Final answer:
The thyrohyoid muscle is predominantly responsible for depression in muscular actions, like moving the scapula downwards during a shrug, while the teres major assists in adduction of the arm.
Step-by-step explanation:
The muscle responsible for primary depression in adduction is the thyrohyoid muscle. This muscle functions to depress the hyoid bone and elevate the larynx's thyroid cartilage, aiding in downward movement which is termed as depression. During activities like shrugging your shoulders, the scapula undergoes depression, and the muscle that plays a significant role in this action is the lower trapezius. Contrary to its role in adduction, where muscles like the teres major assist, depression specifically applies to the downward movement as seen with the scapula or mandible.