(5.32)

Now let's look at the nerves. Between about here and here, the five spinal nerves unite, and divide, unite again, and divide again. The tangle which this produces is called the brachial plexus. It's not really too formidable. At the end of the brachial plexus the four main nerves of the arm emerge: the musculo-cutaneous, the median, the ulnar, and the radial. In the course of the brachial plexus, the nerves that supply the shoulder region are given off. We'll look at the main components of the brachial plexus first, then at the local branches.

Here's the brachial plexus, with several of its small branches removed so we can see the big picture. We'll also remove pectoralis minor. Here are the five roots of the brachial plexus: they are in fact the ventral rami of their respective spinal nerves. They emerge, as we've seen, from between the anterior scalene and middle scalene muscles.

The top two roots join, and the bottom two join, and the middle one, C7, stays alone. These three big units are called the three trunks: upper, middle and lower. Each trunk divides (here's one of them dividing) into an anterior and a posterior division.

Of the three anterior divisions, the upper two unite, and the lower one stays alone. The three posterior divisions all unite, as we'll see in a minute. Once that's all happened, there are again three big units, now called cords: lateral, medial and posterior. They surround the axillary artery.

The lateral cord divides, to become the musculocutaneous nerve, and one half of the median nerve. The medial cord divides, to become the ulnar nerve, and the other half of the median nerve. This arrangement produces an M-shaped pattern of nerves, musculocutaneous, median, and ulnar.

Now let's see the posterior cord. We need to remove the medial cord, the lateral cord, and the artery, to get a good look at it. Here's the posterior cord all by itself. Sometimes it starts dividing before all three of its posterior divisions have united. Its principal branches are the axillary nerve, which we'll see again, and the radial nerve.

Now that we've looked at the main components of the brachial plexus, let's look at the nerves which supply the muscles of the shoulder region. Some of these arise from the cords of the brachial plexus. Some arise in other ways. Let's look at the ones that arise from the cords first. We were looking at a simplified dissection before. Now we'll see the details.

The medial cord gives rise to one local nerve, the lateral cord to two. The one from the medial cord is the medial pectoral nerve. It's one of a pair. Here's its partner, the lateral pectoral nerve, which arises from the lateral cord. The pectoral nerves supply pectoralis major, and pectoralis minor.

Also arising from the lateral cord is the musculocutaneous nerve. It supplies three upper arm muscles, one of which we've seen: coracobrachialis. The other two we'll see in the next section.

The posterior cord (here it is again with all its branches intact) has four branches. The axillary nerve runs round the neck of the humerus, along with the posterior circumflex humeral artery, to supply the deltoid muscle, and also teres minor.

The subscapular nerves, an upper and a lower, supply subscapularis, and teres major. The thoracodorsal nerve supplies latissimus dorsi.

Now let's see the shoulder muscle nerves which don't arise from the cords of the brachial plexus. Of these, one is the branch of a trunk, two arise from the roots of the brachial plexus, and two aren't part of the plexus at all.

Arising from the upper trunk is the suprascapular nerve, which supplies supraspinatus, and infraspinatus. Arising from the C5 root and passing through the middle scalene muscle is the dorsal scapular nerve. It supplies the rhomboid muscles.

Arising from the C5, 6 and 7 roots, the long thoracic nerve emerges through the medial scalene muscle, runs deep to all three trunks of the brachial plexus, and supplies serratus anterior.

Trapezius gets its nerve supply from the spinal accessory nerve. Lastly levator scapulae gets a private nerve supply from the nearby roots of C3, 4 and 5.