The axillary nerve is responsible for both movement and sensation in the shoulder region. It originates from the brachial plexus, receiving fibers from the fifth and sixth cervical spinal nerves (C5 and C6). It is one of the two terminal branches of the posterior cord of the brachial plexus. The nerve’s path through a confined space makes its anatomical location a major factor in its function and vulnerability to injury.
Tracing the Nerve’s Path
The axillary nerve begins its course deep within the armpit, or axilla, as a continuation of the posterior cord of the brachial plexus. From this origin, it travels downward, lying against the subscapularis muscle and passing near the axillary artery. The nerve then reaches a small, confined passageway known as the quadrangular space, which is an opening bounded by several muscles and the upper arm bone.
This quadrangular space serves as a gateway, allowing the nerve and the posterior circumflex humeral artery to move from the front of the shoulder to the back. The boundaries of this space are formed by the teres minor muscle above, the teres major muscle below, the long head of the triceps muscle to the inside, and the surgical neck of the humerus to the outside. This passage places the nerve in close association with the upper end of the humerus.
Once through the quadrangular space, the nerve wraps around the back of the surgical neck of the humerus. This close anatomical relationship means that a fracture of the humerus in this area can easily damage the nerve. After winding around the bone, the nerve divides into its final branches, which supply the muscles and skin of the shoulder.
Motor and Sensory Roles
The primary function of the axillary nerve involves controlling two specific shoulder muscles. The larger of the two muscles it controls is the deltoid, which forms the rounded cap of the shoulder. This innervation allows for the powerful action of shoulder abduction, which is the movement of lifting the arm away from the side of the body.
The nerve also provides motor control to the teres minor muscle, one of the rotator cuff muscles. The teres minor is important for external rotation of the shoulder joint, which helps to turn the arm outward. By supplying these two muscles, the axillary nerve ensures the shoulder can execute a wide range of motion and remain stable during movement.
The axillary nerve provides sensory feedback through the superior lateral cutaneous nerve of the arm. This branch supplies sensation to the skin over the lower, outer portion of the deltoid muscle. Loss of feeling in this specific patch of skin, often called the “regimental badge area,” is a classic sign of axillary nerve damage. The nerve also carries sensory fibers from the shoulder joint itself.
Why This Nerve is Vulnerable to Injury
The axillary nerve is particularly susceptible to trauma due to its tight anatomical path and close proximity to the humerus. The most common cause of injury is an anterior shoulder dislocation, where the head of the humerus is forcibly displaced forward and downward. This movement severely stretches the nerve as it wraps around the bone, causing damage ranging from a temporary block to a complete tear.
Another frequent mechanism of injury is a fracture of the surgical neck of the humerus. Since the nerve courses directly around this part of the bone, a break or displacement can compress or sever the nerve fibers. The nerve can also be damaged by external pressure, sometimes referred to as crutch palsy, which occurs if crutches are used improperly and press too forcefully into the armpit.
When the axillary nerve is injured, symptoms are directly related to its motor and sensory functions. Patients often experience significant weakness when attempting to lift their arm away from the body due to the loss of deltoid muscle function. This weakness is accompanied by a numb or tingling sensation over the lateral shoulder skin supplied by the nerve.