The brachial plexus – explaining its morphology and variability by a generic developmental model

Abstract

In classic anatomy teaching, the brachial plexus generally features as an enigmatic rote-learned structure, leaving the student with a feeling of complexity. The notion of complexity may increase in dissections, where plexuses significantly differing from the standard plexus model are readily found. This raises questions: what determines the existence and prevalence of variants and to what degree should they be considered anomalous? A model linking brachial plexus morphology and its variability to causative morphological parameters which would also standardize plexus description and teaching would be beneficial. The present study aims to provide such a model by analyzing the context of plexus development and applying this model in the analysis of plexus variability in anatomical specimens. Based on a thorough literature review, a generic developmental model was formulated and different factors of variability defined. In 56 plexuses, the proposed generic principles of plexus variability were found consistent with the variations encountered. Summarized, the modeled generic principles are as follows. Brachial plexus axon bundles grow out into an environment of chemical tracer paths in which constraints and obstacles are present: the geometry of the arm bud, cartilaginous bone precursors and vessels. The overall constancy of these factors generates a gross plexus outline, while the variability in these factors gives rise to typical plexus variations. The usefulness of the model derives from the fact that the variability of the main morphologically determining factors is not random but is the expression of the possibilities of the embryological substrate. Within the model, the major plexus morphological determinant is the segmental position of the subclavian artery, which is determined by the segment level of the intersegmental artery from which it develops. Normally, the subclavian artery develops from intersegmental artery i7. However, the subclavian artery can develop from inferior or superior segmental levels, from intersegmental artery i8 or i6, and possibly also from i9 or i5. Each of these arterial variants creates a typical, morphologically distinct, predictable plexus configuration. Superimposed on these basic plexus configurations, the underlying embryological substrate may develop further variability by integrating remnants of other intersegmental arteries into the arterial network. The resulting plexus configurations are further modified by local factors, e.g. the splitting of outgrowing axon bundles around vessels. A large split in the lateral cord around a large vein or veins crossing from lateral to medial, tangentially cranially over the subclavian artery was found in 54% of the 56 investigated BP and therefore might be added to plexus teaching. The distinct plexus morphologies associated with the subclavian artery segmental levels were further found associated with, among others, typical variations in the pectoral nerves and their ansas; these associations were also modeled. The presented models could...

Variant anatomy and codes of the human brachial plexus

“…Обнаруженная закономерность согласуется и объясняется общей эволюционной моделью формирования плечевого сплетения [35]. Согласно этой модели эмбриональный рост аксонов в зачатке руки связан с изменчивостью морфологических структур взрослого человека.…”

Update on the anatomy of the brachial plexus in dogs: Body weight correlation and contralateral comparison in a cadaveric study

“…These differences could be explained by the theory postulated by other authors who considered that the variation in the formation of the anatomy of the brachial plexus was related to embryonal development. In fact, in this phase, the axon forming the brachial plexus grows out from the spine in an environment in which the arm bud, the growing vascular system and the cartilaginous precursor represent obstacles to its advancement, influencing its final conformation [ 1 ].…”

“…The brachial plexus is a complex anatomical structure consisting of a network formed by the ventral roots of the C4 or C5 spinal nerves up to the T1 or T2 spinal nerves, involved in the formation of the brachial nerves [ 1 ]. In past decades, several authors have investigated the anatomy of the brachial plexus in different species.…”

Variant Anatomy of the Brachial Plexus

“…Периодически от C4 и Th2 нервов ПС получает больше волокон, чем от С5 и Th1 нервов [1]. Взаимоотношения корешков и окружающих тканей вместе с питающими их сосудами является клинически значимым аспектом [5,6].…”

“…Occasionally BP receives more fibers from C4 and Th2 nerves than from C5 and Th1 [1]. The relationship between the roots and surrounding tissues, together with the vessels supplying them, is a clinically significant issue [5,6].…”