Chapter 140. General Considerations Related to The Anatomy of The Spinal Cord - Review of Medical Embryology Book

Introduction: the spinal cord occupies the dorsal and median portions of the embryo and is directly caudal to (behind) the medulla. It is surrounded by membranes called meninges and is lodged in the bony vertebral canal. Its axial cavity is called the spinal or central canal
Functional synthesis: the spinal cord of vertebrates is formed by the combination of neuromeres connected to each other, on the one hand, and to the brain on the other. Many interneuronic associations give the spinal cord its unity and coordination. These associations and the great influence of the brain in mammals, particularly in humans, greatly reduce the autonomy of each metamere
General characteristics of the spinal cord

THE SIZE of the spinal cord varies with the age of the fetus
THE RELATIVE VOLUME of the cord, which at first is large, diminishes progressively in relation to the total volume of the central nervous system, especially in relation to the total body size
THE NEURAL TUBE AND VERTEBRAL CANAL initially develop in an almost parallel way. The spinal cord occupies the entire length of the canal, and the spinal nerves emerge between the vertebral bodies through the intervertebral foramina
GROWTH OF THE NEURAL TUBE slows down considerably by month 4 of fetal life, but that of the vertebral canal continues. As a result of this fact

The spinal cord no longer occupies the entire length of the vertebral canal
The roots of the lumbar and sacral nerves, which originally were horizontal, are pulled down by the vertebral canal. They become long and vertical and form the cauda equina (horse's tail) below the spinal cord, which ends at about the second lumbar vertebra in the adult

Somatic and visceral motor neurons, emission centers derived from the basal plates (anterior and lateral horns)
Somatic and visceral sensory neurons, reception centers derived from the alar plates (posterior and lateral horns)
Metameric interneurons
Cell bodies of the intermetameric neurons, derived from the alar plates and particularly numerous in the posterior horns

The neuroglial cells
Fine nerve fibers, slightly myelinated or unmyelinated, coming from the cells listed abov All these elements develop in the mantle layer

A GROUPING OF PATHWAYS OF TRANSIT or of association, the white matter (marginal layer), is formed of myelinated fibers and consists of

Motor fibers from the brain
Sensory fibers from the periphery, passing through the spinal ganglia, making connections in the posterior hom and ascending toward the brain
Axons of intersegmental association neurons
The bundles of fibers in the marginal layer are compressed against each other. The white matter thus is formed from the passage of myelinated fibers in the marginal layer