College of Massage Therapists of British Columbia (CMTBC) Practice Exam

What is the motor innervation of the radial nerve?

1. Triceps brachii

2. Flexor carpi ulnaris

3. Serratus anterior

4. Biceps brachii

The correct answer is: Triceps brachii

The motor innervation of the radial nerve is primarily responsible for the extension of the elbow, and the triceps brachii is a key muscle involved in this action. The radial nerve arises from the posterior cord of the brachial plexus and supplies muscles located primarily in the posterior compartment of the arm and forearm. The triceps brachii, which originates from various points on the scapula and humerus, is innervated by the radial nerve and functions to extend the forearm at the elbow joint. This is a classic example of how the radial nerve's motor innervation facilitates movement involving the extension of the arm. In contrast, the other muscles listed have different innervations: the flexor carpi ulnaris is primarily innervated by the ulnar nerve, the serratus anterior is innervated by the long thoracic nerve, and the biceps brachii is innervated by the musculocutaneous nerve. This anatomical understanding reinforces the role of the radial nerve in motor function, particularly in extension, thereby confirming why the triceps brachii is the correct answer regarding the motor innervation of the radial nerve.