Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Spinal cord
White matter

The WHITE MATTER OF the Spinal Cord is divided into three paired funiculi (columns). The anterior funiculus is located between the anterior median fissure and the exit of the ventral roots, the posterior funiculus is between the glial septum and the dorsal roots, and the lateral funiculus is between the anterolateral and posterolateral sulci.

The white matter of the spinal cord is composed of Cytology/practical/65.html">Myelinated nerve fibers—the axons of Neurons located either in the spinal ganglia or, for the most part, in the Gray matter of the spinal cord. Bundles of nerve fibers directly adjacent to the gray matter form the segmental apparatus of the spinal cord. They belong to phylogenetically older fibers and connect adjacent segments of the spinal cord without extending beyond it. These bundles include the anterior, lateral, and posterior proprietary bundles (propriospinal tracts) (Atl. Fig. 98, 99). For example, they can connect the centers of the lower limb with those of the upper limb. Originating from the Cells of the Reticular Formation and interneurons, the fibers travel up and down 2–3 segments and terminate on the motor neurons of the anterior horns. The primary function of these pathways is to mediate innate Reflexes.

Fibers from the spinal ganglia that enter the spinal cord via the dorsal roots continue along various pathways. Some of these fibers terminate on the motor neurons of the anterior horn of their own segment, on interneurons of the posterior horns on the ipsilateral or contralateral side, on neurons of the lateral horns (Autonomic Nervous system), and on cells of the reticular formation. As a result, the simplest (unconditioned) reflexes are executed at the spinal cord level in response to stimulation of the Skin and Muscles of all body segments and Internal Organs. Other fibers ascend within the posterior funiculi; they belong to the ascending PATHWAYS OF THE spinal cord (Atl. Fig. 99).

The tracts of the spinal cord are located peripheral to its intrinsic bundles. They are formed by the axons of spinal cord interneurons or sensory neurons of the spinal ganglia. In phylogeny, these pathways appear later than the Brain's intrinsic apparatus and develop in parallel with The formation of the brain. Impulses travel along these pathways in an ascending direction from sensory neurons and interneurons to the brain, and in a descending direction from the cells of higher nervous centers to the motor neurons of the spinal cord.

The ascending pathways of the spinal cord include the gracile and cuneate fasciculi, the dorsal and ventral spinocerebellar tracts, the lateral and ventral spinothalamic tracts, and other pathways (Atl. Fig. 99).

The gracile (fasciculus gracilis) and cuneate (f. cuneatus) fasciculi run in the posterior funiculus and are formed by the axons of sensory neurons from the spinal ganglia. These bundles conduct impulses to the Medulla Oblongata from proprioceptors in muscles and joints, as well as from exteroceptors in the skin. The gracile fasciculus conducts impulses from receptors of the lower limbs and the lower half of the body (up to the fifth thoracic (T5) segment); the cuneate fasciculus conducts impulses from the upper limbs and the upper half of the body, which is why it is absent below the T5 segment.

The posterior spinocerebellar tract (tractus spinocerebellaris dorsalis (posterior)) lies in the lateral funiculi. It originates from the cells of the dorsal Nucleus, which is located at the Base of the posterior horn on the ipsilateral side.

The anterior spinocerebellar tract (tractus spinocerebellaris ventralis (anterior)) lies in the lateral funiculi and consists of the axons of interneurons from the posterior horns (laminae V–VI of the gray matter). After decussating at the midline of the spinal cord, the fibers enter the lateral funiculi of the contralateral side.

Both pathways conduct proprioceptive impulses to the Cerebellum.

The lateral spinothalamic tract (tractus spinothalamicus lateralis) is also located in the lateral funiculi and consists of crossed fibers of interneurons from the base of the posterior horn (laminae IV, VI). This pathway transmits pain and Temperature Sensation impulses to the Diencephalon.

The anterior spinothalamic tract (tractus spinothalamicus ventralis (anterior)) runs in the anterior funiculus and conducts tactile sensation impulses.

The decussation of ascending pathways, typically performed by the fibers of interneurons at the level of their own or an adjacent segment, results in impulses reaching the cerebral hemisphere contralateral to the side of the body where the excitation originated.

Descending pathways are represented by fibers traveling from various PARTS OF THE brain to the nuclei of the spinal cord. These include the rubrospinal, lateral and anterior corticospinal, tectospinal, vestibulospinal tracts, the medial longitudinal fasciculus, and others (Atl. Fig. 99).

The rubrospinal tract* (tractus rubrospinalis) begins in the Midbrain (from neurons of the red nucleus), descends through the contralateral lateral funiculus of the spinal cord, and terminates on the motor neurons of the anterior horns. It conducts impulses that control Skeletal Muscle tone and involuntary (automatic) movements.

* Here and below, italics indicate the names of pathways and structures According to the International Anatomical Nomenclature; bold italics indicate the names commonly accepted in modern morphological and physiological literature.

The lateral corticospinal tract (pyramidal) (tractus corticospinalis (piramidalis) lateralis) lies in the lateral funiculus and consists of the axons of pyramidal cells from the Cerebral Cortex. Its fibers terminate on the contralateral motor neurons, Crossing Over within the anterior commissure of the spinal cord. The tract gradually tapers because in each segment of the spinal cord, some of its fibers terminate on the cells of the anterior horns. The pathway conducts voluntary motor impulses—both excitatory and inhibitory—from the cortex.

The anterior corticospinal tract (pyramidal) (tractus corticospinalis (piramidalis) ventralis (anterior)), like the lateral one, consists of fibers from the cells of the cerebral cortex, but lies in the anterior funiculus. Its fibers terminate on the ipsilateral motor neurons of the spinal cord. This pathway has the same function as the lateral corticospinal tract.

Interestingly, corticospinal tracts terminate directly on spinal motor neurons only in humans and primates, whereas in subprimates, and sometimes even in primates, an interneuron is interposed between them. A functional explanation for this phenomenon has not yet been found.

The tectospinal tract (tractus tectospinalis) also lies in the anterior funiculus, originates from the superior and inferior colliculi of the tectum (midbrain roof), and terminates on the cells of the contralateral anterior horns of the spinal cord.

The vestibulospinal tract (tractus vestibulospinalis) lies between the anterior and lateral funiculi. It runs from the medulla oblongata to the anterior horns and conducts impulses that maintain body balance.

The medial longitudinal fasciculus lies in the anterior funiculus and consists mainly of descending fibers; it originates in the Brainstem nuclei and terminates on the cells of the anterior horns. This bundle represents a very ancient fiber system that serves as the most important association pathway of the brain in lower vertebrates. It also includes fibers ascending to the brainstem.

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Fig. 3.6. Successive stages of neural tube development:

1 — neural plate; 2 — endoderm; 3 — mesoderm; 4 — coelom; 5 — neural groove; 6 — neural tube; 7 — neural crest; 8 — somite; 9 — notochord

The reticulospinal tract (tractus reticulospinalis) lies in the anterior funiculus and contains fibers descending from the reticular Formation of the brainstem to the motor neurons of the spinal cord.

Most descending and ascending pathways decussate at different levels of the Central Nervous System. As a result, the impulse undergoes two decussations along its entire path (in the ascending and descending directions) and returns to the side where the stimulus is applied.