Final answer:
The union of the posterior divisions of all three trunks of the brachial plexus forms the Radial nerve, which innervates the posterior compartment of the arm and forearm and is associated with extension movements. The brachial plexus is essential for motor and sensory control of the upper limb.
Step-by-step explanation:
The student's question is regarding the anatomy of the brachial plexus, specifically about the union of the posterior divisions of all three trunks of the brachial plexus. The correct answer to this question is b) Radial nerve.
The brachial plexus is a network of nerves that supplies the arm, and it's formed by the ventral rami of the lower four cervical nerves and the first thoracic nerve (C5-C8, T1). The trunks of the brachial plexus divide into anterior and posterior divisions. Specifically, the union of the posterior divisions of all three trunks -- upper trunk (C5-C6), middle trunk (C7), and lower trunk (C8-T1) -- forms the radial nerve.
The radial nerve is responsible for innervating the posterior compartment of the arm and the forearm, and it is associated with extension movements at the elbow, wrist, and fingers. This nerve is one of the major nerves in the arm, alongside the ulnar, median, musculocutaneous, and axillary nerves, which arise from other parts of the brachial plexus.
The brachial plexus is crucial for motor and sensory control of the upper limb, and understanding its anatomy is important for health professionals. In clinical practice, injuries to the brachial plexus can result in various motor and sensory deficits, depending on which nerves are affected.