ABSTRACT
Respiratory muscles are characterized by their activity pattern, which is correlated with ventilation, and by their mechanical action, which is to influence airflow. These muscles can be classified further as either valvular or pumping muscles, based on their function. Valvular muscles are muscles the activity of which influences airflow resistance. These are the laryngeal, pharyngeal, palatal, and nasal muscles, and consist of both abductors-muscles for which activity decreases airway resistance-and adductors-muscles for which activity increases airway resistance. Generally, abductors are active during inspiration, facilitating airflow and resisting airway collapse (see hypoglossal nerve activity; Fig. 1), whereas adductors are active during early expiration braking airflow (see thyroarytenoid nerve activity; Fig. 1). However, it should be kept in mind that inactivity of abductors is a major contributor to the increase in airway resistance during expiration. Pumping muscles are muscles the activity of which alters thoracic volume and airway pressure. The muscles of the thorax (i.e. , chest wall) represent most of the pumping muscles. Thoracic muscles act both as inspiratory muscles that increase thoracic volume and decrease airway pressure (see phrenic nerve activity, Fig. 1); and as expiratory muscles that decrease thoracic volume and increase airway pressure (see triangularis sterni nerve activity, Fig. 1). Abdominal muscles are another set of pumping muscles that can be activated during expiration to decrease thoracic volume. In this chapter, the control and function of the thoracic pumping muscles will be contrasted with those of the airway valvular muscles.
