Anatomy of the spinal cord

General characteristics and tissue ratio

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 at different levels. The ratio of gray to white matter strictly depends on the region:

• Cervical region: white matter predominates.
• Thoracic region: the amount of white matter is maximal, exceeding the amount of gray matter by 10–12 times.
• Lumbar region: gray matter becomes more abundant than white matter.
• Conus medullaris region: cross-section reveals almost entirely gray matter.

External structure: sulci and fissures

The surface of the spinal cord is uneven, marked by longitudinal grooves that divide the organ into symmetrical parts.

On the anterior surface lies the deepest anterior median fissure (fissura mediana ventralis, or anterior). The intermediate cervical septum (septum cervicale intermedium) penetrates into this fissure.

The posterior median sulcus (sulcus medianus dorsalis, or posterior) runs along the posterior surface. In turn, the posterior median septum (septum medianum dorsale) penetrates into it.

Lateral sulci can be distinguished on each half of the spinal cord:

• Anterolateral sulcus (sulcus ventrolateralis, or anterolateralis).
• Posterolateral sulcus (sulcus dorsolateralis, or posterolateralis).

Additionally, an accessory posterior intermediate sulcus (sulcus intermedius dorsalis, or posterior) is visible exclusively in the cervical and upper thoracic regions.

Segmental structure and enlargements

The organ has a metameric structure. A spinal cord segment (segmentum medullae spinalis) is the region giving rise to precisely one pair of spinal nerves.

There are 31 segments in total, classified by region:

1. 8 cervical (segmenta cervicalia);
2. 12 thoracic (segmenta thoracica);
3. 5 lumbar (segmenta lumbalia);
4. 5 sacral (segmenta sacralia);
5. 1–3 coccygeal (segmenta coccygea).

The diameter of the spinal cord is not uniform throughout its length. It forms two major enlargements, the presence of which is associated with the innervation of the limbs. The cervical enlargement (intumescentia cervicalis) corresponds to the region of innervation of the upper limbs. The lumbosacral enlargement (intumescentia lumbosacralis) functionally corresponds to the innervation zone of the lower limbs.

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.

On the dorsal root lies the sensory spinal ganglion (ganglion spinale, or sensorium). Pseudounipolar neurons are located inside this ganglion. The peripheral process of such a neuron runs toward receptors in the periphery, while the central process goes directly into the dorsal root.

Class and exam questions

What else to study on this topic

• Topography of spinal sulci and fissures
• Differences between anterior and posterior roots
• Structure of the spinal ganglion and pseudounipolar neurons
• Dynamics of gray and white matter ratios across segments

Other topics in the "CNS" section

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