Final answer:
The phrenic nerve is important for diaphragmatic control and is at risk during an interscalene block, while the accessory nerve controls the sternocleidomastoid and trapezius muscles, affecting head and shoulder movements.
Step-by-step explanation:
The phrenic nerve and the accessory nerve have distinct roles in the motor innervation of the neck and are related to the brachial plexus in the context of an interscalene block. The accessory nerve primarily innervates the sternocleidomastoid and trapezius muscles, facilitating movements such as head flexion, extension, and rotation. It receives motor input from cervical spinal nerves that also contribute to movements of the trunk and shoulders. In contrast, the phrenic nerve arises from the cervical plexus at levels C3 to C5 and is crucial for the motor innervation of the diaphragm, essential for breathing. For an interscalene block, which is aimed at anesthetizing the brachial plexus, it is important to understand the proximity of the phrenic nerve to avoid unintentionally impairing diaphragmatic function during procedures targeting the brachial plexus for upper limb surgery or pain relief. Both nerves are associated with cervical spinal nerves, but their function and implications during an interscalene block differ significantly.