Neuraxis

The neuraxis, also known as the neuroaxis is the axis of the central nervous system. It extends from the brain to the spinal cord and denotes the direction in which the central nervous system lies in both development and in mature organisms. Early on in embryological development, the neuraxis begins as a distinctly straight axis, but quickly develops bends by various flexures, most notably the cephalic flexure, which contributes most to the complex mature structure of the spinal cord and brain.

Embryonic development can help in understanding how complex structures form around the neuraxis The embryonic nervous system in vertebrates is highly conserved, meaning its structure and function have stayed the same across species, and generally appear the same. During development, the formation of the neural tube-and later the brain and spinal cord- define the layout of the neuraxis. This establishes the anterior-posterior dimension of the nervous system. The anterior-posterior dimension of the neuraxis overlays the superior-inferior dimension of the body. Depending on the formation of more differentiated structures, this axis may lose its rigid nature, adopting the curvature introduced by encephalic structures. For example, there is a major curve between the brain stem and forebrain, which is called the cephalic flexure. Because of this, the neuraxis starts in an inferior position—the end of the spinal cord—and ends in an anterior position, the front of the cerebrum. This can be illustrated when looking at a four-legged animal standing up on two legs. Without this flexure in the brain stem and at the top of the neck, a bipedal animal would be unable to look directly in front of them.