Shoulder and Arm

The upper and lower limbs (or extremities) are the parts of our bodies we use to interact with the world around us.  Lower extremities are mainly used to support our weight against the force of gravity, while upper extremities are mainly used to manipulate objects in our environment.  

Technically, the “arm” is not the entire upper limb, but is the region of the upper limb between the shoulder and elbow.  Similarly, “leg” is not the entire lower limb, but is the region of the lower limb between the knee and ankle.

The basic design and layout of the extremities is mostly linear and fairly straightforward; bony struts at the core, surrounded by muscles that move those bones, shot through with blood vessels to bring nutrients to the muscles, and nerves that cause the muscles to contract and bring feeling from the overlying skin.

Per usual, the first task at hand as we begin our dissection of the upper extremity is to remove the skin. 

 The superficial veins of the extremities are clinically important in that they are sites of venous access for taking blood samples, as well as providing intravenous access for introducing fluids into the bloodstream, and for access to deeper structures in the body (as with cardiac catheterization).  They are embedded in the fat of the superfical fascia, just deep to the skin.

The two veins to look for are the basilic v. and the cephalic v.  In the arm the basilic v. is medial, while the cephalic v. is more lateral.  

The basilic v. originates down around the pinky, and runs up along the medial aspect of the forearm and arm, eventually piercing the deep fascia of the arm and joining with the deeper brachial v. to form the axillary v.

The cephalic v. originates near the thumb, and runs along the lateral aspect of the forearm and arm, eventually passing between the deltoid and pectoral muscles in the deltopectoral triangle before piercing the deep fascia of the shoulder to join with the axillary v.

The cephalic v. originates near the thumb, and runs along the lateral aspect of the forearm and arm, eventually passing between the deltoid and pectoral muscles in the deltopectoral triangle before piercing the deep fascia of the shoulder to join with the axillary v.  

The cephalic and basilic vv. communicate anterior to the elbow through the median cubital v., in a region called the antecubital fossa.

You will also see multiple cutaneous nerves in the arm.  The regions that these cutaneous nn. innervate are important to know, because a sensory deficit in a given region of the skin can suggest a nerve lesion.  But do not spend time trying to identify any of the cutaneous nn. you come across.  Just appreciate them and move on.  We’ll consider the general pattern of cutaneous n. innervation of the upper limb later in the dissection.

Finally, you’ll notice that deep to the superficial fascia of the arm and forearm there is a strong deep fascia that envelops the muscles.  You’ll need to reflect this deep fascia to identify the muscles, nerves and deep blood vessels in the upper extremity.

Dissection of the limbs typically takes place in a proximal to distal direction, but your group should begin by focusing on the region associated with the clinical correlation you have been assigned for your student prosection presentation. You are expected to complete full dissections of both limbs before the end of the block.

If you do start at the root of the limb that is a fairly tight spot, so you may find it more economical of time to have one person work on the root of the limb while the other starts at the elbow and works their way proximally.

Within the region of the shoulder from the base of the neck to the shoulder you’ll find the roots, trunks, divisions, and cords of the brachial plexus, and mixed in with those nervous system structures you’ll also find the subclavian a. and axillary a., and their accompanying veins.  There are many structures in this region, and they extend out into the armpit, and of course they are covered in connective tissues and fat.  Therefore you need to spend some time in this area, scissor-spreading through the connective tissue structures and removing the fat to clarify the dissection field, while also taking care not to destroy nerves and arteries.

If you have not removed the clavicle yet, you should do so now.  

You will quickly find that the subclavian v. is large and is just right in your way, so after you identify it and appreciate its position and utility as a means of gaining venous access, feel free to transect the vein and reflect the ends medially and laterally to clear the dissection field.  

Brachial Plexus

You will need to spend some time removing connective tissue and adipose from the subclavian and axillary regions to visualize the brachial plexus in all its glory.  Take care to safeguard the subclavian and axillary aa. and their branches, we’ll come back to them after we work through the main parts of the plexus.  DO NOT USE A SCAPEL TO DIG AROUND IN THE AXILLA, YOU WILL CHOP THINGS INTO SMALL PIECES.  Here we start at the roots and work our way laterally.

Roots: Identify the roots of the brachial plexus as they emerge from between the anterior scalene m. and middle scalene m.  Reflect the anterior scalene m. to get a better view of the roots of the brachial plexus.  Ultimately you should be able to identify the 5 ventral rami of spinal nerves C5, C6, C7, C8, and T1 as they exit the intervertebral foramina.

Trunks: Follow the roots laterally, and you’ll see that the upper and lower pairs come together to form 2 of the 3 trunks of the brachial plexus.  The superior trunk is composed of the C5 and C6 ventral rami, and the inferior trunk is composed of the C8 and T1 ventral rami.  The C7 ventral ramus forms the middle trunk by itself.

Divisions: Each of the 3 trunks splits to form 2 divisions, an anterior division and a posterior division, for a total of 6 divisions (3 anterior and 3 posterior).    

Cords: The 6 divisions then form 3 cords; a lateral cord, a medial cord, and a posterior cord.  The cords are named according to their relationship to the axillary a., which runs right through the middle of all this mayhem.  The three posterior divisions all join together to form the posterior cord, which is the largest of the 3 cords.  You’ll have to move the axillary a. around to see the posterior cord, because it is posterior to the axillary a. (hence the name).  The anterior division of the inferior trunk forms the medial cord all by itself.  The anterior divisions of the superior and middle trunks join together to form the lateral cord.  

Branching from the medial and lateral cords are the medial and lateral pectoral nn.  Recall that, as concerns their postions, the medial pectoral n. is actually lateral, and the lateral pectoral n. is actually medial.  That’s because they are named according to the cord they branch from (medial pectoral n. from medial cord...), not their relative positions in the pectoral region.

The medial cord also gives off 2 general sensory (cutaneous) nerves, the medial brachial cutaneous n. and the medial antebrachial cutaneous n.  These are also known, respectively, as the medial cutaneous n. of the arm and the medial cutaneous nerve of the forearm.  We’ll come back to cutaneous innervation of the upper limb.

Terminal branches: Each of the 3 cords splits, and the products of those splits form the terminal branches of the brachial plexus, which are the large named nerves that innervate the upper limb.  The posterior cord splits to from the axillary n. and the radial n.  Both the medial cord and the lateral cord split to form one and one-half nerves.  The medial cord forms the ulnar n. and half of the median n.  The lateral cord forms the musculocutaneous n. and the other half of the median n.  All of the 5 terminal branches of the brachial plexus contain somatic motor, general sensory, and visceral motor fibers.

A good way to begin to orient yourself to the brachial plexus is to find the “M” that is formed by the musculocutaneous, median, and ulnar nerves.  Some look much more like M’s than others.

The axillary artery and its branches:

As you dissected the brachial plexus you had to work your way around the axillary a. and its branches.  The axillary a. is the continuation of the subclavian a.  The subclavian a. changes its name to axillary a. (armpit a.) at the lateral margin of the first rib.  During dissection of the thoracic wall we struggled with the thoracoacromial a. which is the second branch of the axillary a.

The lateral thoracic a. is the next branch off the axillary a.  It follows the lower border of the pectoralis minor m. and supplies blood to the serratus anterior and pectoralis major mm.  In females it gives off an external mammary a. which supplies the breasts.

Next is the largest branch of the axillary a., the subscapular a.  It follows the lower border of the subscapularis m., and gives off a large circumflex scapular a. that branches from the subscapular a. at a right angle and curves around to the posterior aspect of the scapula.  After the circumflex scapular a. branches off, the remaining part of the subscapular a. is called the thoracodorsal a.

The last two branches from the axillary a. form a ring around the superior humerus, the posterior circumflex humeral a., and the anterior circumflex humeral a.  The posterior a. is larger, and travels with the axillary n. through the quadrangular space of the arm.

The triceps brachii m., a single muscle with three heads, is the only muscle of the posterior compartment of the arm.  The posterior compartment is the extensor compartment; the muscles there extend the shoulder and the elbow joints.  

If not already done, reflect the posterior deltoid from the spine of the scapula and the acromion to get a clear view of the 3 heads of the triceps brachii.

The two superficial heads are the lateral head and the long head.  The lateral head is the lateral one.  Between the long and lateral heads of the triceps, and between the teres major and minor, is the quadrangular space, which the axillary n. and the posterior circumflex humeral a. pass through to get to the deep surface of the deltoid.

Separate the long and lateral heads and carefully dissect down toward the humerus.  Running along the posterior surface of the humerus, in the radial groove, are the deep a. of the arm and the radial n.  The deep artery supplies blood to the posterior compartment of the arm, and the radial n. innervates all three heads of the triceps brachii m.

The medial head of the triceps is deep to the lateral head, and originates from the middle third of the humerus.  Widen the separation between the lateral and long heads and look for the medial head.

Sensory innervation of the shoulder and arm:

All 5 of the terminal branches of the brachial plexus have both motor and sensory fibers, and some smaller branches off the brachial plexus are purely sensory nerves (such as the medial cutaneous nerve of the arm).  

It’s important to have an understanding of the sensory territories in the upper limb because a sensory deficit in a given area can help narrow down the potential site(s) of nerve lesions to certain regions of the brachial plexus, or to specific spinal nerves.  The same is true for the motor parts of the nerves.  For example, a deficit in flexion at the elbow indicates a problem with the musculocutaneous n., or the parts of the brachial plexus that give rise to the musculocutaneous n.  

For the sensory territories of the 5 terminal branches of the brachial plexus we only hold you responsible for knowing the territory of each of the 5 branches.  You do not have to know the specific names of the cutaneous branches.  For example, The terminal (cutaneous) part of the axillary n. is called the “superior lateral cutaneous n. of the arm”, but you don’t need to know that name, you just need to know that region is innervated by the cutaneous branch of the axillary n.

Concentrate first on the cutaneous nerves that innervate the arm, and save the forearm and hand for later.

In the shoulder and arm these are the cutaneous nerve territories you should know:

• The superior part of the shoulder has cutaneous innervation from the supraclavicular nn., which are cutaneous branches of the cervical plexus.  If you’ve already dissected the neck you’ve heard of these.

• The lateral aspect of the shoulder has cutaneous innervation from the axillary n.

• A region of the axilla, extending down onto the posterior arm, has cutaneous innervation from the intercostobrachial n., which is part of the second intercostal n.  This is the nerve responsible for referred pain into the arm during a heart attack.

• Branches of the radial n. provide sensory innervation to the lateral and medial aspects of the posterior arm.

• The medial, anterior part of the arm has sensory innervation from the medial cutaneous n. of the arm.

Motor innervation of the shoulder and arm:

Three of the five terminal branches of the brachial plexus supply motor innervation to the shoulder and arm.

The axillary n. is the motor n. to the deltoid m.  Due to its broad attachment to the spine and acromion of the scapula, and to the clavicle, the deltoid m. can cause flexion, extension, and abduction at the shoulder joint.

The musculocutaneous n. is the motor n. to the anterior compartment of the arm, so it innervates muscles that cause flexion, both flexion of the shoulder, and flexion at the elbow.

The radial n. is the motor n. to the posterior compartment of the arm, so it innervates muscles that cause extension, both extension of the shoulder, and extension at the elbow.

The remaining two terminal branches of the brachial plexus, the median n. and ulnar n., have neither sensory nor motor innervation in the arm.  We’ll return to them in the forearm and hand, where they do their work.