Brachial Plexus

(At any time you can rotate the image or add or subtract structures. Clicking the link will always bring you back to a starting point. Remember to use rollovers to see the labels for the various structures. Also, by moving the axial plane to any point in the image on the right, you can see the cross-section on the lower left. Any coloration in the right image will be shown in the cross-sections.)

Objectives

Exercise

Begin with the bones. Use <Ctrl-P> or <Apple-P> to toggle the plane in the dissection window to the on position. Grab the plane and move it to the middle of the blade of the scapula. Note in the cross-section how the blade lies in an oblique rather than a coronal plane. (Look for a thin structure posteriorly that is colored white.)

Add in the subclavian, axial and brachial arteries. Use rollovers to confirm that this is one vessel that changes names at key landmarks. Where does subclavian turn to axial? Right! at the first rib. You might want to rotate the image to 55 degrees to confirm the first rib, and not the scapula, is the transition point. To see a landmark for the change from axillary to brachial, add in the latissimus dorsi and teres major.

Now, let's begin to build the brachial plexus. Start by adding in the spinal nerve roots. To deliver an interscalene nerve block, the anesthesiologist feels with the needle for the transverse process of C6. Use your mouse to identify C6. Notice the distance the anesthetic would have to travel to reach T1 and C8, which is why this block is not used for those nerves.

Now add in the trunks of the brachial plexus. Notice how they all come together as they cross the first rib. We'll remove the clavicle for a better view. By rotating to 65 degrees, we can see the anterior-posterior relationship of the artery and the brachial plexus at the first rib. The nerves and artery travel through a narrow, triangular space bounded by the first rib and the anterior and middle scalene muscles. Now rotate the image back to 0 degrees to review this relationship. To deliver a supraclavicular nerve block, the anesthesiologist feels for the pulse of the axillary artery at the first rib, inserts the needle posterior to the pulse and feels for the first rib. Here all the nerves of the plexus are close together and nearly the entire plexus can be anesthetized (some small branches of the trunks may not be blocked). Note how the lung is placed at risk of a pneumothorax by this procedure.

Now add in the posterior cord. Notice how it lies posterior to the axillary artery. Now add in the lateral cord and confirm it lies lateral to the axillary artery. The medial cord is not properly identified by the program. Let's infer its position by using one of its branches, the medial pectoral nerve. Only the very start of it is seen, "hanging loose" on the medial side of the artery, where the medial cord should be. Note that the lateral pectoral nerve lies "medial" to the medial nerve. That is because the nerves are named according to the cord they branch from and not their relative position on the chest wall.

Now for the terminal branches. Begin with the posterior cord. Add in the radial nerve and note its close relationship to the humerus, which it wraps around. Add in the axillary nerve. It's seen on the donor's left. The ulnar nerve comes from the medial cord. You can zoom out and rotate to see its distribution. Note its close relationship with the elbow. The musculocutaneous nerve comes from the lateral cord. The median nerve comes from the medial and lateral cords. Notice how it passes threw the carpal tunnel (white), but the ulnar nerve does not. Notice how a nerve block in the axilla that is directed toward the pulsing axillary artery, near the base of the axilla, would hit all of the terminal branches except the axillary nerve. Even though this is called an axillary nerve block, the axillary nerve is spared, because it has already separated from the others.

Summary

The brachial plexus coalesces about the axillary artery beginning at the first rib. At this point a nerve block would anesthetize almost the entire plexus, but the lung is at risk of injury. Proximal to this point the plexus is spread apart and an interscalene block would be suitable for shoulder procedures, but not for more distal procedures. In contrast, an axillary block would be suitable for hand or forearm procedures, but not for shoulder procedures.

The brachial plexus surrounds the axillary artery within an axillary sheath and the cords of the plexus are named according to their relationship to the artery.

Due to its close relationshipe with the bone, the radial nerve might be injured by a humeral fracture. Similarly, the ulnar and median nerves may suffer compression injuries at the elbow and wrist, respectively.

 

©2011 Lawrence Rizzolo, Yale School of Medicine