Final answer:
Injury to the axillary nerve can lead to motor deficits in the deltoid and teres minor muscles, affecting shoulder abduction and external rotation.
Step-by-step explanation:
The axillary nerve is a systemic nerve of the arm that arises from the brachial plexus. It innervates the deltoid muscle of the shoulder and the teres minor muscle of the rotator cuff. When this nerve is injured, it can lead to motor deficits in these muscles.
Motor deficits associated with injury to the axillary nerve may include weakness or paralysis of shoulder abduction (lifting the arm away from the body) and external rotation (rotating the arm outward).
For example, if a person falls onto an outstretched hand and injures their shoulder, it can compress or stretch the axillary nerve, leading to damage and subsequent motor deficits. These deficits can result in difficulty lifting the arm or rotating it outward.