Which muscle is innervated by the C5 myotome?

The deltoid muscle is primarily innervated by the axillary nerve, which derives from the C5 nerve root of the brachial plexus. This muscle plays a critical role in shoulder abduction, particularly after the first 15 degrees of movement, and is essential for a variety of arm movements. The C5 myotome is specifically associated with muscle movement and strength testing of the shoulder and upper arm, highlighting the deltoid's importance in these functions.

Additionally, while the biceps brachii is also innervated by the musculocutaneous nerve (which has contributions from the C5, C6, and C7 nerves), its primary functional control pertains more to elbow flexion and forearm supination than to the specific movements controlled under the C5 myotome. The other muscles listed—rhomboids and serratus anterior—are innervated by different nerves (dorsal scapular nerve and long thoracic nerve, respectively) and are associated with different myotomes. Therefore, identifying the deltoid as the muscle innervated by the C5 myotome underscores its role in shoulder movement, which is a crucial aspect of upper extremity function.