Chapter 2. A Summary of the Anatomy of the Nervous System

The spinal cord (medulla spinalis) is located in the spinal canal. At the level of the cervical vertebra I (C₁) and the occipital bone, the spinal cord turns into medulla (medulla oblongata) and descends to the level of the lumbar vertebrae I–II (L_1-2) (Fig. 2.1). Along the length of the spinal cord, there are two enlargements, cervical and lumbosacral.

The spinal cord consists of 31–32 segments:

• 8 cervical (С₁–С₄);
• 12 thoracic (Th₁-Th₁₂);
• 5 lumbar (L₁–L₅);
• 5 sacral (S₁–S₅);
• 1–2 rudimentary coccygeal (Co₁–Co₂).

The cervical enlargement corresponds to the segments С₅–Тh₁ and provides innervation of the upper extremities, while lumbosacral enlargement corresponds to segments L₁–S_1–2 and innervates the lower extremities. Below the lumbar enlargement, the spinal cord thins, forming a conus medullaris, which ends with a terminal filum reaching the coccygeal vertebrae.

Segment refers to a section of the spinal cord corresponding to one pair of spinal roots (ventral and dorsal). While the ventral root of the spinal nerve contains motor fibers, the dorsal spine contains sensory fibers. Connecting in the area of the intervertebral node, they form a mixed spinal nerve.

The spinal cord is slightly shorter than the spinal canal. As result, in the upper parts of the spinal cord, spinal roots pass horizontally. Starting from the thoracic region, they descend slightly downwards before leaving the corresponding intervertebral foramina. In the lower sections, the roots go straight down, forming a cauda equina. On the surface of the spinal cord, the anterior median fissure, posterior median fissure, and the symmetrically located anterior and posterior lateral fissures are seen. Between the anterior median fissure and the anterolateral sulcus, ventral column is located (funiculus anterior); between the anterior and posterior lateral sulci — lateral column (funiculus lateralis); between the posterior lateral fissure and posterior median sulcus — dorsal column (funiculus posterior) divided by shallow intermediate sulcus in the cervical spinal cord into thin bundle (fasciculus gracilis) adjacent to the posterior median sulcus, and wedge-shaped bundle (fasciculus cuneatus) located outward from thin bundle. The columns (funiculi) contain conducting pathways; ventral roots exit from the anterolateral sulcus, while in the area of the posterior lateral sulcus, posterior roots enter the spinal cord.

Fig. 2.1. The structure of the spinal cord: 1 — vertebral body; 2 — intertransverse ligament; 3 — epidural vein; 4 — costovertebral joint; 5 — arachnoid; 6 — rib; 7 — dentate ligament; 8 — ventral root; 9 — spinal nerve; 10 — dorsal branch (skin and muscles); 11 — epidural space; 12 — pia mater; 13 — subarachnoid space; 14 — spinal ganglion; 15 — radicular sleeve; 16 — intervertebral hole; 17 — meningeal branch; 18 — posterior longitudinal ligament; 19 — dura; 20 — intervertebral disc, fibrous ring; 21 — atlas; 22 — sacrum; 23 — coccyx; A — location of the spinal cord in the spinal canal; B — segments of the spinal cord and spinal nerves; C — cervical spinal nerves (C₁–C₈, purple); Th — thoracic spinal nerves (Th₁–Th₁₂, pink); L — lumbar spinal nerves (L₁–L₅, blue); S — sacral spinal nerves (S₁–S₅, yellow); Co — coccygeal spinal nerves (Со_1–2, grey)

On a cross-section (Fig. 2.2), there is gray matter located in the central region of the spinal cord and white matter lying on its periphery in the spinal cord. The gray matter on the cross-section resembles a butterfly with open wings or the letter “H”. In the gray matter of the spinal cord, more massive, wide and short anterior horns and thinner, elongated posterior horns are distinguished. In the thoracic regions, a lateral horn is detected, which is less pronounced in the lumbar and cervical spinal cord. The right and left halves of the spinal cord are symmetrical and connected by commissures consisted of gray and white matter. Anteriorly to the central canal, there is an anterior gray commissure (commissura grisea anterior), then an anterior white commissure (commissura alba anterior); posteriorly to the central canal, there is a posterior gray commissure (comissura grisea posterior) followed by a posterior white commissure (commissura alba posterior).

Fig. 2.2. A cross-section of the spinal cord (scheme): 1 — anterior median fissure; 2 — posterior horn: а — apex; b — head; c — neck; 3 — gelatinous substance; 4 — posterior column; 5 — posterior median sulcus; 6 — fasciculus gracilis; 7 — fasciculus cuneatus; 8 — posterior median septum; 9 — lateral column; 10 — central canal; 11 — anterior horn; 12 — ventral column

Large motor neurons are located in the anterior horns of the spinal cord. Their axons go to the anterior roots and innervate the striated muscles of the neck, trunk and limbs. The primary sensory cells are located in the spinal (intervertebral) ganglions, which are the thickenings of the dorsal roots.