Cardiac plexus

The cardiac plexus is a network of autonomic nerves that innervates the heart, regulating its functions through both sympathetic and parasympathetic fibers. It is composed of nerve fibers from the vagus nerve (parasympathetic) and sympathetic trunk (sympathetic), as well as contributions from the cardiac branches of the thoracic sympathetic ganglia. This plexus is responsible for controlling heart rate, rhythm, and force of contraction, as well as the tone of the coronary blood vessels.

Location

The cardiac plexus is located at the base of the heart, near the aortic arch and tracheal bifurcation. It is divided into two parts: the superficial cardiac plexus, which is situated between the aortic arch and the pulmonary trunk, and the deep cardiac plexus, which lies between the aortic arch and the trachea. From these locations, the plexus distributes nerve fibers to the heart, coronary arteries, and nearby structures.

Structure and Anatomy

The cardiac plexus is a complex network of autonomic nerves responsible for innervating the heart and regulating its activity. The plexus consists of both sympathetic and parasympathetic fibers, providing comprehensive control over the heart’s physiological functions. Below is a detailed description of the anatomy of the cardiac plexus.

Formation and Composition

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

• Sympathetic Fibers: These fibers arise from the cervical and upper thoracic ganglia (T1-T4) of the sympathetic trunk. The sympathetic fibers are postganglionic, meaning they have already synapsed in the sympathetic ganglia before reaching the plexus.
• Parasympathetic Fibers: These fibers are derived from the vagus nerve (cranial nerve X), specifically its cardiac branches. The parasympathetic fibers are preganglionic, synapsing in small ganglia located near or within the walls of the heart.

Nerve Fibers

The cardiac plexus contains both sympathetic and parasympathetic nerve fibers:

• Sympathetic Fibers: These fibers come from the cervical sympathetic ganglia (superior, middle, and inferior) and the upper thoracic ganglia (T1-T4). The fibers from these ganglia contribute to the cardiac nerves that reach the plexus. Sympathetic fibers primarily increase the heart rate and force of contraction.
• Parasympathetic Fibers: These fibers are derived from the vagus nerve, with its cardiac branches innervating the heart. The parasympathetic fibers reach the cardiac plexus and subsequently innervate the heart, especially influencing heart rate by slowing it down.

Branches of the Cardiac Plexus

The cardiac plexus gives rise to several branches that innervate different parts of the heart:

• Cardiac Branches of the Vagus Nerve: These parasympathetic fibers contribute to the control of heart rate by innervating the sinoatrial (SA) and atrioventricular (AV) nodes.
• Cardiac Branches of the Sympathetic Trunk: These branches originate from the cervical and thoracic sympathetic ganglia and provide sympathetic input to the heart, affecting the heart rate, contraction strength, and coronary vessel tone.
• Coronary Plexus: Nerve fibers from the cardiac plexus extend into the coronary plexus, which innervates the coronary arteries. These fibers help regulate the dilation and constriction of the coronary vessels, influencing blood flow to the heart muscle.

Superficial and Deep Plexus Organization

• Superficial Cardiac Plexus: This portion is primarily composed of fibers from the left vagus nerve and superior cervical sympathetic ganglion. The fibers travel toward the base of the heart, particularly supplying the anterior surface of the heart, including the left coronary artery and atrial regions.
• Deep Cardiac Plexus: This portion is more extensive and consists of fibers from both the right and left vagus nerves, as well as the cervical and upper thoracic sympathetic ganglia. It provides innervation to the posterior surface of the heart, including the right coronary artery, and extends to various cardiac structures like the ventricles and the AV node.

Relations to Other Structures

The cardiac plexus is located in close proximity to several important structures in the thoracic cavity:

• Aortic Arch: The plexus lies beneath and around the aortic arch, particularly the superficial portion, which is situated between the arch and the pulmonary trunk.
• Trachea: The deep cardiac plexus is closely related to the bifurcation of the trachea and extends posteriorly toward the bronchi.
• Pulmonary Trunk: The superficial cardiac plexus is located anterior to the right pulmonary artery, which courses near the plexus as it extends toward the heart.
• Esophagus: The esophagus is located posterior to the deep cardiac plexus, highlighting the proximity of autonomic control between the heart and the gastrointestinal system.

Distribution of Nerve Fibers

The cardiac plexus distributes its fibers to various structures within the heart and surrounding regions:

• Sinoatrial (SA) Node: The SA node, the heart’s natural pacemaker, receives both sympathetic and parasympathetic fibers from the plexus. Sympathetic stimulation increases the heart rate, while parasympathetic fibers slow it down.
• Atrioventricular (AV) Node: The AV node, which coordinates the electrical impulses between the atria and ventricles, is also innervated by fibers from the cardiac plexus, allowing the autonomic system to influence the rate of ventricular contraction.
• Coronary Arteries: Fibers from the cardiac plexus reach the coronary arteries through the coronary plexus, regulating blood flow to the heart muscle by controlling the dilation and constriction of these vessels.
• Cardiac Muscle: Autonomic fibers innervate the myocardium (heart muscle) itself, allowing the cardiac plexus to modulate the strength of the heart’s contractions.

Ganglia

While the cardiac plexus contains mainly postganglionic sympathetic fibers, small parasympathetic ganglia are scattered within the plexus. These ganglia are sites where the preganglionic parasympathetic fibers from the vagus nerve synapse before sending postganglionic fibers to the heart. These small ganglia are located near or within the heart walls, particularly around the atria and ventricles.

Histological Structure

Microscopically, the cardiac plexus is composed of myelinated and unmyelinated nerve fibers, both sympathetic and parasympathetic, supported by surrounding connective tissue. Parasympathetic ganglia are found within the plexus, particularly near the heart, where preganglionic fibers synapse with postganglionic fibers.

Function

The cardiac plexus plays a crucial role in regulating the heart’s activity by providing autonomic control through both sympathetic and parasympathetic fibers. These fibers influence the heart rate, the force of contraction, and the tone of the coronary blood vessels. Below is a detailed description of the functions of the cardiac plexus.

Sympathetic Control

The sympathetic component of the cardiac plexus originates from the cervical and thoracic sympathetic ganglia and is responsible for stimulating the heart, particularly during stress or exertion, as part of the “fight or flight” response.

Increase in Heart Rate (Positive Chronotropy)

Sympathetic fibers from the cardiac plexus increase the heart rate by acting on the sinoatrial (SA) node, the heart’s natural pacemaker. When stimulated, the sympathetic fibers release norepinephrine, which binds to β1-adrenergic receptors in the SA node, leading to an increase in the rate at which the heart beats (positive chronotropic effect). This response is essential during physical activity or stress, allowing the body to pump more blood and deliver oxygen to tissues.

Increase in Force of Contraction (Positive Inotropy)

Sympathetic fibers also increase the force of contraction of the heart muscle (myocardium), a function known as positive inotropy. By enhancing the contraction strength of the ventricles, the heart is able to pump more blood with each beat, improving cardiac output. This is particularly important during times of increased demand, such as exercise or emotional stress.

Increase in Conduction Velocity (Positive Dromotropy)

The sympathetic fibers increase the speed of electrical conduction through the heart, especially through the atrioventricular (AV) node and the ventricular conduction system. This function, known as positive dromotropy, ensures that electrical impulses travel quickly and efficiently from the atria to the ventricles, coordinating their contraction and improving cardiac efficiency.

Vasodilation of Coronary Arteries

Sympathetic stimulation of the cardiac plexus leads to vasodilation of the coronary arteries. The increase in coronary blood flow allows the heart to receive more oxygen and nutrients, particularly when the heart is working harder during exercise or stress. This increased blood flow supports the enhanced workload required by the heart under sympathetic stimulation.

Parasympathetic Control

The parasympathetic component of the cardiac plexus is derived from the vagus nerve and acts to counterbalance the sympathetic system, promoting the “rest and digest” response and regulating heart function during periods of relaxation.

Decrease in Heart Rate (Negative Chronotropy)

The parasympathetic fibers from the vagus nerve act on the SA node to decrease the heart rate. Acetylcholine, released by the parasympathetic fibers, binds to muscarinic receptors (M2) on the SA node, slowing the rate of depolarization and reducing the heart rate. This is known as a negative chronotropic effect and is essential for maintaining a slower heart rate during rest or sleep.

Reduction in Force of Contraction (Negative Inotropy)

Parasympathetic stimulation reduces the force of contraction of the heart, especially in the atria. This negative inotropic effect allows the heart to conserve energy and function more efficiently during periods of relaxation, when there is no need for increased cardiac output.

Decrease in Conduction Velocity (Negative Dromotropy)

The parasympathetic fibers also slow down the conduction velocity of electrical impulses through the heart, particularly at the AV node. This negative dromotropic effect delays the transmission of impulses from the atria to the ventricles, ensuring a more controlled and coordinated contraction of the heart during rest.

Vasoconstriction of Coronary Arteries

In contrast to the sympathetic system, parasympathetic fibers induce vasoconstriction of the coronary arteries, reducing blood flow to the heart during periods of rest when oxygen demand is lower. This vasoconstriction ensures that the heart is not overworked and that blood flow is redirected to other organs that require it more during relaxation.

Balance Between Sympathetic and Parasympathetic Control

The cardiac plexus constantly balances sympathetic and parasympathetic inputs to maintain optimal heart function under varying physiological conditions.

Fight or Flight Response

During periods of stress or physical activity, the sympathetic fibers of the cardiac plexus dominate. This leads to increased heart rate, stronger contractions, faster conduction of electrical impulses, and dilation of coronary arteries to support the body’s increased need for oxygen and nutrients. This heightened activity ensures that blood is delivered more quickly to tissues, muscles, and vital organs.

Rest and Digest Response

During periods of rest or relaxation, the parasympathetic fibers of the cardiac plexus take precedence. Heart rate slows, contraction strength diminishes, and conduction through the heart becomes more controlled. These changes allow the body to conserve energy and focus on digestion, repair, and relaxation.

Regulation of Coronary Blood Flow

The cardiac plexus regulates the tone of the coronary arteries, adjusting blood flow to the heart muscle according to physiological needs.

Coronary Vasodilation During Increased Activity

During exercise or stress, the sympathetic fibers of the cardiac plexus stimulate coronary vasodilation to increase blood flow to the myocardium, ensuring that the heart receives sufficient oxygen and nutrients to support its increased workload.

Coronary Vasoconstriction During Rest

During rest, parasympathetic fibers mediate coronary vasoconstriction, reducing blood flow to the heart. This helps to match the reduced oxygen demands of the heart when the body is not under stress.

Sensory (Afferent) Fibers and Reflexes

In addition to motor fibers, the cardiac plexus contains afferent sensory fibers that help regulate reflexes and monitor the heart’s internal environment.

Baroreceptor Reflex

The sensory fibers in the cardiac plexus are involved in the baroreceptor reflex, which monitors blood pressure. When blood pressure increases, sensory fibers relay signals to the brainstem, which activates parasympathetic responses to lower heart rate and blood pressure. Conversely, when blood pressure drops, sympathetic activity is increased to raise heart rate and blood pressure.

Chemoreceptor Reflex

The cardiac plexus also participates in the chemoreceptor reflex, which detects changes in blood oxygen, carbon dioxide, and pH levels. When oxygen levels fall or carbon dioxide levels rise, sensory signals trigger an increase in sympathetic activity, speeding up the heart rate and improving oxygen delivery to tissues.

Clinical Significance

The cardiac plexus plays a vital role in autonomic regulation of the heart, and its dysfunction or damage can lead to several clinical issues:

Arrhythmias

Disturbances in the balance of sympathetic and parasympathetic inputs from the cardiac plexus can lead to arrhythmias (irregular heartbeats). Excessive sympathetic activity can cause tachycardia, while excessive parasympathetic stimulation can lead to bradycardia.

Heart Block

Damage to the parasympathetic fibers of the cardiac plexus, particularly those regulating the atrioventricular (AV) node, can result in heart block, where the electrical impulses from the atria to the ventricles are delayed or blocked entirely, impairing heart function.

Autonomic Dysfunction

Conditions like autonomic neuropathy or trauma to the plexus during thoracic surgeries can disrupt the autonomic regulation of the heart, leading to abnormal heart rates, impaired response to stress, and even heart failure in severe cases.

Cardiac Plexus Block

In some cases, cardiac plexus blocks may be used therapeutically to manage intractable chronic pain from conditions like angina pectoris or post-surgical pain, providing relief by modulating autonomic inputs to the heart.