What Nerve Innervates Pectoralis Major?

The pectoralis major is innervated by two nerves: the lateral pectoral nerve and the medial pectoral nerve. Both arise from the brachial plexus, the network of nerves that emerges from the neck and supplies the upper limb. The naming can be confusing because “lateral” and “medial” refer to where each nerve branches off the brachial plexus, not to its position on the chest wall. Understanding which nerve does what matters far beyond anatomy class, since surgeons routinely need to identify, block, or deliberately cut these nerves during breast reconstruction, flap surgery, and nerve repair procedures.

Where the Two Pectoral Nerves Come From

The lateral pectoral nerve originates from the lateral cord of the brachial plexus. A cadaveric study found that in half the specimens, the lateral pectoral nerve carried nerve fibers from the C5, C6, and C7 spinal roots, and in the other half it carried fibers from C6 and C7 only.¹ Those roots exit the spine in the lower neck and upper back, which makes sense given that the clavicular head of the muscle performs movements like lifting the arm forward and across the body.

The medial pectoral nerve branches off the medial cord. The same study found that about three-quarters of specimens had fibers from C8 and T1, while roughly a quarter carried C8 fibers alone, and a small fraction carried only T1.¹ These lower roots correspond to the muscle’s lower portions, which are responsible for powerful adduction and internal rotation of the arm.

In a reported case variant, both the lateral and medial pectoral nerves arose from the supraclavicular part of the brachial plexus rather than the usual infraclavicular location. In that case the lateral pectoral nerve still supplied the pectoralis major, while the medial pectoral nerve supplied both the pectoralis major and pectoralis minor through two separate branches.² The takeaway is that while textbooks describe a standard origin, real anatomy can deviate.

How Each Nerve Reaches the Muscle

The lateral pectoral nerve takes a relatively straightforward path. After leaving the lateral cord, it passes through the clavipectoral triangle, the space between the pectoralis major and pectoralis minor muscles near the upper chest.³ From there, a separate branch innervates the clavicular portion and the upper sternocostal portion of the pectoralis major. These branches run along a layer of connective tissue on the muscle’s deep surface, superficial to where the blood vessels travel, and most of them end medial to the coracoid process of the shoulder blade.⁴

The medial pectoral nerve’s route is more complicated because it has to get past the pectoralis minor to reach the pectoralis major lying on top of it. In roughly 60 percent of people, the medial pectoral nerve pierces straight through the pectoralis minor, supplying branches to that muscle along the way, before continuing on to innervate the lower half to two-thirds of the pectoralis major.⁴ In the remaining cases, the nerve exits around the lower border of the pectoralis minor instead of piercing through it. One study refined this further, noting that when the nerve pierces the pectoralis minor, it enters as a single trunk in about three-quarters of specimens and as already-divided branches in the rest.⁵

Another study tied the path of the medial pectoral nerve to the physical size of the pectoralis minor. When the pectoralis minor had a narrower attachment to the ribs, the nerve tended to pierce through as a single trunk. When the attachment was wider, the nerve split into branches that passed through the muscle or curved around its lower edge.⁶ For a surgeon operating near this area, these variations change where the nerve can be found and how easily it can be preserved or injured.

Which Nerve Supplies Which Part of the Muscle

The pectoralis major is not one uniform slab. It has a clavicular head originating from the collarbone, a sternocostal head originating from the breastbone and ribs, and a small abdominal portion at its lowest extent. Broadly, the lateral pectoral nerve handles the clavicular head and the upper part of the sternocostal head, while the medial pectoral nerve handles the lower sternocostal fibers and the abdominal portion.

An anatomical study confirmed that a distinct nerve branch, arising from the lateral pectoral nerve, consistently innervated the clavicular part of the muscle separately from the rest.⁷ This separation matters because surgeons can harvest a flap from the lower part of the muscle while preserving the nerve to the clavicular portion, keeping the upper chest functional and maintaining a more normal appearance at the donor site.

Measurements from a cadaveric study focused on tendon transfer procedures found that the medial pectoral nerve pierced the pectoralis major an average of about 10 centimeters medial to the muscle’s attachment on the upper arm bone.⁸ That gives surgeons a predictable zone to look for the nerve during tendon transfers and other procedures around the shoulder.

The Ansa Pectoralis

The lateral and medial pectoral nerves do not operate in complete isolation. They are connected by a nerve loop called the ansa pectoralis. In cadaveric dissections, this loop was found in every specimen and was located at a consistent landmark: roughly the middle third of the distance between the sternoclavicular joint and the acromioclavicular joint, and lateral to the thoracoacromial artery.⁹

The ansa pectoralis allows some cross-communication between the two nerves, which may explain why damage to one nerve does not always produce the degree of weakness you might expect. The loop also creates a reliable surgical landmark. For procedures like selective neurotomy, where a surgeon deliberately cuts certain nerve branches to reduce unwanted muscle contraction, knowing exactly where the ansa pectoralis sits helps avoid damaging the wrong branches.

Common Anatomical Variations

Textbook anatomy describes the typical arrangement, but real bodies deviate frequently. A study of 80 specimens found that the standard pectoralis major structure appeared in only about 64 percent of cases.¹⁰ The most common variant was a distinctly separate clavicular portion. In a few specimens, the clavicular part of the pectoralis major was fused with the deltoid muscle, and in one of those cases, small branches of the lateral pectoral nerve actually crossed over to innervate part of the deltoid.¹⁰ One specimen had an underdeveloped clavicular portion entirely.

Variations also appear in the nerve entry points. Nerve branches may pierce the pectoralis minor anywhere from one to three separate points, or they may run along the muscle’s upper or lower border. The distances from bony landmarks varied widely across individuals, except for the branch running along the upper border of the pectoralis minor, which was reliably located at around 30 percent of the clavicular line measured from the acromioclavicular joint.¹¹

In rare congenital conditions, the lateral pectoral nerve may be entirely absent. A case report of bilateral asymmetric pectoralis major deficiency found that both lateral pectoral nerves were missing while the medial pectoral nerves were present, suggesting the muscle deficiency was a developmental failure rather than a consequence of disease.¹²

Nerve Blocks for Breast Surgery

One of the most direct clinical applications of pectoral nerve anatomy is the pectoral nerve block, commonly called the “Pecs block.” These ultrasound-guided injections place local anesthetic around the pectoral nerves to numb the chest wall. The Pecs I block targets the space between the pectoralis major and pectoralis minor, where the lateral pectoral nerve runs. The Pecs II block goes deeper, targeting the space between the pectoralis minor and the serratus anterior to catch additional nerves that supply the breast area.

A case report demonstrated that combined Pecs I and Pecs II blocks with sedation could serve as the primary anesthetic for breast-conserving cancer surgery, avoiding general anesthesia entirely.¹³ In a larger trial comparing Pecs blocks to standard care in breast augmentation patients, the block group used dramatically less pain medication in the first 24 hours, had lower pain scores at every time point measured, experienced less nausea and vomiting, and went home sooner.¹⁴ The block works precisely because the anatomy is predictable enough to target with ultrasound guidance, even though nerve positions vary somewhat between individuals.

Deliberate Denervation in Breast Reconstruction

When a breast implant is placed beneath the pectoralis major after mastectomy, the intact pectoral nerves can cause the muscle to contract involuntarily, creating a visible “animation deformity” where the reconstructed breast moves unnaturally whenever the patient flexes or uses her arm. Surgeons have explored selectively cutting branches of the pectoral nerves to prevent this.

A study comparing patients who received selective pectoral nerve denervation at the time of implant placement to those who did not found that the denervation group reported higher satisfaction with the appearance of the reconstructed breast, with a trend toward better psychosocial well-being as well.¹⁵ The anatomy study cited earlier, which mapped nerve entry points relative to the pectoralis minor and bony landmarks, was conducted specifically to guide this kind of targeted denervation.¹¹ The challenge is cutting enough nerve branches to stop the animation deformity without causing the muscle to waste away and thin out, which would compromise the soft-tissue coverage over the implant.

Preserving Nerves During Flap Surgery

The pectoralis major is one of the workhorses of reconstructive surgery. Surgeons use it as a pedicled flap, keeping its blood supply attached, to rebuild tissue in the head and neck after cancer removal. The standard technique sacrifices the nerve supply to the transferred muscle, which causes it to shrink over time. That atrophy can affect the final appearance of the reconstructed area.

A long-term study tracking patients who had pectoralis major flaps for jaw reconstruction found that preserving or repairing the medial pectoral nerve made a meaningful difference. Patients whose nerve was left intact or repaired with a nerve suture maintained better facial symmetry for up to five years after surgery, and the muscle showed slower degeneration. Even with nerve preservation, some fatty replacement and fiber loss occurred over time, reaching about 30 percent of the muscle’s cross-sectional area by seven years. But in patients whose nerve was cut, the same degree of degeneration happened within just three months.¹⁶

A different approach preserves function at the donor site rather than the recipient site. By understanding that the clavicular portion has its own distinct nerve supply from the lateral pectoral nerve, surgeons can harvest the lower sternocostal portion of the muscle as a flap while leaving the clavicular head and its nerve intact. The flap is tunneled to the head and neck through the deltopectoral groove, lateral to the blood vessel origin, keeping the upper chest looking and functioning close to normal.⁷

Pectoral Nerves as Donors for Nerve Transfers

When a traumatic injury damages the upper brachial plexus, patients can lose the ability to move the shoulder. One surgical strategy borrows a nerve from a less critical muscle to “rewire” a more critical one. The pectoral nerves, because they are accessible and have a favorable size and fiber count, have emerged as useful donor nerves for these transfers.

An anatomic feasibility study confirmed that the middle and inferior branches of the pectoral nerves have adequate length, diameter, and nerve fiber composition to serve as donors for the axillary nerve, which controls the deltoid and shoulder abduction, and potentially for the suprascapular nerve, which controls the rotator cuff muscles.¹⁷ In clinical practice, transferring the medial pectoral nerve to the axillary nerve has been shown to restore shoulder stability and some degree of shoulder abduction in patients with upper trunk brachial plexus injuries.¹⁸

The trade-off is partial loss of pectoralis major function on the side where the nerve is harvested. For most patients with a devastating shoulder injury, regaining the ability to lift and stabilize the arm is worth sacrificing some chest muscle strength, especially since the lateral pectoral nerve still supplies the upper portion of the muscle. The dual innervation of the pectoralis major is what makes these transfers possible: you can sacrifice one nerve’s contribution and retain partial function through the other.

Electrodiagnostic Testing of the Lateral Pectoral Nerve

When nerve damage to the pectoralis major is suspected, electrodiagnostic testing can help confirm the diagnosis. A study measuring the lateral pectoral nerve’s electrical properties in healthy subjects found an average motor conduction velocity of about 71 meters per second, with an average conduction time of 3.9 milliseconds from Erb’s point at the base of the neck to the muscle.¹⁹ Patients with neuropathy showed measurable differences, providing a diagnostic baseline. These measurements can help distinguish a pectoral nerve injury from a brachial plexus lesion or a cervical spine problem, which matters because the treatment approach differs for each.

In practice, pectoral nerve conduction studies are not commonly requested compared to tests on arm or hand nerves, partly because isolated pectoral nerve injuries are unusual outside of surgery or direct trauma. But for the patient who has unexplained weakness in the chest after an accident, a shoulder operation, or central line placement, having reference values for normal conduction makes a targeted diagnosis possible.