Axilla and Brachial Plexus

Reading resources:
Moore and Agur, Essential Clinical Anatomy
pp 293 - 301

Objectives:

At the end of this laboratory you should be able to:

    1.  Use anatomical terminology for regions of the upper limb: axilla,
         brachium, cubital fossa, antebrachium, carpus, digits.
    2.  Identify the bony landmarks of the axillary region.
    3.  Name the anatomical structures that make up the boundaries of the
         axilla; identify the muscles that contribute to each wall of the axilla.
    4.  Identify the axillary artery and its major branches.
    5.  Identify the veins which contribute to the axillary vein.
    6.  Identify the brachial plexus of nerves and its medial, lateral,
         and posterior cords.
    7.  Identify the innervation of the muscles of the axillary region.

Bony landmarks

    On the skeleton, review the bony features of the scapula, humerus and rib cage.

    Note that the anatomical neck of the humerus is immediately distal to the head, while the surgical neck is located at the junction of the upper end of the humerus and the shaft.

Incisions and reflection of skin

    If you have not already done so, make an incision from the acromion to a point approximately 2 inches above the elbow; another incision encircling the arm (brachium) and remove the skin covering the axilla and arm.

    Clean the fatty connective tissue in the axillary fossa.  There are a number of important lymph nodes in the region, but they are not readily seen in most gross dissections.

Boundaries of the Axillary Fossa

    The axilla is the region between the upper aspect of the arm and the chest.  The axilla, or axillary fossa, includes structures deep to the skin and fascia; hence the term is not exactly the same as the vernacular term 'armpit', which refers to the external space between the arm and the chest wall.  On the skeleton locate the apex of the axilla.  The apex of the axilla is the space between the clavicle, the upper border of the scapula and the first rib.  The base of the axilla is formed by fascia (axillary fascia) extending from the pectoralis major muscle and the latissimusdorsi muscle, and the skin covering it.

    In a previous dissection , the muscles contributing to the anterior wall were dissected: pectoralis major, pectoralis minor, and subclavius (Netter, 399).  The lateral border of the pectoralis major forms the anterior axillary fold.

    Identify the three muscles of the posterior wall of the axilla:  Latissimus dorsi, teresmajor, and subscapularis (Netter, 397).  The posterior axillary fold is formed primarily by the lateral border of the underlying latissimus dorsi.  The lower border of the axilla is described in textbooks as the lower border of the teres major.  Confirm this by dissecting the lower border of the axilla, noting how the latissimus dorsi twists around the teres major to insert into the bicipital groove of the humerus.  Contraction of latissimus dorsi results in a combined movement of adduction, medial rotation and extension of the upper limb.  Teres major has a similar action.  Subscapularis is a medial rotator of the upper limb, the most important muscle for this action.  It extends from the subscapular fossa to the lesser tubercle of the humerus.

    Identify the serratus anterior muscle, which along with the chest wall, forms the medial wall of the axilla (Netter, 174, 395).  Identify the long thoracic nerve, the motor supply to the serratus anterior.  NOTE that the nerve and artery supplying the serratus anterior are found on its superficial surface; for most muscles the nerve will be deep.  Be careful cleaning the surface of the muscle to avoid destroying the nerve and artery.  The lateral wall of the axilla is formed by the humerus and muscles associated with it, the coracobrachialis and the biceps brachii (Netter, 402, 404, 406).  Identify the long and short heads of the biceps and note their origins.  The biceps inserts into the bones of the forearm.  Observe the origin and insertion of the coracobrachialis.

    With the assistance of the skeleton study the attachments of the muscles that comprise the walls of the axilla.  As you observe the origin and insertion of each muscle try to relate this to the action of the muscle.

    Question:  Why do backpackers sometimes lose motor control of serratus anterior?

Dissection of Axillary Vessels

    As the fatty connective tissue is removed from the axilla, blood vessels and nerves will be seen.  Arteries in the region are primarily branches of the axillary artery.  The axillary artery and its branches are shown in Netter (Fig. 398, 400).

    Veins in the region follow the arteries, often accompanying them as vena comitantes, a pair of veins.  Look for the axillary artery.  It is covered by the pectoralis minor muscle anteriorly.  Divide this muscle in the middle and reflect the lateral half downward with its clavipectoral fascia Netter, 398).  The axillary artery may not be immediately obvious, becauase, along with the axillary vein and nerves supplying the upper limb, it is enclosed in a sheath of connective tissue, the axillary sheath (Grant, Fig. 6.17, 6.19), a structure not illustrated in Netter's Atlas.  Make an incision along the sheath to expose the artery, vein and the cordsofthebrachialplexus.  The cords of the brachial plexus will be dissected in detail later.

    NOTE the formation of the axillary vein from the brachial and basilic veins.  Remember that in the previous dissection you found the cephalic vein.  Is it still intact?  Note that it joings the axillary vein in the infraclavicular fossa.  The axillary vein may obscure your view of the axillary artery and its branches.  Remove veins as needed to improve your field of view.

    The axillary artery is divided into three parts (for descriptive anatomical purposes) by the pectoralis minor:  the first part proximal to the muscle, the second part deep to the muscle, and the thrid part distal to the muscle.  Usually there will be a small branch arising from the first part, the supreme thoracic artery, which supplies the first intercostal space.  Two arteries arise from the second part of the axillary artery:  the thoracoacromial trunk and the lateral thoracic artery.  Note the distribution of these arteries (Netter, 400).  The thoracoacromial trunk has four branches:  a small clavicular branch, a pectoral branch (which runs with the lateral pectoral nerve), a deltoid branch (which runs parallel to the cephalic vein), and a small acromial branch (Netter, 405).  Three branches of the third part of the axillary artery are the sub scapular artery, the anterior humeral circumflex artery, and the posterior humeral circumflex artery.  Trace the subscapular artery from the axillary artery to the point at which it divides into two branches, the circumflex scapular artery and the thoraco dorsal artery.  The thoracodorsal artery supplies the latissimus dorsi muscle - you may also find the nerve to the latissimus dorsi, the thoraco dorsal nerve, in close proximity to the artery.  Now look for the two circumflex humeral arteries.  Note that the posterior circumflex humeral artery is usually larger than the anterior circumflex.

     You probably will not be able to identify the axillary lymph nodes.  However check out the atlas (Netter, 169) to see  their relationship to the axillary blood vessels.

The Brachial Plexus

    A good exercise is for someone to draw out the brachial plexus on the blackboard for reference during the dissection.  The brachial plexus is formed by the anterior pimary rami of cervical spinal nerves 5, 6, 7, and 8 and the first thoracic spinal nerve.  Use a diagram (such as the one in Netter, 401) to study the arrangement of spinal nerves, trunks (upper, middle & lower, divisions (anterior and posterior), and cords (medial, lateral and posterior).

    Two nerves arise directly from the ventral primary rami, the long thoracic nerve, and the dorsal scaular nerve.  You have already found the long thoracic nerve.  We will not dissect the dorsal scapular nerve today.  This nerve supplies the rhomboid muscles.

    The upper parts of the brachial plexus lie outside of the boundaries of the present dissection.  Identification of the roots (C5 -  C8, and T1) and trunks (upper, middle, and lower) will occur during the neck dissections.

    The current laboratory session is intended to help you make sense of what on first glance appears to be a complicated and confusing tangle of nerves.

    During this dissection branches of the medial, lateral, and posterior cords will be identified (Netter, 401).  The formation of the median nerve, together with the medial and lateral cords appear as a big letter 'M' overlying the axillary artery (Netter, 400).  The median nerve has a root from the medial cord as well as a root from the lateral cord.  Be prepared for some variability in the brachial plexus.  A common variation is one in which peripheral nerves are formed more distally than expected from the atlas.

        1.  Lateral cord

     In the dissection of the pectoral region you may have found the small lateral pectoral nerve, a branch of the lateral cord.  The lateral pectoral nerve can usually be found passing medial and superior to the medial border of the pectoralis minor muscle.  Clean the nerve between the pectoralis major and the lateral cord.  The lateral cord ends as two large terminal branches, the lateral root of the median nerve and the musculocutaneous nerve.  The musculocutaneous nerves supplies the three muscles of the anterior compartment of the arm and then continues as a cutaneous nerve, the lateral cutaneous nerve of the forearm, which you will find later.

        2.  Medial cord

    From the medial cord, dissect the medial pectoral nerve, the medialbrachialcutaneousnerve, the medial antebrachial cutaneous nerve, the ulnar nerve, and the medial root of the median nerve.

    We know from a previous dissection that the medial pectoral nerve pierces the pectoralis minor muscle.  If you haven't already, trace it to the medial cord.  Since the pectoral nerve are tiny you may have difficulty identifying them. So - not all of you will become surgeons.  The medial brachial cutaneous nerve supplies cutaneous innervation to the medial side of the arm.  It gets destroyed quite often in Med II dissections as it ends in the subcutaneous fascia.  The medial antebrachial cutaneous nerve is larger and longer than the medial brachial cutaneous nerve, and is readily identifiable.  It supplies cutaneous innervation to the forearm (the antebrachium).,  The medial cord ends by dividing into the ulnar nerve and the medial root of the median nerve.  In the anatomy practical exam, a number of students get the ulnar nerve and the medial antebrachial nerve confused - they come off the medial cord next to each other.  Note how the medial antebrachial nerve crosses the ulnar nerve.  Which one is more lateral?  Which one runs behind the 'funny bone' (medial epicondyle of the humerus) of the elbow?

        3.  Posterior cord

    From the posterior cord there are five branches.  Identify the thoracodorsal nerve, the nerve supply to the latissimus dorsi muscle.  Finding the thoracodorsal artery may help you, as the nerve runs along with the artery.  Trace the thoracodorsal nerve to the posterior cord.  Proximal to the originof the thoracodorsal nerve you should find the  upper subscapular nerve (there may actually be more than one branch) and observe that it enters the subscapular muscle, where it supplies the upper part of the muscle.  Distal to the origin of the thoracodorsal nerve is the lower subscapular nerve.  As you might have guessed, this nerve supplies the lower part of the subscapularis muscle.  However, it divides and provides a branch that supplies the teres major muscle as well.  Clean and trace this nerve to the two muscles.

    The terminal branches of the posterior cord are the radial and axillary nerves. They are very large nerves.  Note that the axillary nerve runs along with the posterior humeral circumflex artery, through the quadrangular space (yes, you should know what this space is) to enter the deltoid and teres minor muscles.  The axillary nerve also supplies cutaneous innervation to the skin overlying the deltoid muscle (in case you asked, its the upper lateral cutaneous branch of the axillary nerve).  The radial nerve supplies muscles on the back of the arm and forearm, as well as cutaneous innervation to these areas.  Note that it passes to the back of the arm through the groove for the radial nerve (spiral groove) on the humerus.

    Some of you may be able to identify the intercostobrachial nerve (Netter, 400) emerging from the second intercostal space.  If it is not immediately obvious, do not spend all day looking for it.

Review Questons

1.    The subscapularis muscle is a powerful _________ of the upper limb.

2.    What is the insertion of the latissimus dorsi muscle?  What is its action
       on the upper limb?

3.    From which regions of the body do the axillary lymph nodes receive
       their lymphatic tributaries?

4.    What is the anterior axillary fold?  How does it differ from the anterior
       wall of the axilla?

5.    Which of the following is a correct statement regarding the clavipectoral fascia?
            a.  It invests the pectoralis major muscle.
            b.  It forms a sheath for the axillary artery.
            c.  It is pierced by the cephalic vein.
            d.  The subscapular artery passes through it.

6.    If for surgical reasons, the axillary artery has to be ligated, which part of the
       artery is the most undesirable part for ligation?  Why?

7.    What are the consequences of transection of the long thoracic nerve?

8.    What nerve usually accompanies the thoracodorsal artery?

9.    In relation to which structure are the medial, lateral, and posterior
       cords of the brachial plexus named?

10.   Which partsof the brachial plexus are most likely to be compressed between
       the clavicle and the first rib?  Which blood vessel could be compressed between these two structures?

11.    The lateral cord of the brachial plexus ends by dividing into the _________ and the __________.

12.    The medial cord ends by dividing into the __________ and the ___________.

13.    Which nerves innervate theskin of the shoulder region?  What is the contribution of the brachial plexus?

14.    In this dissection, did you observe any muscular branches of the median and ulnar nerves?  Why not?

15.    Which of the following is not a branch of the posterior cord?
            a.  radial nerve
            b.  long thoracic nerve
            c.  upper subscapular nerve
            d.  thoracodorsal nerve
            e.  axillary nerve

Answers:

1.    Medial rotator
2.    Lesser tubercle and intertubercular groove of humerus.  The lats medially rotate, adduct and extend the arm.
3.    pectoral nodes drain the breast; posterior (subscapular) nodes drain the back; brachial nodes drain the upper limb; central nodes receive    drainage from pectoral, subscapular and brachial nodes; apical nodes receive drainage from all the other axillary nodes.
4.    The anterior axillary fold is bormed by the lateral border of the pectoralis major.  The anterior wall of the axilla is formed by three muscles, p. major, p. minor, and subclavius.
5.    c
6.    It is undesirable to ligate the third part (ie the part distal to the subscapular artery).  Anastomoses to the subscapular artery cannot supply the upper limb when the axillary artery is ligated distal to the subscapular artery.
7.    Cutting the long thoracic nerve would denervate the serratus anterior and result in 'winging' of the scapula.  A person with a denervated serratus anterior would have difficulty abducting  the arm farther than the horizontal position, because of the inability to rotate the scapula.  Also he would have a weakness in pushing with the upper limb on that side.
8.    The thoracodorsal nerve.
9.    The cords of the brachial plexus are named in relation to the axillary artery.
10.  The lower trunk (spinal nerves C8 and T1) (Netter, 405).  The subclavian artery.
11.   Musculocutaneous nerve and the lateral root of the median nerve.
12.   Ulnar nerve and the medial root of the median nerve.
13.   The supraclavicular nerves (from C3 and C4) of the cervical plexus supply cutaneous innervation to the shoulder.  The axillary nerve of the brachial plexus contributes cutaneous innervation to the area over the inferior half of the deltoid muscle.  (The axillary nerve also supplies the motor innervation to this muscle).
14.    The median and ulnar nerves do not have motor branches terminating on muscles of the arm.  You will see muscular branches of these nerves in the forearm and hand.
15.    The long thoracic nerve.

Comments or questions may be directed to:Dr. J. Vriend