Pelvic nerve

Fiber composition and autonomic reflex arc

The pelvic nerve contains a significant number of sensory fibers. Unlike the vagus nerve, where their share reaches 90%, in the pelvic nerve and sympathetic trunks, afferent fibers account for about 50%.

The impulse pathway in the afferent (sensory) link of the autonomic reflex arc follows this scheme: receptor → afferent fiber → cells of spinal or peripheral ganglia. Notably, this afferent link can be shared by both autonomic and somatic reflexes.

The second (associative) link of the arc is represented by preganglionic neurons. The parasympathetic centers associated with the pelvic nerve are located in the sacral region — in the sacral segments of the spinal cord, at the $S_2-S_4$ level, below the $S_1$ level. The processes of these neurons emerge from the spinal cord as part of the ventral roots and proceed to parasympathetic ganglia within the organs for synaptic relay to efferent cells.

Which plexuses does it contribute to?

The fibers of the pelvic splanchnic nerves (nn. splanchnici pelvici) participate in the formation of major autonomic plexuses of the lesser pelvis.

The main one is the inferior hypogastric plexus, or pelvic plexus (plexus hypogastricus inferior, s. plexus pelvicus). It is formed by the combined contribution of parasympathetic fibers from the pelvic splanchnic nerves and sympathetic fibers from the lumbar and sacral splanchnic nerves.

Branches of the pelvic nerve approach the inferior hypogastric plexus. The main plexuses of the lesser pelvis are:

• Uterovaginal plexus (plexus uterovaginalis) — in females, it is 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).

Which organs does it innervate?

Through the system of pelvic plexuses, the parasympathetic fibers of the pelvic nerve participate in the innervation of pelvic organs. 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 internal parts of the fallopian tubes.

Note: The ovaries are innervated by the ovarian plexus, while the external genitalia and pelvic floor are innervated primarily by the pudendal nerve (n. pudendus).

The role of the pelvic nerve in urination

The pelvic nerve plays a critical role in the voiding (micturition) phase. This process is coordinated in the sacral spinal cord, where the spinal micturition center is located.

The mechanism of urine expulsion involves two parallel processes:

1. Parasympathetic activation: Excitation of the center's neurons is transmitted via the pelvic nerve (n. pelvicus). This leads to contraction of the bladder wall muscle (detrusor) and an increase in intravesical pressure. Simultaneously, the internal urethral sphincter relaxes reflexively.
2. Somatic inactivation: Concurrently, an inhibitory neuron is activated in the spinal cord, suppressing the activity of motor neurons in the pudendal nerve (n. pudendus). As a result, impulses along the pudendal nerve cease, causing the relaxation of the external urethral sphincter.

The end result is the relaxation of the sphincters against the background of a contracted bladder, allowing urine to flow freely through the urethra.

Class and exam questions

What else to study on this topic

• Inferior hypogastric plexus (plexus hypogastricus inferior)
• Uterovaginal plexus (plexus uterovaginalis)
• Pudendal nerve (n. pudendus)
• Autonomic reflex arc

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.