Physiological regulation of respiration

Anatomical substrate and control hierarchy

Before discussing receptors, it is necessary to clearly define the structures subject to physiological and pharmacological influence. Anatomically, two key zones are distinguished:

• Airways. This is a strict sequence of structures conducting air: from the nasal cavity, nasopharynx, and larynx to the trachea and bronchi.
• Gas exchange zone. Represented by the pulmonary alveoli.

The logic of regulating these structures always follows a descending principle—from central mechanisms to peripheral (receptor) mechanisms. A full control cycle includes four levels:

1. Central mechanisms (specific CNS centers).
2. Efferent pathways (transmission of commands from the brain to organs).
3. Peripheral mechanisms (direct receptor function locally).
4. Afferent pathways (transmission of information back to the brain).

Higher-level functions are controlled via specific central nervous system centers. These include the respiratory center, the cough reflex center, and the vagus nerve nucleus (n. vagus). Feedback from the respiratory organs enters the CNS via sensory fibers of two cranial nerves: the vagus (n. vagus) and glossopharyngeal (n. glossopharyngeus) nerves.

Somatic nervous system and respiratory muscles

This is the first major block of efferent innervation. The target here is striated muscle, which includes the diaphragm and intercostal muscles.

Their primary function is to execute the act of respiration itself, i.e., physical muscle contraction for inspiration and expiration. Commands from the CNS reach these muscles via motor nerves. Specific targets—$N_M$-cholinergic receptors—are located directly on the muscle fibers. Neuromuscular transmission, which triggers respiratory muscle contraction, is mediated precisely through them.

Autonomic nervous system: bronchi and glands

The second block of efferent innervation controls smooth muscle and glands. This is a critical section for understanding the action of most pulmonary drugs.

Parasympathetic innervation The signal source is the vagus nerve center. The targets are bronchial smooth muscle and bronchial glands. $M_3$-cholinergic receptors are located on these structures. Their physiological activation leads to bronchial narrowing and increased secretion.

Adrenergic regulation (bronchial specifics) This harbors a crucial feature of the respiratory system. Bronchial smooth muscle features $\beta_2$-adrenoceptors. However, they are not innervated—direct sympathetic innervation is completely absent. Their activation mechanism is exclusively extrasynaptic. They are stimulated by adrenaline circulating in the blood. Thus, the regulation of bronchial lumen by the sympathoadrenal system is humoral in nature.

Innervation of accessory structures and vascular tone

In addition to the primary musculature, accessory elements of the respiratory system are subject to physiological control:

• Secretory cells of the mucous membrane. Unlike bronchial smooth muscle, they possess full dual innervation—both sympathetic and parasympathetic.
• Bronchial blood vessels. Vascular tone is regulated by sympathetic fibers. They act on receptors of vascular smooth muscle cells. Two receptor types are present in this zone: $\alpha_1$-adrenoceptors and $\beta_2$-adrenoceptors, the balance of whose activity determines the vascular lumen diameter.

Class and exam questions

What else to study on this topic

• Mechanisms of the respiratory and cough centers of the CNS.
• Pharmacological effects of $M_3$ cholinergic receptor stimulation and blockade.
• The role of $\alpha_1$-adrenoceptors in regulating bronchial vascular tone.
• Humoral regulation of respiration and the effect of adrenaline on $\beta_2$-adrenoceptors.

Other topics in the "Special Pharmacology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.