The splanchnic nerves are specialized nerves forming part of the autonomic nervous system, which controls involuntary bodily functions. These nerves primarily supply organs within the abdominal and pelvic cavities. They transmit signals that regulate various internal processes, such as digestion, blood pressure adjustments, and certain cardiovascular activities.
Anatomy and Types of Splanchnic Nerves
Splanchnic nerves are paired, categorized by their origin along the spinal column. Most carry sympathetic fibers, preparing the body for “fight or flight” responses. Pelvic splanchnic nerves, however, carry parasympathetic fibers, promoting “rest and digest” functions. These nerves pass through the sympathetic trunk without synapsing, traveling instead to prevertebral ganglia closer to their target organs.
Thoracic Splanchnic Nerves
The thoracic splanchnic nerves originate from the lower thoracic sympathetic ganglia and include three main groups: the Greater, Lesser, and Least splanchnic nerves. The Greater splanchnic nerve arises from T5-T9 or T10 spinal segments, synapsing in the celiac ganglia, which innervate upper abdominal organs like the stomach and liver. The Lesser splanchnic nerve originates from T10 and T11, traveling to the superior mesenteric and aorticorenal ganglia, supplying midgut structures. The Least splanchnic nerve, originating from T11 or T12, synapses in the renal ganglia, influencing kidney function.
Lumbar Splanchnic Nerves
Lumbar splanchnic nerves emerge from lumbar sympathetic ganglia, from L1 to L4. These nerves course anteriorly towards preaortic plexuses like the inferior mesenteric and superior hypogastric plexuses. Postganglionic fibers from these plexuses extend along blood vessels, innervating structures in the hindgut, kidneys, adrenal glands, and some pelvic organs.
Sacral and Pelvic Splanchnic Nerves
Sacral splanchnic nerves arise from the sacral part of the sympathetic trunk, from S1 and S2 sympathetic ganglia. These sympathetic fibers travel to the inferior hypogastric plexus, innervating pelvic organs and vessels, including the rectum and bladder. Distinct from these are the Pelvic splanchnic nerves (nervi erigentes), preganglionic parasympathetic fibers from the anterior rami of spinal nerves S2, S3, and S4. These nerves distribute widely to the hindgut and pelvic viscera, providing parasympathetic innervation to organs like the urinary bladder, descending colon, rectum, and reproductive organs.
Primary Functions Within the Autonomic System
Splanchnic nerves regulate internal organ activities, primarily through their efferent, or “motor,” functions. This regulation helps maintain internal balance and respond to physiological demands.
They regulate blood flow to abdominal organs. Sympathetic splanchnic nerves can constrict blood vessels in the digestive system, diverting blood away from the gut to areas like skeletal muscles during situations requiring a rapid response.
Splanchnic nerves also influence gut motility, the movement of food through the digestive tract. Sympathetic splanchnic nerve activity inhibits or slows digestive processes, including peristalsis, the wave-like muscle contractions that move food.
They impact glandular secretion from various organs. For example, some greater splanchnic nerve fibers extend to the adrenal medulla, stimulating the release of catecholamines like adrenaline into the bloodstream. They also influence the pancreas, inhibiting its exocrine function and insulin secretion while stimulating glucagon release to increase blood glucose levels.
The Splanchnic Nerves’ Role in Pain
Beyond their regulatory functions, splanchnic nerves act as sensory pathways, transmitting signals from internal organs to the central nervous system. These sensory fibers, known as visceral afferents, convey sensations, including pain, from the viscera.
Pain originating from internal organs (visceral pain) differs from pain felt in the skin or muscles (somatic pain). Visceral pain is described as dull, aching, cramping, or squeezing, and is poorly localized, making it difficult to pinpoint the exact source. This diffuse nature arises because visceral organs have a lower density of sensory nerve endings compared to somatic tissues, and their afferent fibers converge on fewer neurons in the spinal cord.
The phenomenon of “referred pain” is attributed to splanchnic nerves and other visceral afferent pathways. This occurs when pain from an internal organ is perceived in a different, often distant, body area. For instance, gallbladder pain might be felt in the right shoulder, or cardiac pain in the arm. This mislocalization happens because visceral sensory nerve fibers enter spinal cord segments that also receive somatic sensory input, leading the brain to interpret pain as originating from the more commonly experienced somatic region.
Clinical Relevance and Medical Procedures
Splanchnic nerves are important in clinical practice, particularly in managing chronic pain and aiding in diagnosis. Their role in transmitting visceral pain makes them a target for medical interventions.
One common procedure for severe, persistent abdominal pain is a splanchnic nerve block. This involves injecting a local anesthetic near the splanchnic nerves, around the celiac ganglion, to temporarily block pain signals from reaching the brain. These blocks are used for pain associated with conditions like chronic pancreatitis or abdominal cancers unresponsive to conventional treatments. If a temporary block provides relief, a more permanent solution like neurolysis (using alcohol or phenol to destroy nerve fibers) may be considered.
A more invasive procedure is a splanchnicectomy, involving surgically cutting or resecting parts of the splanchnic nerves. It aims to provide long-term pain relief by permanently disrupting pain pathways. Splanchnicectomy is reserved for intractable abdominal pain, often in cases of advanced pancreatic cancer or severe chronic pancreatitis.
Characteristics of visceral pain transmitted by these nerves are diagnostically important. Understanding how visceral pain presents, including its diffuse nature and potential for referred pain, helps clinicians differentiate internal organ issues from musculoskeletal or other somatic problems. This guides doctors in identifying the affected organ and forming an accurate diagnosis, even when pain is not precisely localized.