Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Nervous System (General Outline of Structure and Development)
Central Nervous System
Spinal Cord

The Spinal cord (medulla spinalis) appears macroscopically as an elongated, cylindrical cord that is flattened anteroposteriorly and features a narrow internal cavity known as the central canal. It is enclosed by three Meninges: the dura mater, arachnoid mater, and pia mater. Located within the vertebral canal, the spinal cord transitions into the Brainstem at the level of the lower margin of the foramen magnum. Inferiorly, it tapers to form the conus medullaris, which terminates at the level of the second lumbar vertebra. The conus medullaris continues as the filum terminale, the upper portion of which still contains neural tissue. Below the level of the second sacral vertebra, this Connective Tissue Structure represents a continuation of the three spinal meninges. The filum terminale ends at the body of the second coccygeal vertebra by fusing with its periosteum. It is surrounded by the long roots of the lower Spinal Nerves, which form a bundle within the vertebral canal known as the cauda equina.

In adults, the spinal cord has an average length of 43 cm (43–45 cm in men and 41–42 cm in women) and weighs about 34–38 g, which is approximately 2% of the Brain's mass.

Two distinct enlargements are visible along the cervical and lumbosacral Regions of the spinal cord: the cervical enlargement and the lumbosacral enlargement. These enlargements are formed due to the presence of A large number of Nerve Cells and fibers in these regions, which innervate the upper and lower limbs.

A deep anterior median fissure is visible on the anterior surface of the spinal cord, although it does not reach the Gray matter. Between the fissure and its floor lies the anterior white commissure, which connects the right and left halves of the spinal cord. The posterior median sulcus runs along the midline of the posterior surface. Deep within this sulcus lies the posterior median glial septum, which penetrates almost the entire thickness of the White matter. The fissure and the sulcus serve as boundaries dividing the spinal cord into right and left symmetrical halves.

On the anterior surface of the spinal cord, on each side of the median fissure, runs the anterolateral sulcus, which marks the exit site of the anterior (motor) roots of the spinal nerves. This sulcus serves as the surface boundary between the anterior and lateral funiculi of the spinal cord. On the posterior surface of each half of the spinal cord lies the posterolateral sulcus, which serves as the entry site for the posterior (sensory) roots. This sulcus acts as the boundary between the lateral and posterior funiculi. The anterior ROOT of a spinal nerve consists of the processes of motor nerve cells whose Cell bodies are located in the anterior horn of the spinal gray matter.

The posterior root of a spinal nerve is formed by the central processes of pseudounipolar (sensory) cells penetrating the spinal cord, whose cell bodies form the spinal ganglion located near the junction of the posterior and anterior roots. Along the entire length of the spinal cord, 31 pairs of spinal nerve roots emerge from each side. A segment of the spinal cord corresponding to two pairs of roots (two anterior and two posterior) is termed a spinal cord segment.

The human spinal cord consists of 31 segments, categorized into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal segments. Each spinal cord segment corresponds to a specific body region innervated by it. Because the spinal cord is significantly shorter than THE Vertebral Column, the numerical designation of a spinal cord segment, starting from the lower cervical region, does not correspond to the number of the vertebra of the same name.

The spinal cord is divided into cervical, thoracic, lumbar, sacral, and coccygeal regions. In adults, the lower boundary of the cervical region corresponds to the seventh cervical vertebra, and the lower boundary of the thoracic region corresponds to the eleventh thoracic vertebra. The lower boundary of the lumbar region is located at the level of the lower margin of the eleventh or the upper margin of the twelfth thoracic vertebra, while the sacral region ends at the level of the first lumbar vertebra. The coccygeal region terminates at the level between the lower margin of the first and the upper margin of the second lumbar vertebrae.

Gray matter of the spinal cord. Throughout the spinal cord, to the right and left of the central canal, lie symmetrical columns of gray matter connected by plates of gray matter known as the anterior and posterior commissures. Within each column of gray matter, anterior and posterior horns (columns) are distinguished, and additionally, lateral horns are present from the eighth cervical to the second lumbar segment.

In cross-section, the columns of gray matter on each side appear as projections of varying widths known as horns (Fig. 97). These include the wider anterior horn and the narrow posterior horn, corresponding to the anterior and posterior columns. The lateral horn corresponds to the lateral column. The gray matter of the posterior horns is heterogeneous, comprising a superficially located marginal zone consisting of small nerve cells. Slightly deeper lies the substantia gelatinosa, formed by The Cell bodies of association (intercalated) Neurons. The substantia gelatinosa is covered by the gelatinous substance, which consists of small nerve and glial cells. Further toward the center of the posterior horn is the proper sensory Nucleus of the posterior horn. It consists of interneurons whose axons project into the anterior horn or cross via the anterior gray commissure to the opposite side of the spinal cord. At the Base of the posterior horn lies the thoracic nucleus (Clark's column), formed by large interneurons with extensively branched dendrites, extending along the entire posterior column as a cellular cord. The cells of all posterior horn nuclei function as interneurons.

The intermediate zone of the spinal gray matter is situated between the anterior and posterior horns. In this region, extending from the eighth cervical to the second lumbar segments, There is a projection of gray matter known as the lateral horn. The lateral horns contain the centers of the Sympathetic division of the Autonomic Nervous system, represented by groups of nerve cells organized into the lateral intermediate substance. The axons of these cells pass through the anterior horn and exit the spinal cord as part of the anterior roots of the spinal nerves. The reticular formation is located in the white matter adjacent to the gray matter between the anterior and posterior horns. This formation consists of small and larger clusters of nerve cells possessing numerous interconnections.

The anterior horns contain large nerve root cells, which are the cell bodies of motor neurons. Their neurites form the bulk of the fibers in the anterior roots. The neurons located in the anterior horns form sizeable nuclear groups, comprising five nuclei: two medial, two lateral, and a central nucleus. The first two nuclei innervate the Muscles of the Trunk and are well-developed throughout the entire length of the spinal cord, whereas the latter two are more prominent in the cervical and lumbar enlargements and innervate the limb muscles.

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Fig. 97. Cross-section of the spinal cord structure:

1 — anterior median fissure; 2 — anterior funiculus; 3 — motor nuclei of the anterior horn; 4 — anterior horn; 5 — lateral funiculus; 6 — intermediolateral (autonomic, sympathetic) nucleus; 7 — lateral horn; 8 — intermediomedial nucleus; 9 — thoracic nucleus; 10 — proper nucleus of the posterior horn; 11 — posterior horn; 12 — posterior funiculus; 13 — fasciculus gracilis; 14 — fasciculus cuneatus; 15 — posterior spinocerebellar tract; 16 — lateral corticospinal (pyramidal) tract; 17 — rubrospinal tract; 18 — lateral spinothalamic tract; 19 — anterior spinothalamic tract; 20 — vestibulospinal tract; 21 — anterior corticospinal (pyramidal) tract

Motor neurons of the anterior horns of the spinal cord are subdivided into two main types: alpha and gamma motor neurons.

Alpha (a) motor neurons are large cells (100–140 µm in diameter) with long dendrites. Their cell bodies and dendrites accommodate up to 10,000–20,000 synapses. The axons of a-motor neurons have a large diameter and a high conduction velocity of nerve impulses (70–120 m/s). They innervate Skeletal Muscle fibers, thereby mediating muscle contractions.

Gamma (y) motor neurons are significantly smaller cells. The diameter of their cell bodies does not exceed 30–40 µm, and their axons are also small in diameter. Consequently, the conduction velocity of impulses along gamma fibers is only 10–40 m/s. Gamma motor neurons lack direct synaptic contacts with the axons of sensory neurons from the spinal ganglia; instead, they are activated by nerve impulses traveling along descending pathways. The coordinated Activation of a- and y-motor neurons ensures the motor control and coordination of skeletal muscle Functions.

Interneurons of the spinal cord comprise a rather heterogeneous group of nerve cells whose cell bodies and dendrites are located entirely within the spinal cord. They form the most dorsal part of the posterior horn, including its spongy layer, substantia gelatinosa, the proper nucleus of the posterior horn, and the base nucleus of the posterior horn (the thoracic nucleus). These structures receive fibers carrying pain (nociceptive), tactile, and proprioceptive sensitivities. The axons of these neurons form ascending pathways (spinothalamic, spinocerebellar, etc.). Interneurons also participate in The formation of the intermediate zone of the gray matter, situated between the anterior and posterior horns. Within this zone, spanning from the eighth cervical (VIII) to the second lumbar (II) segment, there are protrusions of gray matter known as the lateral horns, whose neuronal groups constitute the lateral intermediate substance. The axons of these neurons exit the spinal cord as part of the anterior roots and project peripherally to the autonomic ganglia.

WHITE MATTER OF the spinal cord. The white matter of the spinal cord is conventionally divided into three pairs of funiculi. The anterior funiculus lies between the median fissure medially and the anterolateral sulcus laterally. The posterior funiculus is situated between the posterior median and posterolateral sulci. The lateral funiculus is located between the anterolateral and posterolateral sulci. Deep within all funiculi, in close proximity to the gray matter, lie short intersegmental fibers belonging to the intrinsic Pathways of the spinal cord itself. These fibers establish communication between individual segments, which is why these bundles are classified as the intrinsic apparatus of the spinal cord. Fibers originating from spinal ganglia, which enter the spinal cord via the posterior roots of spinal nerves, pursue various trajectories. Some fibers terminate on interneurons in the posterior horns of the same or the opposite side, on autonomic neurons in the lateral horns, and on Cells of the reticular formation. Other fibers ascend and become part of the posterior funiculi, representing the ascending pathways of the spinal cord.

The long pathways of the spinal cord are located external to its intrinsic bundles; these long pathways appear later in phylogeny than the intrinsic apparatus and develop in parallel with the Formation of the brain. These tracts transmit impulses in an ascending direction—from sensory and interneurons—and in a descending direction, from cells of higher-lying nerve centers to the motor neurons of the spinal cord.

The ascending pathways of the spinal cord include the fasciculus gracilis and fasciculus cuneatus, the posterior and anterior spinocerebellar tracts, the lateral spinothalamic tract, and others. The fasciculus gracilis and fasciculus cuneatus are located in the posterior funiculi and are formed by the neurites of sensory neurons from the spinal ganglia. The nerve fibers of these tracts transmit nerve impulses to the Medulla Oblongata from sensory endings—namely, the proprioceptors of muscles and joints, as well as partially from cutaneous exteroceptors (tactile sensitivity). The fasciculus gracilis transmits impulses from receptors of the lower limbs and the lower half of the body (up to the fifth thoracic segment). The fasciculus cuneatus conveys impulses from the upper limbs and the upper half of the body and is therefore absent below the fifth thoracic segment.

The posterior spinocerebellar tract lies in the posterior part of the lateral funiculus. This tract originates from the cells of the thoracic nucleus, which is located at the base of the posterior horn of the spinal cord on the same side (ipsilateral).

The anterior spinocerebellar tract consists of processes of interneurons belonging to the intermediomedial nucleus. After crossing to the contralateral side of the spinal cord, these nerve fibers form a bundle located in the anterior region of the lateral funiculus. Both of these pathways transmit proprioceptive impulses to the Cerebellum.

The lateral spinothalamic tract is also located in the lateral funiculus and consists of fibers originating from contralateral interneurons of the posterior horn. This pathway conducts impulses of pain and Temperature Sensation from the body to the Diencephalon (thalamus) and subsequently to the Cerebral Cortex. Because ascending pathways formed by interneuronal fibers undergo decussation, nerve impulses reach the cerebral hemisphere opposite to the side of the body from which the stimulus originated.

Descending Neural Pathways include the rubrospinal, lateral and anterior corticospinal, tectospinal, vestibulospinal tracts, and others.

The rubrospinal tract originates in the red nucleus of the Midbrain, descends within the lateral funiculus of the contralateral side of the spinal cord, and terminates on motor neurons in the anterior horns. It transmits involuntary motor impulses.

The lateral corticospinal (pyramidal) tract lies in the lateral funiculus and consists of processes (axons) from the cerebral cortex cells of the opposite hemisphere. The tract gradually tapers as a portion of its fibers terminates on anterior horn cells in each spinal cord segment. This pathway conducts voluntary motor impulses from the cortex.

The anterior corticospinal (pyramidal) tract, much like the lateral one, is composed of fibers from the cerebral cortex cells, but it is located in the anterior funiculus. The fibers of this tract also terminate on the motor cells of the contralateral anterior horn, Crossing Over via the anterior white commissure of the spinal cord. This pathway serves the same function as the lateral corticospinal tract.

The tectospinal tract is likewise located in the anterior funiculus. It originates in the superior and inferior colliculi of the midbrain tectum and terminates on the cells of the anterior horns.

The vestibulospinal tract is also situated in the anterior funiculus of the spinal cord. It extends from the vestibular nuclei of the Pons to the anterior horn cells of the spinal cord, conducting impulses that ensure coordinated movement, balance, and muscle tone.