Review Question - QID 106360

The lateral aspect of the pectoralis major is supplied by the medial pectoral nerve, a branch of the brachial plexus.

The pectoralis major muscle is innervated by branches of the brachial plexus. The medial and lateral pectoral nerves are named for their origin points in the brachial plexus, not for the respective portions of the pectoralis major muscle that they supply. The lateral pectoral nerve, from the lateral cord, crosses the axillary artery and vein and pierces the deep surface of the muscle, supplying the medial and superior portions of the pectorals major. The medial pectoral nerve, from the medial cord, enters the deep surface of the pectoralis minor and then passes around the lateral pectoralis major muscle, where it is most vulnerable to division during mastectomy.

Netter illustrations of the brachial plexus demonstrate the lateral pectoral nerve composed of contributors from C5-7 branching from the lateral cord. The medial pectoral nerve (with contributors from C8, T1) branches from the medial cord of the brachial plexus.

Illustration A demonstrates the medial cord of the brachial plexus branching off to form the medial pectoral nerve, which innervates the lateral aspect of the pectoralis major muscle. The lateral cord of the brachial plexus branches off to form the lateral pectoral nerve, which innervates the medial aspect of the pectoral major muscle.
Illustration B shows the complete brachial plexus anatomy, including the origins of the medial and lateral pectoral nerves from the medial and lateral cords, respectively.

Incorrect Answers:
Answer 1: The long thoracic nerve comes off the lateral and posterior cords of the brachial plexus and supplies the serratus anterior. Damage to this nerve results in a winged scapula.
Answer 2: The intercostobrachial nerve supplies the skin of the axilla and medial arm. It is often transected or transiently injured during axillary dissection.
Answer 4: The lateral pectoral nerve originates from the lateral cord of the brachial plexus and innervates the medial and superior aspects of the pectoralis major muscle.
Answer 5: The intercostal nerves supply the breast; and the fourth intercostal nerve supplies nipple and areola sensation.