Greater splanchnic nerve

The greater splanchnic nerve is a major component of the sympathetic nervous system, arising from the thoracic sympathetic trunk, specifically from the fifth to ninth thoracic ganglia (T5 to T9). It descends through the posterior mediastinum in the thoracic cavity, passing alongside the vertebral column. As it travels downward, the greater splanchnic nerve moves toward the diaphragm, which it typically pierces through the crura (muscular extensions of the diaphragm). After passing through the diaphragm, the nerve enters the abdominal cavity, where it terminates by synapsing in the celiac ganglion, an autonomic nerve center responsible for supplying sympathetic fibers to various abdominal organs.

Structure and Anatomy

Origin

The greater splanchnic nerve arises from the thoracic sympathetic trunk, specifically from the sympathetic ganglia located at the levels of the fifth to ninth thoracic vertebrae (T5 to T9). It may also receive contributions from the T10 ganglion. The sympathetic trunk runs along the lateral sides of the vertebral column, and the greater splanchnic nerve forms as the preganglionic fibers from these thoracic ganglia converge to create a single nerve.

Pathway

After originating from the T5 to T9 sympathetic ganglia, the greater splanchnic nerve descends through the thoracic cavity along the posterior mediastinum, which is the area between the lungs that contains structures such as the heart, esophagus, and major blood vessels. The nerve follows the path of the vertebral column, running close to the thoracic aorta and esophagus as it travels downward.

As it descends, the greater splanchnic nerve continues to run parallel to the vertebral column and alongside the lesser and least splanchnic nerves, which also arise from the thoracic sympathetic trunk but at different levels. The greater splanchnic nerve is the largest and longest of these three nerves.

Diaphragmatic Passage

The greater splanchnic nerve pierces the diaphragm through the crura, which are muscular extensions of the diaphragm that attach to the lumbar vertebrae. It typically passes through the right crus, although it can also pass through the left or both crura. This diaphragmatic passage allows the greater splanchnic nerve to enter the abdominal cavity, where it exerts its sympathetic influence over the abdominal organs.

Termination in the Celiac Ganglion

After passing through the diaphragm, the greater splanchnic nerve terminates in the celiac ganglion, a large autonomic ganglion located near the base of the celiac artery and abdominal aorta. The celiac ganglion is part of the celiac plexus, a major autonomic nerve center that supplies sympathetic and parasympathetic innervation to many of the abdominal organs, including the stomach, liver, spleen, pancreas, and parts of the intestines.

At the celiac ganglion, the preganglionic fibers carried by the greater splanchnic nerve synapse with postganglionic neurons. These postganglionic fibers then travel along the blood vessels of the abdominal organs to regulate their function.

Relationship to Other Structures

The greater splanchnic nerve is closely associated with various anatomical structures as it courses through the thoracic and abdominal cavities:

• Vertebral Column: The nerve runs adjacent to the vertebral column, particularly the thoracic vertebrae, which provide a structural anchor as it descends.
• Thoracic Aorta: The greater splanchnic nerve lies near the thoracic aorta during its descent through the thorax. Its proximity to this major artery places the nerve near critical blood vessels that supply the abdominal organs.
• Esophagus: The nerve runs posterior to the esophagus in the posterior mediastinum. Its close relationship to the esophagus allows it to course through the thorax without being affected by other thoracic structures.
• Diaphragm: The nerve pierces the diaphragm to enter the abdominal cavity, typically passing through the right or left crus of the diaphragm. This passage through the diaphragm is critical for the nerve to reach its target ganglia in the abdominal cavity.
• Celiac Ganglion and Celiac Plexus: The greater splanchnic nerve terminates in the celiac ganglion, where its preganglionic fibers synapse with postganglionic neurons. The celiac ganglion, part of the celiac plexus, is located near the aorta, just below the diaphragm. This plexus is responsible for distributing autonomic innervation to the upper abdominal organs.

Structure

The greater splanchnic nerve consists primarily of preganglionic sympathetic fibers. These fibers originate from neurons in the lateral horn of the spinal cord, specifically in the thoracic segments from T5 to T9. The fibers exit the spinal cord and synapse in the thoracic sympathetic ganglia before joining to form the greater splanchnic nerve.

The nerve is composed of bundles of autonomic nerve fibers surrounded by connective tissue, which helps protect the nerve as it travels through the thoracic and abdominal cavities. The fibers remain preganglionic until they reach the celiac ganglion, where they synapse with postganglionic neurons.

Relationship to Other Splanchnic Nerves

The greater splanchnic nerve is one of three major thoracic splanchnic nerves, all of which carry sympathetic fibers to the abdominal organs:

• Lesser Splanchnic Nerve: The lesser splanchnic nerve arises from the T9 and T10 thoracic ganglia and travels parallel to the greater splanchnic nerve. It typically terminates in the aorticorenal ganglion and provides innervation to the kidneys and adrenal glands.
• Least Splanchnic Nerve: The least splanchnic nerve arises from the T12 thoracic ganglion and is the smallest of the three splanchnic nerves. It terminates in the renal plexus and provides sympathetic innervation to the kidneys and upper ureters.

All three splanchnic nerves descend through the posterior mediastinum and enter the abdominal cavity by piercing the diaphragm. However, they terminate in different ganglia and innervate different target organs.

Blood Supply

The blood supply to the greater splanchnic nerve is provided by small branches of the thoracic arteries, such as the intercostal arteries and the aorta. These arteries supply oxygenated blood to the nerve and surrounding tissues. Venous drainage follows a similar pattern, with small veins draining into the azygos vein and other thoracic venous structures.

Variability

The anatomy of the greater splanchnic nerve can vary among individuals. While it typically arises from the T5 to T9 thoracic ganglia, there may be variations in the exact number of fibers, their origin, or their course. In some individuals, the nerve may receive contributions from the T10 ganglion or show variations in its pathway through the diaphragm.

Function

The greater splanchnic nerve plays a key role in the autonomic nervous system, primarily involved in transmitting sympathetic signals to various abdominal organs. These signals control a wide range of physiological processes, including regulating blood flow, glandular secretions, and smooth muscle contractions in the gastrointestinal and endocrine systems. The greater splanchnic nerve also contributes significantly to the body’s stress response by influencing hormone release and other autonomic functions.

Sympathetic Innervation to the Abdominal Organs

The primary function of the greater splanchnic nerve is to provide sympathetic innervation to multiple abdominal organs, including the stomach, liver, spleen, pancreas, and parts of the intestines. This innervation is delivered through the celiac ganglion and celiac plexus.

• Stomach: Sympathetic fibers from the greater splanchnic nerve reduce the motility of the stomach by inhibiting the contraction of the smooth muscle. This decreases the speed at which food is processed and moved to the intestines. It also reduces the secretion of gastric juices, particularly during the fight-or-flight response when digestive activity is less of a priority for the body.
• Liver: Sympathetic innervation of the liver influences its metabolic functions, including the release of glucose into the bloodstream. During periods of stress, the greater splanchnic nerve signals the liver to release stored glucose to provide the body with immediate energy.
• Spleen: Sympathetic fibers reaching the spleen through the greater splanchnic nerve cause contraction of the smooth muscle in the spleen, leading to the release of stored red blood cells and platelets into the circulation. This function is important in the body’s response to blood loss or stress, as it increases the oxygen-carrying capacity of the blood and aids in clotting.
• Pancreas: The greater splanchnic nerve inhibits the secretion of digestive enzymes from the exocrine pancreas. Sympathetic activation during stress slows down the digestive process by reducing pancreatic enzyme production, conserving energy for more immediate bodily functions.
• Intestines: Sympathetic stimulation from the greater splanchnic nerve inhibits peristalsis and reduces blood flow to the intestines, thereby slowing down the movement of food through the digestive tract. This is particularly important during the fight-or-flight response, where energy is redirected to critical systems such as the cardiovascular and muscular systems.

Modulation of Blood Flow

One of the critical roles of the greater splanchnic nerve is to regulate blood flow to the abdominal organs by controlling the tone of blood vessels in the region.

• Vasoconstriction of Abdominal Blood Vessels: Sympathetic fibers from the greater splanchnic nerve cause vasoconstriction (narrowing) of blood vessels supplying the stomach, liver, intestines, and other abdominal organs. This reduces blood flow to these organs during stressful situations, allowing more blood to be redirected to vital organs like the heart and muscles, which require increased blood supply during the fight-or-flight response.
• Shifting Blood to Vital Organs: By decreasing blood flow to the digestive system, the greater splanchnic nerve ensures that critical organs involved in physical activity, such as the brain, lungs, and muscles, receive adequate blood supply during periods of stress. This shift in blood distribution is essential for the body’s ability to respond to emergencies.

Role in the Fight-or-Flight Response

The greater splanchnic nerve plays a significant role in the body’s fight-or-flight response, which prepares the body to react to threats or stressful situations. This response is mediated by the sympathetic nervous system and involves a series of physiological changes:

• Inhibition of Digestive Activity: The greater splanchnic nerve slows down digestion during stressful situations. By reducing stomach motility, gastric secretions, and pancreatic enzyme release, the body conserves energy for immediate survival needs rather than diverting it toward digestion.
• Glucose Release from the Liver: Sympathetic stimulation causes the liver to release glucose into the bloodstream, providing the body with a quick source of energy to fuel the muscles and brain. This increased glucose availability is crucial for enhanced physical performance in stressful or dangerous situations.
• Splenic Contraction: During the fight-or-flight response, the greater splanchnic nerve stimulates the spleen to contract, releasing stored red blood cells and platelets into the bloodstream. This increases the oxygen-carrying capacity of the blood and improves the body’s ability to respond to physical stress or injury.
• Reduction of Blood Flow to Abdominal Organs: By constricting the blood vessels in the gastrointestinal organs, the greater splanchnic nerve helps prioritize blood flow to the brain, heart, and muscles. This ensures that the body is well-prepared to respond to emergencies, whether through physical exertion or heightened alertness.

Innervation of the Adrenal Medulla

Although the lesser splanchnic nerve primarily innervates the adrenal glands, the greater splanchnic nerve may also contribute to this function in some individuals. Sympathetic fibers from the greater splanchnic nerve can reach the adrenal medulla, where they stimulate the release of catecholamines, such as adrenaline (epinephrine) and noradrenaline (norepinephrine).

Release of Adrenal Hormones: These hormones are released into the bloodstream during times of stress and act to increase heart rate, blood pressure, and blood glucose levels, further enhancing the body’s ability to react to danger. Adrenaline and noradrenaline also support the redistribution of blood flow away from the digestive organs toward more vital systems.

Inhibition of Secretions

The greater splanchnic nerve influences several types of secretions in the abdominal organs, including:

• Reduction of Gastric Secretions: Sympathetic stimulation via the greater splanchnic nerve inhibits the secretion of gastric acid and digestive enzymes in the stomach. This function helps to conserve energy during stressful situations when digestion is not a priority.
• Inhibition of Pancreatic Secretions: The greater splanchnic nerve also inhibits the release of digestive enzymes from the pancreas, further reducing digestive activity. This inhibition helps to slow the digestion of food and conserve energy for more immediate bodily functions.

Modulation of Autonomic Reflexes

The greater splanchnic nerve plays a role in various autonomic reflexes that help maintain homeostasis in the body:

• Gastrointestinal Reflexes: The nerve contributes to reflex pathways that regulate peristalsis and other digestive functions. For example, during periods of stress, the greater splanchnic nerve inhibits peristaltic waves and reduces gut motility, helping to suppress digestion when the body needs to focus on more urgent tasks.
• Blood Pressure Reflexes: The greater splanchnic nerve participates in reflex pathways that maintain blood pressure, especially during changes in posture or periods of stress. By regulating vascular tone in the abdominal organs, the nerve helps maintain blood pressure and ensure adequate perfusion of vital organs.

Clinical Significance

The greater splanchnic nerve is crucial in regulating sympathetic functions in the abdominal organs, and its dysfunction can lead to various clinical conditions. Overactivity of the greater splanchnic nerve can contribute to gastric motility disorders or functional dyspepsia, where patients experience symptoms like bloating, discomfort, and delayed gastric emptying due to increased sympathetic tone.

It also plays a role in the pathophysiology of hypertension through its influence on the liver and spleen, as well as on blood vessel constriction in the abdomen. In pain management, greater splanchnic nerve blocks are used to treat conditions like chronic abdominal pain, pancreatitis, or cancer-related abdominal pain by reducing sympathetic stimulation to the abdominal organs, providing significant relief to patients.