Fiber Composition and Autonomic Reflex Arc
The pelvic nerve contains a significant number of sensory fibers. Unlike the vagus nerve, where sensory fibers account for up to 90%, afferent fibers make up approximately 50% of the pelvic nerve and sympathetic trunks.
In the afferent (sensory) limb of the autonomic reflex arc, the impulse pathway proceeds as follows: receptor → afferent fiber → cells of spinal or peripheral ganglia. Notably, this afferent limb can be shared by both autonomic and somatic reflexes.
The second (intercalated/association) limb of the arc is represented by preganglionic neurons. The parasympathetic centers associated with the pelvic nerve are located in the sacral region — specifically in the sacral spinal cord segments at the level of $S_2-S_4$, inferior to the $S_1$ level. The axons of these neurons emerge from the spinal cord via ventral roots and travel to parasympathetic ganglia within the target organs to synapse on efferent cells.
Contribution to Autonomic Plexuses
The fibers of the pelvic splanchnic nerves (nn. splanchnici pelvici) contribute to the formation of major autonomic plexuses in the lesser pelvis.
The primary plexus is the inferior hypogastric plexus, or pelvic plexus (plexus hypogastricus inferior, s. plexus pelvicus). It is formed by the combination of parasympathetic fibers from the pelvic splanchnic nerves and sympathetic fibers from the lumbar and sacral splanchnic nerves.
Branches of the pelvic nerve project directly to the inferior hypogastric plexus. Major pelvic plexuses include:
- Uterovaginal plexus (plexus uterovaginalis) — in females, located in the parametrial tissue lateral and posterior to the uterus, at the level of the internal os and cervical canal.
- Prostatic plexus (plexus prostaticus) — in males.
- Vesical plexus (plexus vesicalis).
- Rectal plexus (plexus rectalis).
Innervated Organs
Through the pelvic plexus system, parasympathetic fibers of the pelvic nerve innervate the pelvic viscera. Target organs include:
- Large intestine — the distal third.
- Rectum.
- Urinary bladder.
- Genitalia — in females, sympathetic and parasympathetic fibers originating from the uterovaginal plexus innervate the vagina, uterus, and the internal parts of the uterine tubes.
Note: The ovaries receive their innervation from the ovarian plexus, while the external genitalia and pelvic floor are supplied primarily by the pudendal nerve (n. pudendus).
Role of the Pelvic Nerve in Micturition
The pelvic nerve plays a critical role during the emptying phase (micturition). This process is coordinated within the sacral spinal cord, which houses the spinal micturition center.
Urinary expulsion involves two parallel processes:
- Parasympathetic activation: Excitation of center neurons is transmitted via the pelvic nerve (n. pelvicus). This causes contraction of the urinary bladder wall musculature (detrusor muscle) and increases intravesical pressure. Simultaneously, it reflexively relaxes the internal urethral sphincter.
- Somatic inhibition: Concurrently, an inhibitory interneuron is activated in the spinal cord, suppressing the motor neurons of the pudendal nerve (n. pudendus). Consequently, firing along the pudendal nerve ceases, leading to relaxation of the external urethral sphincter.
The net result is sphincter relaxation coupled with detrusor contraction, allowing urine to pass freely through the urethra.