Topography and Relation to the Spinal Cord
During human ontogeny, the spinal cord grows more slowly than the vertebral column. Consequently, it is significantly shorter than the spine and terminates at the level of the upper lumbar vertebrae. The sacral region of the spinal cord consists of 5 segments (S1–S5) and lies superior to the corresponding sacral vertebrae.
To exit through their respective intervertebral foramina, inferior nerve roots must descend a considerable distance within the vertebral canal. This collection of nerve roots surrounding the filum terminale forms the structure known as the cauda equina.
In the sacral spinal cord, descending pathways travel in the posterior funiculus, where they lie adjacent to the medial surface of the fasciculus gracilis.
Connection with the Autonomic Nervous System
- Sympathetic division: The sacral part of the sympathetic trunk lies on the pelvic surface of the sacrum, medial to the pelvic sacral foramina. It consists of 3–4 sacral ganglia (ganglia sacralia). The right and left trunks converge to form the ganglion impar. Sacral splanchnic nerves (nn. splanchnici sacrales) arise from this region and travel to the pelvic plexuses.
- Parasympathetic division (craniosacral system): The centers are located in the gray matter of the lateral horns of the spinal cord at levels S2–S4 (nuclei parasympathici sacrales). Preganglionic fibers are long and emerge as the pelvic splanchnic nerves (nn. splanchnici pelvini, or nn. erigentes). They reach parasympathetic ganglia located intramurally within the walls of organs or close to target organs: the descending colon, rectum, urinary bladder, and reproductive organs. Postganglionic fibers are short.
Clinical and Obstetric Significance
- In obstetrics: During vaginal examinations in physiological labor, the sacrum and individual sacral vertebrae serve as key anatomical landmarks. The IV and V sacral vertebrae are palpable when the plane of the maximum circumference of the fetal head coincides with the plane of the wide part of the pelvic cavity. The sacral concavity is assessed when the fetal head is at the pelvic inlet (either engaged by its small or large diameter).
- In anesthesiology: During lumbar spinal anesthesia, impulse conduction is blocked in sensory fibers entering the lumbosacral region. This produces anesthesia of the lower extremities, lower trunk, and pelvic viscera. This technique is used for surgical procedures involving the pelvic organs and lower limbs.