Situation development

In infants, the spinal cord still fills the spinal canal up to the lower lumbar vertebrae, in children it reaches up to the 4th lumbar vertebra. This must be taken into account when withdrawing nerve fluid; one must then enter the spinal canal further down in order not to endanger the spinal cord. In the following years of life, the spinal cord shifts further and further upwards, since it is “attached” to the brain, but grows more slowly than the spinal column.

This “ascent” of the spinal cord is medically called ascensus medullae spinalis. Originally during embryonic development, a nerve segment lies opposite the associated vertebra. The spinal nerves are therefore forced to grow along with the bony structures; therefore, with increasing age, they have to descend more and more steeply: their course in the spinal canal becomes steeper and more oblique towards the bottom.

In adults, therefore, the height of the spinal cord segment only coincides approximately in the upper cervical medulla with the exit point of the corresponding pair of nerves. The spinal nerves originating from the lower parts of the spinal cord are compressed by “pulling up” the spinal cord to the said cauda equina, the horse’s tail. This developmental process is completed by the age of 12.

Precision engineering

On its front side (ventral or anterior), the coarse cord shows a deep incision, the fissure mediana ventralisanterior, in which the anterior spinal artery (A. spinalis anterior) runs, and on its back side (dorsal or posterior) a shallower furrow, the so-called sulcus medianus dorsalis posterior. This furrow continues towards the inside into a fine septum (septum medianum dorsale). The anterior incision and the posterior septum divide the spinal cord into two mirror-image halves.

When looking at a cross-section of the spinal cord, the inner, butterfly-shaped gray matter can be seen with the naked eye (= macroscopic). From it, one can distinguish the fibrous white substance (substantia alba) surrounding it, which lies outside. The shape of this butterfly figure varies depending on the localization.

Both at the level of the thoracic and lumbar sections of the spinal cord, the grey substance contains a small lateral horn on each side in addition to the front and rear horn. In the middle, the central canal (canalis centralis) runs, visible in cross-section only as a tiny hole; it is filled with liquor and represents the inner liquor space of the spinal cord. A longitudinal section shows that the spinal cord is thicker at these points than in the rest of the spinal canal, since the nerve roots that supply the arms and legs emerge from here – more nerve fibers and also more nerve cell bodies are needed here.

These thickenings are also called intumescences (Intumescentia cervicalis in the cervical marrow or lumbosacralis in the lumbar region). The anterior horn (Cornu anterius) of the gray spinal cord substance is broad and contains nerve cell bodies whose extensions (axons) move towards the muscles (so-called motoneurons). They thus form the origin of the anterior, motor (i.e. movement) part of the spinal nerve root, which extends laterally from the spinal cord.

The posterior horn, on the other hand, is long and narrow and forms the point of entry for the posterior, sensitive part of the spinal nerve roots, which carry “felt” information created in the periphery up to the brain (e.g. pain, temperature, sense of touch). Their nerve cell bodies, however, lie in the so-called spinal ganglion, which is located outside the spinal cord (but still in the spinal canal).Nevertheless, cell bodies lie in the posterior horn, namely those of the long front and side strands of the white matter, the so-called strand cells (see below). The lateral horn contains vegetative nerve cells (neurons) of the sympathetic nervous system (in the thoracic and lumbar marrow) and the parasympathetic nervous system (in the sacral marrow).

These “horns” only appear as “horns” in cross section (“butterfly wings”); they are found – in varying degrees – in the entire spinal cord, no matter where it is cross-cut. Therefore, they are actually columns when viewed three-dimensionally, and they are also referred to as columns or ledges (columnae). The anterior horn column is therefore called the anterior columna, the posterior horn column the posterior columna and the lateral horn column the lateral columna.

These “columns”, in turn, should not be imagined as strands of equal strength everywhere, which run through the entire spinal cord from top to bottom, because they actually consist of groups of cells, usually five, which are attached to each other. These cell groups form short columns that can extend over several segments, i.e. spinal cord segments. They are also called nuclei (nuclei = nuclei).

The cells of such a cell group are then responsible for one muscle each time. If, for example, a cell group extends over three segments, its extensions (axons) leave the spinal cord through three anterior roots. After they have left the spinal cord, they later reassemble to form a nerve, which then pulls to form a muscle.

This is then called a peripheral nerve. If a peripheral nerve is damaged, peripheral paralysis occurs, which means that a muscle fails completely. If, on the other hand, a nerve root is damaged, this leads to radicular paralysis (radix = root), i.e. parts of different muscles fall out.

In the area of the arms and legs, the spinal nerves that emerge from the spinal cord form nerve plexuses, the so-called plexus. The skin area that is supplied by the nerve fibers of a segment is called dermatome. The muscle fibers that are supplied by the nerve fibers of a segment are accordingly called myotoma.

It should be remembered that it is not one segment that supplies a muscle, but “different muscles may supply a little bit of each”. Finally, nerve fibers that connect the two symmetrical halves of the spinal cord with each other run directly around the central channel (commissure fibers; commissura grisea) so that one half of the spinal cord knows what the other half is doing. This alignment is necessary for balancing processes and others. They also belong to the so-called autologous apparatus of the spinal cord. This consists of nerve cells and their fibers that communicate with each other only within the spinal cord and enable processes that can take place without requiring the central circuitry via the brain; this includes, for example, the spinal cord’s own reflexes.