The axillary nerve is a peripheral nerve of the upper limb that plays a crucial role in shoulder function. It is one of the terminal branches of the brachial plexus, specifically originating from the posterior cord.
Structure of Axillary nerve
- Type: The axillary nerve is a mixed nerve, containing both motor and sensory fibers.
- Origin: It originates from the posterior cord of the brachial plexus, primarily from the C5 and C6 nerve roots.
- Branches: The axillary nerve usually has two main terminal branches—the anterior and posterior branches, which innervate different parts of the deltoid muscle. It also gives off a sensory branch known as the superior lateral cutaneous nerve of the arm.
Location
- Origin and Course: The axillary nerve originates in the axillary region, near the axillary artery, from the posterior cord of the brachial plexus.
- Quadrangular Space: It passes through the quadrangular space (or quadrilateral space), a space bounded by the teres minor, teres major, long head of the triceps, and the humerus, along with the posterior circumflex humeral artery.
- Around the Shoulder Joint: After passing through the quadrangular space, the axillary nerve wraps around the surgical neck of the humerus, supplying the deltoid and teres minor muscles as well as providing sensory innervation to the skin over the lower deltoid region.
- Innervation Points:
- Motor: It primarily innervates the deltoid muscle, responsible for shoulder abduction, and the teres minor, which is involved in external rotation of the shoulder.
- Sensory: It provides sensation via the superior lateral cutaneous nerve of the arm, which covers the skin over the lower half of the deltoid muscle.
Axillary Nerve Functions
The axillary nerve has important motor and sensory functions that are crucial for the operation of the shoulder joint and the upper arm.
Motor Functions
- Deltoid Muscle: One of the primary roles of the axillary nerve is to innervate the deltoid muscle, which is responsible for shoulder abduction. The deltoid muscle has three parts—anterolateral, middle, and posterior—which work together to abduct, flex, and extend the shoulder, respectively.
- Teres Minor: In addition to the deltoid, the axillary nerve also innervates the teres minor muscle. The teres minor assists with external (lateral) rotation of the humerus at the shoulder joint, making it critical for actions like reaching backward.
Sensory Functions
Superior Lateral Cutaneous Nerve of the Arm: The axillary nerve provides sensory innervation to the skin over the lower half of the deltoid muscle through this branch. This sensory function plays a role in proprioception, helping you sense the position of your shoulder in space.
Integrated Functions
- Shoulder Mobility: Given its role in shoulder abduction and external rotation, the axillary nerve is integral to many activities involving the upper limb, from lifting and pulling to throwing a ball.
- Sensory Awareness: The sensory fibers of the axillary nerve provide essential feedback about the shoulder’s position, helping in the coordination of complex upper limb movements.
- Muscle Tone: By supplying two important muscles of the shoulder joint, the axillary nerve plays a role in maintaining muscle tone, which is crucial for joint stability.
Clinical significance
The axillary nerve is of great clinical significance due to its essential roles in both the motor and sensory functions of the shoulder. Here’s a brief overview of its clinical importance:
Diagnostic Importance
- Deltoid Weakness: One of the hallmark signs of axillary nerve injury is weakness or paralysis of the deltoid muscle. This manifests as difficulty in abducting the arm, which can be assessed through clinical examination.
- Sensory Deficits: Loss or alteration of sensation over the lower half of the deltoid muscle can serve as a diagnostic clue for axillary nerve dysfunction.
- Shoulder Dislocations: Axillary nerve damage is a common concern in anterior shoulder dislocations and humeral neck fractures. Quick diagnosis is essential to prevent permanent nerve damage.
Clinical Conditions
- Shoulder Surgery: The axillary nerve is at risk during surgical procedures involving the shoulder, such as rotator cuff repairs or shoulder arthroplasty.
- Fractures: Fractures involving the humeral neck can potentially damage the axillary nerve as it wraps around this area.
- Neuropathies: Conditions like brachial plexus injuries or peripheral neuropathies may also involve the axillary nerve, affecting both sensory and motor functions.
Therapeutic Implications
- Physical Therapy: Rehabilitation often involves physical therapy to strengthen the deltoid and teres minor muscles, thereby improving shoulder function.
- Nerve Monitoring: During shoulder surgeries, intraoperative nerve monitoring can be used to minimize the risk of axillary nerve damage.
- Surgical Repair: In severe cases where the nerve is transected, surgical repair or nerve grafting might be necessary.
