Subclavian nerve

The subclavian nerve, also known as the nerve to subclavius, is a small peripheral nerve that arises from the upper trunk of the brachial plexus. It typically receives contributions from the C5 and C6 spinal nerve roots. The subclavian nerve primarily innervates the subclavius muscle, a small muscle located beneath the clavicle (collarbone).

Location

The subclavian nerve originates from the upper trunk of the brachial plexus and travels anteriorly and inferiorly in the posterior triangle of the neck. It courses beneath the clavicle, running closely with the subclavian artery and vein. The nerve reaches its target, the subclavius muscle, which is positioned between the clavicle and the first rib, where it terminates by innervating the muscle. Occasionally, the nerve sends a branch to the sternoclavicular joint.

Anatomy

Origin

The subclavian nerve, also known as the nerve to subclavius, arises from the upper trunk of the brachial plexus, typically from the C5 and C6 spinal nerve roots. It originates in close proximity to other branches of the brachial plexus, such as the suprascapular nerve, at the junction where the C5 and C6 nerve roots merge to form the upper trunk.

Course

Proximal Course

• After its origin from the upper trunk of the brachial plexus, the subclavian nerve descends inferiorly and anteriorly within the posterior triangle of the neck. This region is bordered by the sternocleidomastoid, trapezius, and clavicle, and houses critical neurovascular structures.
• The nerve travels along the posterior surface of the clavicle, running parallel to the subclavian artery and subclavian vein, which are positioned nearby in the lower neck and upper thorax.

Passage Beneath the Clavicle

• The subclavian nerve passes deep to the clavicle, heading toward the subclavius muscle, which lies beneath the clavicle. Its close relationship with the subclavian vessels makes it part of the important neurovascular structures traversing this region.
• The nerve continues to descend through the region below the clavicle, running between the bone and the subclavius muscle, a small but significant muscle that connects the clavicle to the first rib.

Branching

While the subclavian nerve primarily innervates the subclavius muscle, it occasionally gives off small branches:

• Accessory Phrenic Nerve Branch: In some cases, the subclavian nerve sends a small branch, known as the accessory phrenic nerve, to the phrenic nerve. This branch contributes to the innervation of the diaphragm and runs parallel to the main phrenic nerve in the lower neck.
• Sternoclavicular Joint Branch: The nerve may send sensory branches to the sternoclavicular joint, providing innervation to this critical joint between the sternum and the clavicle, which allows for the movement of the clavicle during shoulder and arm movements.

Relations

• Subclavian Vessels: The subclavian nerve travels in close proximity to the subclavian artery and subclavian vein as they course under the clavicle and into the axillary region. These vessels are important for supplying blood to the upper limb, and the nerve’s position near them makes it relevant in surgeries or injuries involving this area.
• Clavicle and Subclavius Muscle: The subclavian nerve runs directly beneath the clavicle, positioned between the clavicle and the subclavius muscle. The subclavius muscle attaches from the first rib to the undersurface of the clavicle, and the nerve innervates this muscle by sending small branches into its fibers.

Termination

The subclavian nerve terminates after innervating the subclavius muscle. Its fibers are distributed within the muscle to control contraction, which helps stabilize the clavicle. The occasional branches to the sternoclavicular joint and accessory phrenic nerve (if present) represent additional points of termination for this nerve.

In some cases, variations in anatomy exist, such as the accessory branch to the phrenic nerve, which can provide additional innervation to the diaphragm. However, these variations do not typically affect the primary motor innervation of the subclavius muscle.

Anatomical Variations

• Accessory Phrenic Nerve: One of the most common anatomical variations of the subclavian nerve is the presence of the accessory phrenic nerve. In this variation, the subclavian nerve gives off a small branch that joins the phrenic nerve and aids in the innervation of the diaphragm.
• Branches to Sternoclavicular Joint: In some individuals, the subclavian nerve may send sensory branches to the sternoclavicular joint, which could play a role in joint stabilization and proprioception.

Function

The subclavian nerve, also known as the nerve to subclavius, serves primarily a motor function, innervating the subclavius muscle, which plays a crucial role in stabilizing the clavicle during shoulder movements. It may also contribute to the sensory innervation of the sternoclavicular joint and provide a small branch to the phrenic nerve, aiding in diaphragm control. Its functions are essential for maintaining proper shoulder mechanics and overall upper limb stability.

Motor Innervation of the Subclavius Muscle

The subclavian nerve’s primary role is to innervate the subclavius muscle, a small muscle located between the clavicle and the first rib.

Functions of the Subclavius Muscle

• Stabilization of the ClavicleThe subclavius muscle, under the control of the subclavian nerve, helps stabilize the clavicle (collarbone) during movements of the shoulder and arm. The muscle keeps the clavicle in place, particularly during overhead or forward arm movements. By securing the clavicle against the sternum and first rib, it minimizes excessive motion or displacement, especially during strong muscle contractions in the upper limb.
• Protection of Subclavian Vessels and NervesThe subclavius muscle also acts as a protective barrier for the subclavian artery and vein, as well as the brachial plexus that passes beneath the clavicle. By stabilizing the clavicle, the subclavius muscle helps shield these vital structures from compression or injury during clavicular motion, such as in overhead lifting or carrying heavy objects.
• Depression of the ClavicleThe subclavius muscle depresses the clavicle, pulling it downward and anteriorly. This action is important when lowering the shoulder, such as during certain arm movements or in tasks that require pulling or reaching downward.
• Absorbing Shock to the ClavicleThe subclavius muscle helps absorb and dissipate shock to the clavicle, such as in cases of blunt trauma or impacts to the shoulder region. This protective function ensures that force transmitted through the clavicle is distributed to the first rib, reducing the risk of injury to the bone or the surrounding neurovascular structures.

Sensory Innervation to the Sternoclavicular Joint

In some cases, the subclavian nerve may provide sensory fibers to the sternoclavicular joint, a joint connecting the clavicle to the sternum.

• Proprioception: Through its sensory fibers, the subclavian nerve helps provide proprioceptive feedback from the sternoclavicular joint. This feedback helps the body maintain awareness of the clavicle’s position during shoulder and arm movements.
• Joint Stability: By innervating the joint capsule, the nerve may contribute to the overall stability of the sternoclavicular joint, which is critical in allowing the clavicle to move freely during upper limb activities while staying properly aligned with the sternum.

Accessory Phrenic Nerve Function

In some anatomical variations, the subclavian nerve contributes to the phrenic nerve through the formation of an accessory phrenic nerve.

Assistance with Diaphragm Control: The small branch from the subclavian nerve that joins the phrenic nerve may provide additional motor fibers to the diaphragm, the primary muscle involved in breathing. While the primary function of the phrenic nerve originates from the C3, C4, and C5 spinal roots, the accessory branch from the subclavian nerve can provide additional input, particularly for motor control during deep breathing or respiratory effort.

Role in Shoulder Movements and Stability

By innervating the subclavius muscle, the subclavian nerve plays an important role in the overall stability of the shoulder girdle:

• Clavicular Stability During Arm Movements: The subclavius muscle helps hold the clavicle in place during a wide range of shoulder and arm movements, such as lifting, throwing, or pushing. Without proper stabilization of the clavicle, the shoulder joint can become less efficient, leading to poor movement mechanics and potential injury.
• Preventing Clavicular Displacement: The subclavius muscle’s ability to depress and stabilize the clavicle prevents excessive upward or forward displacement of the clavicle, especially during strong arm movements like pulling or lifting overhead. The subclavian nerve ensures the subclavius muscle is activated during these movements to keep the clavicle in its proper position.

Clinical Significance

The subclavian nerve, also known as the nerve to subclavius, plays an important role in stabilizing the clavicle and protecting underlying structures, such as the subclavian vessels and the brachial plexus. Its clinical significance arises primarily in cases of trauma, surgical interventions, or anatomical variations that may affect its function.

Common Causes of Injury

• Clavicle Fractures: Injuries such as clavicular fractures or dislocations can impact the subclavian nerve, potentially leading to dysfunction of the subclavius muscle, resulting in compromised clavicle stability.
• Surgical Procedures: The subclavian nerve can be affected during surgeries involving the neck, shoulder, or clavicle, such as during shoulder reconstructions or procedures addressing thoracic outlet syndrome.

Clinical Manifestations

• Clavicular Instability: Injury to the subclavian nerve can lead to weakness or paralysis of the subclavius muscle, resulting in instability of the clavicle. This can affect shoulder mechanics and increase the risk of injury to the subclavian vessels or brachial plexus.
• Sternoclavicular Joint Dysfunction: In cases where the nerve provides sensory innervation to the sternoclavicular joint, damage may result in pain or instability in this joint.

Diagnostic and Treatment Considerations

In cases of injury to the subclavian nerve, diagnosis may involve imaging studies and clinical assessments. Treatment typically focuses on physical therapy to strengthen surrounding muscles or surgical intervention in cases where clavicular stability is compromised. Understanding the nerve’s anatomy is essential for preventing injury during surgical procedures and for managing shoulder and clavicle trauma.