Sympathetic Fibers

Sympathetic fibers form an important component of the autonomic innervation of the respiratory system. They reach the tracheobronchial tree and lungs primarily through branches arising from the thoracic sympathetic trunks and enter the pulmonary plexuses before accompanying the bronchi and pulmonary vessels into the lungs.

Within the respiratory tract, sympathetic influences contribute to regulation of bronchial smooth muscle, pulmonary and bronchial vascular tone, and glandular activity. Sympathetic fibers function together with parasympathetic fibers and visceral afferents within the autonomic nerve plexuses surrounding the roots of the lungs.

Autonomic Innervation of the Lungs

The lungs receive autonomic innervation through the pulmonary plexuses. These networks contain sympathetic fibers, parasympathetic fibers derived from the vagus nerves, and visceral afferent fibers.

The fibers of the pulmonary plexuses follow the branching bronchi and associated blood vessels into the lung substance, creating autonomic networks throughout the bronchial tree.

Origin of Sympathetic Fibers

Sympathetic innervation of the lungs is associated primarily with the upper thoracic segments of the sympathetic system.

Preganglionic sympathetic neurons originate in the intermediolateral cell columns of the thoracic spinal cord. Their axons leave the spinal cord through anterior roots, enter spinal nerves, and pass through white rami communicantes to reach the sympathetic trunks.

Sympathetic Trunks

The sympathetic trunks are paired longitudinal chains positioned along the vertebral column. Within the thorax, they lie near the heads and necks of the ribs and provide sympathetic branches to thoracic viscera.

Preganglionic fibers supplying the respiratory tract synapse in sympathetic ganglia. Postganglionic fibers then leave the sympathetic trunks as visceral branches directed toward the pulmonary plexuses.

Preganglionic and Postganglionic Fibers

Fiber	Course
Preganglionic sympathetic fiber	Passes from the thoracic spinal cord to a sympathetic ganglion
Postganglionic sympathetic fiber	Passes from the sympathetic chain toward the pulmonary plexuses and respiratory structures

Cardiopulmonary Splanchnic Nerves

Postganglionic sympathetic fibers destined for thoracic viscera travel in branches commonly described as cardiopulmonary splanchnic nerves.

These branches arise from sympathetic ganglia and contribute fibers to autonomic plexuses associated with the heart, trachea, bronchi, and lungs.

Pulmonary Plexuses

The pulmonary plexuses are autonomic nerve networks located around the roots of the lungs and major bronchi.

They are commonly described as anterior and posterior pulmonary plexuses according to their relationship to the root of each lung.

Anterior Pulmonary Plexus

The anterior pulmonary plexus lies anterior to the root of the lung. It receives autonomic fibers that distribute along the bronchial tree and pulmonary vessels.

It communicates with other autonomic networks in the thorax and participates in the integrated innervation of pulmonary structures.

Posterior Pulmonary Plexus

The posterior pulmonary plexus lies posterior to the root of the lung and is generally more extensive than the anterior plexus.

Sympathetic, parasympathetic, and visceral afferent fibers intermingle within this plexus before continuing into the lung.

Distribution Along the Bronchial Tree

After entering the pulmonary plexuses, autonomic fibers accompany the bronchi and their branches.

This arrangement allows sympathetic fibers to reach smooth muscle, glands, and vascular structures throughout much of the tracheobronchial tree.

Bronchial Smooth Muscle

Sympathetic influence is associated with relaxation of bronchial smooth muscle and therefore favors widening of the airways.

In humans, circulating catecholamines, particularly epinephrine acting on beta-2 adrenergic receptors, are especially important in producing bronchodilation. Direct sympathetic innervation of airway smooth muscle is relatively limited compared with parasympathetic innervation.

Bronchodilation

Bronchodilation increases airway caliber and decreases resistance to airflow.

This effect is particularly important during sympathetic activation, when increased ventilation may accompany increased metabolic demand.

Adrenergic Receptors

Adrenergic effects within the respiratory system are mediated by receptors responsive to norepinephrine and circulating catecholamines.

Beta-2 adrenergic receptors are abundant on bronchial smooth muscle. Their activation promotes smooth muscle relaxation and bronchodilation.

Sympathetic Effects on Airway Glands

Autonomic innervation influences secretory activity within the respiratory tract. Parasympathetic activity is particularly important in promoting airway gland secretion.

Sympathetic activity generally has less influence on airway secretion than parasympathetic activity, but adrenergic signaling can modify glandular and epithelial function.

Sympathetic Effects on Pulmonary Vessels

Sympathetic fibers also reach vascular smooth muscle associated with pulmonary and bronchial vessels.

Adrenergic effects on vascular tone vary according to vessel type, receptor distribution, and physiological conditions. The pulmonary circulation is strongly regulated by local factors, particularly alveolar oxygen tension, in addition to autonomic influences.

Bronchial Circulation

The bronchial arteries are part of the systemic circulation and supply oxygenated blood to supporting tissues of the bronchi and lungs.

Sympathetic vasomotor fibers can influence the smooth muscle of these systemic vessels and contribute to regulation of bronchial vascular tone.

Relationship with Parasympathetic Innervation

Sympathetic and parasympathetic components converge within the pulmonary plexuses but have different physiological influences on respiratory structures.

Feature	Sympathetic Influence	Parasympathetic Influence
Bronchial smooth muscle	Favors bronchodilation	Promotes bronchoconstriction
Airway secretions	Generally less prominent secretory influence	Promotes glandular secretion
Major pathway	Thoracic sympathetic trunks and pulmonary plexuses	Vagus nerves and pulmonary plexuses

Parasympathetic Fibers in the Pulmonary Plexus

Parasympathetic fibers reaching the lungs arise from the vagus nerves.

Unlike sympathetic postganglionic fibers, vagal preganglionic fibers travel toward small ganglia located within or near the walls of the respiratory passages, where they synapse with short postganglionic neurons.

Visceral Afferent Fibers

The pulmonary plexuses also contain visceral afferent fibers carrying sensory information from the respiratory tract.

These afferents participate in reflexes associated with airway irritation, bronchial tone, respiratory mechanics, and other visceral responses.

Autonomic Reflexes

Autonomic control of the respiratory tract operates partly through reflex pathways involving sensory receptors, central nervous system integration, and autonomic efferent output.

These mechanisms contribute to adjustments in airway caliber and secretion in response to changing physiological conditions and inhaled stimuli.

Airway Resistance

Airway resistance is strongly influenced by bronchial diameter. Because resistance rises markedly as airway radius decreases, changes in bronchial smooth muscle tone can substantially alter airflow.

Adrenergic bronchodilation therefore facilitates airflow by increasing airway caliber.

Fight-or-Flight Response

During generalized sympathetic activation, circulating catecholamines contribute to respiratory adjustments that support increased physical activity.

Bronchodilation facilitates movement of air through the conducting passages while cardiovascular sympathetic responses increase delivery of oxygenated blood to active tissues.

Clinical Relevance of Beta-2 Receptors

The presence of beta-2 adrenergic receptors on bronchial smooth muscle has major clinical importance.

Beta-2 adrenergic agonists can produce bronchodilation and are widely used in the management of reversible bronchoconstriction, including that associated with asthma.

Sympathetic versus Somatic Innervation

Sympathetic fibers innervating pulmonary structures are part of the autonomic nervous system and regulate involuntary visceral functions.

They should be distinguished from somatic motor fibers supplying skeletal muscles involved in respiration, including the diaphragm and intercostal muscles.

Relationship to the Root of the Lung

The pulmonary plexuses lie in close anatomical relationship to the structures forming the root of each lung.

From this region, autonomic fibers follow bronchovascular pathways into the lung, allowing their distribution to progressively smaller branches of the respiratory tree.

Course of Sympathetic Innervation

1. Preganglionic neurons arise in the thoracic spinal cord.
2. Preganglionic axons leave through anterior spinal roots.
3. Fibers enter spinal nerves and pass through white rami communicantes.
4. They enter the thoracic sympathetic trunks.
5. Preganglionic fibers synapse in sympathetic ganglia.
6. Postganglionic fibers leave the sympathetic chain as visceral branches.
7. These fibers contribute to the pulmonary plexuses.
8. Fibers accompany bronchi and blood vessels into the lungs.
9. Adrenergic influences contribute to regulation of airway and vascular smooth muscle.

Key Anatomical Relationships

• Respiratory sympathetic fibers are associated primarily with thoracic sympathetic pathways.
• Preganglionic sympathetic neurons originate in the thoracic spinal cord.
• Preganglionic fibers reach the sympathetic trunks through white rami communicantes.
• Postganglionic fibers arise after synapsing in sympathetic ganglia.
• Cardiopulmonary splanchnic branches carry sympathetic fibers toward thoracic visceral plexuses.
• Sympathetic fibers contribute to the anterior and posterior pulmonary plexuses.
• Pulmonary plexuses are situated around the roots and major bronchi of the lungs.
• Autonomic fibers follow bronchovascular structures into the lung.
• Sympathetic influence favors bronchodilation.
• Beta-2 adrenergic receptors are important in bronchial smooth muscle relaxation.
• Circulating epinephrine contributes substantially to adrenergic bronchodilation in humans.
• Sympathetic fibers also influence vascular and glandular structures.
• Parasympathetic pulmonary innervation is supplied primarily through the vagus nerves.
• Sympathetic, parasympathetic, and visceral afferent fibers intermingle within pulmonary plexuses.

Clinical Importance

Sympathetic pathways form part of the autonomic network regulating the respiratory tract. Their fibers reach the lungs through the thoracic sympathetic system and pulmonary plexuses, where they intermingle with parasympathetic and visceral afferent fibers before following the bronchial tree and pulmonary vessels.

Adrenergic signaling is particularly important for regulation of bronchial smooth muscle. Activation of beta-2 adrenergic receptors promotes bronchodilation, although circulating catecholamines have a major role in this response in humans. This receptor mechanism is also the anatomical and physiological basis for the use of beta-2 agonists as bronchodilator medications.

Understanding sympathetic respiratory innervation provides a foundation for studying pulmonary plexuses, bronchial autonomic control, airway resistance, adrenergic receptors, bronchodilation, bronchial circulation, and interactions between sympathetic and parasympathetic pathways.