Pulmonary plexus

Medically Reviewed by Anatomy Team

The pulmonary plexus is a network of autonomic nerves that innervates the lungs and bronchi, regulating various functions such as bronchial muscle contraction and dilation, as well as pulmonary vascular tone. It is made up of both sympathetic and parasympathetic fibers. The parasympathetic fibers primarily come from the vagus nerve, while the sympathetic fibers originate from the upper thoracic sympathetic ganglia. The plexus helps coordinate involuntary control over respiratory functions.

Location

The pulmonary plexus is located around the roots of the lungs, specifically at the level of the hilum of each lung. There are two main divisions of the pulmonary plexus: the anterior pulmonary plexus, located in front of the hilum, and the posterior pulmonary plexus, situated behind the hilum. These nerve networks extend into the lung tissue and bronchi, providing autonomic innervation throughout the pulmonary system.

Structure and Anatomy

The pulmonary plexus is a complex network of autonomic nerves responsible for innervating the lungs and bronchi. It is a combination of sympathetic and parasympathetic fibers that interact to regulate respiratory functions. Below is a detailed explanation of the anatomy of the pulmonary plexus.

Formation and Composition

The pulmonary plexus is formed by contributions from both the sympathetic and parasympathetic nervous systems:

Sympathetic fibers: These arise from the upper thoracic sympathetic ganglia (T1 to T5). These sympathetic nerves synapse in the ganglia and send postganglionic fibers to the pulmonary plexus, where they participate in the regulation of smooth muscle and blood vessels in the lungs.
Parasympathetic fibers: These are primarily derived from the vagus nerve (cranial nerve X). The preganglionic parasympathetic fibers travel directly from the brainstem to the lungs without synapsing until they reach their target organ, where they synapse in small ganglia near the bronchi.

Nerve Fibers

Sympathetic Fibers: The sympathetic fibers that contribute to the pulmonary plexus originate from the thoracic spinal cord and reach the plexus after synapsing in the paravertebral sympathetic chain ganglia. These fibers are responsible for influencing smooth muscle tone in the bronchial tree and pulmonary blood vessels.
Parasympathetic Fibers: The parasympathetic fibers from the vagus nerve course directly to the lungs, synapsing in small ganglia located along the bronchi. These fibers are involved in controlling bronchial smooth muscle, glandular secretion, and vascular tone within the lungs.

Ganglia

The pulmonary plexus contains small parasympathetic ganglia located near the walls of the bronchi. These ganglia are the sites where preganglionic parasympathetic fibers synapse with postganglionic fibers, which then project into the bronchi and surrounding lung tissue. Sympathetic fibers typically bypass these ganglia, synapsing earlier in the sympathetic chain.

Branches and Distribution

The nerve fibers of the pulmonary plexus distribute throughout the lungs, supplying various components of the pulmonary system:

Bronchial Branches: The plexus sends fibers to the bronchi, regulating smooth muscle contraction and relaxation. These branches extend deep into the bronchial tree, down to smaller bronchioles.
Vascular Branches: Nerve fibers from the plexus also accompany the pulmonary arteries and veins, influencing the tone of the blood vessels and controlling blood flow within the lungs.
Alveolar Innervation: The plexus provides autonomic fibers that help regulate the smooth muscle surrounding the alveolar ducts and sacs, although the direct innervation of alveoli is limited.

Relations

The pulmonary plexus is closely related to several key anatomical structures:

Lung Root (Hilum): The plexus is located around the hilum of each lung, where the main bronchi, pulmonary arteries, veins, and lymphatic vessels enter and leave the lung.
Vagus Nerve: The parasympathetic fibers of the plexus are direct branches of the vagus nerve, which runs along the lateral aspect of the trachea and then down the esophagus, contributing fibers to both the anterior and posterior pulmonary plexuses.
Esophagus: The posterior pulmonary plexus is located near the esophagus, and fibers from the esophageal plexus (also derived from the vagus nerve) are in close proximity, allowing for interaction between respiratory and gastrointestinal autonomic functions.
Sympathetic Trunk: The sympathetic chain runs lateral to the spine and gives rise to the sympathetic fibers that contribute to the pulmonary plexus.

Vascular Supply

The blood supply to the pulmonary plexus is derived from small branches of the bronchial arteries and pulmonary arteries. These vessels ensure that the nerve network within the plexus remains well-nourished to support its autonomic functions.

Histological Structure

Microscopically, the pulmonary plexus consists of a network of myelinated and unmyelinated nerve fibers, which are supported by connective tissue. The parasympathetic ganglia embedded in the plexus contain neuronal cell bodies where preganglionic and postganglionic synapses occur.

Function

The pulmonary plexus plays a vital role in the autonomic regulation of the respiratory system. It is composed of both sympathetic and parasympathetic fibers that work together to control various functions, such as bronchial muscle tone, secretion of mucus, and vascular regulation within the lungs. Below is a detailed explanation of the functions of the pulmonary plexus.

Sympathetic Control

The sympathetic fibers in the pulmonary plexus originate from the thoracic spinal cord (T1-T5) and are responsible for regulating aspects of lung function during stress or physical activity, aligning with the body’s “fight or flight” response.

Bronchodilation

The primary function of sympathetic fibers in the pulmonary plexus is to induce bronchodilation. Sympathetic stimulation causes the relaxation of the smooth muscles in the bronchi and bronchioles, widening the airways to allow more air to flow into the lungs. This is particularly important during times of stress or exercise, when the body requires increased oxygen intake.

Vasoconstriction

Sympathetic stimulation causes vasoconstriction of the pulmonary blood vessels. By constricting the vessels, the pulmonary plexus helps regulate blood flow within the lungs, directing it to areas of the lung that are better ventilated. This process ensures more efficient gas exchange during times of heightened sympathetic activity.

Inhibition of Glandular Secretions

The sympathetic fibers also inhibit the secretion of mucus in the bronchial glands. During stress, the inhibition of excessive mucus production ensures that the airways remain clear, preventing obstruction of airflow and optimizing breathing efficiency.

Parasympathetic Control

Parasympathetic fibers from the vagus nerve are also part of the pulmonary plexus, playing a crucial role in regulating lung function during periods of rest and recovery, supporting the “rest and digest” state of the body.

Bronchoconstriction

The parasympathetic fibers induce bronchoconstriction, which involves the contraction of the smooth muscles in the bronchi and bronchioles. This narrows the airways, reducing airflow. Bronchoconstriction is part of the body’s natural regulation of breathing during periods of rest, when oxygen demand is lower.

Stimulation of Mucus Secretion

Parasympathetic stimulation increases the production of mucus in the bronchial glands. This secretion helps to moisten and lubricate the respiratory tract, trapping dust, microbes, and other particles that are inhaled, and assisting in their clearance from the respiratory system.

Vasodilation

Parasympathetic fibers also cause vasodilation of the pulmonary blood vessels, increasing blood flow through the lungs. This is important during normal respiration and at rest, ensuring that the lungs receive an adequate blood supply for gas exchange, thereby maintaining efficient oxygenation of the blood.

Coordination with Other Autonomic Plexuses

The pulmonary plexus does not function in isolation but works in coordination with other autonomic plexuses to ensure a harmonized autonomic response across different systems in the body.

Esophageal Plexus Coordination

The esophageal plexus, also derived from the vagus nerve, is anatomically close to the pulmonary plexus. This close relationship allows for synchronized regulation of both respiratory and digestive processes. For example, during swallowing, the lungs and bronchi adjust their function to prevent aspiration of food or liquids into the airway.

Cardiac Plexus Coordination

The cardiac plexus, which regulates heart rate and contraction strength, also interacts with the pulmonary plexus. Sympathetic and parasympathetic fibers that influence both the lungs and the heart work together to ensure that heart rate and respiratory rate are appropriately adjusted during periods of physical activity or rest.

Vascular Regulation

The pulmonary plexus plays an important role in regulating the vascular tone within the lungs, ensuring efficient gas exchange and maintaining optimal oxygenation levels in the blood.

Regulation of Pulmonary Blood Flow

Through vasoconstriction and vasodilation, the pulmonary plexus regulates blood flow in the lungs, adapting to different physiological demands. During periods of increased oxygen demand, such as exercise, the pulmonary plexus ensures that blood is directed to well-ventilated areas of the lungs, maximizing oxygen absorption and carbon dioxide elimination.

Response to Changes in Oxygen Levels

The pulmonary plexus also responds to changes in blood oxygen levels. When oxygen levels are low, the sympathetic component of the plexus can constrict pulmonary vessels to divert blood to areas of the lungs where gas exchange is more efficient. Conversely, during times of rest, parasympathetic activity allows for more relaxed pulmonary blood flow to ensure adequate oxygenation.

Response to Physiological States

The pulmonary plexus adjusts its functions depending on the physiological needs of the body, such as during periods of stress or rest, ensuring that respiration is appropriately modulated.

Fight or Flight Response

During the fight or flight response, the sympathetic fibers of the pulmonary plexus are activated. This results in bronchodilation to increase the airflow into the lungs and vasoconstriction of pulmonary vessels to optimize blood flow to well-ventilated areas. Additionally, mucus secretion is inhibited to ensure the airways remain clear, maximizing oxygen intake during times of physical exertion or stress.

Rest and Digest Response

In contrast, during the rest and digest response, parasympathetic fibers of the pulmonary plexus become more active. This results in bronchoconstriction to reduce airflow, increased mucus production to protect and cleanse the respiratory tract, and vasodilation to promote increased blood flow for gas exchange. This balance allows for normal respiration and efficient oxygen exchange during restful periods.

Reflex Control

The pulmonary plexus is also involved in various reflex mechanisms that help maintain respiratory health and protect the lungs.

Cough Reflex

The sensory fibers within the pulmonary plexus are involved in triggering the cough reflex. When irritants, such as dust or smoke, are detected in the airways, the sensory fibers send signals to the brain, which then activates the motor fibers in the plexus to initiate a cough, expelling the irritants from the lungs.

Bronchoconstriction Reflex

The parasympathetic fibers can trigger reflex bronchoconstriction in response to irritants, allergens, or other foreign particles inhaled into the lungs. This reflex helps to protect the lungs by reducing the amount of air and potential harmful substances entering the respiratory system.

Clinical Significance

The pulmonary plexus plays a crucial role in autonomic control over the lungs and airways, and its dysfunction or involvement in disease can have important clinical consequences.

Asthma and Bronchospasm

Dysregulation of the parasympathetic component of the pulmonary plexus can lead to excessive bronchoconstriction, contributing to conditions like asthma and bronchospasm. In these conditions, parasympathetic overactivity causes excessive contraction of the bronchial smooth muscles, narrowing the airways and making breathing difficult.

Chronic Obstructive Pulmonary Disease (COPD)

In diseases like COPD, the balance between sympathetic and parasympathetic control is disturbed, often leading to excessive mucus production and airway constriction. Dysregulation of the pulmonary plexus contributes to airway obstruction and poor lung function.