Topography and relation to the spinal cord
During ontogenesis, the human spinal cord grows slower than the vertebral column. As a result, it is significantly shorter than the spine and ends at the level of the upper lumbar vertebrae. The sacral spinal cord comprises 5 segments (S1–S5) and is located superior to the corresponding sacral vertebrae.
To exit through their intervertebral foramina, the inferior nerve roots must descend a considerable distance within the vertebral canal. This cluster of roots surrounding the terminal filum (filum terminale) forms a structure known as the cauda equina (cauda equina).
In the sacral spinal cord, descending branches of the pathways run in the posterior funiculus, where they adjoin the medial surface of the gracile fasciculus.
Connection with the autonomic nervous system
- Sympathetic part: The sacral part of the sympathetic trunk lies on the pelvic surface of the sacrum, medially to the sacral foramina. It consists of 3–4 sacral ganglia (ganglia sacralia). The right and left trunks join to form the ganglion impar (ganglion impar). Sacral splanchnic nerves (nn. splanchnici sacrales) arise from this part and head toward the lesser pelvis plexuses.
- Parasympathetic part (craniosacral system): The centers are located in the gray matter, in the lateral horns of the spinal cord, at the level of segments S2–S4 (nuclei parasympathici sacrales). Preganglionic fibers are long and emerge as part of the pelvic splanchnic nerves — nn. splanchnici pelvini (or nn. erigentes). They reach parasympathetic ganglia located intramurally, in the walls of organs, or near target organs: the descending colon, rectum, urinary bladder, and genital organs. Postganglionic fibers are short.
Clinical and obstetric significance
- In obstetrics: During vaginal examination in physiological labor, the sacrum and individual sacral vertebrae serve as landmarks. The IV and V sacral vertebrae are determined when the plane of the largest diameter of the fetal head coincides with the plane of the wide part of the pelvic cavity. The sacral concavity is determined when the head is at the inlet of the lesser pelvis with its small or large diameter.
- In anesthesiology: During spinal anesthesia at the lumbar level, impulse conduction along sensory fibers entering the lumbosacral region is blocked. This causes anesthesia of the lower extremities, lower trunk, and internal organs located there. The method is used for surgical interventions on the pelvic organs and lower extremities.
Class and exam questions
Where is the sacral part of the sympathetic trunk located?
It is located on the pelvic surface of the sacrum. Its ganglia lie medially to the sacral foramina, and the right and left trunks merge into the ganglion impar (ganglion impar).
Which organs are innervated by the pelvic splanchnic nerves?
Target organs of the pelvic splanchnic nerves include the descending colon, rectum, urinary bladder, and genital organs.
Why do the levels of the sacral spinal cord segments not coincide with the sacral vertebrae?
During development, the spinal cord grows slower than the spine and terminates at the level of the upper lumbar vertebrae. Therefore, the inferior nerve roots have to descend down the vertebral canal; their cluster around the terminal filum forms the cauda equina.
What else to study on this topic
- Sacrum (os sacrum)
- Autonomic nervous system of the pelvis
- Spinal cord: segmental structure
- Physiological labor: obstetric examination
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