Lesser splanchnic nerve

The lesser splanchnic nerve is part of the sympathetic nervous system and typically arises from the ninth and tenth thoracic ganglia (T9 and T10), though it may also include fibers from the eleventh thoracic ganglion (T11). This nerve descends through the thoracic cavity along the vertebral column and pierces the diaphragm to enter the abdominal cavity. It is located posteriorly in the thorax and runs parallel to the greater and least splanchnic nerves. The lesser splanchnic nerve terminates by joining the aorticorenal ganglion, where it contributes sympathetic fibers to the innervation of the kidneys, adrenal glands, and portions of the gastrointestinal tract.

Structure and Anatomy

Origin

The lesser splanchnic nerve arises from the thoracic sympathetic trunk, specifically from the ninth and tenth thoracic ganglia (T9 and T10), although it can also receive contributions from the eleventh thoracic ganglion (T11) in some individuals. These thoracic ganglia are part of the sympathetic chain, which runs along both sides of the vertebral column. The nerve carries preganglionic sympathetic fibers that originate in the spinal cord’s lateral horn at the same thoracic levels.

Pathway

After originating from the thoracic ganglia, the lesser splanchnic nerve travels inferiorly along the posterior thoracic wall. It runs parallel to the vertebral column and courses alongside the greater splanchnic nerve, which is larger and located more medially. As it descends, the lesser splanchnic nerve passes through the posterior mediastinum, the space between the lungs that contains the heart, esophagus, and major blood vessels.

The nerve continues its path toward the diaphragm, eventually piercing this muscular structure to enter the abdominal cavity. It typically passes through the diaphragm via one of the openings, such as the crura (muscular extensions of the diaphragm attached to the lumbar vertebrae).

Location Relative to Other Structures

The lesser splanchnic nerve lies in close proximity to several important anatomical structures:

• Vertebral Column: The nerve runs adjacent to the vertebral bodies, parallel to the greater and least splanchnic nerves. Its proximity to the spine allows it to remain anchored as it travels through the thorax.
• Aorta: The nerve courses near the descending thoracic aorta, which supplies blood to the abdominal organs. Its location close to this major artery places it near critical structures that it ultimately innervates.
• Diaphragm: The lesser splanchnic nerve pierces the diaphragm to reach the abdominal cavity. It typically passes through the diaphragm’s crura, similar to the greater and least splanchnic nerves, though in a more lateral and inferior position.

Termination in the Aorticorenal Ganglion

Once the lesser splanchnic nerve enters the abdominal cavity, it terminates by synapsing in the aorticorenal ganglion. The aorticorenal ganglion is a collection of sympathetic nerve cell bodies located near the origin of the renal arteries from the abdominal aorta, close to the adrenal glands.

At this point, the preganglionic fibers carried by the lesser splanchnic nerve synapse with postganglionic neurons. The postganglionic fibers then distribute to the kidneys, adrenal glands (particularly the medulla), and parts of the gastrointestinal system.

Structure

The lesser splanchnic nerve is a mixed nerve composed primarily of preganglionic sympathetic fibers. These fibers originate from the lateral horn of the spinal cord at the T9 to T11 levels. The nerve fibers are bundled together within a sheath of connective tissue, which helps protect them as they travel from the thoracic cavity into the abdomen.

The lesser splanchnic nerve is typically smaller than the greater splanchnic nerve but larger than the least splanchnic nerve. Its role in the autonomic nervous system is to convey sympathetic outflow to the abdominal viscera.

Relationship to Other Splanchnic Nerves

The lesser splanchnic nerve is one of three major thoracic splanchnic nerves:

• Greater Splanchnic Nerve: This nerve arises from the T5 to T9 ganglia and travels alongside the lesser splanchnic nerve but is larger in size. The greater splanchnic nerve generally terminates in the celiac ganglion, where it provides innervation to many abdominal organs.
• Least Splanchnic Nerve: The least splanchnic nerve is smaller than both the greater and lesser splanchnic nerves and arises from the T12 ganglion. It often terminates in the renal plexus, supplying the kidneys and ureters.

All three splanchnic nerves pass through the diaphragm and enter the abdominal cavity, but they terminate in different ganglia and have slightly different targets for sympathetic innervation.

Blood Supply

The lesser splanchnic nerve, like other nerves in the region, is supplied by small branches of the thoracic and abdominal arteries. These arteries, including branches from the aorta and intercostal arteries, provide oxygenated blood to the nerve and surrounding tissues. Venous drainage follows a similar pattern, with small veins draining into the azygos and hemiazygos veins in the thoracic region.

Variability

Anatomical variability in the lesser splanchnic nerve is common. While it typically arises from the T9 and T10 ganglia, the nerve may also receive fibers from the T11 ganglion. The number of fibers, their size, and the precise pathway may vary between individuals. However, the general course of the nerve from the thoracic ganglia through the diaphragm and its termination in the aorticorenal ganglion remains consistent.

Function

The lesser splanchnic nerve plays a crucial role in the autonomic nervous system, particularly in the sympathetic regulation of abdominal organs. Its fibers are primarily preganglionic sympathetic nerves that contribute to various autonomic processes. The functions of the lesser splanchnic nerve can be divided into several categories based on its target organs and systems, including the adrenal glands, kidneys, and portions of the gastrointestinal system.

Sympathetic Innervation to the Adrenal Glands

One of the key functions of the lesser splanchnic nerve is to provide sympathetic innervation to the adrenal medulla via its connection to the aorticorenal ganglion.

• Stimulation of the Adrenal Medulla: The lesser splanchnic nerve transmits preganglionic sympathetic fibers that synapse in the aorticorenal ganglion, from where postganglionic fibers travel to the adrenal medulla. Unlike most organs, the adrenal medulla is directly innervated by preganglionic sympathetic fibers. Upon stimulation, these fibers induce the release of catecholamines, such as adrenaline (epinephrine) and noradrenaline (norepinephrine), into the bloodstream. These hormones play a significant role in the fight-or-flight response, increasing heart rate, blood pressure, and blood glucose levels, and preparing the body to respond to stress.
• Regulation of Stress Response: By stimulating the adrenal medulla to release these hormones, the lesser splanchnic nerve plays a direct role in regulating the body’s response to stress. The rapid release of catecholamines helps mobilize energy stores, enhance physical performance, and prepare the body for immediate action.

Sympathetic Control of Renal Function

The lesser splanchnic nerve also provides sympathetic innervation to the kidneys, mainly through the renal plexus. The nerve influences several important functions related to renal physiology:

• Regulation of Renal Blood Flow: The lesser splanchnic nerve helps control the diameter of blood vessels within the kidneys. Sympathetic stimulation leads to vasoconstriction of the renal arteries, reducing blood flow to the kidneys. This reduction in blood flow can decrease glomerular filtration rate (GFR), which in turn conserves water and helps maintain blood pressure, particularly in situations of stress or dehydration.
• Influence on Renin Secretion: The lesser splanchnic nerve contributes to the regulation of renin secretion from the juxtaglomerular cells of the kidney. Sympathetic stimulation triggers the release of renin, which activates the renin-angiotensin-aldosterone system (RAAS). This hormonal cascade is critical in regulating blood pressure and fluid balance. By influencing renin release, the lesser splanchnic nerve helps control blood volume and vascular resistance, which are essential in maintaining homeostasis, especially during stress or changes in blood pressure.
• Modulation of Urine Production: Sympathetic input from the lesser splanchnic nerve affects the amount of urine produced by the kidneys. By reducing renal blood flow and affecting the function of renal tubules, the nerve helps decrease urine output in response to stress, dehydration, or low blood pressure. This is a key survival mechanism that helps the body conserve fluids in critical situations.

Regulation of Gastrointestinal Functions

The lesser splanchnic nerve contributes to the sympathetic innervation of parts of the gastrointestinal system, particularly via the aorticorenal and superior mesenteric plexuses. The nerve’s effects on gastrointestinal function include:

• Inhibition of Peristalsis: The lesser splanchnic nerve inhibits the smooth muscle contractions that drive peristalsis, the process that moves food through the intestines. Sympathetic activation reduces intestinal motility, slowing down the movement of contents through the gastrointestinal tract. This function is particularly important during stress, where the body prioritizes energy conservation for vital systems like the cardiovascular and muscular systems over digestion.
• Constriction of Blood Vessels: The sympathetic fibers of the lesser splanchnic nerve cause vasoconstriction of blood vessels supplying the gastrointestinal tract. This reduces blood flow to the intestines and other abdominal organs during periods of heightened sympathetic activity, such as in the fight-or-flight response. The redirection of blood to more critical organs like the heart and skeletal muscles helps optimize the body’s performance in stressful situations.
• Reduced Secretions: Sympathetic innervation from the lesser splanchnic nerve reduces the secretions of digestive enzymes and fluids in the gastrointestinal tract. This further contributes to the suppression of digestion during stress, allowing the body to conserve energy for more immediate survival functions.

Contribution to the Fight-or-Flight Response

The lesser splanchnic nerve plays an essential role in the fight-or-flight response, which prepares the body to respond to stressful or dangerous situations. This response is orchestrated by the sympathetic nervous system and involves several key changes in bodily function:

• Release of Catecholamines: Through its stimulation of the adrenal medulla, the lesser splanchnic nerve triggers the release of adrenaline and noradrenaline. These hormones rapidly increase heart rate, blood pressure, and blood glucose levels, enhancing physical and mental alertness and preparing the body for immediate action.
• Reduction in Digestive Activity: As part of the fight-or-flight response, the lesser splanchnic nerve reduces digestive activity, allowing blood flow and energy to be redirected to more critical systems. This suppression of gastrointestinal function ensures that the body can focus its resources on responding to the immediate threat.
• Fluid and Electrolyte Conservation: By reducing renal blood flow and urine production, the lesser splanchnic nerve helps the body conserve fluids and maintain blood pressure during stress. This is crucial in preventing dehydration and ensuring that the cardiovascular system remains stable in critical situations.

Modulation of Autonomic Reflexes

The lesser splanchnic nerve is involved in several autonomic reflexes that help maintain homeostasis in the body. These reflexes include:

• Baroreceptor Reflexes: The nerve contributes to reflex pathways that regulate blood pressure. By controlling the tone of blood vessels in the kidneys and gastrointestinal tract, the lesser splanchnic nerve helps maintain vascular resistance and blood pressure, particularly during changes in posture or stress.
• Fluid Balance Reflexes: The lesser splanchnic nerve plays a role in reflexes that regulate fluid and electrolyte balance. In situations where the body needs to conserve fluids, sympathetic activation reduces renal function, promoting water and sodium retention.

Clinical Significance

The lesser splanchnic nerve plays a critical role in the sympathetic regulation of the adrenal glands, kidneys, and gastrointestinal organs. Overactivity or dysfunction of this nerve can contribute to clinical conditions such as hypertension, particularly due to its influence on renin secretion and the activation of the renin-angiotensin-aldosterone system (RAAS). This can lead to high blood pressure and fluid retention.

Additionally, the nerve’s involvement in the adrenal medulla can lead to heightened release of catecholamines during stress, contributing to disorders like pheochromocytoma, where excess adrenaline and noradrenaline are released, causing episodes of severe hypertension, palpitations, and anxiety. In cases of chronic abdominal pain or visceral dysfunction, targeting the lesser splanchnic nerve with nerve blocks may provide relief by reducing sympathetic outflow to the affected organs.