Human Anatomy, Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovskyi 2008

Special Section
Nervous system, systema nervosum - Special Section
Spinal Cord - Internal Structure

Class="center">Gray matter

In a cross-section of the Spinal Cord, the gray matter, substantia grisea, occupies the central part and resembles a butterfly with outspread wings (lower segment) (Fig. 126) or the letter H (upper segment). The gray matter is composed of nerve Cell processes and their perikarya, which form clusters of nuclei cytoarchitectonically organized into spinal cord laminae or Rexed laminae, laminae spinales (laminae Rexedi I-X), (Fig. 127). The ten laminae of the spinal cord gray matter, laminae spinales (or Rexed laminae), are arranged horizontally along the entire length of the spinal cord. The Topography of the nuclei corresponds to the topography of the laminae, although they do not always coincide. In the center of the gray matter lies a narrow cavity—the central canal, canalis centralis, which varies in size and shape at different levels of the spinal cord. The central canal extends through the entire spinal cord, continuing superiorly into the cavity of the Fourth ventricle. Inferiorly, in the region of the conus medullaris, it widens to an average of 1 mm; this region of the central canal is called the terminal ventricle, ventriculus terminalis. With age, the central canal narrows and in some places becomes completely obliterated, so that after 40 years of age, in 90% of cases, it ceases to be a continuous canal (V. L. Kurkovsky).

Fig. 126. Spinal cord, medulla spinalis (horizontal section of the lumbar region).

Carmine staining

In cross-section, the gray matter of the spinal cord consists of projections of two columns, the anterior and posterior columns, columnae griseae anteriores et posteriores, located in each half of the spinal cord (Fig. 126). The columns are connected to each other by the intermediate Column, columna intermedia (spinal lamina VII), in which the following are distinguished: the central intermediate substance, substantia intermedia centralis, located around the central canal, and the lateral intermediate substance, substantia intermedia lateralis, which wedges between the anterior and posterior horns. The right and left halves of the gray matter are connected to each other by the gray commissure, commissura grisea, which lies within lamina X.

In a cross-section of the spinal cord, the columns of gray matter are represented as the anterior horns, cornu anterius, posterior horns, cornu posterius, and lateral horns, cornu laterale. The lateral horns, extending from segment C VIII to segment L I–II, are projections of the substantia intermedia lateralis.

Fig. 127. Division of the spinal cord gray matter into nuclear groups and Rexed laminae (diagram)

In the region of the lower cervical and upper thoracic PARTS OF THE spinal cord, in the angle between the lateral horn and the lateral margin of the posterior horn, the gray matter projects into the White matter as a process, forming the reticular formation, formatio reticularis, of the spinal cord, with white matter located within its meshwork.

The anterior horn is significantly thicker but shorter than the posterior horn, corresponding to the Location of laminae VIII–IX. The posterior horn, cornu posterius, includes laminae I–IV, which are well developed in the cervical and lumbar regions. In the posterior horns, the apex is distinguished, which is the narrower part of the dorsal region of the posterior horns. The apex of the posterior horns is bordered by an area called Lissauer's marginal zone (zona Lissaueri)—lamina I. The substantia gelatinosa (of Rolando), lamina II, is separated from the marginal zone by the spongy zone, zona spongiosa.

Fig. 128. Spinal cord, medulla spinalis in cross-section. The right side shows the main pathways, the left side shows the structures of the gray matter (nuclei). (Diagram after R. D. Sinelnikov, Ya. R. Sinelnikov)

Nerve Cells of the gray matter form clusters of various SHAPES AND SIZES—the nuclei of the spinal cord. Located in the posterior horns are the somatosensory nuclei (Fig. 128):

- marginal zone, zona marginalis;

- spongy zone, zona spongiosa;

- substantia gelatinosa (Rolandi);

- scattered cells, cellulae disseminatae;

- proper Nucleus of the posterior horn, nucl. proprius cornus posterioris.

The somatosensory nuclei of the posterior horns are formed by funicular (tract) cells and interneurons. Funicular cells vary in size. Their axons form fiber tracts that terminate in the nuclei of both the spinal cord and the Brain. Interneurons establish connections between the cells of the spinal cord gray matter. The proper nucleus, nucl. proprius, is formed from large funicular cells, but their axons cross over to the lateral funiculus of the opposite side. Nerve Cells of the substantia gelatinosa and interneurons connect the sensory cells of the spinal ganglia with the motor cells of the anterior horns, closing local reflex arcs. The thoracic nucleus, nucl. thoracicus (Clarke's column)*, is located in the medial part of the Base of the posterior horn (lamina VII). It extends along the posterior column of gray matter at the level of segments C VIII–L II; the axons of this nucleus form the posterior spinocerebellar tract (Flechsig's tract).

Located in the anterior horns of the spinal cord gray matter are the somatomotor nuclei (Fig. 128):

- anterolateral nucleus, nucl. anterolateralis;

- anteromedial nucleus, nucl. anteromedialis;

- posterolateral nucleus, nucl. posterolateralis;

- retroposterolateral nucleus, nucl. retroposterolateralis:

- posteromedial nucleus, nucl. posteromedialis^-,

- central nucleus, nucl. centralis;

- Accessory nerve nucleus, nucl. nervi accessorii, located in segments (CI-CV):

- phrenic nerve nucleus, nucl. nervi phrenici, located in segments (CIII-CV).

These nuclei are formed by large radicular cells (N. M. Yakubovych; V. M. Bekhterev; I. Ye. Kefeli) (Fig. 129), whose processes form a significant portion of the anterior ROOT fibers and provide motor innervation to striated Muscles**. Neurons of the anteromedial and posteromedial nuclei innervate the trunk muscles, while the anterolateral and central nuclei are involved in the Innervation of the Muscles of the proximal limb segments. The posterolateral nucleus innervates the muscles of the forearm and lower leg, and the Muscles of the hand and FOOT are innervated by the retroposterolateral nucleus.

In the lateral horns (intermediate zone), neurons of lamina VII form the following nuclei:

- Intermediolateral nucleus, nucleus intermediolateralis, located at the level of segments Th_I-L_II (L_III);

- Intermediomedial nucleus, nucleus intermediomedialis, located at the level of Th_I-L_III lateral to the central canal. The Nucleus receives visceral afferents from the Internal Organs;

- Sacral parasympathetic nuclei, nuclei parasympathici sacrales, and the reticular formation, formatio reticularis.

* Clarke, J.A.L. (1817-1880), a London physician and anatomist.

**These nerve cells (motor neurons) account for approximately 3% (V. S. Gurfinkel, Ya. M. Kots, M. L. Shik, 1965). A distinction is made between large alpha (A) and small alpha (a) motor neurons, and smaller gamma motor neurons. Large alpha motor neurons innervate muscles predominantly composed of white striated fibers. Small alpha motor neurons innervate red striated Muscle fibers. Gamma motor neurons send impulses to proprioceptors.

Fig. 129, 1, 2, 3. Large radicular cells of the anterior horns (after I. Ye. Kefeli)

The lateral horns, along segments CV_III-L_III on each side, form the lateral intermediate column, columna intermediolateralis (autonomica), which contains central sympathetic neurons of oval or spindle shape, ranging in size from 12 to 45 μm. The dendrites of these cells branch within the lateral horns, while their axons enter the anterior roots of the Spinal Nerves.

The sacral parasympathetic nuclei, nuclei parasympathici sacrales, are located in the gray matter of the sacral segments (SII-SIV) of the spinal cord. These nuclei belong to the Parasympathetic division of the Autonomic Nervous system. The axons of their nuclear cells are preganglionic parasympathetic fibers that exit the spinal cord via the anterior roots and participate in the innervation of the pelvic organs.

White matter

The white matter, substantia alba, is located external to the gray matter (Figs. 126, 128). Its bulk consists of myelinated and, partially, Cytology/practical/64.html">Unmyelinated nerve fibers of various lengths and thicknesses, neuroglia, and Blood Vessels surrounded by a small amount of Connective Tissue. The WHITE MATTER OF one half of the spinal cord is connected to the white matter of the opposite half by the white commissure, commissura alba. The nerve fibers of the white matter are grouped into bundles, fasciculi, or tracts, tracti (see Fig. 128).

The sulci of the spinal cord divide the white matter into three symmetrically arranged columns on the right and left sides:

1. The anterior column, funiculus anterior, is bounded by the anterior median fissure and the anterolateral sulcus;

2. The lateral column, funiculus lateralis, is located between the anterolateral and posterolateral sulci;

3. The posterior column, funiculus posterior, is located between the posterolateral and posterior median sulci.

In the upper Regions of the thoracic and cervical parts of the spinal cord, the posterior intermediate sulcus divides the posterior column into two bundles: the gracile fasciculus, fasciculus gracilis, (Goll's tract)* and the cuneate fasciculus, fasciculus cuneatus, (Burdach's tract)**. In the lower regions of the spinal cord, the columns merge on the outer surface of the spinal cord.

Each column forms several bundles of nerve fibers with different functional significance—the Pathways of the spinal cord. These pathways are divided into short and long. The short pathways (associative) belong to the intrinsic apparatus of the spinal cord, which mediates one of its main Functions: the reflex function. The presence of long pathways is associated with another crucial function of the spinal cord: conduction. Long pathways are divided into ascending (sensory) pathways, which conduct nerve impulses to the brain, and descending (motor) pathways, which run from various brain centers to the spinal cord.

The short pathways include the anterior, posterior, and lateral proper fasciculi, fasciculi proprii anteriores, posteriores et laterales. They are formed by the processes of tract cells and cells of the gelatinous substance, which establish connections between segments, as well as between the sensory cells of the spinal ganglia and the motor cells of the anterior horns. The proper fasciculi are part of local reflex arcs, which mediate innate and unconditioned Reflexes.

The long pathways represent the apparatus of two-way communication between the brain and the spinal cord, which mediates complex reflex acts.

The posterior funiculus, funiculus posterior, consists of fibers that are the central processes of sensory cells of the spinal ganglia—namely, the gracile fasciculus, fasciculus gracilis (Goll's tract), the cuneate fasciculus, fasciculus cuneatus (Burdach's tract), and the posterior proper fasciculi, fasciculi proprii posteriores. The gracile fasciculus occupies the medial part of the posterior funiculus. It is formed by the processes of spinal ganglion cells from the 19 lower segments of the spinal cord. Located laterally is the cuneate fasciculus, formed by the processes of spinal ganglion cells from the 12 upper segments of the spinal cord. The fibers of these tracts synapse with cells in the gracile and cuneate nuclei of the Medulla Oblongata, which house the second-order neurons of conscious proprioceptive, vibratory, and tactile sensitivity.

*Goll Friedrich (1829-1904) was a Swiss anatomist who described the pathways of deep sensation in the posterior funiculi of the spinal cord (the gracile fasciculus of Goll).

**Burdach Karl Friedrich (1776-1848) was a German anatomist. The cuneate fasciculus of the spinal cord is named after him.

The lateral funiculi, funiculus lateralis, are formed by the axons of the sensory nuclei of the spinal cord, which conduct nerve impulses from lower to higher centers (ascending tracts), the axons of efferent neurons from the Brainstem nuclei and the Cerebral Cortex, along which nerve impulses travel from higher to lower centers (descending tracts), and the lateral proper fasciculi, fasciculi proprii laterales.

The ascending tracts include:

- posterior spinocerebellar tract, tr. spinocerebellaris posterior,

- anterior spinocerebellar tract, tr. spinocerebellaris anterior.

- spinotectal tract, tr. spinotectalis;

- lateral spinothalamic tract, tr. spinothalamicus lateralis.

The posterior spinocerebellar tract, tr. spinocerebellaris posterior, is a direct pathway that conducts sensory impulses from receptors in the Skin, muscles, and tendons to the Cerebellum. The first-order neuron is located in the spinal ganglion, while The Cell bodies of the second-order neurons are situated throughout the length of the spinal cord in the thoracic nucleus, nucl. thoracicus, of the posterior horn.

The anterior spinocerebellar tract, tr. spinocerebellaris anterior, conducts sensory impulses from receptors in the skin, muscles, and tendons to the cerebellum. The first-order neuron is located in the spinal ganglion, and the second-order neuron is in the intermediate nucleus, nucl. intermediomedialis, sending one portion of its fibers through the white commissure, commissura alba, into the lateral columns of the contralateral side, and the other portion into the lateral columns of the ipsilateral side.

The spinotectal tract, tr. spinotectalis, runs adjacent to the spinothalamic tract in the lateral funiculi and terminates in the tectal plate.

The fibers of the lateral spinothalamic tract are divided into two parts: anterior and posterior. Pain Sensation is transmitted along the anterior part, while Temperature Sensation is transmitted along the posterior part. The cell bodies of the first-order neurons are located in the spinal ganglia. The processes of the second-order neurons, originating from the cells of the proper nucleus of the posterior horn, pass through the white commissure into the lateral funiculus of the contralateral side.

The descending (efferent) tracts of the lateral funiculus include:

- lateral corticospinal (pyramidal) tract, tr. corticospinalis (pyramidalis) lateralis;

- rubrospinal tract, tr. rubrospinalis;

- reticulospinal tract, tr. reticulospinalis;

- olivospinal tract, tr. olivospinalis, and others.

The lateral corticospinal (pyramidal) tract, tr. corticospinalis (pyramidalis) lateralis, originates from the giant pyramidal cells (Betz cells) of the motor cortex, descends as part of the corona radiata through the posterior limb of the internal capsule, and enters the crus cerebri. At the boundary between the medulla oblongata and the spinal cord, in the region of the decussation of pyramids, decussatio pyramidum, the fibers undergo partial decussation: one portion of the fibers crosses to the contralateral side, forming the lateral corticospinal tract that runs in the lateral funiculi of the spinal cord's white matter, while the remaining fibers descend uncrossed into the anterior funiculus of the white matter, forming the anterior corticospinal tract.

The rubrospinal tract, tr. rubrospinalis, originates in the red nucleus, nucleus ruber, and projects to the spinal cord. Descending fibers arising from the cells of the red nucleus decussate in the Midbrain with the corresponding fibers of the opposite side and descend through the brainstem (crus cerebri, Pons, and medulla oblongata). This tract connects the extrapyramidal system and the cerebellum with the spinal cord.

The reticulospinal tract, tr. reticulospinalis, is located between the anterior and lateral Pyramidal Tracts. It is formed by fibers from the cells of the brainstem reticular Formation of the contralateral side and terminates segmentally on the motor cells of the anterior horns. It functions in the subconscious regulation of movements.

The olivospinal tract, tr. olivospinalis, is a group of descending olivary fibers that descend within the lateral funiculi of the spinal cord's white matter and terminate throughout its cervical segments on the cells of the anterior horns of the gray matter.

The anterior funiculi are mainly formed by the axons of efferent neurons from the brainstem and cerebral cortex, which transmit nerve impulses from higher to lower centers. They include:

- tectospinal tract, tr. tectospinalis;

- anterior corticospinal (pyramidal) tract, tr. corticospinalis (pyramidalis) anterior,

- anterior spinothalamic tract, tr. spinothalamicus anterior,

- vestibulospinal tract, tr. vestibulospinalis;

- anterior proper fasciculi, fascc. proprii anteriores.