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//CENTRAL NERVOUS SYSTEM

CENTRAL NERVOUS SYSTEM

SPINAL CORD

In the future spinal cord the median roof plate (dorsal lamina) and floor plate (ventral lamina) of the neural tube do not participate in the cellular proliferation which occurs in the lateral walls and so remain thin. Their cells contribute largely to the formation of the ependyma.

The neuroblasts of the lateral walls of the tube are large and at first round or oval (apolar). Soon they develop processes at opposite poles and become bipolar neurons. However, one process is withdrawn and the neuroblast becomes unipolar, although this is not invariably so in the case of the spinal cord. Further differentiation leads to the development of dendritic processes and the cells become typical multipolar neurones. In the developing cord they occur in small clusters representing clones of neurones. The development of a longitudinal sulcus limitans on each side of the central canal of the cord divides the ventricular and intermediate zones in each lateral wall into a basal (ventrolateral) plate or lamina and an alar (dorsolateral) plate or lamina (Fig. 24.17). This separation indicates a fundamental functional difference. Neural precursors in the basal plate include the motor cells of the anterior (ventral) and lateral grey columns, while those of the alar plate exclusively form ‘interneurones’ (which possess both short and long axons), some of which receive the terminals of primary sensory neurones. Caudally the central canal of the cord ends as a fusiform dilatation, the terminal ventricle.

Anterior (ventral) grey column

The cells of the ventricular zone are closely packed at this stage and arranged in radial columns (Fig. 24.6). Their disposition may be determined in part by contact guidance along the earliest radial array of glial fibres which cross the full thickness of the early neuroepithelium. The cells of the intermediate zone are more loosely packed. They increase in number initially in the region of the basal plate. This enlargement outlines the anterior (ventral) column of the grey matter and causes a ventral projection on each side of the median plane: the floor plate remains at the bottom of the shallow groove so produced. As growth proceeds these enlargements, which are further increased by the development of the anterior funiculi (tracts of axons passing to and from the brain), encroach on the groove until it becomes converted into the slit-like anterior median fissure of the adult spinal cord (Fig. 24.17). The axons of some of the neuroblasts in the anterior grey column cross the marginal zone and emerge as bundles of ventral spinal nerve rootlets on the anterolateral aspect of the spinal cord. These constitute, eventually, both the α-efferents which establish motor end plates on extrafusal striated muscle fibres and the γ-efferents which innervate the contractile polar regions of the intrafusal muscle fibres of the muscle spindles.

Lateral grey column

In the thoracic and upper lumbar regions some intermediate zone neuroblasts in the dorsal part of the basal plate outline a lateral column. Their axons join the emerging ventral nerve roots and pass as preganglionic fibres to the ganglia of the sympathetic trunk or related ganglia, the majority eventually myelinating to form white rami communicantes. The axons within the rami synapse on the autonomic ganglionic neurones, and axons of some of the latter pass as postganglionic fibres to innervate smooth muscle cells, adipose tissue or glandular cells. Other preganglionic sympathetic efferent axons pass to the cells of the suprarenal medulla. An autonomic lateral column is also laid down in the midsacral region. It gives origin to the preganglionic parasympathetic fibres which run in the pelvic splanchnic nerves.

The anterior region of each basal plate initially forms a continuous column of cells throughout the length of the developing cord. This soon develops into two columns (on each side): one is medially placed and concerned with innervation of axial musculature, and the other is laterally placed and innervates the limbs. At limb levels the lateral column enlarges enormously, but regresses at other levels.

Axons arising from ventral horn neurones, i.e. α-, β- and γ-efferent fibres, are accompanied at thoracic, upper lumbar and midsacral levels by preganglionic autonomic efferents from neuroblasts of the developing lateral horn. Numerous interneurones develop in these sites (including Renshaw cells): it is uncertain how many of these differentiate directly from ventrolateral lamina (basal plate) neuroblasts and how many migrate to their final positions from the dorsolateral lamina (alar plate).

In the human embryo, the definitive grouping of the ventral column cells, which characterizes the mature cord, occurs early, and by the 14th week (80 mm) all the major groups can be recognized. As the anterior and lateral grey columns assume their final form the germinal cells in the ventral part of the ventricular zone gradually stop dividing. The layer becomes reduced in thickness until ultimately it forms the single-layered ependyma which lines the ventral part of the central canal of the spinal cord.

Posterior (dorsal) grey column

The posterior (dorsal) column develops later; consequently the ventricular zone is for a time much thicker in the dorsolateral lamina (alar plate) than it is in the ventrolateral lamina (basal plate) (Fig. 24.6).

While the columns of grey matter are being defined, the dorsal region of the central canal becomes narrow and slit-like, and its walls come into apposition and fuse with each other (Fig. 24.17). In this way the central canal becomes relatively reduced in size and somewhat triangular in outline.

About the end of the fourth week advancing axonal sprouts invade the marginal zone. The first to develop are those destined to become short intersegmental fibres from the neuroblasts in the intermediate zone, and fibres of dorsal roots of spinal nerves which pass into the spinal cord from neuroblasts of the early spinal ganglia. The earlier dorsal root fibres that invade the dorsal marginal zone arise from small dorsal root ganglionic neuroblasts. By the sixth week they form a well-defined oval bundle near the peripheral part of the dorsolateral lamina (Figs 24.6 and 24.7). This bundle increases in size and, spreading towards the median plane, forms the primitive posterior funiculus of fine calibre. Later, fibres derived from new populations of large dorsal root ganglionic neuroblasts join the dorsal root: they are destined to become fibres of much larger calibre. As the posterior funiculi increase in thickness, their medial surfaces come into contact separated only by the posterior medial septum, which is ependymal in origin and neuroglial in nature. It is thought that the displaced primitive posterior funiculus may form the basis of the dorsolateral tract or fasciculus (of Lissauer).

Maturation of the spinal cord

Long intersegmental fibres begin to appear at about the third month and corticospinal fibres are seen at about the fifth month. All nerve fibres at first lack myelin sheaths. Myelination starts in different groups at different times, e.g. the ventral and dorsal nerve roots about the fifth month, the corticospinal fibres after the ninth month. In peripheral nerves the myelin is formed by Schwann cells (derived from neural crest cells) and in the CNS by oligodendrocytes (which develop from the ventricular zone of the neural tube). Myelination persists until overall growth of the CNS and PNS has ceased. In many sites, slow growth continues for long periods, even into the postpubertal years.

The cervical and lumbar enlargements appear at the time of the development of their respective limb buds.

In early embryonic life, the spinal cord occupies the entire length of the vertebral canal and the spinal nerves pass at right angles to the cord. After the embryo has attained a length of 30 mm the vertebral column begins to grow more rapidly than the spinal cord and the caudal end of the cord gradually becomes more cranial in the vertebral canal. Most of this relative rostral migration occurs during the first half of intrauterine life. By the 25th week the terminal ventricle of the spinal cord has altered in level from the second coccygeal vertebra to the third lumbar, a distance of nine segments. As the change in level begins rostrally, the caudal end of the terminal ventricle, which is adherent to the overlying ectoderm, remains in situ, and the walls of the intermediate part of the ventricle and its covering pia mater become drawn out to form a delicate filament, the filum terminale. The separated portion of the terminal ventricle persists for a time, but it usually disappears before birth. It does, however, occasionally give rise to congenital cysts in the neighbourhood of the coccyx. In the definitive state, the upper cervical spinal nerves retain their position roughly at right angles to the cord. Proceeding caudally, the nerve roots lengthen and become progressively more oblique.

During gestation the relationship between the conus medullaris and the vertebral column changes, such that the conus medullaris gradually ascends to lie at higher vertebral levels. By 19 weeks of gestation the conus is adjacent to the fourth lumbar vertebra, and by full term (40 weeks) it is at the level of the second lumbar vertebra. By 2 months postnatally the conus medullaris has usually reached its permanent position at the level of the body of the first lumbar vertebra.

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