Sacral Parasympathetic Nerves Control the Bladder Emptying Reflex
The sacral parasympathetic nerves (pelvic nerves) control the bladder emptying reflex, which operates through a spinal cord reflex arc that can be triggered by stimulation near the coccyx.
Neuroanatomical Basis of the Micturition Reflex
The bladder emptying mechanism operates through a bladder-to-bladder contraction reflex with its primary reflex center located in the rostral pontine tegmentum (pontine micturition center), but the fundamental spinal reflex arc involves sacral parasympathetic pathways 1, 2.
Spinal Reflex Components
In the first 2 years of life, voiding occurs in a purely reflex fashion: bladder distention triggers detrusor contraction and sphincter relaxation via a spinal cord reflex arc at the sacral level 3.
The sacral parasympathetic (pelvic) nerves provide excitatory input (both cholinergic and purinergic) to the bladder detrusor muscle during voiding, while simultaneously providing inhibitory input (nitrergic) to the urethra 2.
Sacral spinal interneurons located within the region of parasympathetic preganglionic neurons coordinate the reciprocal activity between bladder contraction and sphincter relaxation 4, 5.
The Coccygeal Stimulation Phenomenon
The ability to trigger bladder emptying through coccygeal or perineal stimulation relates to the sacral reflex pathways:
Perineal electrophysiological reflexes involve sacral nerve pathways, with bulbo-cavernosus reflexes demonstrating sacral latencies that can be altered in conditions affecting these pathways 3.
Segmental sensory information from the perineum reaches sacral spinal segments where parasympathetic preganglionic neurons and sphincter motoneurons are located, allowing for reflex activation 4.
Hierarchical Control System
While the basic reflex is sacral, normal adult micturition involves hierarchical control:
The pontine micturition center (PMC) in the dorsolateral pons serves as the primary micturition center that, when activated, induces bladder contraction and reciprocal urethral relaxation 1, 2.
Efferent pathways from the PMC descend through the lateral funiculus of the spinal cord to promote sacral parasympathetic nucleus activity while inhibiting the thoracolumbar sympathetic nucleus and sacral pudendal nerve nucleus 1.
This spinobulbospinal parasympathetic reflex pathway represents the major component of the micturition switching circuit, with essential connections in the periaqueductal gray and pontine micturition center 6.
Clinical Relevance
Diabetic cystopathy demonstrates impaired detrusor contractions through dysfunction of these parasympathetic pathways, with bulbo-cavernosus reflexes showing altered sacral latencies 3.
Spinal cord injury can disrupt descending control while preserving the basic sacral reflex arc, leading to involuntary reflex voiding 6, 4.
The sacral parasympathetic system remains the final common pathway for bladder emptying regardless of whether voiding is voluntary (brain-initiated) or reflexive (spinal-mediated) 2, 6.