physiology
The nasal cavity is found between the ethmoid bone superiorly and the palate inferiorly. It is divided into two sections by the nasal septum. The nasal cavity is essential for conditioning the air that passes into the lungs as well as acting as a resonance chamber to enhance a person’s speech. Within the nasal cavity lie the nasal and olfactory epithelia. This entry will focus on the olfactory mucosa and epithelium.
The olfactory mucosa contains the olfactory epithelium (also called the neuroepithelium to describe the olfactory sensory neurons within it) and the lamina propria, which lies just below the olfactory epithelium. The olfactory mucosa is located in the posterior and dorsal portion of the nasal cavity. The cell bodies within the olfactory epithelium are aligned, making the tissue appear stratified. There are four main cell types within the epithelium: (1) sustentacular cells (located near the apical surface), (2) mature olfactory sensory neurons (which have cilia that project into the external environment of the nasal cavity), (3) globose basal cells, and (4) horizontal basal cells (which are closest to the basal lamina). Sustentacular cells are support cells that express proteins associated with detoxifying mechanisms. This suggests that sustentacular cells assist in protecting olfactory sensory neurons from toxins and other foreign bodies. Mature olfactory sensory neurons are bipolar in shape with a ciliated dendrite that extends to the apical portion of the epithelium. These cilia contain receptors that bind specific odorants, converting the chemical signal to an electrical impulse (action potential), which begins the process of odor perception. The axons of the olfactory sensory neurons leave the epithelium through the basal layer and project to the olfactory bulb, carrying the action potential. There is a ratio of immature to mature olfactory sensory neurons within the epithelium, which allows a constant and/or continual replacement of neurons that become damaged. Immature sensory neurons have been shown to mature in about a week, including reinnervating the olfactory bulb. Lastly, the globose and horizontal basal cells proliferate at different rates, globose at a higher rate than horizontal cells, which maintains the normal function of the epithelium.
Within the lamina propria of the olfactory mucosa are the Bowman’s glands. These are specialized cells that protrude through the olfactory epithelium so that its duct can release fluid onto the apical surface, allowing the cilia of the olfactory sensory neurons to be bathed in a protective discharge. Secretions from Bowman’s glands, including lysozyme (an enzyme that damages bacterial cell walls to fight infection), amylase (an enzyme that breaks up starches), and immunoglobulin A (an antibody that is necessary for mucosal immunity), help moisten the epithelium in the nasal passages.
Regenerating the Olfactory Mucosa
There are two general types of injuries to the olfactory system: (1) direct damage to the olfactory epithelium via exposure to toxic chemicals that damage several different cell types, and (2) damage to the axons of the olfactory sensory neuron, which prevents the electrical signal from reaching the olfactory bulb. In cases where the epithelium is damaged, the toxin may not spread evenly throughout the nasal cavity, resulting in several regions of the olfactory mucosa being spared. The damaged area will have some recovery, but it may be incomplete. Severely damaged regions can recover but generally become respiratory epithelium instead of olfactory. In cases where the olfactory nerve is damaged, this signals an increase in maturing of immature olfactory sensory neurons so that the olfactory bulb can be reinnervated. Full recovery may or may not occur depending on the severity of the original lesion.
Jennifer L. Hellier
See also: Anosmia; Bowman’s Glands; Dysosmia; Olfactory Bulb; Olfactory Sensory Neurons; Olfactory System
Farbman, Albert I. (1992). Cell biology of olfaction. New York, NY: Cambridge University Press.
Schwob, James E. (2002). Neural regeneration and the peripheral olfactory system. Anatomical Record, 269(1), 33—49.