Why does the brachial plexus exist

In the quadriceps nerves, our control nerve, a bimodal distribution of fibers is observed Figure 3C. Figure 3. Morphometrical analysis results. Myelinated fibers distribution spectra: A medial anterior thoracic nerve; B lateral anterior thoracic nerve; C Quadriceps nerve.

The percentages of each single fiber-diameter class are shown in Table 2. Table 2. Distribution spectra of myelinated fibers of pectoral and quadriceps nerves. Inspective analysis revealed that motor, ChAT-positive fibres were mainly represented by intermediate and large diameter fibers, while most sensory, ChAT negative fibers were of low and intermediate diameter. ChAT-positive and ChAT-negative fibers were grouped together within the fascicles, thus displaying a non-uniform distribution within the endonerium.

The mean percentage of ChAT-positive fibers was Figure 4. Immunohistochemical analysis. Quantitative analysis of the percentage of ChAT positive fibers in study nerves. We report for the first time a systematic description, including morphometrical and immunohistochemical analysis, of the branches to the pectoral muscles of the rat brachial plexus, showing that these nerves are predominantly composed of large motor nerve fibers.

Isolation of the MATN and LATN proved a simple and reproducible procedure and the nerves could be easily identified and harvested in all study animals. Since in the rat, contrary to humans, the entire brachial plexus is located infraclavicularly Bertelli and Ghizoni, , our surgical approach allowed the exposure of the entire plexus with maintenance of all muscular attachments of the MANT and LATN.

The MATN, harvested distal to the branches to the cutaneous maximus nerve, proved the smallest of the nerves studied regarding both endoneurial area and number of fibers.

While no terms of comparison are available for the MATN and the LATN, our data concerning quadriceps nerve fiber numbers are in line with previous literature reports Peyronnard et al.

The LATN is considered to correspond in human anatomy to the lateral pectoral nerve, which originates from the lateral cord of the brachial plexus, while the MATN corresponds to the medial pectoral nerve, from the medial cord of the brachial plexus. In a human autopsy study, however, it has been shown that the pectoral nerves would exist at the trunk level as 3 distinct nerves: the superior, middle, and inferior pectoral nerves; the mean fiber diameter was similarly high in the middle and inferior pectoral nerves 8.

In our study, performed on the rat brachial plexus, we could not identify the existence of three pectoral nerves, thus confirming previous reports Greene, , and suggesting that in the rat the middle pectoral nerve of human anatomy could be absent or fused to the LATN or the MATN.

In both species, however, these nerves are composed of large-diameter fibers. Myelinated fibers distribution spectra visual analysis confirmed a shift to the right for the MATN and the quadriceps nerve, similarly to ventral motor roots Dyck et al.

As large diameter fibers mainly derive from alpha motor neurons, the above-mentioned findings support the motor nature of the nerves studied. The LATN showed a significantly higher percentage of 6. Although immunohistochemical staining for ChAT confirmed that all nerves studied are predominantly composed of motor fibers, MATN shows the higher percentage. The quadriceps nerve is commonly considered a predominantly motor nerve and is frequently used in order to assess the motor fiber involvement at hind-limbs Wrabetz et al.

The MATN, however, contained a significantly higher percentage of motor fibers In conclusion, our study yields a systematic description of the branches of the brachial plexus to the pectoral muscles, and might provide reference values for future experimental studies in rats. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We thank Stefano Grassi for technical assistance and Lawrence Wrabetz for helpful discussion.

Aszmann, O. The anatomy of the pectoral nerves and their significance in brachial plexus reconstruction. Hand Surg. Bertelli, J. Concepts of nerve regeneration and repair applied to brachial plexus reconstruction.

Microsurgery 26, — De Medinacelli, L. Interpreting nerve morphometry data after experimental traumatic lesions. Methods 58, 29— Pubmed Abstract Pubmed Full Text. Dyck, P. The morphometric composition of myelinated fibers by nerve, level and species related to nerve microenvironment and ischaemia. This is the stage where peripheral nerves are developed from the growing brachial plexus into the mesenchyme of the developing upper limb. Spinal nerves develop to both dorsal and ventral aspects of the limb in the form of segmental bands.

This embryogenic development reveals the development and innervation of the brachial plexus. Segmental bands of the spinal nerves form following the proximal and distal gradient, which indicate that the muscles proximal to the brachial plexus receive innervation by the higher segmental band of C5 and C6.

In contrast, the muscles distal to the brachial plexus obtain nerve supply from the lower segmental band of C8 and T1. The brachial plexus receives the majority of its blood supply from the subclavian artery and its derivatives.

The vertebral artery, through its anterior and posterior spinal branches, provides blood supply to the cervical rootlets. The trunks receive blood supply from the superior intercostal and deep cervical arteries, or directly from the subclavian artery. The cords are supplied by the subclavian, subscapular, and axillary arteries.

The musculocutaneous nerve arises from C5 and C6 spinal nerves. It innervates all the muscles of the arm anteriorly, enabling motor functions such as flexion of the elbow and supination by the biceps brachii.

It primarily innervates the anterior forearm with a section innervated by the ulnar nerve and the hand thenar and central sections. The median nerve allows pronation of the forearm and flexion of the wrist and digits, together with the opposition of the thumb. It constitutes the innervation of the anterior forearm with a section innervated by the median nerve and the hand hypothenar and central sections.

The central section, which involves the palmar and dorsal aspects, is responsible for the adduction and abduction of second to fifth digits, respectively. Unlike the median nerve, which allows opposition, the ulnar nerve is responsible for the adduction of the thumb. The axillary nerve is the result of the network of C5 and C6 spinal nerves, which arise toward the deltoid muscle allowing for abduction; and the teres minor allowing for external rotation of the shoulder.

It provides the function of extending the wrist, elbow, and metacarpophalangeal joints of digits and supination by the supinator muscle. Collateral nerves of the brachial plexus include the following [21] :. The long thoracic nerve is known for allowing the protraction and superior rotation of the scapula, while the suprascapular nerve allows for shoulder abduction by the supraspinatus muscle and lateral rotation of the shoulder by the infraspinatus muscle.

There have been several reports regarding anatomical variations of the brachial plexus. Knowledge of potential anatomic variations is important to mitigate the risks of iatrogenic injuries. For example, a split median nerve pattern was observed with an anomalous muscle on the right forearm of a year-old male cadaver in one study.

An anatomical variation was found because of the splitting of the median nerve into two nerves in the proximal third of the forearm, then reuniting at the medial third as a single nerve.

Moreover, a study on anatomical variations of the brachial plexus suggested that there is a higher risk of injury for variated nerve branches on fetal cadavers used in the study. Surgeons need to have a better understanding of the possible anatomical variations of the brachial plexus, including extensions, whenever they provide surgical intervention to the upper limb. Other variations include direct branches from C6 to the Pectoralis Minor and Latissimus dorsi muscles, innervation of the deltoid muscle by C6 and C7 roots, and the lateral pectoral nerve arising from the posterior division of the upper trunk.

The median nerve was also observed to be lateral to the axillary artery. Patients typically lose sensation, motor power, and often experience disabling neuropathic pain following traumatic brachial plexus injuries.

Lesions of the brachial plexus can generally divide into upper and lower lesions. These lesions indicate important landmarks to determine the specific spinal nerves of the brachial plexus that are affected. Erb-Duchenne palsy is due to a lesion in the upper brachial plexus.

Erb-Duchenne palsy usually occurs when both the head and the shoulder of the patient are separated by force in cases of birth injury, disk herniation, or accident. The resulting trauma will damage the C5 and C6 spinal nerves affecting the axillary, suprascapular, and musculocutaneous nerves.

The loss of axillary and suprascapular nerves is observable when the arm is medially rotated and adducted at the shoulder. Another described variation that may have similar consequences is the phrenic nerve arising entirely from the BP.

Figure 7. Cadaveric dissection showing a two trunked brachial plexus. Nerve stimulation does not allow clinicians to account for and adapt to the presence of anatomical variations.

Ultrasound on the other hand facilitates real-time visualisation, recognition of any anatomical variants and modification of a block technique based on the visualised anatomical variation. In the case where all the nerve roots and trunks are not found to pass between anterior and middle scalene muscles on ultrasound, the interscalene block can be successfully completed by injecting anaesthetic in the interscalene groove as well as in areas adjacent to the variant roots.

In an ultrasound guided supraclavicular BP block, the superior trunk was noted to be medial to the subclavian artery whereas the middle and inferior trunks were in the usual location lateral to the subclavian artery. Anatomical variations at this level are present in ADMT can connect with the medial cord, lateral or both roots of the median nerve or can give rise to the lateral root of the median nerve with no connection to the lateral cord which is then formed solely by the anterior division of the upper trunk.

Multiple variations in the formation of the cords of the BP have been noted. The ventral ramus of T1 can fail to contribute to the posterior cord and it may provide the sole contribution to the medial cord. A pair of cords, one small and one large, can replace the usual pattern of three cords.

Cases of formation of a single cord have even been reported. Significant variation has been described in the origin of the branches of the BP too Figure 8. Figure 8. Anatomical variations in the origin of the branches of the brachial plexus.

Brachial plexus variations in its formation and main branches. Acta Cir Bras ; Normally, the three cords of the BP encircle the axillary artery but, in some cases, all of these cords can lie lateral to the axillary artery. Clinically, if this anatomical variation is visualised prior to an infraclavicular block, the anaesthetist could insert the needle lateral to the axillary artery and entirely avoid the artery.

The BP demonstrates a constant relationship to certain vessels which can be explained from a embryological perspective. If however it is derived from the sixth, eighth or ninth intersegmental branch of the dorsal aorta, as it sometimes is, the relationship between the axillary artery and BP is correspondingly altered. In an ultrasound study which evaluated the arrangement of the median, ulnar, radial and musculocutaneous nerves around the axillary artery, ten different arrangements of the four nerves were found Figures 9 and The classical anatomical picture was present in only An accessory axillary artery, running parallel to the axillary artery, was observed in 2.

Classically, the musculocutaneous nerve is described as lying in the coracobrachialis muscle or between the coracobrachialis and biceps muscles at the level of axillary block. If joined to the median nerve, it can run distally for a variable distance before separating from it. In these two described ultrasound studies, the incidence of anatomical variation was different to that previously reported from the results of anatomical dissection.

In view of these possible anatomical variations, the axillary block procedure must be modified to accommodate them.

Nerves around the axillary artery can be individually traced with ultrasound as they course distally from the axilla to confirm their nature, identity and relationship. In the case where the musculocutaneous nerve does not lie in its classical location, ultrasound guidance can be used to locate the aberrant nerve so that it may be blocked separately at its new location. Moreover, in the case where the musculocutaneous nerve fuses with the median nerve, there is potentially no need for an additional anaesthetic injection since the target nerve, the musculocutaneous nerve, is ensheathed along with the other nerves of interest for the axillary block.

Figure 9. Schematic drawing of the most common arrangement of four nerves of the brachial plexus M, median nerve; U, ulnar nerve; R, radial nerve; MC, musculocutaneous nerve around the axillary artery AA.