I. The post triangle of the neck.
 

An entire lecture will be dedicated to the triangles of the neck during the head and neck unit. It is however important to describe here the posterior triangle of the neck in order to understand thoroughly the brachial plexus.
The boundaries of the posterior triangle are (Netter 22):
 -anterior boundary: posterior border of the sternocleidomastoid muscle
 -posterior boundary: anterior border of the trapezius muscle
 -inferior boundary : middle third of the clavicle

The lower portion of the posterior triangle is further divided into an occipital (upper part) and supraclavicular triangles (lower part) by the omohyoid muscle (inferior belly).  The entire posterior triangle is covered by the skin, the superficial fascia of the neck which incloses the platysma muscle (Netter 21) and by the investing layer of the deep fascia. The brachial plexus can be palpated in the lower portion of the posterior triangle, between the clavicle and the trapezius muscle (Netter 22).

II. The axilla.
 

The axilla (or armpit) is a space between the upper part of the arm and the upper part of the lateral chest. (Netter 400). This space provides a passage for nerves, blood vessels, lymph vessels travelling from (or to) the base of the neck and to (or from) the upper limb.  Classically, the axilla is described as a pyramid-shaped space with an apex , a base and four walls. The apex is the upper end of the axilla, the base the lower end of the axilla. The walls are (Netter 399):
 - anterior wall : pectoralis major m., pectoralis minor m. and subclavius m. as well as the clavipectoral fascia (see below for details)
 - posterior wall: subscapularis m. (above),  teres major m. and latissimus dorsi m. below
 - medial wall: first four (or five) ribs, intercostals m., upper part of the serratus anterior muscle
 - lateral wall: medial upper portion of the humerus

The clavipectoral fascia is a thick sheet of connective tissue attached to the inferior aspect of the clavicle (Netter 399). It splits to enclose the subclavius m. superiorly and the pectoralis minor m. inferiorly.  A thickening of the fascia below the subclavius is called the costocoracoid ligament. The portion immediately inferior to the costocoracoid ligament is the costocoracoid membrane. The clavipectoral fascia ends inferiorly with the suspensory ligament of the axilla joining the fascial floor of the armpit.   The clavipectoral fascia is pierced by 4 structures:
    -the cephalic vein, returning to the axillary vein
    -the thoracoacromial artery (described in previous lecture)
    -the lymph vessels
    -the lateral pectoral nerve supplying the pectoralis major m.

III . The Brachial plexus (Netter 401).
 

The 5 roots (ventral rami) of the brachial plexus (C5 to T1) forms trunks (3), then divisions  (3 anterior and 3 posterior), then cords (3). The brachial plexus has 5 terminal branches and numerous other nerves. One must understand that the term "root" when applied to the brachial plexus refers to the ventral rami of spinal nerves C5 to T1. It must not be confused with the spinal roots of the spinal cord (Netter 156).  The brachial plexus provides motor, cutaneous and autonomic innervation to the upper extremity. Please study carefully below the organization of the brachial plexus (Netter 401):
 
3.1 Trunks: the ventral rami of C5 and C6  form the superior trunk of the plexus. The ventral rami of C7  alone forms the middle trunk of the plexus. The ventral rami of C8 and T1 unite to form the inferior trunk.  The 3 trunks run downward and laterally, under the protection of the clavicle.
 
3.2 Divisions: just posterior and superior to the clavicle, each of the three trunks divides into a posterior and anterior division.
 
3.3 Cords: the anterior divisions of the superior and middle trunks form the lateral cord of the plexus. The anterior division of the inferior trunk alone forms the medial cord.  The 3 posterior divisions together form the posterior cord of the plexus.
 
3.4 Terminal branches: each cord splits into two terminal branches. From top to bottom, the lateral cord divides into the musculocutaneous nerve and the lateral portion of the median nerve. The posterior cord splits into the axillary and radial nerves. Finally, the medial cord provides the ulanr nerve and medial portion of the median nerve.

Note that:
  - the posterior divisions, forming the posterior cord, provide innervation to the extensor   muscles whereas the anterior divisions, forming the medial and lateral cords, supply the flexor muscles of the upper extremity.
  - in terms of relationship, the plexus exits the cervical region between the anterior scalene    muscle and the middle scalene mm. (Netter 25 and 27).

As already described, the roots and the trunks of the brachial plexus can be palpated in the lower portion of the posterior triangle. The divisions are found posterior to the medial 2/3 of the clavicle. (Netter 400).  The cords lie in the axilla posteriorly to the pectoralis minor mm. At that level, the cords are named according to their position relative to the second part of the axillary artery: the lateral cord is lateral to the artery, the medial cord is medial and so on. Finally, the terminal branches originate from the cords at the level of the inferior border of the pectoralis minor m.

 3.5 Other branches (Netter 401):
 
A. Supraclavicular branches:
1. from the roots:
         - the dorsal scapular nerve (Netter 401): arises from C5, pierces the medius scalenus and passes posteriorly to supply the levator scapulae           m. and the rhomboid major and minor mm
         - the long thoracic nerve (Netter 401): from C5 C6 C7,  also pierces the middle scalene m. and passes laterally downward to supply the serratus anterior m.
 2. from the upper trunk:
         -the nerve to the subclavius m. (Netter 401) : from C5 C6,  supplies the subclavius m.
         -the suprascapular nerve (Netter 397): from C5 C6,  passes laterally and deep to the trapezius m., supplies the supraspinatus  m. by entering the supraspinous fossa through the scapular notch, continues  to supply the infraspinatus m. in the infraspinous fossa
 
B. Infraclavicular branches
1. from the lateral cord:
         - the musculocutanous nerve (Netter 443):  from C5 C6 C7,  one of the two terminal branches of the lateral cord, pierces and supplies the coracobrachialis m, continues to supply the biceps brachi  and brachialis mm in the anterior compartment of the arm, continues to become the lateral cutaneous nerve of the forearm (see later)
         - the lateral root of the median nerve (Netter 401): from C5 C6 C7, is joined by the medial root to form the median nerve, has no branch in the axilla
         - the lateral pectoral nerve: from C5 C6 C7,  pierces the clavipectoral fascia and supplies the pectoralis major
 
C. from the medial cord:
         - the ulnar nerve: (Netter 401): from C7C8 T1,  one of the two terminal branches of the medial cord, provides motor innervation to the forearm and hand as well as cutaneous innervation to the hand (see later)
         - the medial root of the median nerve (Netter 400): from C8 T1,  crossed over the axillary artery to join the lateral root to form the median nerve, has no branch in the axilla
         - the medial pectoral nerve (Netter 400): from C8 T1,  pierces and supplies the pectoralis minor, continues to supply the pectoralis major m.
         - the medial cutaneous nerve of the forearm (Netter 404):  from C8 T1,  cutaneous nerve only, passes down the arm, supplies a small portion on the lower anterior aspect of the forearm, supplies the skin on the anterior and medial aspect of the forearm (Netter 450)
     - the medial cutaneous nerve of the arm (Netter 404):  from T1,  receives the intercostobrachial nerve (lateral cutaneous branch of the second intercostal nerve), supplies the skin on the medial and distal aspect of the arm (Netter 450)
 
D. from the posterior cord:
  - the axillary nerve (Netter 446): from C5C6, passes posteriorly under the shoulder joint, through the quadrangular space, supplies the shoulder joint, the deltoid and teres minor mm, and the skin of the shoulder (superior lateral cutaneous nerve of the arm) (Netter 450)
  - the upper subscapular nerve (Netter 401): from C5C6, passes downward to supply the upper part of the subscapularis m.
  - the lower subscapular nerve (Netter 401): from C5C6, passes downward to supply the lower part of the subscapularis  and teres major mm.
  - the thoracodorsal nerve (Netter 401): from C6C7C8, passes down laterally to supply the latissimus dorsi m.
  - the radial nerve (Netter 401, 446 - 447): from C5C6C7C8 T1, passes posteriorly to the axilla and brachial arteries, supplies innervation to the triceps of the arm and some muscles of the forearm (see later), provides also cutaneous innervation  to the posterior aspect of the arm, forearm and hand
 
E. Autonomic innervation:
    1. No parasympathetic innervation in the upper extremity
    2. The neurons of the sympathetic system providing innervation to the upper extremity starts in the intermediolateral cell columns of the upper thoracic segment of the spinal cord (T1 to T3 or T4) (Netter 154). The pre-ganglionic fibers synapse in the cervicothoracic (stellate) and middle cervical ganglia (Netter 124). The post-ganglionic fibers reach the nerves of the brachial plexus or the arteries of the upper extremity via the grey rami. The post-ganglionic fibers then reach and control the activities of the smooth muscles in the blood vessels walls, the arrectores pilorum muscles and the sweat glands of the upper extremity.

IV. Clinical Correlations.
 

 In the radiology CD, under the upper extremity section, under soft tissues, click on
 
 neurovascular bundle: read and study the MRI.
 

V.  Internet Sites.
 
 You will not be tested on the contents of the site as this is for illustration only. Please visit the following internet site created by Dr. Thomas Louis: The Brachial Plexus 98 at http://anatomy.med.ecu.edu/anat5k/books97/Bp98/index.htm
 
 You can also visit a site dedicated to mnemonics related to brachial plexus (and other anatomical structures) at http://uhsweb.edu/rb/mn.htm
 

VI.  Surface Anatomy.

 Please study the surface anatomy terms that are relevant to this lecture.
 

VII.  Computer Software
 
 Please study the laboratory 3, 4 and 5 in the human anatomy 2.2 software at http://edweb.library.musc.edu/haonline3/labs/upperex.htm