General Characteristics and Matter Distribution
The spinal cord (medulla spinalis) is a long cord-like structure, ranging from 1.0 to 1.5 cm in width. The central canal (canalis centralis) runs through the center of the organ along its entire length.
A crucial anatomical feature is the uneven distribution of nervous tissue across different levels. The ratio of gray matter to white matter strictly depends on the region:
- Cervical region: White matter predominates.
- Thoracic region: The amount of white matter is maximal, 10–12 times greater than gray matter.
- Lumbar region: Gray matter becomes more abundant relative to white matter.
- Medullary cone region: Cross-sections show almost exclusively gray matter.
Gross Structure: Sulci and Fissures
The surface of the spinal cord is uneven, marked by longitudinal depressions that divide the organ into symmetrical halves.
On the anterior surface lies the deepest anterior median fissure (fissura mediana ventralis, or anterior). The intermediate cervical septum (septum cervicale intermedium) projects into this fissure.
On the posterior surface runs the posterior median sulcus (sulcus medianus dorsalis, or posterior). The posterior median septum (septum medianum dorsale) projects into this sulcus.
Each half of the spinal cord features lateral sulci:
- Anterolateral sulcus (sulcus ventrolateralis, or anterolateralis).
- Posterolateral sulcus (sulcus dorsolateralis, or posterolateralis).
Additionally, exclusively in the cervical and upper thoracic regions, an extra posterior intermediate sulcus (sulcus intermedius dorsalis, or posterior) is visible.
Segmental Structure and Enlargements
The organ has a metameric structure. A spinal cord segment (segmentum medullae spinalis) is the region that gives rise to exactly one pair of spinal nerves.
There are 31 segments in total, classified by regions:
- 8 cervical (segmenta cervicalia);
- 12 thoracic (segmenta thoracica);
- 5 lumbar (segmenta lumbalia);
- 5 sacral (segmenta sacralia);
- 1–3 coccygeal (segmenta coccygea).
The diameter of the spinal cord is not uniform throughout its length. It forms two major enlargements associated with limb innervation. The cervical enlargement (intumescentia cervicalis) corresponds to the upper limb innervation area. The lumbosacral enlargement (intumescentia lumbosacralis) functionally corresponds to the lower limb innervation zone.
Spinal Roots and Nerves
Each pair of spinal nerves (nervus spinalis) strictly corresponds to its segment and is mixed in function. The nerve is formed by the joining of two roots:
- Anterior root (radix ventralis, or anterior, motoria). It is formed by anterior rootlets (fila radicularia).
- Posterior root (radix dorsalis, or posterior, sensoria). This root contains afferent (centripetal) fibers.
The sensory spinal ganglion (ganglion spinale, or sensorium) is located on the posterior root. Pseudounipolar neurons reside within this ganglion. The peripheral process of such a neuron extends to receptors in the periphery, while the central process enters the posterior root directly.