All of the following muscles of the pectoral girdle are innervated by branches from the brachial plexus: except:
**Core Concept**
The pectoral girdle muscles are primarily involved in movements of the shoulder joint and are innervated by nerves originating from the brachial plexus, with the exception of one muscle. The **brachial plexus** is a complex network of nerves that arises from the spinal cord in the neck.
**Why the Correct Answer is Right**
The correct answer is based on the understanding that the **serratus anterior** muscle, which is part of the pectoral girdle, is actually innervated by the **long thoracic nerve**, which arises directly from the spinal roots (C5-C7) and not from the brachial plexus itself. This unique innervation pattern sets it apart from other muscles in the region.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice would be incorrect because the muscle in question is indeed innervated by a branch of the brachial plexus.
**Option B:** Similarly, this option is incorrect as it also receives innervation from the brachial plexus.
**Option D:** This choice is incorrect because, like options A and B, it is innervated by a branch of the brachial plexus.
**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the **long thoracic nerve** innervates the **serratus anterior**, which plays a key role in forward elevation of the arm and in preventing winging of the scapula. Damage to this nerve can lead to noticeable disability.
**Correct Answer:** D. Serratus Anterior