Central apparatus of the sacral division
The center of the sacral parasympathetic nervous system is strictly localized within the spinal cord. Autonomic neurons giving rise to parasympathetic pathways lie in the gray matter, specifically in its lateral horns.
Topographically, this center encompasses the sacral segments of the spinal cord, from the second to the fourth (SII–SIV). The clusters of these neuronal cell bodies form the sacral parasympathetic nuclei, designated in anatomical nomenclature as nuclei parasympathici sacrales. It is from here that impulses directed to the target organs of the caudal half of the trunk are generated.
Peripheral pathways and fibers
From the nuclei located in the lateral horns of the SII–SIV segments, preganglionic fibers originate. They leave the central nervous system and travel to the periphery as part of specific nerve trunks — the pelvic splanchnic nerves (nn. splanchnici pelvici). Historically, in the literature, these nerves may also be found under the name nn. erigentes.
The further pathway of the pelvic parasympathetic fibers, running within the nn. splanchnici pelvici, proceeds in two main directions:
- Toward the inferior hypogastric plexus (plexus hypogastricus inferior), where they interact with other autonomic structures.
- Directly to the intramural ganglia embedded in the walls of the pelvic organs themselves. In these ganglia, synapses are formed onto postganglionic fibers.
Target organs and parasympathetic plexuses
The fibers of the sacral parasympathetic center innervate a strictly defined pool of internal organs. Their area of responsibility includes:
- Urinary bladder.
- Distal segments of the large intestine.
- Internal reproductive organs.
It is worth noting separately that this same functional group includes vasodilator fibers innervating the vessels of the corpora cavernosa.
A complex network of autonomic ganglia and nerve cords is formed within the pelvic cavity. The pelvic parasympathetic plexuses are not formed in isolation: both branches of nn. splanchnici pelvici and sympathetic fibers take part in their formation. As a result, a mixed autonomic network is created.
The baseline and largest collector is the inferior hypogastric plexus (plexus hypogastricus inferior). Organ plexuses are formed from it and related structures:
- plexus vesicalis — vesical plexus;
- plexus rectalis — rectal plexus;
- plexus uterovaginalis — uterovaginal plexus (characteristic of the female body);
- plexus prostaticus — prostatic plexus (characteristic of the male body).
To remember the segments of the sacral center (SII-SIV) and their function, you can use the rule: «Segments Two, Three, Four — control the pelvis evermore» (S2-S4 innervate the pelvic organs).
Class and exam questions
In which matter of the spinal cord is the center of the sacral parasympathetic division located?
The center is located in the gray matter of the spinal cord, specifically in its lateral horns at the level of segments SII–SIV.
What are the names of the nerves that contain the preganglionic fibers?
Preganglionic fibers emerge as part of the pelvic splanchnic nerves, which are called nn. splanchnici pelvici (or nn. erigentes) in Latin.
With which fibers do nn. splanchnici pelvici combine when forming the pelvic plexuses?
Pelvic parasympathetic fibers form the pelvic plexuses with the participation of sympathetic fibers.
To which ganglia do the parasympathetic fibers in the organ walls head?
The fibers head toward the intramural ganglia of the pelvic organs, as well as to the inferior hypogastric plexus.
What else to study on this topic
- Morphology of intramural ganglia of the pelvic organs
- Anatomy and connections of the inferior hypogastric plexus (plexus hypogastricus inferior)
- Pathway of vasodilator fibers to the corpora cavernosa
- Organ plexuses: plexus prostaticus and plexus uterovaginalis
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