The neuraxis, also known as the neuroaxis is the axis of the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

. 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 In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, ...

-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 The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...

and

forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain controls body temperature, reproductive functions, eating, sleeping, and the display of emotions. Ve ...

, which is called the

cephalic flexure Three flexures form in the part of the embryonic neural tube that develops into the brain. At four weeks gestational age in the human embryo, the neural tube has developed at the cranial end into three swellings – the primary brain vesicles. Th ...

. 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 The cerebrum (: cerebra), telencephalon or endbrain is the largest part of the brain, containing the cerebral cortex (of the two cerebral hemispheres) as well as several subcortical structures, including the hippocampus, basal ganglia, and olfac ...

. 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.

Anatomical and clinical significance

The neuraxis holds a highly relevant role in anatomical and clinical settings. As a key feature of anatomical orientation, it provides a strong framework for identifying other structures, determining directionality, and applying these aspects towards clinical practice. In humans, neuraxis formation is marked by the emergence of a curved axis, increasing the complexity of neurological features. This added axis defies typical anatomical terminology, necessitating adapted terms to accurately describe new features. Once accurately established, the neuraxis provides a reference point to allow clinicians and researchers to accurately make diagnoses, conceptualize brain structures, and enables educators to explain spatial relationships between other parts of the body.

Anatomical orientation and origin of the neuraxis

The neuraxis holds great importance anatomically, establishing the localization of the central nervous system. Through development, it establishes the anterior-posterior axis through which other anatomical terms can be applied in different CNS regions. Early on in

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

, during

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals, the blastocyst, is reorganized into a two-layered or three-layered embryo known as ...

, the beginnings of the neuraxis become defined. Here the

primitive streak The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians, the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the ...

is formed, a precursor the neural tube. The primitive streak marks the beginning of gastrulation and is a transient feature that exists primarily in vertebrates.Following the neural tube formation, a defined axis can be established and an initial neuraxis is formed. The originally straight axis adopts a curve as the brain enlarges, allowing for the formation and distinction of complex structures such as the brain stem and cerebral cortex. The terminology of these structures remains the same after development is complete. Dorsal refers to the back of the body as well as the top of the head, while ventral denotes the front of the body and the region under the neuraxis, the bottom of the head.

Clinical applications of the neuraxis

The neuraxis provides an importance reference for clinicians, particularly in the fields of neurology, neurosurgery, and radiology. Due to the curved nature of the human neuraxis, common anatomical terms such as rostral, caudal, dorsal, and ventral hold different meanings at the head, underscoring the importance of the neuraxis as a reference point. In radiology, interpretations of the neuraxis allows for accurate image interpretations for cross-sectional imaging and MRIs. Neurosurgery also heavily relies on a strong understanding of the neuraxis, particularly for entry-point localization and risk assessments. Atypical alignment of the neuraxis has been documented as a sign of underlying disease. Insufficient mechanical support and altered vasculature due to misaligned neuraxes causes extreme pathology in several conditions such as scoliosis, Arnold-Chiari malformation, and even some cases of Ehlers-Danlos Syndromes (EDS). This pathology is often marked by a disproportionate degree of neurological dysfunction. Extreme conditions of

androgen insensitivity syndrome Androgen insensitivity syndrome (AIS) is a condition involving the inability to respond to androgens, typically due to androgen receptor dysfunction. It affects 1 in 20,000 to 64,000 XY (karyotype, karyotypically male) births. The condition result ...

(AIS) can also lead to neuraxis deviation, increasing the chance of further pathogenesis. The neuraxis also plays a relevant role in rehabilitation and treatment, underscoring the importance of its anatomical role.

References

*''This article incorporates text from the Creative Commons Attribution 3.0 Licensed edition of Anatomy & Physiology by OpenStax College'' {{Authority control Neuroanatomy

Anatomical and clinical significance

Anatomical orientation and origin of the neuraxis

Clinical applications of the neuraxis

See also

References