Clinical anatomy of the nerve supply to the upper limb

Learning objectives.

By reading this article, you should be able to:

• •Know the neural anatomy of the upper limb.
• •Apply this knowledge to clinical practice of appropriate regional anaesthesia techniques and to prevent iatrogenic injury.
• •Identify the anatomy on ultrasound in relation to the three-dimensional structure of the upper limb.

Key points.

• •The brachial plexus is a complex network of nerves originating from spinal roots C5–T1.
• •The brachial plexus is a tethered superficial structure with a narrowing between the clavicle and the first rib. Its superficial nature facilitates reliable ultrasound identification, but also renders it vulnerable to injury.
• •Understanding the key anatomy will improve the safety and efficacy of regional anaesthesia.
• •A working knowledge of the anatomical course of the nerves of the upper limb enables protection of nerves during anaesthesia and correct placement of skin electrodes for neuromuscular monitoring, and can facilitate the diagnosis of nerve injury.

In 1963, Dr David Little noted the clinical relevance of the brachial plexus to the anaesthetist: ‘Man uses his arms and hands constantly. As a result, he exposes his arms and hands to injury constantly. The combination of man's prehensility and his unflagging appetite keeps a steady flow of patients with injured upper extremities and full stomachs streaming into hospital emergency rooms. This is why the brachial plexus is so frequently the anaesthesiologist's favourite group of nerves’.¹

To deliver comprehensive care to patients, the anaesthetist must have an understanding of the anatomical implications of the brachial plexus. This allows for holistic care of the patient, including suitable pain relief with regional block and avoidance of iatrogenic injury.

Anatomy of the brachial plexus

The brachial plexus is a complex network of nerves originating from spinal nerve roots (C5–T1). Its course can be described as superficial, narrowing to a ‘pinch point’ between the clavicle and first rib forming an almost hourglass shape (Fig. 1). The plexus is expanded at each end, yet is also fixed to the intervertebral foramen and axillary sheath. This makes it vulnerable to avulsion, compression and surgical trauma as potential mechanisms of injury.² However, its course also enables us to reliably visualise its position using ultrasound and exploit this for regional anaesthesia. Direct needle trauma from regional anaesthesia is another potential source of plexus injury.

Fig 1.

Simplified brachial plexus anatomy demonstrating the different points at which it divides with the anatomical locations of plexus blocks and subsequent cutaneous sensory coverage. Inset image demonstrates the ‘hourglass’ shape of the plexus. Note that the nerve roots in the cervical region (C5–8) emerge above the corresponding vertebra, whilst the T1 nerve root appears below. A, axillary nerve; M, median nerve; Mc, musculocutaneous nerve; R, radial nerve; U, ulnar nerve.

A working knowledge of the anatomical course of the nerves of the upper limb enables their protection during anaesthesia and the correct placement of skin electrodes for neuromuscular monitoring, and can facilitate diagnosis of nerve injury. The plexus initially forms from the anterior primary rami from spinal nerve roots C5–T1, with variable contributions from C4 and T2.³ Having joined, the trunks pass through the interscalene groove, a palpable anatomical landmark between the anterior and middle scalene muscles. Most commonly, the C5 and C6 rami join to form the superior trunk near the medial border of middle scalene. The C7 ramus continues to solely form the middle trunk, whilst the C8 and T1 rami form the inferior trunk (Fig. 1).

The trunks emerge from the interscalene groove passing the lateral border of the first rib where they each divide into anterior (flexor) and posterior (extensor) divisions, which run behind the clavicle.³ These are then further reorganised into cords taking their name based on their arrangement around the axillary artery in the axilla, which they accompany along with the subclavian-axillary vein (Fig. 2). Each possesses two major terminal nerve branches alongside a number of intermediary branches.³

Fig 2.

Computer-generated image from cadaveric specimen demonstrating the dissected brachial plexus, including its relationship to the major arteries.

All the posterior divisions unite to form the posterior cord, which mostly supplies the posterior/extensor aspect of the arm, forearm and hand via the main terminal branches: the radial and axillary nerves. The anterior divisions of the superior and middle trunks join, forming the lateral cord, whilst the anterior division of the inferior trunk forms the medial cord. The lateral cord supplies the anterior/flexor compartment of the arm and sensation across the lateral and anterior aspects of both the arm and forearm primarily via the musculocutaneous nerve (Fig. 3). It also provides the lateral root of the median nerve. The medial cord provides the medial root of the median nerve, which supplies the medial side of the arm and forearm via the medial cutaneous nerve(s) of the arm and forearm. The other terminal branch, the ulnar nerve, supplies both sensation and motor functions to the anterior/flexor compartment of the forearm and hand (Fig. 3).

Fig 3.

Cutaneous sensory distribution of the arm and forearm Original image courtesy of Professor Alice Roberts. inf., inferior; lat., lateral; n., nerve.

Superior and middle trunks

Clinical scenario 1:

A 52 yr old patient undergoes a laparoscopic hysterectomy in a marked Trendelenburg position with a shoulder brace to ensure safe positioning on the table. It is a prolonged procedure and following emergence from anaesthesia the patient is unable to abduct her left shoulder and has reduced left elbow flexion. There is no distinguishable sensory deficit.

The upper plexus is at risk of stretch in prolonged procedures where excessive shoulder depression exists, this is exacerbated by contralateral neck flexion. In a marked Trendelenburg position, continued stretching and compression of the superior and middle trunk by a brace depressing the shoulder may result in nerve injury (Table 1).

Damage to the upper portion of the brachial plexus usually manifests with motor deficit in the C5/6 myotomes, and commonly there is no sensory impairment (Table 1).⁴ This clinical picture is described as Erb's palsy (waiter's tip position), where injury to the upper brachial plexus leads to the arm being held by the patient's side, medially rotated and pronated.²

Table 1.

Superior/middle trunk injury and clinical manifestations.

Nerve root	Trunk	Nerve affected	Sign/symptom
C5/6	Superior	Musculocutaneous	Loss of biceps brachii function (elbow flexion) Potential paraesthesia: lateral forearm
C7	Middle	Axillary	Loss of deltoid function (shoulder abduction) Potential paraesthesia: lateral shoulder and arm
C5	Superior	Suprascapular	Loss of supraspinatus function (shoulder abduction) and infraspinatus (external rotation)

The long thoracic nerve (C5/6/7) and dorsal scapular nerve (C5/6) originate directly from the nerve roots, and therefore can be used to differentiate between injuries of the plexus or a proximal nerve root injury.⁵ These two nerves will be spared (preservation of scapula abduction and elevation) if the damage has occurred at the plexus rather than the nerve roots.

Measures to reduce plexus stretching and avoid compression by the clavicle and humeral head include:⁶

• (i)avoiding prolonged use of the Trendelenburg position and shoulder compression
• (ii)careful positioning to avoid shoulder abduction of >90°
• (iii)ensuring a neutral neck position

Inferior trunk

The inferior trunk is formed by the C8 and T1 rami, and the anterior division of this continues on to become the medial cord. This gives rise to the ulnar and median nerves. However, as the median nerve also has contributions from the anterior roots of C5–7, an injury to the inferior trunk does not result in any median nerve clinical manifestations.

This is not the case for the ulnar nerve, and should the inferior trunk be damaged the resulting injury exhibits an ulnar nerve distribution (Table 2). This is classically described as Klumpke's palsy with a claw hand deformity, radial deviation of the wrist and hand weakness. The ‘claw deformity’ occurs because of the unopposed action of the long flexor and extensor muscles, resulting in hyperextension of the metacarpophalangeal joints with concurrent flexion of the interphalangeal joints (Fig. 4).⁷ Radial deviation is caused by loss of flexor carpi ulnaris function.

Table 2.

Inferior trunk injury and clinical manifestations.

Nerve root	Trunk	Nerve affected	Sign/symptom
C8/T1	Inferior	Ulnar	Loss of function of intrinsic muscles of the hand (lumbricals and hypothenar muscles) Long-term: loss of flexors of the fingers and wrist (claw hand deformity) Paraesthesia: medial (ulnar) portion of palm and 1.5 fingers
C8/T1	Inferior	Medial cutaneous nerve of the arm	Paraesthesia: medial and partial anterior aspects of the arm
C8/T1	Inferior	Medial cutaneous nerve of the forearm	Paraesthesia: ulnar border of the forearm
T1	NA	Sympathetic fibres	Manifestations of Horner's syndrome

NA, not applicable.

Fig 4.

Ulnar claw hand deformity. Reprinted from Hussain and Winterton, with permission.⁷

Patients with a lower trunk injury can develop Horner's syndrome from concurrent injury to the sympathetic fibres that leave the spinal cord at T1. Inferior trunk damage may occur (including sympathetic fibres) in cases where there is prolonged or excessive shoulder abduction (>90°) and if the arms are below the height of the torso, exacerbated by contralateral rotation of the head.⁶

In cardiothoracic surgery, the retractors used for surgical access to the chest can potentially cause the first rib to place pressure against the lower trunk of the plexus, causing injury. This demonstrates the vulnerability of the plexus because of its shape and ‘pinch point’ at the rib and clavicle (Fig. 1). Inferior trunk injuries complicate 5–10% of cardiac surgical cases, resulting in a mostly sensory impairment in the ulnar nerve distribution. This impairment recovers fully in 96–98% of cases, but lasts for up to 4 weeks.⁸

Regional anaesthesia and the brachial plexus

The choice of regional block of the brachial plexus depends on the site of surgery, factors related to the patient and, to an extent, preference of the operator. A detailed anatomical knowledge is required to select and perform a successful block.

Figure 1 demonstrates the position and sensory block achieved with each regional technique. A block at the axilla does not block the plexus per se, but rather the individual terminal branches (see later).

Traditionally, the brachial plexus used to be considered as a dense tubular structure that extended from above the first rib to the axilla. Subsequently, it was felt an injection of local anaesthetic anywhere along this site would result in spread both distally and proximally, with volume being essential for good spread.^3,9 This is partly true because the plexus structures are connected. However, it is more complicated in practice, as the brachial plexus is multicompartmental with discrete fascial septa that limit spread of local anaesthetic.³

Ultrasound is now regarded as a vital tool to enable visualisation of the plexus and needle, and thereby permits safe deposition of local anaesthetic in the correct location. The plexus anatomy is well visualised by ultrasound, as it is both superficial and consistent in location (although there is significant anatomical variation both between individuals and between opposite sides in the same individual).³ Challenges with plexus blocks arise when identifying anatomy via landmarks or ultrasound imaging becomes difficult. Common examples include obese or muscular patients, in which supraclavicular or infraclavicular nerve blocks can be more challenging.

Interscalene block

This approach is classically regarded as ulnar sparing, as the C8, T1 nerve roots and lower trunk may not be covered; this results in the ulnar nerve or medial cutaneous nerve of the arm being missed in up to 50% of cases.³ Care must be taken with the lateral approach, as the dorsal scapular nerve lies within the middle scalene and can be accompanied by the long thoracic nerve. When needling, it is important to avoid this structure if visualised.

The plexus proximal to the clavicle is beneath the prevertebral (deep cervical) fascia, notably in conjunction with the phrenic nerve, which is found on the surface of the anterior scalene muscle. Local anaesthetic within this fascial plane can track around the muscle involving the phrenic nerve, resulting in hemidiaphragmatic paralysis, with higher volumes increasing the likelihood.¹⁰ The result is a loss of lung capacity of 15–30%, which may impair respiratory function in some patients.¹¹ Alternatives to the interscalene block (ISB) to minimise phrenic nerve paralysis include the recently described superior trunk block, or a combination of axillary and suprascapular nerve block.¹²

The ISB is used for awake shoulder surgery in combination with a superficial cervical plexus block (to block the supraclavicular nerves) to provide anaesthesia over the cape of the shoulder (Fig. 3). However, local infiltration is often required for the posterior port site when surgery is performed arthroscopically. A superficial cervical plexus block can be used either solely or in combination with a general anaesthetic for carotid artery surgery.

The ISB most reliably provides anaesthesia and analgesia for surgeries involving the shoulder joint, the proximal humerus and the lateral two-thirds of the clavicle.¹³ Figure 5 shows the ultrasound anatomy visualised at this point, and can be found by scanning up from the clavicle or from identifying the cricoid cartilage at the level of C6 and scanning laterally.

Fig 5.

Cross-sectional and ultrasound interscalene brachial plexus anatomy (right arm). The roots are seen in the interscalene groove between the middle scalene muscle (MSM) and the anterior scalene muscle (ASM). The C6 root can commonly divide into two equally hypoechoic structures at this point on ultrasound. CA, carotid artery; DSN, dorsal scapular nerve; IJV, internal jugular vein; TP, transverse process of C7; VA, vertebral artery. The likely positions of the phrenic nerve (PN) are marked.

Supraclavicular block

The plexus is found at the first rib superficial to the subclavian artery, typically in a posterolateral position (although an anterolateral position can be seen).¹³ At this location, it is separating into anterior and posterior divisions, which are easily identified using the ultrasound transducer behind the clavicle (Supplementary Fig. S1). This block provides a rapid dense block covering a significant proportion of the upper limb dermatomes (Fig. 1). It provides reliable analgesia or anaesthesia of surgical sites from the proximal humerus distally to the hand.

The popularity of this block has increased with ultrasound, as direct visualisation of the pleura reduces the complication of pneumothorax.¹⁴

Infraclavicular block

This block is often the selected site for catheter insertion should continuous infusion of local anaesthetic be required, and provides the same dermatomal cover as a supraclavicular approach (Fig. 1).¹³ Increasing availability of ultrasound has improved the safety and efficacy of the ISB, given the proximity of the pleura and axillary artery. Historically, larger volumes of local anaesthetic were required to achieve an effective block, which, despite being a more distal plexus block, did have a reasonable incidence of phrenic nerve palsy. Ultrasound guidance has reduced the risk of axillary vessel and pleural puncture. It has also increased efficacy with smaller local anaesthetic volumes in a portion of the plexus that demonstrates wide anatomical variation and spread as it begins to fan out from its divisions to medial, lateral and posterior cords (Supplementary Fig. S2).

Anatomy of the axilla

Once the brachial plexus reaches the axilla, it has divided into terminal branches. This occurs at the apex of the axilla at the inferior border of pectoralis minor.¹⁵ One of the two terminal branches of the posterior cord, the axillary nerve, has already divided at this stage to supply the deltoid muscle and skin across the shoulder and upper portion of the arm, the so-called regimental badge area (Fig 1, Fig 3).

The terminal branches of the plexus contained within the axilla are found within an axillary sheath. This connective tissue surrounds the neurovascular structures of the plexus and is a continuation of the prevertebral fascia separating the anterior and middle scalene muscles.³ Any damage or excessive stretching (e.g. prolonged abduction of the arm beyond 90° under anaesthesia) of the structures here will manifest itself as damage to a terminal nerve.

The terminal nerves maintain a similar orientation around the axillary artery to the cords from which they originate. The anatomical variation within this portion of the brachial plexus is significant and identifying nerves may be difficult, as only four separate nerves will be seen in 78% of cases.¹⁶ Additionally, the vascular relationships of the nerves are affected by changes in applied external pressure and arm position.³

The axilla also contains the origins of the medial and lateral pectoral nerves that arise from the anterior aspect of the medial and lateral cords, respectively.¹⁵ These nerves supply the pectoralis muscles and may be blocked more distally as part of the anaesthetic management of breast surgery with a reduction in postoperative opioid requirement.¹⁷

Axillary block

When depositing local anaesthetic around the terminal nerves in the axilla, it is suggested that only 65% of cases demonstrate ‘normal’ anatomy described in textbooks (Fig. 6). The configuration of nerves around the axillary artery is extremely variable.¹⁶ Positioning of the axillary vein is equally varying, although it is commonly found beside the median and ulnar nerves; in fact, there can be multiple vessels around the axillary artery (Fig. 7). When performing an axillary block, care must be taken to avoid pressure from the ultrasound transducer and intravascular injection of local anaesthetic.

Fig 6.

Schematic diagram showing the common locations of the nerves around the axillary artery (AA). The orientations are in relation to the AA as if the patient's left arm was slightly abducted. Viewing it like a clock face, the most common locations are median nerve (M) positioned at 11–12 (81%), ulnar nerve (U) at 2–3 (85%), and the radial nerve (R) at 4–6 (89%). The musculocutaneous nerve (Mc) is found laterally between the biceps brachii (BB) and coracobrachialis (CB) muscles at 8–9 (90%).¹⁴

Fig 7.

Cross-sectional view of left axillary fossa with corresponding ultrasound image. The axillary artery (AA) and axillary veins (AV) are enclosed within adipose tissue. BB, biceps brachii; CB, coracobrachialis muscle; McN, musculocutaneous nerve; MN, median nerve; RN, radial nerve; UN, ulnar nerve.

Terminal branches of the arm and forearm

Ulnar nerve (C8/T1)

Figure 8 demonstrates a schematic representation of the ulnar nerve through the arm and forearm terminating in its deep and superficial branches over the pisiform bone. Upon emerging from the medial cord, the ulnar nerve passes deep through the arm medial to the brachial artery and provides no sensory or motor branches until it arrives in the forearm.

Fig 8.

Schematic representation of the ulnar nerve route through the arm and forearm.

The nerve becomes very superficial as it passes behind the medial epicondyle of the humerus and enters the forearm between the two heads of flexor carpi ulnaris giving off its first sensory branch to supply proprioception of the elbow joint. Its course continues in the forearm deep to flexor carpi ulnaris and laterally over flexor digitorum profundus, giving motor supply to both muscles. It continues medial to the ulnar artery and crosses over the flexor retinaculum before giving its terminal branches in the hand supplying the muscles and sensory areas (Table 2).

Clinical scenario 2:

A 33 yr old patient with Crohn's disease demonstrates evidence of a caecal perforation on a CT scan and he undergoes a right hemicolectomy. He last received rocuronium 20 mg 60 minutes ago; when a nerve stimulator is applied to his wrist he has 2 twitches with minimal fade with double burst stimulation.

Commonly nerve stimulator electrodes are placed at the wrist to assess degree of neuromuscular blockade. In this scenario the negative electrode is placed to stimulate the ulnar nerve and should be placed on the anterior medial aspect of wrist as close to the course of the nerve as possible. What is then observed is contraction of the adductor pollicis muscle resulting in adduction of the thumb.

Because of its superficial location, ulnar nerve injury at the medial epicondyle can be caused by direct pressure or forearm flexion, and is three times more common in men.² The resulting cubital tunnel syndrome can easily be avoided with careful positioning of the patient during anaesthesia. Exploring the clinical symptoms and signs of the lesion will differentiate the location of the ulnar nerve lesion, especially as Klumpke's palsy and cubital tunnel syndrome can be hard to distinguish (Table 3).

Table 3.

Differentiating ulnar nerve damage.

Site of injury/damage	Syndrome/palsy	Reasons for differentiation
Inferior trunk	Klumpke's palsy	Lesion affects medial cord: signs/symptoms of cubital tunnel syndrome plus involvement of the medial cutaneous nerve of the arm/forearm
Medial epicondyle	Cubital tunnel syndrome	Paraesthesia in an ulnar distribution and reduced grip strength/hand weakness; prolonged injury will result in a claw hand deformity
Guyon's canal (wrist)	Guyon canal syndrome	Paraesthesia and motor loss as mentioned previously: will not develop a claw hand deformity (lesion below the supply to flexor digitorum profundus and flexor carpi ulnaris)

Conversely, the superficial nature of the ulnar nerve here can help with ultrasound identification of the nerve for regional technique, albeit local anaesthetic should not be injected directly into the tight cubital fossa space (Fig. 9), but rather elsewhere along its course.

Fig 9.

Cross-sectional view of right antecubital fossa with ultrasound image. The ulnar nerve (UN) is visible behind the medial epicondyle. The median nerve (MN) is medial to the brachial artery (BA), whilst the radial nerve is found between the brachioradialis muscle (BRM) and brachialis muscle (BM). PT, pronator teres muscle; RN, radial nerve.

Median nerve (C5–T1)

The median nerve has contributions from all the anterior rami of the brachial plexus with contributions from the medial and lateral cords just prior to the axilla. Figure 10 shows a schematic view of the course of the median nerve from the axilla and its position in the arm and forearm. It supplies no structures in the arm itself, but once in the forearm it supplies multiple muscles in the hand and forearm, and a sensory supply to the hand (Fig. 3).

Fig 10.

Schematic representation of the median nerve route through the arm into the forearm.

Radial head fixation can be associated with median nerve damage either from initial injury, carpal tunnel compression or surgical fixation. Regional anaesthesia should also be excluded as a cause of any injury.² Median nerve injury results in a loss of sensation in the hand along with loss of pronator teres function (hand pronation). As any potential median nerve lesion becomes more distal, there is less forearm involvement, and the signs and symptoms are predominantly in the hand.

Regional block of this nerve can be performed at multiple points. Ultrasound imaging at the elbow demonstrates the median nerve's close proximity to the medial aspect of the brachial artery (Fig. 9). From here, it is possible to scan proximally and visualise the nerve continuing from the brachial plexus. Visualisation of the nerve improves by scanning it distally as it appears more hyperechoic between flexor digitorum profundus and superficialis.

Radial nerve (C5–T1)

The radial nerve continues from the posterior cord of the brachial plexus and remains primarily within the posterior compartment, supplying sensory and motor innervation to both the posterior arm and forearm. Initially, the nerve remains posterior to the axillary artery and continues with the profunda brachii vessels through two heads of the triceps muscle before running around the spiral groove of the humerus. It then runs more anteriorly between the brachioradialis and brachialis muscles to terminate over the lateral epicondyle in two branches (superficial radial nerve and posterior interosseous nerve). These branches then supply the extensor compartment of the forearm, sensation of the overlying skin and posterior hand (Fig 3, Fig 11).

Fig 11.

Schematic representation of the radial nerve route through the arm terminating at the lateral epicondyle.

Prolonged tourniquet pressure across the humerus and spiral groove can cause damage to the radial nerve, resulting in loss of the forearm extensors and wrist drop. In addition, the sensory distribution of the superficial radial nerve may be disrupted. Radial nerve injury may also be caused by compression against a patient screen or arm board, resulting in paraesthesia along the posterior surface of the lower part of the arm/forearm.²

When using ultrasound to identify the nerve for regional block, it is usually performed above the lateral epicondyle to ensure the terminal branches are covered. Placing the transducer 3–4 cm above the elbow crease ensures an adequate view of the nerve, which appears hyperechoic and can be traced upwards to demonstrate its course within the spiral groove (Supplementary Fig. S3).

Conclusions

Applied knowledge and understanding of the anatomy of the nerves of the upper limb are essential for clinical practice, in planning and providing regional anaesthesia and perioperative analgesia. It should also help the anaesthetist to avoid iatrogenic injury, and improve the diagnosis and management of nerve injuries.

Acknowledgements

The authors would like to thank Anatomage, Inc. for allowing the use of their cadaveric images from the Anatomage Table used in this publication (Figs 2, 5, 7 and 9, and all supplementary figures); Professor Alice Roberts for the use of Figure 3; and Mr Zaf Naqui (consultant hand, wrist and upper limb surgeon, Salford Royal NHS Foundation Trust) who confirmed the accuracy of this article.

Biographies

Nathan Betteridge FRCA is a specialty trainee in HEE North West. He organises and runs the North West deanery primary and final FRCA anatomy courses.

Robert Hartley BSc (Hons) FRCA is a consultant anaesthetist within the Royal Cornwall Hospitals NHS Trust. He has a postgraduate certificate in regional anaesthesia and was previously clinical lead for regional anaesthesia. He is an organiser of the SonoAnaesthesia course in the North West, focusing on the applied use of ultrasound.

Adam Taylor BSc (Hons) PhD SFHEA is the director of the Clinical Anatomy Learning Centre and professor in anatomy at Lancaster Medical School. He has pioneered the integration of student ultrasound teaching with anatomy, increasing their proficiency in anatomy, use of imaging and interpretation on their entry into clinical practice.

Matrix codes: 1A01, 1H02, 2G01, 2G03, 2G04, 3A09, 3I00

Declaration of interests

The authors declare that they have no conflicts of interest.

MCQs

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Supplementary Material

The following are the Supplementary material to this article:

Supplementary Fig. 1.

Supraclavicular cross-section of brachial plexus with similar ultrasound image (right arm) - the probe is placed behind the clavicle (Cl), parallel to it initially then the lateral aspect of the probe is rotated posteriorly to optimise the image. This gives an oblique ultrasound view of the plexus and the first rib and is why it appears more compact. The brachial plexus (BP) is seen in relation to the subclavian artery (SA). With permission from Anatomage, Inc.

Supplementary Fig. 2.

Infraclavicular brachial plexus cross-section with concurrent ultrasound view (right arm). The brachial plexus (medial cord - MC, posterior cord - PC and lateral cord - LC) can be seen surrounding the axillary artery (AA) with the axillary vein (AV). The plexus lies below the fascia (orange line) of the pectorals minor muscle (PMinM); PMajM - pectoralis major muscle.

Supplementary Fig. 3.

Cross-sectional view of the humerus as the radial nerve (RN) leaves the spiral groove accompanying the profunda brachii artery (PB) with concurrent sonoanatomy (right arm). MN - Median nerve; BB - Biceps Brachii; TM - Triceps muscle; BA - Brachial Artery.