Extremities Lab 4.1

Lab 4.1: Shoulder and Arm

Learning objectives:

Understand the flow of blood both from and to the upper extremity
Identify the roots, trunks, divisions, cords, and terminal branches of the brachial plexus
Identify the muscles and neurovasculature in the anterior and posterior compartments of the arm

Short List of Structures - 4.1

Before you Begin...

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.

Shoulder and Rotator Cuff Review

Rotator Cuff Musculature

Four of the scapulohumeral muscles—Supraspinatus, Infraspinatus, Teres minor, and Subscapularis (also referred to as SITS muscles)—are called the rotator cuff muscles because they form a musculotendinous rotator cuff around the glenohumeral joint. All except the supraspinatus are rotators of the humerus. The supraspinatus, besides being part of the rotator cuff, initiates and assists the deltoid in the first 15 degrees of abduction of the arm.

The tendons of the SITS or rotator cuff muscles blend with the joint capsule of the glenohumeral joint, reinforcing it as the musculotendinous rotator cuff, which protects the joint and gives it stability. Tonic contraction of these muscles holds the relatively large head of the humerus firmly against the small and shallow glenoid cavity during arm movements. You can carefully reflect the deltoid m. to visualize the attachments of the rotator cuff musculature to the humerus.

Shoulder and Back Musculature

The teres major is a thick rounded muscle that lies on the inferolateral third of the scapula. It adducts and medially rotates the arm, but along with the deltoid and rotator cuff muscles, it is an important stabilizer of the humeral head in the glenoid cavity during movement.

The deltoid is a thick powerful muscle forming the rounded contour of the shoulder. It courses from the spine of the scapula, acromion, and lateral one third of the clavicle to the deltoid tuberosity of the humerus. The muscle is divided into anterior, middle and posterior parts that can act separately or as a whole. When all three parts contract simultaneously, the shoulder joint is abducted.

The serratus anterior muscle overlies the lateral part of the thorax and forms the medial wall of the axilla. This broad sheet of thick muscle was given its name because of the sawtooth (serrated) appearance of its fleshy slips or digitations.

Trapezius m. - identify its attachments to the base of the skull, nuchal ligament, spines of the C7 - T12 vertebrae, spine of the scapula, acromial process of the scapula, and the lateral part of the clavicle.

- Observe the superior, medial, and inferior divisions (their muscle fibers course in different directions).

Latissimus dorsi m. - identify its attachments to the vertebrae from T7 to the sacrum via the thoracolumbar fascia (the muscle inserts laterally into the humerus).

Thoracolumbar fascia - a thick layer of deep fascia that is the fused aponeuroses of several mm. It attaches to the spinous processes from the T7 vertebra to the sacrum.

Rhomboid major and minor mm. - observe muscle attachments along the spines of C7-T5 vertebrae and the medial margin of the scapula; the two muscles may be fused and appear as one

Levator scapulae m. - observe its attachments on the superior angle of the scapula and transverse processes of C1 -C4 vertebrae

Skin Removal and Superficial Veins

Per usual, the first task at hand as we begin our dissection of the upper extremity is to remove the skin. The image to the right shows a red line, today's dissection will focus on the area proximal to that line.

As you begin you will encounter cutaneous nerves and superficial veins.

These 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 adipose of the superficial fascia, just deep to the skin, and they can be very difficult to find in most cadavers. Do not spend much time trying to find them if they dont stand out, you may likely need to remove them before moving on.

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 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. 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.

The Axilla

Dissection of the limbs will take place in a proximal to distal direction. You are expected to complete full dissections of both limbs before the end of the block.

The root (most proximal part) of the limb that is a fairly tight spot, so you may find it more economical 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 a large portion of the clavicle yet, you can 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

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. 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, although keep in mind that it is possible they were accidentally cut when your cadavers thorax was sectioned.

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.

Smaller branches that arise from the plexus

We’ve already mentioned some of the smaller nerves that branch off the brachial plexus; the medial and lateral pectoral nn., and the medial cutaneous n. of the arm and forearm. All three of those “medial” nerves arise from the medial cord.

All the way back at the roots of the plexus you’ll find the dorsal scapular n arising from the C5 ventral ramus. We saw this nerve in the back, it innervates the rhomboids and the levator scapulae mm.

The ventral rami of C5, C6, and C7 all contribute to the long thoracic n. The long thoracic n. was seen during dissection of the lateral thoracic wall, it innervates the serratus anterior m.

The subclavian n. is a small branch from the superior trunk that innervates the subclavius m. Since the muscle is usually taken off with the clavicle, chances of seeing this nerve are slim.

Also arising from the superior trunk is the suprascapular n. This is another nerve we saw during dissection of the back, it innervates the supraspinatus m. and infraspinatus m.

3 small nerves emerge from the posterior cord. The upper subscapular n. and the lower subscapular n. both innervate the subscapularis m., and the lower subscapular n. also innervates the teres major m. The thoracodorsal n. runs along the deep surface of the latissimus dorsi m. and innervates it.

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. and it continues down the arm to form the Brachial 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 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 deep brachial a. (profunda brachii a.) is a branch from the brachial a. that passes posterior to the humerus and runs with the radial n. in the radial groove to supply blood to the posterior compartment of the arm.

Figure 1.9 Branches of the 3 parts of the axillary artery.

Anterior Compartment of the Arm

There are three muscles in the anterior compartment of the arm that flex the elbow and shoulder. The biceps brachii is the most superficial. The short head of the biceps is the more medial and attaches to the coracoid process of the scapula. The long head is the more lateral and appears shorter, because the tendon of the long head disappears as it passes through the glenohumeral joint capsule via the intertubercular groove to attach to the supraglenoid tubercle of the scapula. The two heads of the biceps come together and insert by a single distal tendon onto the radius and the superficial fascia of the forearm by the bicipital aponeurosis. Because this muscle crosses the shoulder, it can aid in flexing the glenohumeral joint. However, its main function is as a powerful flexor of the elbow.

The coracobrachialis is an elongated muscle in the superomedial part of the arm. It attaches proximally to the coracoid process of the scapula and distally to the middle third of the humerus. This muscle’s sole function is flexion of the glenohumeral joint.

The brachialis is a flattened fusiform muscle that lies deep to the biceps brachii. It is the workhorse of elbow flexion, flexing the elbow in all positions, and during slow and quick movements. It attaches to the humerus just inferior to the coracobrachialis muscle, crosses the elbow joint, and inserts into the ulna.

Posterior Compartment 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 (= profunda brachii a.) 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. This muscle lies deep the lateral and long heads and can be quite difficult to see.

Innervation of the Upper Limb

Sensory Innervation

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.

Motor Innervation

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.

Clinical Correlations

Brachial Plexus Injury

Introduction – Brachial plexus injuries (BPI) can occur at any level of the plexus and can be functionally devastating. They are broadly classified regarding their mechanism of injury as either:

Traumatic - associated with high energy injuries such as a car crash or a fall.
Obstetric - complicated child birth (shoulder dystocia).

As concerns the severity of the injury, there are 3 types:

1. complete plexus injuries

2. upper plexus injuries (Erb’s palsy)

3. lower plexus injuries (Klumpke’s palsy)

Erb’s palsy (upper plexus injury) – caused by over-stretching of the C5 and C6 roots, as when the neck is laterally flexed or the shoulder is pushed inferiorly. For example (see figure to the right) a fall where you land on your shoulder, or if the head is pulled too forcefully when the shoulder is caught on the pubic symphysis (shoulder dystocia).

Klumpke’s palsy (lower plexus injury) – caused by overstretching the C8-T1 roots, as when the shoulder is forcibly abducted. For example (see figure to the right), if you were falling from a high place and grabbed something to stop yourself, or if the arm is pulled too forcefully during delivery.

Effects of the lesions:

Upper lesion (Erb’s palsy): Primarily affects the following nerves:

Axillary n. (C5 and C6) - weakness in the deltoid m. (abduction at the shoulder) and the teres minor m. (external rotation at the shoulder)

Suprascapular n. (C5 and C6) - weakness in the supraspinatus m. (abduction at the shoulder) and infraspinatus m. (external rotation at the shoulder)

Musculocutaneous n. (C5-C7) - weakness in biceps m. (flexion at the shoulder and elbow, supination of the forearm), coracobrachialis m. (flexion at the shoulder) and brachialis m. (flexion at the elbow)

Radial n. (C5-T1) is not as strongly affected , but there is variable weakness in the brachioradialis, supinator, and the extensors of the elbow, wrist, and digits

Lower lesion (Klumpke’s palsy): Primarily affects the following nerves:

Median n. (C5-T1) - weakness in anterior compartment of the forearm, which leads to wrist extension, weakness in the lumbricals 1 and 2, which leads to extension of MCP joints and flexion of IP joints.

Ulnar n. (C8-T1) – Weakness in lumbricals 3 and 4.

Taken together, lower lesions cause "claw hand" = extension at the wrist, extension at the MCP joint, and flexion at the IP joints.

Humeral Fracture

Introduction – Fractures involving the humerus can occur in the proximal, middle, and distal portions of the bone, with each fracture location representing a different potential nerve palsy syndrome:

Proximal humeral fracture: The location of the axillary nerve near the surgical neck of the humerus makes deltoid muscle paralysis most common in proximal humerus fractures. Patients may present with a “flat” deltoid and weak arm abduction. Also at risk are the anterior and posterior circumflex humeral arteries.

Mid-shaft humeral fracture: The radial nerve is closely applied to the posterior surface of the humerus, as is the deep brachial artery. Damage to the radial nerve at this point would lead to weakness or lack of extension at the wrist and in the digits

Distal humeral fracture: The median and ulnar nerves are both in danger in a distal humeral fracture. Damage to these nerves would lead to weakness in flexion at the wrist and digits.

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