inspiration and expiration is different. With forced or exertional inspiration the scalenes and the sternocleidomastoid (SCM) muscles are active. They impact the first and second ribs as well as the sternum, and this results in the elevation of the bony cage in an effort to increase lung volume. Forced expiration is primarily an action of the intercostal muscles that pull the thoracic cage inward and force air out of the lungs.
Figure 3.1 Evolution of SRBD based on the acquisition of speech and the loss of the epiglottic‐soft palatal lockup. This allows the tongue base to occupy more of the oropharynx.
Anatomy and Function of the Airway
The understanding and focus is related to the pathophysiology of SRBD that begins with an understanding of the anatomy of the airway most often implicated in the onset and perpetuation of these conditions. The anatomy of the airway here focuses mainly on the upper airway, primarily the musculature that directly controls airway function, which has been broken down by anatomic location [7]. These same muscles are also involved in the function of speech and eating, hence breathing while awake is likely impacted by these functions but not during sleep and this then increases the likelihood for the SRBD.
The airway may be divided into three regions: the retropalatal area (nasopharynx), the retroglossal area (oropharynx), and the hypopharynx, also known as the laryngopharynx [8]. The oropharynx is at the level of the second and third cervical vertebrae (C‐2 to C‐3). The hypopharynx is at the level of C‐4 to C‐6, is a continuation of the oropharynx demarcated by the epiglottis, and starts at the level of the hyoid bone. In these three regions there are structures that need to be considered that have an impact on the airway as well as breathing (Figure 3.2).
Soft Palate
The muscles involved here are mainly designed to elevate and tense the soft palate. In addition, during the act of swallowing these muscles close off the nasopharynx (Table 3.1).
The palatopharyngeus and palatoglossus muscles are included with the soft palate because their origin is from the palatine aponeurosis, and the control of these muscles is the same as the musculus uvulae and levator veli palatini. The palatoglossus muscle is one of the four extrinsic muscles of the tongue, the only one that is not innervated by hypoglossal nerve (cranial nerve XII), and is innervated by the pharyngeal branch of the vagus nerve (cranial nerve X) [9]. The palatopharyngeus muscle goes from the area of the palate to the pharynx. Its primary function is associated with swallowing and is also innervated by the same nerve as the palatoglossis.
Figure 3.2 Illustration of the three regions of the upper airway: N, nasopharynx; O, oropharynx; and H, hypopharynx; E, is the epiglottis.
Table 3.1 Muscles involved with the soft palate.
| Muscle | Action |
|---|---|
| Levator veli palatini | Elevates the soft palate |
| Pulls soft palate posteriorly to close the nasopharynx | |
| Tensor veli palatini | Pulls soft palate laterally |
| Musculus uvulae | Elevates the uvula |
| Pulls uvula laterally | |
| Palatopharyngeus | Helps close the nasopharynx |
| Elevates pharynx and larynx | |
| Palatoglossus | Elevates posterior portion of tongue |
| Narrows oropharyngeal isthmus during swallowing |
Table 3.2 Muscles involved with the oropharynx.
| Muscle | Action |
|---|---|
| Extrinsic | |
| Genioglossus | Protracts tongue |
| Depresses tongue | |
| Hyoglossus | Depresses tongue |
| Styloglossus | Retracts tongue |
| Elevates tongue | |
| Palatoglossus | Elevates tongue |
| Narrows oropharyngeal isthmus during swallowing | |
| Intrinsic | |
| Superior longitudinal | Shortens tongue |
| Curls tongue apex upward | |
| Inferior longitudinal | Shortens tongue |
| Curls tongue apex downward | |
| Transverse | Narrows tongue |
| Lengthens tongue | |
| Vertical | Broadens tongue |
| Flattens tongue | |
Oropharynx
These muscles involve the posterior portion of the airway and also involve the tongue, especially the posterior aspect of the tongue. The tongue muscles are further divided into two groups, the extrinsic and the intrinsic. The extrinsic muscles have some boney support and the intrinsic have no boney support (Table 3.2).
The human tongue is a unique structure. The tongue functions as what is known as a muscular hydrostat [10]. This means that the tongue as a structure is filled with compressible liquid. As such the tongue can go through many different movements while always maintaining a constant volume. This explains how the tongue, especially the base of the tongue, when it is laying flat, has the potential to occupy more of the oropharynx and hence be a factor in airway compromise.
The Hyoid
