Sympathetic

The sympathetic innervation of the urinary tract forms part of the thoracolumbar autonomic outflow and supplies the kidneys, ureters, urinary bladder, and portions of the urethral outlet. Sympathetic fibers reach these structures through a series of prevertebral and pelvic autonomic plexuses.

Sympathetic activity has different functions at different levels of the urinary tract. In the kidney, it influences renal vascular resistance, renin secretion, and tubular handling of sodium. In the bladder, sympathetic activity is particularly associated with the storage phase of the micturition cycle by promoting detrusor relaxation and increasing smooth muscle activity at the bladder outlet.

Autonomic Innervation of the Urinary Tract

The urinary tract receives both sympathetic and parasympathetic autonomic fibers.

These pathways are accompanied by visceral afferent fibers that convey sensory information from urinary organs toward the central nervous system.

Sympathetic Outflow

Sympathetic preganglionic neurons supplying the urinary tract originate in the thoracolumbar spinal cord.

The exact spinal levels vary with the target organ, with renal and upper ureteric pathways generally arising more superiorly than pathways supplying the bladder and pelvic urinary tract.

Preganglionic Sympathetic Neurons

Cell bodies of preganglionic sympathetic neurons are located in the intermediolateral cell column of the spinal cord.

Their axons leave the spinal cord through anterior roots, enter spinal nerves, and then pass through white rami communicantes into the sympathetic trunks.

Sympathetic Trunks

The paired sympathetic trunks extend longitudinally alongside the vertebral column.

Preganglionic fibers supplying abdominal and pelvic viscera can pass through sympathetic trunk ganglia without synapsing and continue into splanchnic nerves.

Splanchnic Nerves

Thoracic and lumbar splanchnic nerves convey sympathetic fibers toward prevertebral plexuses associated with the abdominal aorta and its branches.

From these plexuses, sympathetic fibers are distributed to the kidneys, ureters, and pelvic autonomic pathways.

Renal Sympathetic Innervation

The kidneys receive a rich sympathetic nerve supply through the renal plexus.

Fibers within this plexus accompany the renal arteries and their branches into the renal parenchyma.

Renal Plexus

The renal plexus is an autonomic network surrounding the renal arteries.

It receives sympathetic contributions from neighboring prevertebral plexuses and splanchnic pathways.

Renal Ganglia

Sympathetic pathways associated with the kidney involve prevertebral ganglia located near the origins of major abdominal arterial branches.

Postganglionic fibers then travel along vascular pathways toward the kidney.

Renal Blood Vessels

Sympathetic fibers innervate renal vascular smooth muscle.

Increased sympathetic activity can produce renal vasoconstriction and thereby alter renal blood flow and glomerular filtration.

Renin Secretion

Sympathetic stimulation influences renin secretion from juxtaglomerular cells.

Beta-1 adrenergic receptor activation is an important mechanism through which renal sympathetic activity promotes renin release.

Renal Tubular Function

Renal sympathetic activity can influence tubular sodium reabsorption.

These neural effects operate together with renal hemodynamic and hormonal mechanisms in the regulation of extracellular fluid volume and arterial pressure.

Functional Effects in the Kidney

• Regulation of renal vascular resistance
• Modification of renal blood flow
• Influence on glomerular filtration under strong sympathetic activation
• Stimulation of renin secretion
• Influence on tubular sodium reabsorption

Ureteric Sympathetic Innervation

The ureters receive sympathetic fibers through autonomic plexuses along their retroperitoneal and pelvic course.

These include renal, aortic, superior hypogastric, and inferior hypogastric plexuses.

Upper Ureter

The upper ureter receives autonomic fibers associated with the renal and abdominal aortic plexuses.

Nerve fibers frequently travel alongside the ureteric blood vessels.

Middle Ureter

As the ureter descends through the abdomen, it receives contributions from autonomic networks associated with the abdominal aorta.

These fibers form part of the extrinsic neural supply accompanying the ureter toward the pelvis.

Pelvic Ureter

The distal ureter receives autonomic fibers through the superior and inferior hypogastric plexuses.

These pelvic plexuses contain mixtures of sympathetic, parasympathetic, and visceral afferent fibers.

Ureteric Peristalsis

Urine is propelled through the ureters by coordinated contractions of ureteric smooth muscle.

This peristaltic activity is primarily generated intrinsically and can continue without intact extrinsic autonomic innervation, although autonomic nerves can modulate ureteric activity.

Visceral Pain from the Ureter

Visceral afferent fibers conveying pain from the ureter generally travel centrally alongside sympathetic pathways.

Distension or powerful contraction of the ureter, as may occur with an obstructing calculus, can produce severe visceral pain.

Referred Ureteric Pain

Ureteric pain can be referred to somatic regions corresponding to spinal cord segments receiving the visceral afferent input.

The perceived distribution may extend from the flank toward the lower abdomen, groin, and external genital region as an obstructing calculus moves distally.

Urinary Bladder

The urinary bladder receives sympathetic fibers through the hypogastric and pelvic autonomic plexuses.

Sympathetic activity is particularly important during the period when the bladder is filling and urine must be stored.

Sympathetic Pathway to the Bladder

Preganglionic sympathetic fibers arise from thoracolumbar spinal segments and travel through sympathetic and splanchnic pathways toward prevertebral and hypogastric plexuses.

Postganglionic fibers ultimately reach the bladder through the inferior hypogastric and vesical plexuses.

Superior Hypogastric Plexus

The superior hypogastric plexus is an autonomic network located anterior to the lower abdominal aorta and sacral promontory region.

It contains predominantly sympathetic fibers descending toward the pelvis.

Hypogastric Nerves

The right and left hypogastric nerves descend from the superior hypogastric plexus into the pelvis.

They convey sympathetic fibers to the inferior hypogastric plexuses.

Inferior Hypogastric Plexus

The paired inferior hypogastric plexuses lie along the lateral aspects of the pelvic viscera.

They contain sympathetic fibers arriving through hypogastric and sacral sympathetic pathways as well as parasympathetic fibers from the pelvic splanchnic nerves.

Vesical Plexus

Autonomic fibers supplying the bladder are distributed from the inferior hypogastric plexus through vesical plexuses associated with the bladder wall and its blood vessels.

These networks distribute sympathetic and parasympathetic fibers to smooth muscle and other target tissues.

Sympathetic Role in Urine Storage

During bladder filling, sympathetic pathways help maintain conditions favorable for urinary storage.

They contribute to relaxation of the detrusor and increased resistance at the proximal urinary outlet.

Detrusor Muscle

The detrusor muscle is the smooth muscle forming the principal muscular layer of the bladder wall.

Sympathetic signaling promotes detrusor relaxation during the storage phase, allowing bladder volume to increase without triggering immediate voiding.

Beta-Adrenergic Receptors

Beta-adrenergic signaling contributes to relaxation of bladder smooth muscle.

The beta-3 adrenergic receptor is particularly important in human detrusor relaxation and is a pharmacological target in the treatment of some bladder storage disorders.

Bladder Neck

Sympathetic activity can increase smooth muscle tone in the region of the bladder neck and proximal urethra.

This contributes to increased outlet resistance during urinary storage.

Alpha-Adrenergic Receptors

Alpha-adrenergic receptors, particularly alpha-1 receptors, contribute to contraction of smooth muscle in the bladder outlet and proximal urethral region.

This effect complements detrusor relaxation during the storage phase.

Internal Urethral Sphincter

The term internal urethral sphincter is commonly applied to smooth muscle at the bladder neck and proximal urethra.

Sympathetic stimulation increases smooth muscle tone in this region, although the anatomical organization and functional importance of the outlet differ between males and females.

Male Bladder Neck

In males, sympathetic contraction of smooth muscle at the bladder neck has additional importance during ejaculation.

Closure of the bladder neck helps limit retrograde passage of seminal fluid into the urinary bladder.

External Urethral Sphincter

The external urethral sphincter is skeletal muscle and is not directly controlled by sympathetic motor fibers.

It receives somatic motor innervation principally through the pudendal nerve.

Storage Phase

During the storage phase, the nervous system coordinates bladder relaxation with maintenance of urethral outlet resistance.

Sympathetic activity contributes to both components while parasympathetic detrusor activity remains relatively suppressed.

Voiding Phase

During normal micturition, sympathetic storage pathways are reduced while parasympathetic activity to the bladder increases.

The detrusor contracts and urethral outlet resistance decreases, allowing urine to pass through the urethra.

Storage and Voiding

Feature	Storage Phase	Voiding Phase
Sympathetic activity	Relatively increased	Reduced
Detrusor	Relaxed	Contracts primarily through parasympathetic activity
Bladder outlet	Resistance maintained	Resistance decreases
Urine flow	Prevented	Facilitated

Sympathetic and Parasympathetic Comparison

Feature	Sympathetic	Parasympathetic
General origin	Thoracolumbar spinal cord	Sacral spinal cord S2-S4
Major pelvic pathway	Hypogastric pathways	Pelvic splanchnic nerves
Detrusor effect	Promotes relaxation during storage	Promotes contraction during voiding
Bladder outlet effect	Promotes smooth muscle tone	Coordinates with outlet relaxation during voiding
Primary bladder role	Storage	Emptying

Somatic Innervation

Somatic motor control complements autonomic control of the urinary bladder.

The pudendal nerve supplies the external urethral sphincter and allows voluntary maintenance and release of continence.

Integrated Continence Mechanism

Urinary continence depends on coordinated autonomic and somatic activity rather than on a single sphincter or nerve.

During filling, detrusor relaxation, smooth muscle outlet tone, and contraction of the external urethral sphincter work together to prevent urine loss.

Visceral Afferent Fibers

Visceral sensory fibers travel alongside autonomic pathways but are functionally distinct from sympathetic motor fibers.

They transmit information concerning distension, chemical conditions, and pain from urinary organs toward the spinal cord.

Pain Pathways

Pain afferents from the kidneys and much of the ureters generally accompany sympathetic fibers toward thoracolumbar spinal levels.

This arrangement contributes to characteristic patterns of referred pain associated with renal and ureteric disease.

Renal Pain

Distension of the renal capsule or collecting system can activate visceral sensory pathways.

Renal pain is commonly perceived in the flank and may be referred according to the spinal segments receiving the afferent input.

Autonomic Plexuses of the Urinary Tract

Plexus	Major Urinary Relationship
Renal plexus	Kidney and proximal ureter
Abdominal aortic plexus	Ureteric autonomic pathways
Superior hypogastric plexus	Descending pelvic sympathetic pathways
Inferior hypogastric plexus	Bladder, distal ureter, urethral and pelvic visceral innervation
Vesical plexus	Direct autonomic distribution to urinary bladder

Clinical Pharmacology

Adrenergic receptors within the lower urinary tract are important pharmacological targets.

Manipulation of sympathetic signaling can alter detrusor relaxation and urethral outlet resistance.

Beta-3 Adrenergic Agonists

Activation of beta-3 adrenergic receptors promotes relaxation of the detrusor during bladder filling.

This mechanism is used therapeutically in some patients with symptoms related to excessive bladder activity.

Alpha-1 Adrenergic Antagonists

Alpha-1 adrenergic antagonists reduce smooth muscle tone in portions of the bladder outlet and urethral pathway.

They are particularly relevant where increased smooth muscle outlet resistance contributes to difficulty with urinary flow.

Pelvic Surgery

Sympathetic fibers traveling through the superior and inferior hypogastric plexuses may be affected during extensive pelvic or retroperitoneal surgery.

Damage to these pathways can contribute to disturbances of urinary, sexual, and other pelvic autonomic functions.

Retroperitoneal Surgery

The renal and aortic autonomic plexuses lie close to major vessels and retroperitoneal organs.

Operations in these regions can therefore encounter autonomic pathways associated with the kidneys and ureters.

Neurogenic Bladder

Neurogenic bladder can result from lesions affecting autonomic, somatic, spinal, or supraspinal pathways controlling the lower urinary tract.

The clinical pattern depends on which components of storage and voiding circuitry are disrupted.

Spinal Cord Injury

Spinal cord injury can disturb the normal balance between sympathetic storage pathways, parasympathetic voiding pathways, somatic sphincter control, and higher regulatory centers.

The resulting bladder dysfunction varies according to the neurological level and completeness of injury.

Sympathetic Pathway Summary

1. Preganglionic sympathetic neurons originate in the thoracolumbar spinal cord.
2. Axons leave through anterior spinal roots.
3. Fibers pass through spinal nerves and white rami communicantes.
4. They enter the sympathetic trunks.
5. Visceral fibers can continue through splanchnic nerves.
6. Fibers reach prevertebral and autonomic plexuses.
7. Postganglionic fibers follow vascular and plexus pathways toward urinary organs.
8. Renal fibers enter through the renal plexus.
9. Pelvic fibers descend through hypogastric pathways.
10. Bladder fibers are distributed through the inferior hypogastric and vesical plexuses.

Key Anatomical Features

• Sympathetic urinary innervation originates from the thoracolumbar spinal cord.
• Preganglionic neurons are located in the intermediolateral cell column.
• Sympathetic fibers reach abdominal urinary organs through splanchnic and prevertebral pathways.
• The renal plexus supplies the kidneys and contributes fibers to the upper ureter.
• Renal sympathetic activity influences vascular tone, renin secretion, and tubular sodium handling.
• The ureters receive sympathetic fibers from several autonomic plexuses along their course.
• Ureteric peristalsis is primarily intrinsic but can be modulated by autonomic nerves.
• The superior hypogastric plexus provides a major pathway for sympathetic fibers descending into the pelvis.
• Hypogastric nerves connect the superior and inferior hypogastric plexuses.
• The inferior hypogastric plexus distributes autonomic fibers to the bladder and other pelvic viscera.
• Sympathetic activity supports urinary storage.
• Beta-adrenergic signaling promotes detrusor relaxation.
• Beta-3 receptors have an important role in human detrusor relaxation.
• Alpha-1 adrenergic signaling promotes smooth muscle tone in the bladder outlet.
• The external urethral sphincter is controlled by somatic rather than sympathetic motor fibers.

Clinical Importance

Sympathetic innervation of the urinary tract has important functions from the kidney to the urinary bladder. Renal sympathetic fibers regulate vascular resistance, influence renin release, and modify tubular sodium handling. Sympathetic fibers accompanying the ureters participate in autonomic modulation and provide pathways alongside which many visceral pain afferents travel.

In the lower urinary tract, sympathetic activity is particularly important during urinary storage. Beta-adrenergic signaling promotes detrusor relaxation, while alpha-adrenergic signaling contributes to smooth muscle tone at the bladder outlet. During micturition, these storage mechanisms are reduced while parasympathetic detrusor activity increases and outlet resistance falls.

Understanding sympathetic innervation of the urinary tract provides the anatomical foundation for studying renal autonomic innervation, the renal plexus, ureteric pain pathways, superior and inferior hypogastric plexuses, bladder storage, adrenergic receptors, urinary continence, neurogenic bladder, and autonomic complications of retroperitoneal or pelvic surgery.