Anatomic Substrate and Control Hierarchy
Before discussing receptors, it is essential to clearly define the structures subject to physiological and pharmacological intervention. Anatomically, two key zones are distinguished:
- Airways: A strict sequence of structures conducting air, extending from the nasal cavity, nasopharynx, and larynx to the trachea and bronchi.
- Gas Exchange Zone: Represented by the pulmonary alveoli.
Regulation of these structures follows a top-down hierarchy, extending from central mechanisms to peripheral (receptor) structures. A complete control loop comprises four levels:
- Central mechanisms (specific CNS centers)
- Efferent pathways (command transmission from the brain to target organs)
- Peripheral mechanisms (direct receptor activity on-site)
- Afferent pathways (feedback transmission back to the brain)
High-level function is coordinated through specific central nervous system centers, including the respiratory center, the cough reflex center, and the vagus nerve nucleus (n. vagus). Afferent feedback from the respiratory tract reaches the CNS via sensory fibers of two cranial nerves: the vagus nerve (n. vagus) and the glossopharyngeal nerve (n. glossopharyngeus).
Somatic Nervous System and Respiratory Muscles
This represents the first major block of efferent innervation, targeting striated skeletal muscles such as the diaphragm and intercostal muscles.
Their primary function is driving the mechanical act of breathing—physically contracting muscles for inspiration and expiration. Commands from the CNS reach these muscles via motor nerves. The muscle fibers feature specific targets: $N_M$-cholinergic receptors. Neuromuscular transmission at these sites triggers respiratory muscle contraction.
Autonomic Nervous System: Bronchi and Glands
The second block of efferent innervation controls smooth muscle and glands, serving as a cornerstone for understanding pulmonary pharmacology.
Parasympathetic Innervation Signals originate from the vagus nerve center. Targets include bronchial smooth muscle and bronchial glands, which express $M_3$-cholinergic receptors. Physiological activation of these receptors leads to bronchoconstriction and increased mucus secretion.
Adrenergic Regulation (Bronchial Specificity) A crucial respiratory system feature lies here: bronchial smooth muscles express $\beta_2$-adrenergic receptors, but they lack direct sympathetic innervation.
Activation is strictly extrasynaptic. These receptors are stimulated by circulating epinephrine in the bloodstream. Consequently, sympathetic-adrenal regulation of the bronchial lumen is purely humoral.
Innervation of Accessory Structures and Vascular Tone
Beyond the primary musculature, auxiliary respiratory elements are under physiological control:
- Mucosal Secretory Cells: Unlike bronchial smooth muscle, secretory cells receive dual autonomic innervation (both sympathetic and parasympathetic).
- Bronchial Blood Vessels: Vascular tone is regulated by sympathetic fibers acting on receptors in vascular smooth muscle cells. Two receptor types are present in this zone: $\alpha_1$-adrenergic receptors and $\beta_2$-adrenergic receptors, with their balance of activity determining vessel caliber.