Section 1 Sino-Nasal Disorders

**3**

**Chapter 1**

**1. Introduction**

Introductory Chapter:

Dysfunction of the Olfactory

Our sensory systems are continuously exposed to external stimuli that are processed in the neural pathways of the nervous system in order to maintain bodily homeostasis and to provide appropriate behavioral responses. While some of our senses are more readily recognized for their role in guiding our daily lives and routine behaviors, such as vision and hearing, other senses are noticed primarily when they fail to work or are impaired during disease. This is the case for our chemical senses, taste and smell [1]. Recent estimates suggest that more than 12% of the U.S. population experiences taste or smell (chemosensory) dysfunction [2, 3]. Therefore, it is critical to identify treatments for smell and taste disorders [4]. Olfaction is increasingly acknowledged for its predictive value as an indicator of disorders. Olfactory deficits are evident early on in certain disorders such as Alzheimer's Disease and Parkinson's Disease. More generally, olfactory dysfunction is found in diseases that cause degenerative neuropathology, progressive loss of memory and communication function, normal age-based decline of physiological functions, intellectual challenges, depressive and anxiety disorders, as well as post-traumatic stress disorders. The relevance of olfaction as a predictor of disease has come to the forefront during the Covid-19 pandemic. Many Covid-19 patients experience smell and taste dysfunctions that are not related to blockage of nasal

System and Nasal Disorders

*Thomas Heinbockel and Balwant S. Gendeh*

passages as seen in the upper respiratory tract infections [5–8].

This chapter briefly introduces the structures and functions of the nose and olfactory pathway since they form the basis of dysfunctions of the olfactory system and nasal disorders. The nasal passages start with the nostrils or nares separated by a septum. The nasal passages include the vestibule which is the most anterior part of the nasal cavity. The nasal cavity is enclosed by an elastic cartilage and lined by a stratified squamous, keratinized epithelium. The back part of the nasal cavity is lined by the respiratory epithelium, which is a pseudostratified, ciliated, and columnar epithelium. Likewise, this respiratory epithelium is found further down the airways including the trachea and bronchi. Our organ of smell is a specialized epithelium, the olfactory epithelium, which is also a pseudostratified, ciliated, and columnar epithelium. This epithelium covers the superior nasal concha and presents as the olfactory area. Each nasal cavity has an olfactory area in the roof of the nose. The term olfactory mucosa describes the olfactory epithelium and the underlying connective tissue (lamina propria). The olfactory mucosa contains several

**2. The olfactory epithelium**
